Patent Publication Number: US-2023155864-A1

Title: Systems and Methods for Enhancing Efficient Uplink MIMO Performance and Implementation for O-RAN-Based Radio Access Networks

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Pat. Application No. 63/244,774, filed on Sep. 16, 2021, U.S. Provisional Pat. Application No. 63/257,811, filed on Oct. 20, 2021, and U.S. Provisional Pat. Application No. 63/300,108, filed on Jan. 17, 2022, the entirety of each of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
     The present disclosure relates to systems and methods for operating the Radio Access Network (RAN) design for 4G- and 5G-based mobile networks, and relates more particularly to systems and methods to enable efficient uplink mMIMO performance for Open-RAN (O-RAN). 
     Traditionally, the radio access networks were built as an integrated unit where the entire RAN was processed. The RAN network traditionally uses application-specific hardware for processing, making them difficult to upgrade and evolve. As future networks evolve to have massive densification of networks to support increased capacity requirements, there is a growing need to reduce the CAPEX/OPEX costs of RAN deployment and make the solution scalable and easy to upgrade. 
     In the field of cloud-based Radio Access Networks (RAN), a significant portion of the RAN layer processing is performed at a central unit (CU) and a distributed unit (DU). Both CUs and DUs are also known as the baseband units (BBUs). CUs are usually located in the cloud on commercial off-the-shelf servers, while DUs can be distributed. Also, the RF and real-time functions can be processed in the remote radio unit (RU). 
     The context of the present disclosure is related to systems and methods to enable efficient uplink mMIMO performance for O-RAN-based RANs. 
     3GPP has defined multiple split options across the entire radio access network (RAN). There are various factors affecting the selection of the fronthaul split option, such as bandwidth, latency, implementation cost, virtualization benefits, complexity of the fronthaul interface, expansion flexibility, computing power, and memory requirement. 
     One of the most common splits that are standardized recently by the O-RAN alliance is split option 7-2x (Intra-PHY split). This split has multiple advantages such as simplicity, transport bandwidth scalability, beamforming support, interoperability, support for advanced receivers and inter-cell coordination, lower O-RU complexity, future proof-ness, and interface and functions symmetry. 
     One of the technologies that use the O-RAN 7-2x specifications is 4G/5G massive MIMO (mMIMO). In SU-MIMO, the gNB serves a single user on a set of time-frequency resources, whereas in MU-MIMO setup, the gNB serves multiple users on the same time-frequency resources. In UL, UEs send SRS signals over a relatively long period of time, which are sent to the O-DU from the O-RU via the fronthaul interface. Using the SRS signal, the O-DU then obtains the UL channel estimates of the UEs, using which gNB performs user pairing (in the case of MU-MIMO) and calculates UL combining/digital beamforming matrix for the scheduled user(s). The O-DU sends the combining/digital beamforming matrix elements or weights to the O-RU, which in return applies these elements to the frequency-domain PUSCH IQ samples received at a later time and sends the resulting IQ samples to the O-DU for the rest of PHY processing. One aspect of the combining operation (using the combining/digital beamforming matrix) at O-RU is that it enables the O-RU to reduce the number of streams transferred from O-RU to O-DU to a value smaller than the number of RX antennas. In mMIMO systems, the number of RX antennas is large, and transferring all received streams for PUSCH to O-DU incurs significant and prohibitive front-haul bandwidth consumption. 
     Although split option 7-2x enables multiple advanced features, such as beamforming and UL CoMP, the system performance may degrade in certain scenarios, such as UL mMIMO for high-speed UEs and/or interference-limited scenarios, where there is significant inter-cell interference (ICI) from neighboring cells contaminating the UL signal from the desired UEs. 
     Performance degradation in high mobility scenarios: The reason for such degradation in high-speed UEs scenarios is channel aging. Specifically, the SRS signals are sent over a relatively long period of time (e.g., 10 s of milliseconds). By the time the O-RU applies the combining/digital beamforming matrix elements to the PUSCH symbols, these matrix elements become inaccurate since the SRS signals are outdated (i.e., SRS to UL-data TTI delay is long). In other words, the channel gains between the UEs and the gNB at the time PUSCH is received are no longer reflected accurately by the SRS signals (since UEs are moving fast in such a scenario), which cause interference and hence throughput degradation during the combining process (using the combining/digital beamforming matrix) in the UL chain. 
     Performance degradation in the presence of inter-cell interference (ICI): The main reason for performance degradation in the presence of ICI is signal dimension reduction at O-RU. In particular, squeezing desired signal and interference signal into a smaller number of dimensions causes performance degradation of the interference rejection at O-DU. 
     One way to mitigate the performance degradation in high mobility scenarios is to configure SRS transmissions with shorter periodicities. However, shorter SRS periodicity may decrease the SRS capacity, i.e., the number of UEs that can be configured to transmit SRS, because more resources are occupied by each UE for SRS transmission. Also, in a time-division duplex (TDD) system, there are certain limitations on SRS periodicity due to the TDD frame structure adopted by the system. 
     Another approach to mitigate the performance degradation in high mobility scenarios is to predict the UL channel of the PUSCH/PUCCH slot at DU based on UL channels of a number of previous SRS transmissions. In addition to the computation complexity of the underlying prediction algorithm, this approach has its own limitations on performance improvement depending on UE speeds. 
     For scenarios with inter-cell interference, a typical O-DU processing uses a minimum mean-squared error receiver with interference rejection combining (MMSE-IRC) to mitigate the impact of the interference. However, since O-DU has access to the compressed PUSCH signal transferred from O-RU, the performance of MMSE-IRC may be degraded compared to a typical 7-3 receiver wherein MMSE-IRC is computed based on and applied to the full-dimension signal. 
     There are four planes specified in the O-RAN specs, namely user-plane (U-plane), control plane (C-plane), synchronization plane (S-plane), and management plane (M-plane). The existing O-RAN spec explains the exact method of sending the C-plane messages in the downlink (DL) direction and sending/receiving the U-plane messages between the O-DU and the O-RU. 
     M-plane: 
     The M-plane specifies the management plane protocols used over the fronthaul interface linking the O-RU (O-RAN Radio Unit) with other management plane entities, which can include the O-DU (O-RAN Distributed Unit), the O-RAN defined Service Management and Orchestration (SMO) functionality as well as other generic Network Management Systems (NMS). 
     In contrast to C-plane (described in the next section), Management Plane refers to non-real-time management operations between the O-DU and the O-RU. Specifically, The Lower-Layer Split M-plane (LLS-M) facilitates the initialization, configuration, and management of the O-RU to support the stated functional split 
     A NETCONF/YANG-based M-Plane is used for supporting the management features, including “startup” installation, software management, configuration management, performance management, fault management, and file management towards the O-RU. The M-Plane supports two architectural models: 
     1. Hierarchical model: The O-RU is managed entirely by one or more O-DU(s) using a NETCONF based M-Plane interface. When the O-RU is managed by multiple O-DUs, it is typically for enabling O-DU and/or transport connectivity redundancy capabilities.   2. Hybrid model: The hybrid architecture enables one or more direct logical interface(s) between management system(s) and O-RU in addition to a logical interface between O-DU and the O-RU. Based on the transport topology, various modes of network connectivity are possible between O-RU and O-DU and SMO. The basic requirement for M-Plane is to have end-to-end IP connectivity between the O-RU and the elements managing it (O-DU, SMO, or so-called “O-RU Controllers”).   

     The M-Plane provides the following major functionalities to the O-RU. These features are implemented using the NETCONF provided functions such as “Startup” installation, SW management, Configuration management, Performance management, Fault Management, and File Management. The data models representing the M-Plane are organized as a set of reusable YANG modules. 
     C-plane: 
     The purpose of the C-plane messages is to transmit data-associated control information required for the processing of user data (e.g., scheduling and beamforming commands). These messages are sent separately for DL-related commands and UL-related commands. 
     A common frame format is used for C-Plane messages, having a transport layer and an application layer. The application layer is within the transport layer payload and consists of a common header for time reference, followed by information and parameters dependent and specific to the Section Type in use. Multiple sets of section data of the same Section Type value can be lined up one after another within the payload. To minimize packet rate over the interface, a transmitter should fill messages with as many subsequent sections (with or without sequential section IDs) as possible. 
     Section Type 1 is used for most Downlink and Uplink radio channels. In accord with O-RAN specifications, the fields of Section type 1 can be explained as follows: 
     Transport Layer   Application Layer 
   Common Header Fields 
   dataDirection (data direction (gNB Tx/Rx)) field: 1 bit   payloadVersion (payload version) field: 3 bits 
   value = “1” shall be set (1st protocol version for payload and time reference format)   
   filterIndex (filter index) field: 4 bits   frameId (frame identifier) field: 8 bits   subframeId (subframe identifier) field: 4 bits   slotID (slot identifier) field: 6 bits   startSymbolid (start symbol id) field: 6 bits   numberOfsections (number of sections) field: 8 bits   sectionType (section type) field: 8 bits 
   value = “1” shall be set   
   udCompHdr (user data compression header) field: 8 bits 11   reserved (reserved for future use) field: 8 bits   
   Section Fields 
   sectionId (section identifier) field: 12 bits   rb (resource block identifier) field: 1 bit   symInc (symbol number increment command) field: 1 bit   startPrbc (starting PRB of data section description) field: 10 bits   numPrbc (number of contiguous PRBs per data section description) field: 8 bits   reMask (resource element mask) field: 12 bits   numSymbol (number of symbols) field: 4 bits   ef (extension flag) field: 1 bit   beamId (beam identifier) field: 15 bits   
   
   

       FIG.  1 A  illustrates the conventional UL functional split in O-RAN split 7-2x for Cat A O-RU.  FIG.  1 B  illustrates the processing blocks of UL processing in a conventional ORAN split 7-2x for Cat A O-RU. As illustrated in  FIG.  1 B , the operations in O-RAN split 7-2x for UL processing for Cat A O-RU are as follows: 
     O-RU receives the sounding reference signal (SRS) signal (s) from the UE(s) at slot n-M, where M is a positive integer.   O-RU applies ADC, CP removal, and FFT and transfers the frequency-domain (FD) IQ samples of the SRS over all the antennas to the O-DU using O-RAN U-plane message(s).   O-DU performs channel estimation on the IQ samples of SRS received from O-RU. Also, using the estimated channels, it performs user scheduling (and user pairing in the case of MU-MIMO) to indicate which UEs are scheduled to transmit PUSCH over the same time-frequency resources at a later time.   O-RU applies ADC, CP removal, and FFT and transfers the frequency-domain (FD) IQ samples of the received PUCCH/PUSCH over all the antennas to the O-DU using O-RAN U-plane message(s).   O-DU performs DM-RS channel estimation on the signal stream(s) of slot n received from O-RU. Subsequently, O-DU performs UL MIMO processing by performing channel equalization using the estimated channel (with optional inter-cell interference rejection) and demodulation of the equalized signal stream(s) and performs the rest of L1/L2 processing on the resulting signal stream(s).   

     One drawback of the O-RAN operation with Cat A O-RU is its large fronthaul throughput requirement, especially in massive MIMO scenarios. 
       FIG.  2 A  illustrates the conventional UL functional split in O-RAN split 7-2x for Cat B O-RU  FIG.  2 B  illustrates the conventional processing blocks of UL processing in ORAN split 7-2x for Cat B O-RU. As illustrated in  FIG.  2 B , the operations in O-RAN split 7-2x for UL processing for Cat B O-RU are as follows: 
     1) O-RU receives the sounding reference signal (SRS) signal(s) from the UE(s) at slot n-M, where M is a positive integer. 
   O-RU applies ADC, CP removal, and FFT and transfers the frequency-domain (FD) IQ samples of the SRS over all the antennas to the O-DU using O-RAN U-plane message(s).   O-DU performs channel estimation on the IQ samples of SRS received from O-RU. Also, using the estimated channels, it performs the following: 
   O-DU performs user scheduling (and user pairing in the case of MU-MIMO) to indicate which UEs are scheduled to transmit PUSCH over the same time-frequency resources at a later time.   O-DU calculates combining/digital beamforming matrix elements to be applied on the PUSCH received from UE(s) scheduled at a later time, i.e., at slot n.   
   O-DU transfers the combining/digital beamforming matrix elements calculated at Step 3b to O-RU sometime earlier than slot n.   O-RU applies ADC, CP removal, and FFT and then applies the combining/digital beamforming matrix elements on the received PUCCH/PUSCH signal of slot n and transfers the resulting layers to the O-DU.   O-DU performs DM-RS channel estimation on the signal stream(s) of slot n received from O-RU. Subsequently, O-DU performs UL MIMO processing by performing channel equalization using the estimated channel (with optional inter-cell interference rejection) and demodulation of the equalized signal stream(s) and performs the rest of L1/L2 processing on the resulting signal stream(s).   
   

     One drawback of O-RAN operation with Cat B O-RU is that the combining/digital beamforming matrix elements are calculated using SRS channel estimates (at O-DU) at a slot n-M which is earlier than the current slot n. Since the channel may change during the M slots between slot n-M and slot n, the combining/digital beamforming matrix may not properly match the UL channel of slot n. This can result in some performance degradation, particularly for UEs with higher mobility. Also, since O-DU operates on a combined version of the signal received from O-RU in slot n (not the full-dimension signal), the interference rejection performance in O-DU may be degraded. 
     SUMMARY 
     Implementations as described herein provide methods to enable an efficient mMIMO system for O-RAN based RANs. 
     Implementations as described herein address the aforementioned issues of performance degradation of the O-RAN 7-2x based UL MIMO. Aspects of the implementation include the following: 
     Asymmetrical functional split (using different functional splits in DL and UL): In implementations, the functional split point in the UL is changed so that further signal processing can be done at O-RU.In one embodiment, O-RU performs a first level of DM-RS channel estimation on PUSCH/PUCCH and uses the result of this channel estimation to calculate the combining/digital beamforming matrix, and after performing combining/digital beamforming on the received signal, transfers the resulting signal to O-DU for the rest of processing. In another embodiment, O-RU performs both DM-RS channel estimation and equalization on PUSCH/PUCCH and transfers the equalized signal to O-DU for the rest of the processing. In yet another embodiment, O-RU performs DM-RS channel estimation, equalization, and demodulation on PUSCH/PUCCH and transfers the resulting demodulated signal to O-DU for the rest of the processing. Described are multiple signaling (between O-DU and O-RU) and processing options for functional splits. 
     Signaling mechanisms to enable asymmetrical functional splits: Described are C-plane message modifications and M-plane enhancements to enable different split architectures. To enable different asymmetrical functional splits mentioned above and allow interoperability across different O-RU and O-DU vendors, several M-plane and C/U-plane enhancements are described. In one embodiment, the O-RU reports, via the M-plane, to the O-RU controller as part of its capability the supported split points and supported techniques (e.g., channel estimation techniques, MIMO detection techniques, etc.). In another embodiment, the O-RU controller statically configures, via the M-plane, the O-RU with the static parameters and static configurations based on the reported capabilities to operate using specific methods. In another embodiment, new sections and section extensions to the C-plane messages are proposed to carry, in real-time, the parameters and configurations at the O-RU to enable operation in an innovative split architecture. 
     Described is an Open Radio Access Network (0-RAN) system, comprising: 
     a baseband unit (BBU) having an O-RAN centralized unit (O-CU) and an O-RAN distributed unit (O-DU);   an O-RAN radio unit (O-RU) remote from the BBU; and   a fronthaul interface between the O-RU and the BBU;   wherein a functional split of 0-RAN functions respectively assigned to O-RU and O-DU for the fronthaul interface between the BBU and the O-RU is different for downlink (DL) and uplink (UL), and wherein at least one of i) demodulation reference signal (DM-RS)-based channel estimation is performed by the O-DU in the DL and by the O-RU in the UL, ii) equalization is performed by the O-DU in the DL and by the O-RU in the UL, and iii) demodulation is performed by the O-DU in the DL and by the O-RU in the UL.   

     In an embodiment where the O-DU transfer DM-RS information to the O-RU, the DM-RS information including DM-RS configuration parameters of the scheduled or paired UEs/layers are received by the O-RU, and the O-RU generates a DM-RS sequence using the DM-RS configuration parameters received from O-DU. In an implementation, the O-DU transfers DM-RS channel estimation method information to the O-RU to indicate a channel estimation method. In an implementation, the O-DU indicates a combining/digital beamforming matrix calculation method to the O-RU. In an implementation, the O-DU transfers a combining/digital beamforming matrix enhancement information to the O-RU. In an implementation, the O-DU indicates a combining/digital beamforming matrix calculation method to the O-RU. The O-RU estimates the UL channels of the UE(s) and/or layer(s) transmitted at a current slot n using the DM-RS information. In an implementation, the O-DU indicates a combining/digital beamforming matrix calculation method to the O-RU. In an implementation, the O-RU calculates the combining/digital beamforming matrix. In an implementation, the O-RU applies the calculated combining/digital beamforming matrix to a received signal and transfers the calculated combining/digital beamforming matrix to the O-DU. In an implementation, the O-DU performs a L1/L2 processing on the resulting signal. 
     In an embodiment where i) a demodulation reference signal (DM-RS)-based channel estimation is performed by the O-DU in the DL and by the O-RU in the UL, and ii) an equalization is performed by the O-DU in the DL and by the O-RU in the UL. In an implementation, where the O-DU transfers DM-RS information to the O-RU, the DM-RS information including DM-RS configuration parameters of the scheduled or paired UEs/layers are received by the O-RU, and the O-RU generates a DM-RS sequence using the DM-RS configuration parameters received from O-DU. In an implementation, the O-RU estimates the UL channels of the UE(s) and/or layer(s) transmitted at a current slot n using the DM-RS information. In an implementation, prior to the estimation the UL channels of the UE(s) and/or layer(s) transmitted at a current slot n using the DM-RS information, the O-DU indicates ta DM-RS channel estimation method information to the O-RU. In an implementation, the O-DU transfers DM-RS channel estimation method information to the O-RU to indicate the channel estimation method. In an implementation, the O-RU equalizes the UL channels of the one or more UEs, the one or more layers, or both transmitted at a current slot n. In an implementation, the O-RU calculates a supplementary demodulation information and transfers the supplementary demodulation information along with an equalized signal stream to the O-DU. In an implementation the O-DU performs a demodulation on a signal of slot n transferred from the O-RU and a L1/L2 processing on the demodulated signal. 
     In an embodiment, i) a demodulation reference signal (DM-RS)-based channel estimation is performed by the O-DU in the DL and by the O-RU in the UL, ii) an equalization is performed by the O-DU in the DL and by the O-RU in the UL, and iii) demodulation is performed by the O-DU in the DL and by the O-RU in the UL. In an implementation, the O-DU transfer DM-RS information to the O-RU, the DM-RS information including DM-RS configuration parameters of the scheduled or paired UEs/layers are received by the O-RU, and the O-RU generates a DM-RS sequence using the DM-RS configuration parameters received from O-DU. In an implementation, the the O-RU estimates the UL channels of the UE(s) and/or layer(s) transmitted at a current slot n using the DM-RS information. In an implementation, the, prior to the estimation the UL channels of the UE(s) and/or layer(s) transmitted at a current slot n using the DM-RS information, the O-DU indicates ta DM-RS channel estimation method information to the O-RU. In an implementation, the O-DU transfers DM-RS channel estimation method information to the O-RU to indicate the channel estimation method. In an implementation, the O-RU equalizes the UL channels of the one or more UEs, the one or more layers, or both, transmitted at a current slot n. In an implementation, prior to a slot n, the DU transfers modulation information to the RU and the O-RU performs demodulation on an equalized signal. In an implementation, the O-DU performs a descrambling operation on the equalized signal of slot n transferred from the O-RU and a L1/L2 processing on the descrambled signal. 
     In an embodiment, i) the demodulation reference signal (DM-RS)-based channel estimation is performed by the O-DU in the DL and by the O-RU in the UL, ii) the equalization is performed by the O-DU in the DL and by the O-RU in the UL, and iii) demodulation is performed by the O-DU in the DL and by the O-RU in the UL. 
     In an embodiment, described is a method of operating an Open Radio Access Network (O-RAN) system, comprising: 
     providing a baseband unit (BBU) having an 0-RAN centralized unit (O-CU) and an O-RAN distributed unit (O-DU);   providing an O-RAN radio unit (O-RU) remote from the BBU; and   providing a fronthaul interface between the O-RU and the BBU;   wherein a functional split of 0-RAN functions respectively assigned to O-RU and O-DU for the fronthaul interface between the BBU and the O-RU is different for downlink (DL) and uplink (UL), and wherein at least one of i) demodulation reference signal (DM-RS)-based channel estimation is performed by the O-DU in the DL and by the O-RU in the UL, ii) equalization is performed by the O-DU in the DL and by the O-RU in the UL, and iii) demodulation is performed by the O-DU in the DL and by the O-RU in the UL. In an implementation of the method, i) the demodulation reference signal (DM-RS)-based channel estimation is performed by the O-DU in the DL and by the O-RU in the UL, and ii) equalization is performed by the O-DU in the DL and by the O-RU in the UL. In an implementation of the method i) demodulation reference signal (DM-RS)-based channel estimation is performed by the O-DU in the DL and by the O-RU in the UL, ii) equalization is performed by the O-DU in the DL and by the O-RU in the UL, and iii) demodulation is performed by the O-DU in the DL and by the O-RU in the UL.   

     In an embodiment, a method of demodulation reference signal (DM-RS)-based channel estimation is performed by the O-RU in an OFDM system with N subcarriers, comprising at least one of:
     a Least-Squares (LS) Channel Estimation;   a Minimum Mean-Square Error (MMSE) Channel Estimation;   a Discrete Fourier Transform (DFT) based Channel Estimation; or   a Discrete Cosine Transform (DCT) based Channel Estimation, where a received signal in the frequency domain is expressed as           Y   =   XH   +   Z            where Y is a received signal vector, X is a diagonal matrix where each diagonal element represents a DM-RS signal in subcarrier k, H is a channel vector, and Z is a noise vector. In an implementation, the method further comprises:   after extracting DM-RS symbols of the received DM-RS signal in the frequency domain, the LS channel estimation is:             H     L   S             k   i     ,     l   j     ,     r   m     ,     t   n         =         Y     R   S             k   i     ,     l   j     ,     r   m     ,     t   n               X     D   M   R   S             k   i     ,     l   j     ,     r     m   ,         t   n                      where k i  represents a subcarrier index, l j  represents a symbol index, r m  is a receiver antenna index, and t n  is a DM-RS transmission port index; and and de-spreading the channel estimation as:                 H   ^     ′       L   S           D   M   R     S     g   ,         r     m   ,         t   n         =       ∑     i   ,   j   =   1     2             H     L   S             k   i     ,     l   j     ,     r   m     ,     t   n           c     i   j             t   n           L                  where c ij (t n ) represents a code in symbol i, the subcarrier j, and the DM-RS transmission port n.   

     In an implementation, the method further comprises: the MMSE channel estimation being calculated as follows: 
     
       
         
           
             
               
                 
                   
                     
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      where k i  represents a subcarrier index, l j  represents a symbol index, r m  is a receiver antenna index, and t n  is a DM-RS transmission port index. 
     In an implementation, the method further comprises: 
     the DFT channel estimation being calculated as follows: calculating the LS channel estimate (Ĥ LS ) or an MMSE channel estimate as an initial estimate;   converting the LS channel estimate or MMSE channel estimate to a time domain with an IDFT operation:               h   ^       L   S       D   F   T             n   i     ,     l   j     ,     r   m     ,     t   n         =   I   D   F   T             H   ^     ′       L   S             k   i     ,     l   j     ,     r   m     ,     t   n             ;   and           filtering noise outside maximum delay of the channel:               h   ^       f   i   l   t   e   r   e   d       D   F   T             n   i     ,     l   j     ,     r   m     ,     t   n         =                       h   ^       L   S       D   F   T                   0             n   i     ,     l   j     ,     r   m     ,     t   n                     o   t   h   e   r   w   i   s   e       0   ≤         n   i     ≤       w                    where w denotes the window length.   

     In an implementation, the method further comprises: 
     the DCT channel estimation being calculated as follows: calculating the LS channel estimate (H LS ) or the MMSE estimate as an initial estimate;   converting the LS or MMSE estimate to a transform domain with a DCT operation:               h   ^       L   S       D   C   T             n   i     ,     l   j     ,     r   m     ,     t   n         =   D   C   T             H   ^     ′       L   S             k   i     ,     l   j     ,     r   m     ,     t   n                     filtering noise outside maximum delay of the channel:               h   ^       f   i   l   t   e   r   e   d       D   C   T             n   i     ,     l   j     ,     r   m     ,     t   n         =                       h   ^       L   S       D   C   T                       0             n   i     ,     l   j     ,     r   m     ,     t   n                     o   t   h   e   r   w   i   s   e       0   ≤             n   i             ≤   w                    where w denotes the window length.   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  illustrates the UL functional split in O-RAN split 7-2x for Cat A O-RU. 
         FIG.  1 B  illustrates the processing blocks of UL processing in ORAN split 7-2x for Cat A O-RU. 
         FIG.  2 A  illustrates the UL functional split in 0-RAN split 7-2x for Cat B O-RU. 
         FIG.  2 B  illustrates the processing blocks of UL processing in ORAN split 7-2x for Cat B O-RU. 
         FIG.  3 A  illustrates an embodiment of a UL functional split of an O-RU Cat C1. 
         FIG.  3 B  illustrates an embodiment of a UL functional split of an O-RU Cat C2. 
         FIG.  3 C  illustrates an embodiment of a UL functional split of an O-RU Cat C3. 
         FIG.  4 A  is a flowchart of UL Split in O-RAN with Cat C1 O-RU. 
         FIG.  4 B  is a flowchart of UL Split in O-RAN with Cat C1 O-RU. 
         FIG.  4 C  is a flowchart of UL Split in O-RAN with Cat C1 O-RU. 
         FIG.  4 D  is a flowchart of UL Split in O-RAN with Cat C1 O-RU. 
         FIG.  5 A  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 1 to enable O-RU Cat C1 a signaling mechanism. 
         FIG.  5 B  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 1 to enable O-RU Cat C1 a signaling mechanism, 
         FIG.  5 C  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 1 to enable O-RU Cat C1 a signaling mechanism. 
         FIG.  5 D  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 1 to enable O-RU Cat C1 a signaling mechanism. 
         FIG.  5 E  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 1 to enable O-RU Cat C1 a signaling mechanism. 
         FIG.  5 F  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 1 to enable O-RU Cat C1 a signaling mechanism. 
         FIG.  5 G  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 1 to enable O-RU Cat C1 a signaling mechanism. 
         FIG.  5 H  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 1 to enable O-RU Cat C1 a signaling mechanism. 
         FIG.  6 A  is a flowchart of UL Split in O-RAN with Cat C2 O-RU. 
         FIG.  6 B  is a flowchart of UL Split in O-RAN with Cat C2 O-RU. 
         FIG.  6 C  is a flowchart of UL Split in O-RAN with Cat C2 O-RU. 
         FIG.  7 A  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 2 to enable O-RU Cat C2 a signaling mechanism. 
         FIG.  7 B  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 2 to enable O-RU Cat C2 a signaling mechanism. 
         FIG.  7 C  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 2 to enable O-RU Cat C2 a signaling mechanism. 
         FIG.  8 A  is a flowchart of UL Split in O-RAN with Cat C3 O-RU. 
         FIG.  8 B  is a flowchart of UL Split in O-RAN with Cat C3 O-RU. 
         FIG.  8 C  is a flowchart of UL Split in O-RAN with Cat C3 O-RU. 
         FIG.  9 A  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 3 to enable O-RU Cat C3 a signaling mechanism. 
         FIG.  9 B  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 3 to enable O-RU Cat C3 a signaling mechanism. 
         FIG.  9 C  illustrates an exemplary implementation of a signaling mechanism according to Embodiment 3 to enable O-RU Cat C3 a signaling mechanism. 
         FIG.  10 A  shows an exemplary 5G NR DM-RS structure. 
         FIG.  10 B  shows an exemplary 5G NR DM-RS structure. 
         FIG.  10 C  shows an exemplary 5G NR DM-RS structure. 
         FIG.  10 D  shows an exemplary 5G NR DM-RS structure. 
         FIG.  10 E  shows an exemplary 5G NR DM-RS structure. 
         FIG.  11 A  shows a channel estimation technique that can be applied at the O-RU. 
         FIG.  11 B  shows a channel estimation technique that can be applied at the O-RU. 
         FIG.  11 C  shows a channel estimation technique that can be applied at the O-RU. 
         FIG.  11 D  shows a channel estimation technique that can be applied at the O-RU. 
         FIG.  11 E  is a flowchart showing channel estimation techniques that can be applied at the O-RU described with respect ot  FIGS.  11 A- 11 D . 
     
    
    
     DETAILED DESCRIPTION OF THE IMPLEMENTATIONS 
     Implementations as described herein provide methods to enable an efficient mMIMO system for O-RAN based RANs. 
       FIGS.  3 A,  3 B, and  3 C  illustrate innovative embodiments of UL functional splits O-RU categories. Three different O-RU categories, namely, O-RU Cat C1, C2, and C3, are as follows: 
     O-RU Cat C1 ( FIG.  3 A ): 0-RU performs a first level of DM-RS channel estimation on PUSCH/PUCCH and uses the result of this channel estimation to calculate the combining/digital beamforming matrix, and after performing precoding/digital beamforming on the received signal, transfers the resulting signal to O-DU for the rest of processing.   O-RU Cat C2 ( FIG.  3 B ): O-RU performs both DM-RS channel estimation and equalization on PUSCH/PUCCH and transfers the equalized signal stream(s) along with the supplementary demodulation information to O-DU for the rest of the processing.   O-RU Cat C3 ( FIG.  3 C ): O-RU performs DM-RS channel estimation, equalization, and demodulation on PUSCH/PUCCH and transfers the resulting demodulated signal to O-DU for the rest of the processing.   

     In order to mitigate the issues of UL 7-2x, an asymmetrical functional split between DL and UL is proposed. The DL processing follows the O-RAN split 7-2x (either Cat A or Cat B) to balance the tradeoff between O-RU complexity and fronthaul throughput, where the UL processing follows a new split architecture. Three different UL functional splits corresponding to three different innovative O-RU categories are summarized in Table 1. 
     
       
         
          TABLE 1
           
               
               
               
             
               
                 A summary of possible options for DL and ULfunctional splits 
               
               
                 DL 
                 UL 
                 Brief Description 
               
             
            
               
                 Cat A 
                 Cat C1 
                 O-RU performs “DM-RS based Channel Estimation” in the UL. 
               
               
                 Cat A 
                 Cat C2 
                 O-RU performs “DM-RS based Channel Estimation” and “Equalization” in the UL. 
               
               
                 Cat A 
                 Cat C3 
                 O-RU performs “DM-RS based Channel Estimation”, “Equalization”, and “Demodulation” in the UL. 
               
               
                 Cat B 
                 Cat C1 
                 O-RU performs “DM-RS based Channel Estimation” in the UL. 
               
               
                 Cat B 
                 Cat C2 
                 O-RU performs “DM-RS based Channel Estimation” and “Equalization” in the UL. 
               
               
                 Cat B 
                 Cat C3 
                 O-RU performs “DM-RS based Channel Estimation”, “Equalization”, and “Demodulation” in the UL. 
               
            
           
         
       
     
     The following describes detailed processing steps for UL functional splits with O-RU Cat C1, O-RU Cat C2, O-RU Cat C3. 
     Embodiment 1: Processing Steps for UL Functional Split With O-RU Cat C1 
     Embodiment 1 describes the processing steps for UL functional split with O-RU Cat C1, in which O-RU performs a “DM-RS based Channel Estimation” in the UL.  FIGS.  4 A,  4 B,  4 C, and  4 D  are flowcharts of UL Split in O-RAN with Cat C1 O-RU. 
     As shown in  FIGS.  4 A,  4 B,  4 C and  4 D , in this embodiment, O-RU performs the following steps to process the received signal: 
     As shown in  FIG.  4 A , at  1 - 1 ), prior to slot n, O-DU transfers “DM-RS information” to O-RU so that O-RU can obtain the DM-RS signal and perform channel estimation. Some options for the “DM-RS information” are as follows: 
     Option  1 - 1 -A: “DM-RS configuration parameters” of the scheduled or paired UEs/layers. O-RU generates the “DM-RS sequence” using the “DM-RS configuration parameters” received from O-DU. The “DM-RS configuration parameters” are summarized in Table 4. Also, the details of the “DM-RS sequence” generation and mapping using the “DM-RS configuration parameters” are provided below under section Embodiment 6: 5G NR DM-RS. 
     Option  1 - 1 -B: “DM-RS sequence” of the scheduled or paired UEs/layers. 
     As shown in  FIG.  4 A , at  1 - 2  (Optional) Prior to slot n, O-DU transfers the “DM-RS channel estimation method information” to O-RU to indicate the “channel estimation method” and possibly “details of the indicated channel estimation method”. Exemplary channel estimation methods are listed below:
     Least-squares (LS) channel estimation   Minimum mean-squared error (MMSE) channel estimation   Discrete Fourier transform (DFT) based channel estimation   Discrete cosine transform (DCT) based channel estimation   

     The above channel estimation methods are described in more detail in Embodiment 7: Channel Estimation. The “details of the indicated channel estimation method” depends on the indicated channel estimation method and may include any of the following:
     RMS delay spread of the channel (τ RMS )   Maximum Doppler spread of the channel (Δf max )   Initial SINR estimation (SINR 0 )   MMSE matrix elements   Channel Estimation granularity in the frequency domain   Window type   Window size   

     Indication of the “channel estimation method” and/or “details of the indicated channel estimation method” can be done based on one of the following options: 
     Option  1 - 2 -A: O-DU indicates the “channel estimation method” to the O-RU using a particular bitfield in the signaling from O-DU to O-RU. O-RU determines the channel estimation details, including parameters and processing steps based on its own implementation. An example of a mapping between the bitfield content and corresponding channel estimation methods is provided in Table 5. 
     Option  1 - 2 -B: O-DU indicates “channel estimation method” and “details of the indicated channel estimation method” to the O-RU using particular bitfields in the signaling from O-DU to O-RU. O-RU uses a table, such as Table 5, to find the channel estimation method and uses other look-up tables, such as Table 6 to Table 9, to obtain the channel estimation details using these indications from O-DU. A summary of channel estimation methods and their corresponding look-up table(s) is provided in Table 10. 
     Option  1 - 2 -C: O-DU provides “partial channel estimation details” to O-RU, using which O-RU calculates full channel estimation details. In this option, the “DM-RS channel estimation method information” includes the channel estimation method and its partial details as listed in Table 11 to Table 14. O-RU calculates the channel estimation details, such as MMSE matrix, CE granularity in the frequency-domain, and window size, based on the channel estimation method and its received details from O-DU. The details of these channel estimation calculations can be found in Embodiment 7: Channel Estimation. 
     Option  1 - 2 -D: O-DU provides “full channel estimation details” to O-RU. In this option, the “DM-RS channel estimation method information” includes the channel estimation method and its full details as listed in Table 15. O-DU transfers the channel estimation details by either indicating bits in a bitfield for a particular parameter or directly transferring IQ/integer values of relevant parameters to O-RU. 
     As shown in  FIG.  4 B , at  1 - 3  (Optional) O-DU transfers “combining/digital beamforming matrix enhancement information” to O-RU prior to slot n. This information improves the quality of the combining/digital beamforming matrix when it is calculated by the O-RU. Some options for the “combining/digital beamforming matrix enhancement information” are as follows: 
     Option  1 - 3 -A: “Combining/digital beamforming matrix enhancement information” includes SRS channel estimates of the scheduled or paired UEs/layers. Later in Step  1 - 6 , O-RU combines SRS and DM-RS channel estimates using weighting coefficients. Options for the weighting coefficients of Option  1 - 3 -A: 
     Option  1 - 3 -A- 1 : O-RU either uses pre-configured weighting coefficients or determines the weighting coefficients based on other parameters. 
     Option  1 - 3 -A- 2 : O-DU provides the weighting coefficients to 0-RU as part of “combining/digital beamforming matrix enhancement information”. 
     Option  1 - 3 -B: “Combining/digital beamforming matrix enhancement information” includes preliminary combining/digital beamforming matrix elements corresponding to the scheduled or paired UEs/layers. This preliminary combining/digital beamforming matrix is calculated at O-DU prior to slot n using SRS channel estimates. Later in Step  1 - 6 , O-RU combines its own calculated combining/digital beamforming matrix elements with the preliminary combining/digital beamforming matrix using weighting coefficients. Options for the weighting coefficients of Option  1 - 3 -B: 
     Option  1 - 3 -B- 1 : O-RU either uses pre-configured weighting coefficients or determines the weighting coefficients based on other parameters. 
     Option  1 - 3 -B- 2 : O-DU provides the weighting coefficients to O-RU as part of “combining/digital beamforming matrix enhancement information”. 
     As shown in  FIG.  4 B , at  1 - 4  O-RU estimates the UL channels of the UE(s) and/or layer(s) transmitted at the current slot n using the “DM-RS information” obtained in Step  1 - 1 . Depending on whether the optional Step  1 - 2  is executed, one of the following options are taken by O-RU regarding the channel estimation method and its details: 
     Option  1 - 4 -A: (Step  1 - 2  is not executed): O-RU uses a pre-configured channel estimation method and its details. 
     Option  1 - 4 -B: (Step  1 - 2  is executed): O-RU uses “DM-RS channel estimation method information” provided by O-DU to obtain the channel estimation method and its details. 
     As shown in  FIG.  4 C , at  1 - 5  (Optional) O-DU indicates “combining/digital beamforming matrix calculation method” to O-RU. Some examples for the combining/digital beamforming matrix calculation method are as follows:
     Conjugate combining   Zero forcing   MMSE   MMSE-IRC   Null-space dimension increase based   DFT-compression   

     An example of a mapping between the bitfield contents and corresponding combining/digital beamforming matrix calculation methods is provided in Table 16. 
     As shown in  FIG.  4 C , at  1 - 6  O-RU calculates the “combining/digital beamforming matrix” either based on the calculation method provided by O-DU in Step  1 - 5  (if Step  1 - 5  is executed) or based on a pre-configured method (if Step  1 - 5  is not executed). Depending on whether the optional Step  1 - 3  is executed, one of the following options are taken by O-RU: 
     Option  1 - 6 -A (Step  1 - 3  is not executed): O-RU calculates the “combining/digital beamforming matrix” based on the estimated DM-RS channel(s). 
     Option  1 - 6 -B (Step  1 - 3  with Option  1 - 3 -A is executed): O-RU combines SRS channel estimates provided by O-DU with its own estimated DM-RS channel estimates using weighting coefficients. O-RU calculates the “combining/digital beamforming matrix” based on the combined DM-RS and SRS channel estimates. 
     Option  1 - 6 -C (Step  1 - 3  with Option  1 - 3 -B is executed): O-RU calculates the “combining/digital beamforming matrix” based on the estimated DM-RS channel(s), and using weighting coefficients, combines it with the SRS-based preliminary combining/digital beamforming matrix received from O-DU. 
     As shown in  FIG.  4 D , at  1 - 7 , O-RU applies the calculated “combining/digital beamforming matrix” to the received signal in slot n and transfers the resulting signal to O-DU through the fronthaul link. 
     As shown in  FIG.  4 D , at  1 - 8 , O-DU performs DM-RS channel estimation on the signal of slot n transferred from O-RU and subsequently performs UL MIMO processing by equalizing the channel using the estimated channel (with optional inter-cell interference rejection). Afterward, O-DU performs the rest of L1/L2 processing on the resulting signal. 
       FIGS.  5 A,  5 B,  5 C,  5 D,  5 E,  5 F,  5 G, and  5 H  illustrate different variations of Embodiment 1 to enable O-RU Cat C1 through different signaling mechanisms. 
     Depending on the type of information provided by O-DU to O-RU, different implementations are described, as illustrated in  FIGS.  5 A to  5 H  and summarized in table 2 
     
       
         
          TABLE 2
           
               
               
               
             
               
                 Implementations from embodiment 1 
               
               
                 Embodiment # 
                 Executed steps from Embodiment 1 
                 Block diagram 
               
             
            
               
                 1-A 
                 ( 1 - 1 ), ( 1 - 4 ), ( 1 - 6 ), ( 1 - 7 ), ( 1 - 8 ) 
                 
                   FIG.  5 A 
                 
               
               
                 1-B 
                 ( 1 - 1 ), ( 1 - 4 ), ( 1 - 5 ), ( 1 - 6 ), ( 1 - 7 ), ( 1 - 8 ) 
                 
                   FIG.  5 B 
                 
               
               
                 1-C 
                 ( 1 - 1 ), ( 1 - 2 ), ( 1 - 4 ), ( 1 - 6 ), ( 1 - 7 ), ( 1 - 8 ) 
                 
                   FIG.  5 C 
                 
               
               
                 1-D 
                 ( 1 - 1 ), ( 1 - 2 ), ( 1 - 4 ), ( 1 - 5 ), ( 1 - 6 ), ( 1 - 7 ), ( 1 - 8 ) 
                 
                   FIG.  5 D 
                 
               
               
                 1-E 
                 ( 1 - 1 ), ( 1 - 3 ), ( 1 - 4 ), ( 1 - 6 ), ( 1 - 7 ), ( 1 - 8 ) 
                 
                   FIG.  5 E 
                 
               
               
                 1-F 
                 ( 1 - 1 ), ( 1 - 3 ), ( 1 - 4 ), ( 1 - 5 ), ( 1 - 6 ), ( 1 - 7 ), ( 1 - 8 ) 
                 
                   FIG.  5 F 
                 
               
               
                 1-G 
                 ( 1 - 1 ), ( 1 - 2 ), ( 1 - 3 ), ( 1 - 4 ), ( 1 - 6 ), ( 1 - 7 ), ( 1 - 8 ) 
                 
                   FIG.  5 G 
                 
               
               
                 1-H 
                 ( 1 - 1 ), ( 1 - 2 ), ( 1 - 3 ), ( 1 - 4 ), ( 1 - 5 ), ( 1 - 6 ), ( 1 - 7 ), ( 1 - 8 ) 
                 
                   FIG.  5 H 
                 
               
            
           
         
       
     
     Embodiment 2: Processing Steps for UL Functional Split With O-RU Cat C2 
     Embodiment 2 describes the processing steps for UL functional split with O-RU Cat C2, in which O-RU performs the “DM-RS based Channel Estimation” and “Equalization” in the UL.  FIGS.  6 A,  6 B, and  6 C  present a flowchart of UL Split in O-RAN with Cat C2 O-RU. 
     As shown in  FIGS.  6 A,  6 B, and  6 C , in this embodiment, O-RU performs the following steps to process the received signal: 
     As shown in  FIG.  6 A , at  2 - 1  (Mandatory) Prior to slot n, O-DU transfers “DM-RS information” to O-RU so that O-RU can obtain the DM-RS signal and perform channel estimation. Some options for the “DM-RS information” are as follows: 
     Option  2 - 1 -A: “DM-RS configuration parameters” of the scheduled or paired UEs/layers. O-RU generates the “DM-RS sequence” using the “DM-RS configuration parameters” received from O-DU. These “DM-RS configuration parameters” are summarized in Table 4. Also, the details of the “DM-RS sequence” generation and mapping using the “DM-RS configuration parameters” are provided in Embodiment 6: 5G NR DM-RS. 
     Option  2 - 1 -B: “DM-RS sequence” of the scheduled or paired UEs/layers. 
     As shown in  FIG.  6 A , at  2 - 2  (Optional) prior to slot n, O-DU transfers the “DM-RS channel estimation method information” to O-RU to indicate the “channel estimation method” and possibly “details of the indicated channel estimation method”. Exemplary channel estimation methods are listed below:
     Least-squares (LS) channel estimation   Minimum mean-squared error (MMSE) channel estimation   Discrete Fourier transform (DFT) based channel estimation   Discrete cosine transform (DCT) based channel estimation   

     The details of the above channel estimation methods are described in Embodiment 7: Channel Estimation below. The “details of the indicated channel estimation method” depends on the indicated channel estimation method and may include any of the following:
     RMS delay spread of the channel (τ RMS )   Maximum Doppler spread of the channel (Δf max )   Initial SINR estimation (SINR 0 )   MMSE matrix elements   Channel Estimation granularity in the frequency domain   Window type   Window size   

     Indication of the “channel estimation method” and/or “details of the indicated channel estimation method” can be done based on one of the following options: 
     Option  2 - 2 -A: O-DU indicates the “channel estimation method” to the O-RU using a particular bitfield in the signaling from O-DU to O-RU. O-RU determines the channel estimation details, including parameters and processing steps based on its own implementation. An example of a mapping between the bitfield content and corresponding channel estimation methods is provided in Table 5. 
     Option  2 - 2 -B: O-DU indicates “channel estimation method” and “details of the indicated channel estimation method” to the O-RU using particular bitfields in the signaling from O-DU to O-RU. O-RU uses a table, such as Table 5, to find the channel estimation method and uses other look-up tables, such as Table 6 to Table 9, to obtain the channel estimation details using these indications from O-DU. A summary of channel estimation methods and their corresponding look-up table(s) is provided in Table 10. 
     Option  2 - 2 -C: O-DU provides “partial channel estimation details” to O-RU, using which O-RU calculates full channel estimation details. In this option, the “DM-RS channel estimation method information” includes the channel estimation method and its partial details as listed in Table 11 to Table 14. O-RU calculates the channel estimation details, such as MMSE matrix, CE granularity in the frequency-domain, and window size, based on the channel estimation method and its received details from O-DU. The details of these channel estimation calculations can be found in Embodiment 7: Channel Estimation. 
     Option  2 - 2 -D: O-DU provides “full channel estimation details” to O-RU. In this option, the “DM-RS channel estimation method information” includes the channel estimation method and its full details as listed in Table 15. O-DU transfers the channel estimation details by either indicating bits in a bitfield for a particular parameter or directly transferring IQ/integer values of relevant parameters to O-RU. 
     As shown in  FIG.  6 B , at  2 - 3  O-RU estimates the UL channels of the UE(s) and/or layer(s) transmitted at the current slot n using the “DM-RS information” obtained in Step  2 - 1 . Depending on whether the optional Step  2 - 2  is executed, one of the following options are taken by O-RU regarding the channel estimation method and its details: 
     Option  2 - 3 -A: (Step  2 - 2  is not executed): O-RU uses a pre-configured channel estimation method and its details. 
     Option  2 - 3 -B: (Step  1 - 2  is executed): O-RU uses “DM-RS channel estimation method information” provided by O-DU to obtain the channel estimation method and its details. 
     As shown in  FIG.  6 B , at  2 - 4  (Optional) prior to slot n, O-DU transfers the “channel equalization information” to O-RU to indicate the “channel equalization method” and possibly further details of the channel equalization procedure through the “additional channel equalization information”. Exemplary channel equalization methods are listed below:
     ZF   MMSE   MMSE-IRC   

     The details of the channel equalization are described in Embodiment 8: Channel Equalization below. The “additional channel equalization information” of the demodulation procedure during the log-likelihood ratio (LLR) calculation can be provided in several ways. Based on the indicated “additional channel equalization information”, which may be a part of the “channel equalization information”, the supplementary demodulation information is calculated. Exemplary “additional channel equalization information” options are listed below:
     Equalization Gain and Post-processed Noise plus Interference Power   Post-processed Noise plus Interference Power divided by Equalization Gain   Equalization Gain divided by Post-processed Noise plus Interference Power   Equalization Gain   Post-processed Noise plus Interference Power   Channel Estimates and Estimated DM-RS Noise plus Interference Power   

     Indication of the “channel equalization method” and “additional channel equalization information” can be done based on selecting one or both of the following options below: 
     Option  2 - 4 -A: O-DU indicates the “channel equalization method” to the O-RU using a particular bitfield in the signaling from O-DU to O-RU. An example of a mapping between the bitfield content and corresponding channel equalization methods is provided in Table 17. 
     Option  2 - 4 -B: O-DU indicates “additional channel equalization information” to the O-RU using particular bitfields in the signaling from O-DU to O-RU. Examples of mapping between the bitfield content and corresponding “additional channel equalization information”, which determines the “supplementary demodulation information” calculation method, are provided in Table 18 and Table 19. 
     As shown in  FIG.  6 B , at  2 - 5  (Mandatory) O-RU equalizes the UL channels of the UE(s) and/or layer(s) transmitted at the current slot n. Depending on whether the optional Step  2 - 4 -A is executed, one of the following options are taken by O-RU regarding the “channel equalization method information”: 
     Option  2 - 5 -A: (Step  2 - 4 -A is not executed): O-RU uses a pre-configured channel equalization method and its details. 
     Option  2 - 5 -B: (Step  2 - 4 -A is executed): O-RU uses “channel equalization method information” provided by O-DU to obtain the channel equalization method and its details. 
     As shown in  FIG.  6 C , at  2 - 6  O-RU calculates the “supplementary demodulation information” and transfers it along with the equalized signal stream(s) to O-DU through the fronthaul link. Depending on whether the optional  2 - 4 -B is executed, one of the following options are taken by O-RU regarding the “additional channel equalization information”: 
     Option  2 - 6 -A: (Step  2 - 4 -B is not executed): O-RU uses a pre-configured “supplementary demodulation information” calculation method. 
     Option  2 - 6 -B: (Step  2 - 4 -B is executed): O-RU uses the “supplementary demodulation information” calculation method provided by O-DU. 
     As shown in  FIG.  6 C , at  2 - 7  O-DU performs demodulation on the signal of slot n transferred from O-RU and subsequently performs the rest of L1/L2 processing on the resulting signal. 
       FIGS.  7 A,  7 B, and  7 C  illustrate different variations of Embodiment 2 to enable O-RU Cat C2 through different signaling mechanisms, as summarized Table 2. 
     
       
         
          TABLE 3
           
               
               
               
             
               
                 Summary of selected embodiments from embodiment 2 
               
               
                 Embodiment # 
                 Executed steps from Embodiment 2 
                 Block diagram 
               
             
            
               
                 2-A 
                 ( 2 - 1 ), ( 2 - 3 ), ( 2 - 5 ), ( 2 - 6 ), ( 2 - 7 ) 
                 
                   FIG.  7 A 
                 
               
               
                 2-B 
                 ( 2 - 1 ), ( 2 - 2 ), ( 2 - 3 ), ( 2 - 5 ), ( 2 - 6 ), ( 2 - 7 ) 
                 
                   FIG.  7 B 
                 
               
               
                 2-C 
                 ( 2 - 1 ), ( 2 - 2 ), ( 2 - 3 ), ( 2 - 4 ), ( 2 - 5 ), ( 2 - 6 ), ( 2 - 7 ) 
                 
                   FIG.  7 C 
                 
               
            
           
         
       
     
     Embodiment 3: Processing Steps for UL Functional Split With O-RU Cat C3 
     Embodiment 3 describes the processing steps for UL functional split with O-RU Cat C3, in which O-RU performs the “DM-RS based Channel Estimation”, “Equalization”, and “Demodulation” in the UL.  FIGS.  8 A,  8 B, and  8 C  are a flowchart of UL Split in O-RAN with Cat C3 O-RU. 
     In this embodiment, O-RU performs the following steps to process the received signal: 
     As shown in  FIG.  8 A , at  3 - 1 , prior to slot n, O-DU transfers “DM-RS information” to O-RU so that O-RU can obtain the DM-RS signal and perform channel estimation. Some options for the “DM-RS information” are as follows: 
     Option  3 - 1 -A: “DM-RS configuration parameters” of the scheduled or paired UEs/layers. O-RU generates the “DM-RS sequence” using the “DM-RS configuration parameters” received from O-DU. These “DM-RS configuration parameters” are summarized in Table 4. Also, the details of the “DM-RS sequence” generation and mapping using the “DM-RS configuration parameters” are provided in Embodiment 6: 5G NR DM-RS. 
     Option  3 - 1 -B: “DM-RS sequence” of the scheduled or paired UEs/layers. 
     As shown in  FIG.  8 A , at  3 - 2  (Optional) Prior to slot n, O-DU transfers the “DM-RS channel estimation method information” to O-RU to indicate the “channel estimation method” and possibly “details of the indicated channel estimation method”. Exemplary channel estimation methods are listed below:
     Least-squares (LS) channel estimation   Minimum mean-squared error (MMSE) channel estimation   Discrete Fourier transform (DFT) based channel estimation   Discrete cosine transform (DCT) based channel estimation   

     The details of the above channel estimation methods are described in Embodiment 7: Channel Estimation. The “details of the indicated channel estimation method” depends on the indicated channel estimation method and may include any of the following:
     RMS delay spread of the channel (τ RMS )   Maximum Doppler spread of the channel (Δƒ max )   Initial SINR estimation (SINR 0 )   MMSE matrix elements   Channel Estimation granularity in the frequency domain   Window type   Window size   

     Indication of the “channel estimation method” and/or “details of the indicated channel estimation method” can be done based on one of the following options: 
     Option  3 - 2 -A: O-DU indicates the “channel estimation method” to the O-RU using a particular bitfield in the signaling from O-DU to O-RU. O-RU determines the channel estimation details, including parameters and processing steps based on its own implementation. An example of a mapping between the bitfield content and corresponding channel estimation methods is provided in Table 5. 
     Option  3 - 2 -B: O-DU indicates “channel estimation method” and “details of the indicated channel estimation method” to the O-RU using particular bitfields in the signaling from O-DU to O-RU. O-RU uses a table, such as Table 5, to find the channel estimation method and uses other look-up tables, such as Table 6 to Table 9, to obtain the channel estimation details using these indications from O-DU. A summary of channel estimation methods and their corresponding look-up table(s) is provided in Table 10. 
     Option  3 - 2 -C: O-DU provides “partial channel estimation details” to O-RU, using which O-RU calculates full channel estimation details. In this option, the “DM-RS channel estimation method information” includes the channel estimation method and its partial details as listed in Table 11 to Table 14. O-RU calculates the channel estimation details, such as MMSE matrix, CE granularity in the frequency-domain, and window size, based on the channel estimation method and its received details from O-DU. The details of these channel estimation calculations can be found in Embodiment 7: Channel Estimation. 
     Option  3 - 2 -D: O-DU provides “full channel estimation details” to O-RU. In this option, the “DM-RS channel estimation method information” includes the channel estimation method and its full details as listed in Table 15. O-DU transfers the channel estimation details by either indicating bits in a bitfield for a particular parameter or directly transferring IQ/integer values of relevant parameters to O-RU. 
     As shown in  FIG.  8 B , at  3 - 3  (Mandatory) O-RU estimates the UL channels of the UE(s) and/or layer(s) transmitted at the current slot n using the “DM-RS information” obtained in Step  3 - 1 . Depending on whether the optional Step  3 - 2  is executed, one of the following options are taken by O-RU regarding the channel estimation method and its details: 
     Option  3 - 3 -A: (Step  3 - 2  is not executed): O-RU uses a pre-configured channel estimation method and its details. 
     Option  3 - 3 -B: (Step  3 - 2  is executed): O-RU uses “DM-RS channel estimation method information” provided by O-DU to obtain the channel estimation method and its details. 
     As shown in  FIG.  8 B , at  3 - 4  (Optional) Prior to slot n, O-DU transfers the “channel equalization method information” to O-RU to indicate the type and, possibly, further details of the channel equalization method to be adopted by O-RU. In the following, “channel equalization method” may refer to any of the following example channel equalization techniques:
     ZF   MMSE   MMSE-IRC   
 An example of a mapping between the bitfield contents and the corresponding channel equalization method is provided in Table 17.
     As shown in  FIG.  8 B , at  3 - 5  (Mandatory) O-RU equalizes the UL channels of the UE(s) and/or layer(s) transmitted at the current slot n. Depending on whether the optional Step  3 - 4  is executed, one of the following options are taken by O-RU regarding the “channel equalization method information”: 
     Option  3 - 5 -A: (Step  3 - 4  is not executed): O-RU uses a pre-configured channel equalization method and its details. 
     Option  3 - 5 -B: (Step  3 - 4  is executed): O-RU uses “channel equalization method information” provided by O-DU to obtain the channel equalization method and its details. 
     As shown in  FIG.  8 C , at  3 - 6  (Mandatory) Prior to slot n, O-DU transfers “modulation information” to O-RU. Afterward, O-RU performs demodulation on the equalized signal in Step  3 - 5  during slot n and transfers the resulting signal to O-DU through the fronthaul link. 
     As shown in  FIG.  8 C , at  3 - 7  (Mandatory) O-DU performs descrambling on the signal of slot n transferred from O-RU and subsequently performs the rest of L1/L2 processing on the resulting signal. 
       FIGS.  9 A,  9 B, and  9 C  illustrate different variations of Embodiment 3 to enable O-RU Cat C3 through different signaling mechanisms, as shown in Table 4. 
     
       
         
          TABLE 4
           
               
               
               
             
               
                 Summary of selected embodiments from embodiment 3 
               
               
                 Embodiment # 
                 Executed steps from Embodiment 2 
                 Block diagram 
               
             
            
               
                 3-A 
                 ( 3 - 1 ), ( 3 - 3 ), ( 3 - 5 ), ( 3 - 6 ), ( 3 - 7 ) 
                 
                   FIG.  9 A 
                 
               
               
                 3-B 
                 ( 3 - 1 ), ( 3 - 2 ), ( 3 - 3 ), ( 3 - 5 ), ( 3 - 6 ), ( 3 - 7 ) 
                 
                   FIG.  9 B 
                 
               
               
                 3-C 
                 ( 3 - 1 ), ( 3 - 2 ), ( 3 - 3 ), ( 3 - 4 ), ( 3 - 5 ), ( 3 - 6 ), ( 3 - 7 ) 
                 
                   FIG.  9 C 
                 
               
            
           
         
       
     
     Embodiment 4: Example Tables for the Signaling Mechanisms that Enables Asymmetrical Functional Splits 
       FIGS.  10 A,  10 B,  10 C,  10 D, and  10 E  clarify the 5G NR DM-RS structures with examples for a better understanding. Example Tables 5-20 show exemplary parameters for the signaling mechanisms that enables asymmetrical functional splits as shown in  FIGS.  10 A,  10 B,  10 C,  10 D, and  10 E . 
     
       
         
          TABLE 4
           
               
               
               
               
             
               
                 DM-RS configuration parameters 
               
               
                 Purpose 
                 Parameter 
                 Brief Description 
                 Value Range 
               
             
            
               
                 Waveform Selection 
                 TransformPrecoding 
                 Enable/disable transform precoding 
                 {0,1} 
               
               
                 DMRS Sequence Generation 
                 
                   
                     
                       
                         
                           N 
                           
                             I 
                             D 
                           
                           
                             
                               n 
                               
                                 S 
                                 C 
                                 I 
                                 D 
                               
                             
                           
                         
                       
                     
                   
                 
                 DMRS Scrambling Identity (for CP-OFDM) 
                 {0,1, ..., 65535} 
               
               
                 n SCID 
 
                 DMRS Scrambling Initialization 
                 {0,1} 
               
               
                 
                   
                     
                       
                         
                           N 
                           
                             s 
                             y 
                             m 
                             b 
                           
                           
                             s 
                             l 
                             o 
                             t 
                           
                         
                       
                     
                   
                 
                 Number of OFDM Symbols in a Slot 
                 {12,14} 
               
               
                 
                   
                     
                       
                         
                           n 
                           
                             s 
                             , 
                             f 
                           
                           u 
                         
                       
                     
                   
                 
                 Slot Number in a Radio Frame 
                 {0, ..., 159} 
               
               
                 I 
                 Symbol Number within a Slot 
                 {0,1, ...,13} 
               
               
                 
                   
                     
                       
                         
                           n 
                           
                             I 
                             D 
                           
                           
                             R 
                             S 
                           
                         
                       
                     
                   
                 
                 DM-RS Scrambling Identity (for DFT-s-OFDM) 
                 {0,1, ..., 65535} 
               
               
                 dmrsGH 
                 Group Hopping 
                 {0,1} 
               
               
                 dmrsSH 
                 Sequence Hopping 
                 {0,1} 
               
               
                 DMRS Configuration 
                 dmrs- Type 
                 DMRS Configuration Type 
                 {1,2} 
               
               
                 maxLength 
                 Single Symbol or Double Symbol 
                 {1,2} 
               
               
                 dmrs- TypeA- Position 
                 DMRS Start Pos for Type A 
                 {pos2,pos3} 
               
               
                 dmrs- AdditionalPosition 
                 Additional DMRS Position 
                 {0,1,2,3} 
               
               
                 p̃ 
                 Antenna Port Index 
                 {0,1, ...,11} 
               
            
           
         
       
     
     
       
         
          TABLE 5
           
               
               
             
               
                 An example look-up table for the channel estimation methods 
               
               
                 Bitfield 
                 Channel estimation method 
               
             
            
               
                 00 
                 LS 
               
               
                 01 
                 MMSE 
               
               
                 10 
                 DFT based 
               
               
                 11 
                 DCT based 
               
            
           
         
       
     
     
       
         
          TABLE 6
           
               
               
             
               
                 An example look-up table for MMSE channel estimation matrix elements. 
               
               
                 Bitfield 
                 MMSE matrix 
               
             
            
               
                 000 
                 ∅ 1 
 
               
               
                 001 
                 ∅ 2 
 
               
               
                 010 
                 ∅ 3 
 
               
               
                 011 
                 ∅ 4 
 
               
               
                 100 
                 ∅ 5 
 
               
               
                 101 
                 ∅ 6 
 
               
               
                 110 
                 ∅ 7 
 
               
               
                 111 
                 ∅ 8 
 
               
            
           
         
       
     
     
       
         
          TABLE 7
           
               
               
             
               
                 An example look-up table for the channel estimation granularity in the frequency domain. 
               
               
                 Bitfield 
                 CE Granularity in the Frequency Domain [RBs] 
               
             
            
               
                 00 
                 N 1 
 
               
               
                 01 
                 N 2 
 
               
               
                 10 
                 N 3 
 
               
               
                 11 
                 N 4 
 
               
            
           
         
       
     
     
       
         
          TABLE 8
           
               
               
             
               
                 An example look-up table for window type 
               
               
                 Bitfield 
                 Window type 
               
             
            
               
                 00 
                 Type 1 
               
               
                 01 
                 Type 2 
               
               
                 10 
                 Type 3 
               
               
                 11 
                 Type 4 
               
            
           
         
       
     
     
       
         
          TABLE 9
           
               
               
             
               
                 An example look-up table for window size 
               
               
                 Bitfield 
                 Window size 
               
             
            
               
                 00 
                 Window size 1 
               
               
                 01 
                 Window size 2 
               
               
                 10 
                 Window size 3 
               
               
                 11 
                 Window size 4 
               
            
           
         
       
     
     
       
         
          TABLE 10
           
               
               
             
               
                 Summary of relevant look-up tables for channel estimation 
               
               
                 Channel estimation method 
                 Relevant look-up table(s) 
               
             
            
               
                 LS 
                 Table 5, Table 7 
               
               
                 MMSE 
                 Table 5, Table 6, Table 7 
               
               
                 DFT based 
                 Table 5, Table 7, Table 8, Table 9 
               
               
                 DCT based 
                 Table 5, Table 7, Table 8, Table 9 
               
            
           
         
       
     
     
       
         
          TABLE 11
           
               
               
               
               
               
               
               
             
               
                 A summary of channel estimation methods and their corresponding details for option  1 - 2 -C (Example-1) 
               
               
                 Channel estimation method 
                 CE Granularity in the Frequency Domain (Integer Values for #RBs) 
                 SINR 0  (IQ Values) 
                 τ RMS  (IQ Values) 
                 Δƒ max  (IQ Values) 
                 Window type (Indicator for a row in a table) 
                 Window size (Integer Values) 
               
             
            
               
                 LS 
                 ü 
                 - 
                 - 
                 - 
                 - 
                 - 
               
               
                 MMSE 
                 ü 
                 ü 
                 ü 
                 ü 
                 - 
                 - 
               
               
                 DFT based 
                 ü 
                 - 
                 - 
                 - 
                 ü 
                 ü 
               
               
                 DCT based 
                 ü 
                 - 
                 - 
                 - 
                 ü 
                 ü 
               
            
           
         
       
     
     
       
         
          TABLE 12
           
               
               
               
               
               
               
             
               
                 A summary of channel estimation methods and their corresponding details for option  1 - 2 -C (Example-2) 
               
               
                 Channel estimation method 
                 SINR 0  (IQ Values) 
                 τ RMS  (IQ Values) 
                 Δƒ max  (IQ Values) 
                 Window type (Indicator for a row in a table) 
                 Window size (Integer Values) 
               
             
            
               
                 LS 
                 - 
                 ü 
                 - 
                 - 
                 - 
               
               
                 MMSE 
                 ü 
                 ü 
                 ü 
                 - 
                 - 
               
               
                 DFT based 
                 - 
                 ü 
                 - 
                 ü 
                 ü 
               
               
                 DCT based 
                 - 
                 ü 
                 - 
                 ü 
                 ü 
               
            
           
         
       
     
     
       
         
          TABLE 13
           
               
               
               
               
             
               
                 A summary of channel estimation methods and their corresponding details for option  1 - 2 -C (Example-3) 
               
               
                 Channel estimation method 
                 SINR 0  (IQ Values) 
                 τ RMS  (IQ Values) 
                 Δƒ max  (IQ Values) 
               
             
            
               
                 LS 
                 - 
                 ü 
                 - 
               
               
                 MMSE 
                 ü 
                 ü 
                 ü 
               
               
                 DFT based 
                 - 
                 ü 
                 - 
               
               
                 DCT based 
                 - 
                 ü 
                 - 
               
            
           
         
       
     
     
       
         
          TABLE 14
           
               
               
               
             
               
                 A summary of channel estimation methods and their corresponding details for option  1 - 2 -C (Example-4) 
               
               
                 Channel estimation method 
                 τ RMS  (IQ Values) 
                 Δƒ max  (IQ Values) 
               
             
            
               
                 LS 
                 ü 
                 - 
               
               
                 MMSE 
                 ü 
                 ü 
               
               
                 DFT based 
                 ü 
                 - 
               
               
                 DCT based 
                 ü 
                 - 
               
            
           
         
       
     
     
       
         
          TABLE 15
           
               
               
               
               
               
             
               
                 A summary of channel estimation methods and their corresponding details for option  1 - 2 -D 
               
               
                 Channel estimation method 
                 CE Granularity in the Frequency Domain (Integer Values for #RBs) 
                 MMSE matrix (IQ Values) 
                 Window type (Indicator for a row in a table) 
                 Window size (Integer Values) 
               
             
            
               
                 LS 
                 ü 
                 - 
                 - 
                 - 
               
               
                 MMSE 
                 ü 
                 ü 
                 - 
                 - 
               
               
                 DFT based 
                 ü 
                 - 
                 ü 
                 ü 
               
               
                 DCT based 
                 ü 
                 - 
                 ü 
                 ü 
               
            
           
         
       
     
     
       
         
          TABLE 16
           
               
               
             
               
                 An example look-up table for the combining/digital beamforming matrix calculation methods 
               
               
                 Bitfield 
                 Combining/digital beamforming matrix calculation method 
               
             
            
               
                 000 
                 Conjugate combining 
               
               
                 001 
                 Zero forcing 
               
               
                 010 
                 MMSE 
               
               
                 011 
                 MMSE-IRC 
               
               
                 100 
                 Null-space dimension increase based 
               
               
                 101 
                 DFT-compression 
               
            
           
         
       
     
     
       
         
          TABLE 17
           
               
               
             
               
                 An example look-up table for the channel equalization methods 
               
               
                 Bitfield 
                 Channel equalization method 
               
             
            
               
                 00 
                 Zero forcing 
               
               
                 01 
                 MMSE 
               
               
                 10 
                 MMSE-IRC 
               
            
           
         
       
     
     
       
         
          TABLE 18
           
               
               
             
               
                 An example look-up table for the additional channel equalization information 
               
               
                 Bitfield 
                 Supplementary Demodulation Information Calculation Method 
               
             
            
               
                 000 
                 Equalization Gain and Post-processed Noise plus Interference Power 
               
               
                 001 
                 Post-processed Noise plus Interference Power divided by Equalization Gain 
               
               
                 010 
                 Equalization Gain divided by Post-processed Noise plus Interference Power 
               
               
                 011 
                 Equalization Gain 
               
               
                 100 
                 Post-processed Noise plus Interference Power 
               
               
                 101 
                 Channel Estimates and Estimated DM-RS Noise plus Interference Power 
               
            
           
         
       
     
     
       
         
          TABLE 19
           
               
               
             
               
                 Another example look-up table for the additional channel equalization information 
               
               
                 Bitfield 
                 Supplementary Demodulation Information Calculation Method 
               
             
            
               
                 00 
                 Equalization Gain and Post-processed Noise plus Interference Power 
               
               
                 01 
                 Post-processed Noise plus Interference Power divided by Equalization Gain 
               
               
                 10 
                 Equalization Gain 
               
               
                 11 
                 Post-processed Noise plus Interference Power 
               
            
           
         
       
     
     Embodiment 5: C-Plane and M-Plane Messages for the UL functional splits with O-RU Cat C1, O-RU Cat C2, O-RU Cat C3: 
     M-plane Embodiments for the O-RU to Report Its Capabilities About the Supported O-RU Categories 
     In one embodiment, the O-RU exposes its ability to support category C1, category C2, or category C3 to the SMO or the O-RU controller in the O-RAN module-cap.yang. One example of the RU-supported-category leaf under the RU-capabilities structure in the O-RAN-module-cap.yang can be shown as follows:  
     
       
         
           
               
            
               
                 grouping ru-capabilities { 
               
               
                 description 
               
               
                 “Structure representing set of capabilities.”; 
               
               
                 leaf ru-supported-category { 
               
               
                 type enumeration { 
               
               
                 enum CAT_A { 
               
               
                  description 
               
               
                   “Informs that precoding is not supported in O-RU”; 
               
               
                 enum CAT_B { 
               
               
                  description 
               
               
                   “Informs that precoding is supported in O-RU”; 
               
               
                 enum CAT_A_C1 { 
               
               
                  description 
               
               
                   “Informs that precoding is not supported in O-RU for DL and DM-RS based 
               
               
                 channel estimation is supported in the O-RU for UL”; 
               
               
                 } 
               
               
                 enum CAT_A_C2 { 
               
               
                  description 
               
               
                   “Informs that precoding is not supported in O-RU for DL and DM-RS based 
               
               
                 channel estimation and equalization are supported in the O-RU for UL ”; 
               
               
                 } 
               
               
                 enum CAT_A_C3 { 
               
               
                  description 
               
               
                   “Informs that precoding is not supported in O-RU for DL and DM-RS based 
               
               
                 channel estimation, equalization, and demodulation are supported in the O-RU for 
               
               
                 UL”; 
               
               
                 } 
               
               
                 enum CAT_B_C1 { 
               
               
                  description 
               
               
                   “Informs that precoding is supported in O-RU for DL and DM-RS based 
               
               
                 channel estimation is supported in the O-RU for UL ”; 
               
               
                 } 
               
               
                 enum CAT_B_C2 { 
               
               
                  description 
               
               
                   “Informs that precoding is supported in O-RU for DL and DM-RS based 
               
               
                 channel estimation and equalization are supported in the O-RU for UL ”; 
               
               
                 } 
               
               
                 enum CAT_B_C3 { 
               
               
                  description 
               
               
                   “Informs that precoding is supported in O-RU for DL and DM-RS based 
               
               
                 channel estimation, equalization, and demodulation are supported in the O-RU for 
               
               
                 UL ”; 
               
               
                 } 
               
               
                 } 
               
               
                 description 
               
               
                 “Informs about which category O-RU supports”; } 
               
               
                   
               
            
           
         
       
     
     M-Plane Embodiments to Cover DM-RS Based Channel Estimation Features Support at the O-RU 
     The O-RU exposes its ability to support DM-RS-based channel estimation locally and the dependent features by supporting of the feature DMRS-BASED-CHANNEL-ESTIMATION-SUPPORTED in o-ran-module-cap.yang module.  
     
       
         
           
               
            
               
                 feature DMRS-BASED-CHANNEL-ESTIMATION-SUPPORTED { 
               
               
                 description 
               
               
                 “This is used to indicate O-RU’s support for demodulation reference signal 
               
               
                 (DMRS)-based channel estimation.”; 
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     If feature DMRS-BASED-CHANNEL-ESTIMATION-SUPPORTED is true, the O-RU reports its support for the following dependent features: 
     dmrs-based-channel-estimation-configuration-type 
   static-dmrs-based-channel-estimation-configuration-type   dynamic-dmrs-based-channel-estimation-configuration-type   
   dmrs-generation-method 
   dmrs-local-generation   dmrs-sequence-reception   
   dmrs-channel-estimation-technique 
   Is-channel-estimation   mmse-channel-estimation   dft-channel-estimation   dct-channel-estimation   
   dmrs-channel-estimation-fh-method 
   static-channel-estimation-type-only-indication-method   static-full-id-based-channel-estimation-method   real-time-channel-estimation-method   partial-real-time-channel-estimation-method   
   combining-digital-bf-matrix-enhancement-information 
   srs-channel-estimates 
   weighting-coefficients-local-generation   weighting-coefficients-reception   
   preliminary-combining-digital-bf-matrix 
   weighting-coefficients-local-generation   weighting-coefficients-reception   
   
   combining-matrix-calculation-method 
   conjugate-combining   zero-forcing   dft-compression   null-space-dimension-increase-based   mmse-irc-based   
   

     M-plane Embodiments to Cover DM-RS Based Channel Estimation Features Support at the O-RU 
     One example implementation in the o-ran-module-cap.yang can be described as follows.  
     
       
         
           
               
            
               
                 grouping dmrs-based-channel-estimation-features { 
               
               
                 description 
               
               
                 “O-RU’s capabilities related to supported DMRS-based channel estimation”; 
               
               
                 - container dmrs-based-channel-estimation-configuration-type { 
               
               
                    description 
               
               
                    “O-RU’s capabilities related to DMRS-based channel estimation configuration 
               
               
                 type. ”; 
               
               
                        o leaf static-dmrs-based-channel-estimation-configuration-type { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU supports static configurations, via M-plane, for the DMRS-based 
               
               
                    channel estimation parameters.”; 
               
               
                             } 
               
               
                        o leaf dynamic- dmrs-based-channel-estimation-configuration-type{ 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU supports real-time configurations, via C-plane, for the DMRS- 
               
               
                    based channel estimation parameters.”; 
               
               
                    } } 
               
               
                 - container dmrs-generation-method { 
               
               
                    description 
               
               
                    “O-RU’s capabilities related to DMRS signal generation. O-DU transfers DM-RS 
               
               
                 information to O-RU so that O-RU can obtain DM-RS signal and perform channel 
               
               
                 estimation”; 
               
               
                        o leaf dmrs-local-generation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU supports reception of the DM-RS configuration parameters of 
               
               
                    the scheduled or paired UEs/layers. O-RU supports local generation of the DM- 
               
               
                    RS sequence using the DM-RS configuration parameters received from O-DU. 
               
               
                    ”; 
               
               
                             } 
               
               
                        o leaf dmrs-sequence-reception { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU supports DMRS sequence reception from O-DU.”; 
               
               
                    } } 
               
               
                 - container dmrs-channel-estimation-technique { 
               
               
                    description 
               
               
                    “O-RU’s capabilities related to supported channel estimation techniques”; 
               
               
                        o leaf ls-channel-estimation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Least-squares (LS) channel estimation ”; 
               
               
                             } 
               
               
                        o leaf mmse-channel-estimation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Minimum mean-squared error (MMSE) channel estimation ”; 
               
               
                             } 
               
               
                        o leaf dft-channel-estimation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Discrete Fourier transform (DFT) based channel estimation ”; 
               
               
                             } 
               
               
                        o leaf dct-channel-estimation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                    }“Discrete cosine transform (DCT) based channel estimation ”; 
               
               
                 - container dmrs-channel-estimation-fh-method { 
               
               
                    description 
               
               
                    “O-RU’s capabilities related to supported fronthaul indication methods of 
               
               
                 channel estimation parameters”; 
               
               
                        o leaf static-channel-estimation-type-only-indication-method { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Static channel estimation indication method refers to the case where 
               
               
                    the O-DU only indicates channel estimation type/method to the O-RU. O-RU 
               
               
                    determines the channel estimation details, including coefficients and 
               
               
                    processing steps based on its own implementation”; 
               
               
                             } 
               
               
                        o leaf static-full-id-based-channel-estimation-method { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “static ID-based channel estimation method refers to the case where 
               
               
                    the O-DU indicates the channel estimation type/method as well as other 
               
               
                    channel estimation parameters needed at the O-RU. O-RU uses look-up tables 
               
               
                    to obtain the channel estimation details”; 
               
               
                             } 
               
               
                        o leaf real-time-channel-estimation-method { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “real-time-channel-estimation-method refers to the case where the O- 
               
               
                    DU provides full channel estimation details to O-RU in real-time”; 
               
               
                             } 
               
               
                        o leaf partial-real-time-channel-estimation-method { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Partial-real-time-channel-estimation-method refers to the case where 
               
               
                    the O-DU provides partial channel estimation parameters to O-RU, using 
               
               
                    which O-RU calculates full channel estimation details. In this option, the 
               
               
                    channel estimation method information includes the channel estimation 
               
               
                    method and other parameters. O-RU calculates the channel estimation details, 
               
               
                    such as MMSE matrix, CE granularity in the frequency-domain, and window 
               
               
                    size, based on the channel estimation method and its received details from O- 
               
               
                    DU.”; 
               
               
                    } } 
               
               
                 - container combining-digital-bf-matrix-enhancement-information { 
               
               
                    description 
               
               
                    “O-RU’s capabilities related to supporting of utilizing combining/digital 
               
               
                 beamforming matrix enhancement information, which improves the quality of the 
               
               
                 combining/digital beamforming matrix.”; 
               
               
                        o container srs-channel-estimates { 
               
               
                    description 
               
               
                    “O-RU supports combining/digital beamforming matrix enhancement 
               
               
                    information which includes SRS channel estimates of the scheduled or paired 
               
               
                    UEs/layers. The O-RU combines SRS and DM-RS channel estimates using 
               
               
                    weighting coefficients. Two options exist for the weighting coefficients:”; 
               
               
                                 ■ leaf weighting-coefficients-local-generation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU supports either using pre-configured weighting coefficients or 
               
               
                             determining the weighting coefficients based on other parameters.”; 
               
               
                              } 
               
               
                                 ■ leaf weighting-coefficients-reception { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU supports reception of the weighting coefficients from the O-DU.”; 
               
               
                        o container preliminary-combining-digital-bf-matrix { 
               
               
                    description 
               
               
                    “O-RU supports combining/digital beamforming matrix enhancement 
               
               
                    information which includes preliminary combining/digital beamforming 
               
               
                    matrix elements corresponding to the scheduled or paired UEs/layers. This 
               
               
                    preliminary combining/digital beamforming matrix is calculated at O-DU 
               
               
                    prior to slot n using SRS channel estimates. Later, the O-RU combines its own 
               
               
                    calculated combining/digital beamforming matrix elements with the 
               
               
                    preliminary combining/digital beamforming matrix using weighting 
               
               
                    coefficients. Two options exist for the weighting coefficients.”; 
               
               
                                 ■ leaf weighting-coefficients-local-generation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU supports local generation of the weighting coefficients”; 
               
               
                              } 
               
               
                                 ■ leaf weighting-coefficients-reception { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU supports reception of the weighting coefficients from the O-DU”; 
               
               
                  } } } 
               
               
                 - container combining-matrix-calculation-method { 
               
               
                    description 
               
               
                    “O-RU capabilities related to combining/digital beamforming matrix 
               
               
                 calculation method.”; 
               
               
                        o leaf conjugate-combining { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU’s support for Conjugate combining method.”; 
               
               
                             } 
               
               
                        o leaf zero-forcing { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU’s support for Zero-forcing combining method”; 
               
               
                             } 
               
               
                        o leaf dft-compression { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU’s support for Discrete Fourier transform compression based 
               
               
                    combining method. ”; 
               
               
                             } 
               
               
                        o leaf null-space-dimension-increase-based { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU’s support for Null-space dimension increase based combining 
               
               
                    calculation method.”; 
               
               
                             } 
               
               
                        o leaf mmse-irc-based { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU’s support for Minimum mean square error - Interference 
               
               
                    rejection combining (MMSE-IRC) based combining calculation method.”; 
               
               
                 } } } 
               
               
                   
               
            
           
         
       
     
     Another example implementation in the o-ran-uplane-conf.yang can be described as follows, where dependent features can be marked as read-write for capabilities reporting and configurations  
     
       
         
           
               
            
               
                 grouping dmrs-based-channel-estimation-features { 
               
               
                 description 
               
               
                 “Dependent features on DMRS-based channel estimation”; 
               
               
                 - container dmrs-based-channel-estimation-configuration-type { 
               
               
                    description 
               
               
                    “DMRS-based channel estimation configuration type.”; 
               
               
                        o leaf static-dmrs-based-channel-estimation-configuration-type { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Static configurations, via M-plane, for the DMRS-based channel 
               
               
                    estimation parameters.”; 
               
               
                             } 
               
               
                        o leaf dynamic- dmrs-based-channel-estimation-configuration-type{ 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Real-time configurations, via C-plane, for the DMRS-based channel 
               
               
                    estimation parameters.”; 
               
               
                              } 
               
               
                     } 
               
               
                 - container dmrs-generation-method { 
               
               
                    description 
               
               
                    “DMRS signal generation. O-DU transfers DM-RS information to O-RU so that 
               
               
                 O-RU can obtain DM-RS signal and perform channel estimation”; 
               
               
                        o leaf dmrs-local-generation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU’s reception of the DM-RS configuration parameters of the 
               
               
                    scheduled or paired UEs/layers. O-RU’s local generation of the DM-RS 
               
               
                    sequence using the DM-RS configuration parameters received from O-DU. ”; 
               
               
                             } 
               
               
                        o leaf dmrs-sequence-reception { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU’s DMRS sequence reception from O-DU.”; 
               
               
                             } 
               
               
                    } 
               
               
                 - container dmrs-channel-estimation-technique { 
               
               
                    description 
               
               
                    “Supported channel estimation techniques”; 
               
               
                        o leaf Is-channel-estimation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “ Least-squares (LS) channel estimation ”; 
               
               
                              } 
               
               
                        o leaf mmse-channel-estimation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Minimum mean-squared error (MMSE) channel estimation ”; 
               
               
                             } 
               
               
                        o leaf dft-channel-estimation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Discrete Fourier transform (DFT) based channel estimation ”; 
               
               
                             } 
               
               
                        o leaf dct-channel-estimation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Discrete cosine transform (DCT) based channel estimation ”; 
               
               
                     } } 
               
               
                 - container dmrs-channel-estimation-fh-method { 
               
               
                    description 
               
               
                    “Fronthaul indication methods of channel estimation parameters”; 
               
               
                        o leaf static-channel-estimation-type-only-indication-method { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Static channel estimation indication method refers to the case where 
               
               
                    the O-DU only indicates channel estimation type/method to the O-RU. O-RU 
               
               
                    determines the channel estimation details, including coefficients and 
               
               
                    processing steps based on its own implementation”; 
               
               
                             } 
               
               
                        o leaf static-full-id-based-channel-estimation-method { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “static ID-based channel estimation method refers to the case where 
               
               
                    the O-DU indicates the channel estimation type/method as well as other 
               
               
                    channel estimation parameters needed at the O-RU. O-RU uses look-up tables 
               
               
                    to obtain the channel estimation details”; 
               
               
                             } 
               
               
                        o leaf real-time-channel-estimation-method { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “real-time-channel-estimation-method refers to the case where the O- 
               
               
                    DU provides full channel estimation details to O-RU in real-time”; 
               
               
                             } 
               
               
                        o leaf partial-real-time-channel-estimation-method { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Partial-real-time-channel-estimation-method refers to the case where 
               
               
                    the O-DU provides partial channel estimation parameters to O-RU, using 
               
               
                    which O-RU calculates full channel estimation details. In this option, the 
               
               
                    channel estimation method information includes the channel estimation 
               
               
                    method and other parameters. O-RU calculates the channel estimation details, 
               
               
                    such as MMSE matrix, CE granularity in the frequency-domain, and window 
               
               
                    size, based on the channel estimation method and its received details from O- 
               
               
                    DU.”; 
               
               
                     } } 
               
               
                 - container combining-digital-bf-matrix-enhancement-information { 
               
               
                    description 
               
               
                    “O-RU’s utilizing combining/digital beamforming matrix enhancement 
               
               
                 information, which improves the quality of the combining/digital beamforming 
               
               
                 matrix.”; 
               
               
                        o container srs-channel-estimates { 
               
               
                    description 
               
               
                    “Combining/digital beamforming matrix enhancement information which 
               
               
                    includes SRS channel estimates of the scheduled or paired UEs/layers. The O- 
               
               
                    RU combines SRS and DM-RS channel estimates using weighting coefficients. 
               
               
                    Two options exist for the weighting coefficients:”; 
               
               
                                 ■ leaf weighting-coefficients-local-generation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU uses pre-configured weighting coefficients or determines the 
               
               
                             weighting coefficients based on other parameters.”; 
               
               
                              } 
               
               
                                 ■ leaf weighting-coefficients-reception { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU’s reception of the weighting coefficients from the O-DU.”; 
               
               
                     } } 
               
               
                        o container preliminary-combining-digital-bf-matrix { 
               
               
                    description 
               
               
                    “Combining/digital beamforming matrix enhancement information which 
               
               
                    includes preliminary combining/digital beamforming matrix elements 
               
               
                    corresponding to the scheduled or paired UEs/layers. This preliminary 
               
               
                    combining/digital beamforming matrix is calculated at O-DU prior to slot n 
               
               
                    using SRS channel estimates. Later, the O-RU combines its own calculated 
               
               
                    combining/digital beamforming matrix elements with the preliminary 
               
               
                    combining/digital beamforming matrix using weighting coefficients. Two 
               
               
                    options exist for the weighting coefficients.”; 
               
               
                                 ■ leaf weighting-coefficients-local-generation { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU’s local generation of the weighting coefficients”; 
               
               
                             } 
               
               
                                 . leaf weighting-coefficients-reception { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “O-RU’s reception of the weighting coefficients from the O-DU”; 
               
               
                 }}} 
               
               
                 - container combining-matrix-calculation-method { 
               
               
                    description 
               
               
                    “O-RU’s combining/digital beamforming matrix calculation method.”; 
               
               
                        o leaf conjugate-combining { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Conjugate combining method.”; 
               
               
                             } 
               
               
                        o leaf zero-forcing { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Zero-forcing combining method”; 
               
               
                             } 
               
               
                        o leaf dft-compression { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Discrete Fourier transform compression based combining method. ”; 
               
               
                             } 
               
               
                        o leaf null-space-dimension-increase-based { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Null-space dimension increase based combining calculation method.”; 
               
               
                             } 
               
               
                        o leaf mmse-irc-based { 
               
               
                             type boolean; 
               
               
                             description 
               
               
                             “Minimum mean square error - Interference rejection combining 
               
               
                    (MMSE-IRC) based combining calculation method.”; 
               
               
                             } 
               
               
                 }} 
               
            
           
         
       
     
     C-plane Embodiment Related to Real-Time DMRS Sequence Generation at the O-RU 
     DMRS Configuration Parameters 
     C-plane Embodiment: If dmrs-local-generation and dynamic-dmrs-based-channel-estimation-configuration-type features are supported by the O-RU and configured by the SMO/O-RU controller, the O-DU appends the UL C-plane message sent in the DL to the O-RU with the following section extension and includes the DM-RS configuration parameters to enable the O-RU to locally generate the DM-RS signal. 
     ExtType=21: Section description for DMRS sequence generatio 
     This section extension applies to DMRS signal generation. This enables the O-DU to provide the DMRS configuration parameters at the O-RU to generate the DMRS signal locally. This section extension applies to section types 1 and 3. 
     
       
         
          TABLE 20
           
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 Section Extension Type 21 
               
               
                 ef 
                 extType = 0x15 
                 1 
                 Octet N 
               
             
            
               
                 extLen 
                 1 
                 N+1 
               
               
                 TP 
                 scramblingInit 
                 GH 
                 SH 
                 configType 
                 maxLen 
                 pos 
                 reserved 
                 1 
                 N+2 
               
               
                 scramblingID[15:8] 
                 1 
                 N+3 
               
               
                 scramblingID [7:0] 
                 1 
                 N+4 
               
               
                 addPos[1:0] 
                 antPortInd[3:0] 
                 reserved 
                 1 
                 N+5 
               
               
                 zero pad to 4-byte boundary 
                 1 
                 N+6 
               
               
                 zero pad to 4-byte boundary 
                 1 
                 N+7 
               
            
           
         
       
     
     TP (transformPrecoding) 
     
         
         Description: This parameter is used to indicate if Transform precoding is enabled or disabled. The default is that transform precoding is disabled. 
         Value range: {0b: Disabled; 1b: Enabled}. 
         Type: binary bit. 
         Field length: 1 bit. 
         Default Value: 0 
       
    
     Description of the remaining fields in the above section extension follows the same definitions in Table 2 above. 
     M-plane Embodiment Related to Static DMRS Sequence Generation at the O-RU 
     M-plane Embodiment for static DMRS sequence generation: If dmrs-local-generation and static-dmrs-based-channel-estimation-configuration-type features are supported by the O-RU and configured by the SMO/O-RU controller, the SMO/O-RU controller statically configures the DM-RS parameters to enable the O-RU to locally generate the DM-RS signal and execute DMRS-based channel estimation.  
     
       
         
           
               
            
               
                 list dmrs-based-channel-estimation-configuration { 
               
               
                 if-feature mcap: DMRS-BASED-CHANNEL-ESTIMATION-SUPPORTED; 
               
               
                 key dmrs-ce-config-id; 
               
               
                 description 
               
               
                 “List of DMRS-based channel estimation configurations. An O-RU shall reject any 
               
               
                 configuration 
               
               
                 modification which exceed the maximum permitted configurations supported 
               
               
                 by 
               
               
                 the O-RU”; 
               
               
                 leaf dmrs-ce-config-id { 
               
               
                 type uint8; 
               
               
                 description 
               
               
                  “Supplementary parameter acting as key in list of DMRS based channel 
               
               
                 estimation configurations.”; 
               
               
                 } 
               
               
                 uses dmrs-based-channel-estimation-configuration; 
               
               
                 grouping dmrs-based-channel-estimation-configuration { 
               
               
                 description 
               
               
                 “Set of parameters related to DMRS-based channel estimation configuration”; 
               
               
                 list dmrs-sequence-generation { 
               
               
                 key dmrs-sequence-id; 
               
               
                 min-elements 1; 
               
               
                 description 
               
               
                  “Provides configuration parameters by the O-RU to generate the DMRS 
               
               
                 sequence locally to be used for channel estimation.”; 
               
               
                 leaf dmrs-sequence-id { 
               
               
                 type uint16; 
               
               
                  mandatory true; 
               
               
                  description 
               
               
                    “Supplementary parameter acting as key for dmrs-sequence list.”; 
               
               
                 } leaf transform-recoding { 
               
               
                 type boolean{ 
               
               
                 } 
               
               
                  mandatory true; 
               
               
                  description 
               
               
                   “This parameter enable or disable transform precoding. True means 
               
               
                 transform precoding is enabled.”; 
               
               
                 } 
               
               
                 leaf dmrs-scrambling-id{ 
               
               
                 type uint16{ 
               
               
                 } range 0..65535; 
               
               
                  mandatory true; 
               
               
                  description 
               
               
                 } “DMRS scrambling Identity.”; 
               
               
                 leaf dmrs-scrambling-initialization{ 
               
               
                 type uint8{ 
               
               
                   range 0,1; 
               
               
                 } 
               
               
                  mandatory true; 
               
               
                  description 
               
               
                 } “DMRS scrambling initialization”; 
               
               
                 leaf number-of-ofdm-symbols-in-slot{ 
               
               
                 type uint8{ 
               
               
                   range 12, 14; 
               
               
                 } 
               
               
                  mandatory true; 
               
               
                  description 
               
               
                 } “Number of OFDM symbols per slot”; 
               
               
                 leaf slot-index-in-radio-frame{ 
               
               
                 type uint8{ 
               
               
                 } range 0..159; 
               
               
                  mandatory true; 
               
               
                  description 
               
               
                 } “DMRS slot index in radio frame”; 
               
               
                 leaf symbol-index-within-slot{ 
               
               
                 type uint8{ 
               
               
                   range 0..13; 
               
               
                 } 
               
               
                  mandatory true; 
               
               
                  description 
               
               
                 } “Symbol index within a slot”; 
               
               
                 leaf dmrs-group-hopping{ 
               
               
                 type uint8{ 
               
               
                   range 0,1; 
               
               
                 } 
               
               
                  mandatory true; 
               
               
                  description 
               
               
                 } “DMRS group hopping”; 
               
               
                 leaf dmrs-sequence-hopping{ 
               
               
                 type uint8{ 
               
               
                   range 0,1; 
               
               
                 } 
               
               
                  mandatory true; 
               
               
                  description 
               
               
                 } “DMRS sequence hopping”; 
               
               
                 leaf dmrs-configuration-type{ 
               
               
                 type uint8{ 
               
               
                   range 1,2; 
               
               
                 } 
               
               
                  mandatory true; 
               
               
                  description 
               
               
                 } “DMRS configuration type”; 
               
               
                 leaf dmrs-max-length{ 
               
               
                 type uint8{ 
               
               
                   range 1,2; 
               
               
                 } 
               
               
                  mandatory true; 
               
               
                  description 
               
               
                   “DMRS maximum length: Single symbol or double symbol”; 
               
               
                 } 
               
               
                 leaf dmrs-typeA-position{ 
               
               
                 type uint8{ 
               
               
                   range 2,3; 
               
               
                 } 
               
               
                  mandatory true; 
               
               
                  description 
               
               
                   “DMRS TypeA position: DMRS start position for Type A. 2 refers to pos2 and 3 
               
               
                 refers to pos3”; 
               
               
                 } 
               
               
                 leaf additional-dmrs-position{ 
               
               
                 type uint8{ 
               
               
                   range 0,1,2,3; 
               
               
                 } 
               
               
                  mandatory true; 
               
               
                  description 
               
               
                 } “Additional DMRS position”; 
               
            
           
         
       
     
     C-Plane Embodiment Related to O-DU Transmission in Real-Time DMRS Sequence to the O-RU 
     C-plane Embodiment: If dmrs-sequence-reception and dynamic-dmrs-based-channel-estimation-configuration-type features are supported by the O-RU and configured by the SMO/O-RU controller, the O-DU appends the UL C-plane message sent in the DL to the O-RU with the following section extension and includes the DM-RS sequence to enable the O-RU to do DMRS-based channel estimation. 
     
       
         
          TABLE 22
           
               
               
               
               
               
             
               
                 Section Extension Type 22 
               
               
                 ef 
                 extType = 0x16 
                 1 
                 Octet N 
               
             
            
               
                 extLen 
                 1 
                 N+1 
               
               
                 re 
                 startRec (4 bits) 
                 reserved 
                   
                   
               
               
                 udCompHdr (not always present) 
                 1 
                 N+2 
               
               
                 reserved (not always present) 
                   
                   
               
               
                 udCompLen (not always present) 
                   
                   
               
               
                 udCompParam (not always present) 
                   
                   
               
               
                 iSample (1 st  RE in the PRB) 
                 1 
                 N+3 
               
               
                 qSample (1 st  RE in the PRB) 
                 1 
                 N+4 
               
               
                 ... 
                 1 
                 N+5 
               
               
                 iSample (12 th  RE in the PRB) 
                   
                   
               
               
                 qSample (12 th  RE in the PRB) 
                   
                   
               
               
                 udCompParam (not always present) 
                   
                   
               
               
                 iSample (1 st  RE in the PRB) 
                   
                   
               
               
                 qSample (1 st  RE in the PRB) 
                   
                   
               
               
                 ... 
                 1 
                 N+6 
               
               
                 iSample (12 th  RE in the PRB) 
                 1 
                 N+7 
               
               
                 qSample (12 th  RE in the PRB) 
                   
                   
               
            
           
         
       
     
     ExtType=22: Section description for DMRS sequence transmission 
     This section extension applies to DMRS signal transmission. This enables the O-DU to provide the DMRS sequence needed at the O-RU to execute DMRS-based channel estimation. This section extension applies to section types 1 and 3. 
     Re (Resource Element Indicator) 
           Description: This parameter is used to indicate if every RE is used or every other RE is used. The starting RE is defined by startRec   Value range: {0 b=every RE used; 1 b=every other RE used}.   Type: binary bit.   Field length: 1 bit.   Default Value: 0 b (every RE used) startRec (start RE)   Description: This parameter is used to indicate the first RE within a PRB.   Value range: {0 -11}.   Type: uint.   Field length: 4 bits.   Default Value: 0   
iSample and qSample represent the I and Q samples, respectively of the DMRS signal.
     C-Plane Embodiment Related to Real-Time Configuration of the O-RU by the O-DU for the Channel Estimation Method and Parameters 
     If dynamic-dmrs-based-channel-estimation-configuration-type and real-time-channel-estimation-method features are supported by the O-RU and configured by the SMO/O-RU controller, the O-DU configures the O-RU in real time with the channel estimation method and needed parameters to enable the O-RU to do channel estimation locally. 
     ExtType=23: Section description for channel estimation 
     This section extension applies to channel estimation at the O-RU. This enables the O-DU to provide the channel estimation technique and relevant configuration parameters needed at the O-RU to locally perform channel estimation. This section extension applies to section types 1 and 3. 
     
       
         
          TABLE 23
           
               
               
               
               
               
               
               
             
               
                 Section Extension Type 23 
               
               
                 ef 
                 extType = 0x17 
                 1 
                 Octet N 
               
             
            
               
                 extLen 
                 1 
                 N+1 
               
               
                 chEstMeth 
                 mmseMatrixId 
                 chEstGranularity 
                 1 
                 N+2 
               
               
                 windowType 
                 windowSize 
                 reserved 
                 1 
                 N+3 
               
            
           
         
       
     
     chEstMeth (Channel Estimation Method) 
     
         
         Description: This parameter is used to indicate the channel estimation method. The default is the LS method.. 
         Value range: {00 b: LS; 01 b: MMSE; 10 b: DFT based; 11 b: DCT based}. 
         Type: unsigned integer. 
         Field length: 2 bits. 
         Default Value: 00 
       
    
     mmseMatrixId (MMSE Matrix ID) 
     
         
         Description: This parameter is used to indicate the channel estimation matrix ID. This field is feasible only if the chEstMeth = 00 b (i.e., LS channel estimation). For other channel estimation methods, the O-DU shall set this field to the default value and the O-RU shall ignore this field. 
         Value range: {000 b: ∅ 1  - 111 b: ∅ 8 }. 
         Type: unsigned integer. 
         Field length: 3 bits. 
         Default Value: 000 
       
    
     chEstGranularity (Channel Estimation Granularity) 
     
         
         Description: This parameter is used to indicate the channel estimation granularity in the frequency domain. 
         Value range: {00 b: N 1 ; 01 b: N 2 ; 10 b: N 3 ; 11 b: N 4 }. 
         Type: unsigned integer. 
         Field length: 2 bits. 
         Default Value: 00 
       
    
     windowType (Channel Estimation Window Type) 
     
         
         Description: This parameter is used to indicate the channel estimation window type. This field is feasible only if the chEstMeth = 10 b or 11 b(i.e., DFT or DCT based channel estimation). For other channel estimation methods, the O-DU shall set this field to the default value and the O-RU shall ignore this field. 
         Value range: {00 b: Type 1; 01 b: Type 2; 10 b: Type 3; 11 b: Type 4}. 
         Type: unsigned integer. 
         Field length: 2 bits. 
         Default Value: 00 
       
    
     windowSize (Channel Estimation Window Size) 
     
         
         Description: This parameter is used to indicate the channel estimation window size. This field is feasible only if the chEstMeth = 10 b or 11 b(i.e., DFT or DCT based channel estimation). For other channel estimation methods, the O-DU shall set this field to the default value and the O-RU shall ignore this field. 
         Value range: {00 b: Size 1; 01 b: Size 2; 10 b: Size 3; 11 b: Size 4}. 
         Type: unsigned integer. 
         Field length: 2 bits. 
         Default Value: 00 
       
    
     Embodiment 6: 5G NR DM-RS 
     In 5G NR, DM-RS is used for channel estimation as part of coherent demodulation of the physical channels (i.e., PUSCH/PDSCH). It is designed to support various use cases, channel conditions, and deployment scenarios. DM-RS configuration depends on other PHY layer parameters such as the resource allocation type and utilized waveform (i.e., CP-OFDM or DFT-s-OFDM), and these are detailed in the following parts. In order for O-RU to be able to perform DM-RS channel estimation, either DM-RS sequence or DM-RS configuration parameters, which are required to generate DM-RS sequence at O-RU, need to be provided by O-DU to O-RU. This part explains the latter option and lists the parameters with detailed instructions for DM-RS sequence generation, precoding, and mapping.  FIGS.  11 A,  11 B,  11 C,  11 D, and  11 E  demonstrate the possible channel estimation techniques that can be applied at the O-RU. 
     DM-RS Sequence Generation 
     For CP-OFDM 
     A length-31 Gold sequence is used as the DM-RS sequence for CP-OFDM (i.e., when transform precoding is disabled). This pseudo-random sequence, c(n), with a length of M PN  is defined as follows: 
     
       
         
           
             c 
             
               n 
             
             = 
             
               
                 
                   x 
                   1 
                 
                 
                   
                     n 
                     + 
                     
                       N 
                       C 
                     
                   
                 
                 + 
                 
                   x 
                   2 
                 
                 
                   
                     n 
                     + 
                     
                       N 
                       C 
                     
                   
                 
               
             
             m 
             o 
             d 
             2 
           
         
       
     
     
       
         
           
             
               x 
               1 
             
             
               
                 n 
                 + 
                 31 
               
             
             = 
             
               
                 
                   x 
                   1 
                 
                 
                   
                     n 
                     + 
                     3 
                   
                 
                 + 
                 
                   x 
                   1 
                 
                 
                   n 
                 
               
             
             m 
             o 
             d 
             2 
           
         
       
     
     
       
         
           
             
               x 
               2 
             
             
               
                 n 
                 + 
                 31 
               
             
             = 
             
               
                 
                   x 
                   2 
                 
                 
                   
                     n 
                     + 
                     3 
                   
                 
                 + 
                 
                   x 
                   2 
                 
                 
                   
                     n 
                     + 
                     2 
                   
                 
                 + 
                 
                   x 
                   2 
                 
                 
                   
                     n 
                     + 
                     1 
                   
                 
                 + 
                 
                   x 
                   2 
                 
                 
                   n 
                 
               
             
             m 
             o 
             d 
             2 
           
         
       
     
      where n = 0,1, ..., M PN  - 1 and N c  is 1600. The first m-sequence, x 1 (n), can be initialized with x 1 (0) = 1 and x 1 (n) = 0 for n = 1,2, ...,30. Also, the second m-sequence, x 2 (n) is initialized according to 
     
       
         
           
             
               c 
               
                 i 
                 n 
                 i 
                 t 
               
             
               
               
             = 
               
               
             
               
                 ∑ 
                 
                   i 
                   = 
                   0 
                 
                 
                   30 
                 
               
               
                 
                   x 
                   2 
                 
                 
                   i 
                 
                 
                   2 
                   i 
                 
                 . 
               
             
           
         
       
     
     Using the pseudo-random sequence, the DM-RS sequence for CP-OFDM, r(n), can be generated as follows: 
     
       
         
           
             r 
             
               n 
             
             = 
             
               1 
               
                 
                   2 
                 
               
             
             
               
                 1 
                 − 
                 2. 
                 c 
                 
                   
                     2 
                     n 
                   
                 
               
             
             + 
             j 
             
               1 
               
                 
                   2 
                 
               
             
             
               
                 1 
                 − 
                 2. 
                 c 
                 
                   
                     2 
                     n 
                     + 
                     1 
                   
                 
               
             
             . 
           
         
       
     
     The initialization of the random sequence is performed as follows: 
     
       
         
           
             
               c 
               
                 i 
                 n 
                 i 
                 t 
               
             
             = 
             
               
                 
                   2 
                   
                     17 
                   
                 
                 
                   
                     
                       N 
                       
                         s 
                         y 
                         m 
                         b 
                       
                       
                         s 
                         l 
                         o 
                         t 
                       
                     
                     
                       n 
                       
                         s 
                         , 
                         f 
                       
                       μ 
                     
                     + 
                     l 
                     + 
                     1 
                   
                 
                 
                   
                     2 
                     
                       N 
                       
                         I 
                         D 
                       
                       
                         
                           n 
                           
                             S 
                             C 
                             I 
                             D 
                           
                         
                       
                     
                     + 
                     1 
                   
                 
                 + 
                 2 
                 
                   N 
                   
                     I 
                     D 
                   
                   
                     
                       n 
                       
                         S 
                         C 
                         I 
                         D 
                       
                     
                   
                 
                 + 
                 
                   n 
                   
                     S 
                     C 
                     I 
                     D 
                   
                 
               
             
             m 
             o 
             d 
             
               2 
               
                 31 
               
             
           
         
       
     
      where I is the OFDM symbol number within the slot and  
     
       
         
           
             
               n 
               
                 s 
                 , 
                 f 
               
               μ 
             
           
         
       
     
     is the slot number within a frame. Furthermore,  
     
       
         
           
             
               N 
               
                 I 
                 D 
               
               
                 
                   n 
                   
                     S 
                     C 
                     I 
                     D 
                   
                 
               
             
           
         
       
     
     denotes the DM-RS scrambling identity and n SCID  represents DM-RS scrambling initialization parameter. 
     For DFT-s-OFDM 
     The Zadoff-Chu sequence is used as the DM-RS sequence for DFT-s-OFDM (i.e., when transform precoding is enabled). This low-PAPR sequence,  
     
       
         
           
             
               r 
               
                 u 
                 , 
                 v 
               
               
                 
                   
                     α 
                     , 
                     δ 
                   
                 
               
             
             
               n 
             
             , 
           
         
       
     
     (n), is defined by a cyclic shift a of a base sequence r̅ u , v  (n) as follows: 
     
       
         
           
             
               r 
               
                 u 
                 , 
                 v 
               
               
                 
                   
                     α 
                     , 
                     δ 
                   
                 
               
             
             
               n 
             
             = 
             
               e 
               
                 j 
                 α 
                 n 
               
             
             
               
                 r 
                 ¯ 
               
               
                 u 
                 , 
                 v 
               
             
             
               n 
             
           
         
       
     
      where  
     
       
         
           
             n 
             = 
             0 
             , 
             1 
             , 
             ... 
             , 
             
               M 
               
                 Z 
                 C 
               
             
             − 
             1 
               
               
             and 
               
               
             
               M 
               
                 Z 
                 C 
               
             
             = 
             
               
                 m 
                 
                   N 
                   
                     s 
                     c 
                   
                   
                     R 
                     B 
                   
                 
               
               / 
               
                 
                   2 
                   δ 
                 
               
             
           
         
       
     
     denotes the length of the sequence. Also,  
     
       
         
           
             
               N 
               
                 s 
                 c 
               
               
                 R 
                 B 
               
             
           
         
       
     
     represents the number of subcarriers in a resource block. Multiple orthogonal sequences can be generated from a base sequence through different values of a and δ. The base sequences, r̅ u,v  (n), are divided into groups, where u ∈ {0,1,2, ..., 29} specifies the group number and ν ∈ {0,1} represents the base sequence number within the group. 
     The base sequence definition depends on the sequence length as follows:
     1) When M ZC  ≥ 36:                                           r   ¯       u   ,   v         n     =     x   q         n           m   o   d         N     Z   C                             x   q       m     =     e     −   j       π   q   m       m   +   1             N     Z   C                                   where                                           q       =             q   ¯     +     1   /   2         +   v         −   1             2     q   ¯                                   q   ¯         =         N     Z   C               u   +   1         /     31               

     The length N ZC  is defined as the largest prime number, which is less than the base sequence length M ZC .
     2) When M ZC  ∈ {6,12,18,24}:             r   ¯             u   ,   v         n     =     e     j   φ     n       π   /   4                  where n = 0,1, ...,M ZC  - 1 and the value of φ(n) is provided by Tables 5.2.2.2-1 to 5.2.2.2-4 in TS 38.211.   3) When M ZC  = 30:                 r   ¯         u   ,   v         n     =     e     −   j       π       u   +   1           n   +   1           n   +   2           31                    where n = 0,1, ..., M ZC  - 1.   

     Using the Zadoff-Chu sequence, the DM-RS sequence for DFT-s-OFDM, r(n), can be generated as follows: 
     
       
         
           
             r 
             
               n 
             
             = 
             
               r 
               
                 u 
                 , 
                 v 
               
               
                 
                   
                     α 
                     , 
                     δ 
                   
                 
               
             
             
               n 
             
           
         
       
     
      where  
     
       
         
           
             n 
             = 
             0 
             , 
             1 
             , 
             ... 
             , 
             
               
                 
                   
                     
                       M 
                       
                         s 
                         c 
                       
                       
                         P 
                         U 
                         S 
                         C 
                         H 
                       
                     
                   
                 
               
               / 
               
                 
                   2 
                   δ 
                 
                 − 
                 1 
               
             
             . 
           
         
       
     
     The low-PAPR sequence parameters δ is given as 1 and the cyclic shift, α, is set to 0 for a PUSCH transmission. The sequence group number, u, equals to  
     
       
         
           
             
               
                 
                   f 
                   
                     g 
                     h 
                   
                 
                 + 
                 
                   n 
                   
                     I 
                     D 
                   
                   
                     R 
                     D 
                   
                 
               
             
             m 
             o 
             d 
             30 
             , 
           
         
       
     
     where  
     
       
         
           
             
               n 
               
                 I 
                 D 
               
               
                 R 
                 S 
               
             
           
         
       
     
     denotes the DM-RS scrambling identity. Furthermore, the values of f gh  and the base sequence number, ν, are given as follows: 
     1) When neither group nor sequence hopping is enabled:             f     g   h       =   0       a   n   d       v       =       0           2) When group hopping is enabled:             f     g   h       =           ∑     m   =   0     7         2   m     c       8         N     s   y   m   b       s   l   o   t         n     s   ,   f     μ     +   l       +   m               m   o   d   30            and           v   =   0            where the pseudo-random sequence, c(i), should be initialized with c init  =   3) When sequence hopping is enabled:             f     g   h       =   0       a   n   d       v   =               c         N     s   y   m   b       s   l   o   t         n     s   ,   f     μ     +   l               i   f         M     Z   C       ≥   6     N     s   c       R   B                 0         O   t   h   e   r   w   i   s   e                        where the pseudo-random sequence, c(i), should be initialized with c init  =              n     I   D       R   S               .   

     DM-RS Precoding and Mapping to Physical Resources 
     The orthogonality of the DM-RS sequences that are allocated to the different antenna ports is achieved by multiplexing in time, frequency, and code domains. To realize this, the DM-RS sequence is multiplied with the orthogonal cover codes in time and frequency domains (i.e., w t (k) and w f (k) respectively). Also, shifts in the frequency domain are included to support more antenna ports. The DM-RS sequence, r(m) is mapped to an intermediate quantity  
     
       
         
           
             
               
                 a 
                 ˜ 
               
               
                 k 
                 , 
                 l 
               
               
                 
                   
                     
                       
                         p 
                         ˜ 
                       
                       j 
                     
                     , 
                     μ 
                   
                 
               
             
           
         
       
     
     as follows: 
     
       
         
           
             
               
                 
                   
                     a 
                     ˜ 
                   
                   
                     k 
                     , 
                     l 
                   
                   
                     
                       
                         
                           
                             p 
                             ˜ 
                           
                           j 
                         
                         , 
                         μ 
                       
                     
                   
                 
                 = 
                 
                   w 
                   f 
                 
                 
                   
                     k 
                     ′ 
                   
                 
                 
                   w 
                   t 
                 
                 
                   
                     l 
                     ′ 
                   
                 
                 r 
                 
                   
                     2 
                     n 
                     + 
                     
                       k 
                       ′ 
                     
                   
                 
               
             
             
               
                 k 
                 = 
                 
                   
                     
                       
                         
                           
                             4 
                             n 
                             + 
                             2 
                             
                               k 
                               ′ 
                             
                             + 
                             Δ 
                               
                               
                             C 
                             o 
                             n 
                             f 
                             i 
                             g 
                             u 
                             r 
                             a 
                             t 
                             i 
                             o 
                             n 
                               
                             T 
                             y 
                             p 
                             e 
                               
                             1 
                           
                         
                       
                       
                         
                           
                             6 
                             n 
                             + 
                             
                               k 
                               ′ 
                             
                             + 
                             Δ 
                               
                               
                             C 
                             o 
                             n 
                             f 
                             i 
                             g 
                             u 
                             r 
                             a 
                             t 
                             i 
                             o 
                             n 
                               
                             T 
                             y 
                             p 
                             e 
                               
                             2 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 
                   k 
                   ′ 
                 
                 = 
                 0 
                 , 
                 1 
               
             
             
               
                 l 
                 = 
                 
                   l 
                   ¯ 
                 
                 + 
                 
                   l 
                   ′ 
                 
               
             
             
               
                 n 
                 = 
                 0 
                 , 
                 1 
                 , 
                 … 
               
             
             
               
                 j 
                 = 
                 0 
                 , 
                 1 
                 , 
                 … 
                 , 
                 v 
                 − 
                 1 
               
             
           
         
       
     
      where w f (k′), w t (l′), and Δ are defined by Tables 6.4.1.1.3-1 and Tables 6.4.1.1.3-2 in TS .. Also, p̃ j  represents the antenna port for transmission layer j. However, DFT-s-OFDM supports only single-layer transmission, and j is equal to 0 when transform-precoding is enabled. 
     DM-RS configuration type determines the DM-RS density in the frequency domain, which also impacts the available number of orthogonal sequences. DM-RS configuration Type 1 has a higher density in the frequency domain and provides a more accurate channel estimation. On the other hand, DM-RS configuration Type 2 has a lower density in the frequency domain, and it allows supporting more antenna ports. Type 1 can support up to 8 antenna ports, whereas Type 2 can support up to 12 antenna ports. These are illustrated in  FIGS.  10 A and  10 B , along with the associated mapping and orthogonal cover codes obtained from Tables 6.4.1.1.3-1 and Tables 6.4.1.1.3-2 in TS 38.211. 
     The reference point for k depends on the waveform type. For CP-OFDM (i.e., when transform precoding is disabled), k is subcarrier 0 in the common resource block. On the other hand, for DFT-s-OFDM (i.e., when transform precoding is enabled), k is subcarrier 0 of the lowest-numbered RB of the scheduled PUSCH allocation. 
     The reference point of l and the position of the first DM-RS symbol (l o ) depends on the PUSCH mapping type and activation of the frequency hopping scheme. For PUSCH Mapping Type A, l is defined relative to the start of the slot if frequency hopping is disabled, and it is defined relative to the start of each hop if frequency hopping is enabled. l 0  is indicated by dmrs — TypeA — Position parameter, and it refers to either the third or fourth OFDM symbol in a slot. On the other hand, for PUSCH Mapping Type B, l is defined relative to the start of the scheduled PUSCH resources if frequency hopping is disabled, and it is defined relative to the start of each hop if frequency hopping is enabled. In this case, l 0  equals to 0; in other words, it refers to the first symbol in the scheduled data symbol duration. Exemplary DM-RS mappings are illustrated in  FIGS.  10 C and  10 D  for PUSCH Mapping Type A and PUSCH Mapping Type B accordingly. 
     The position(s) of the DM-RS symbols is provided by l̃ parameter and it depends on the PUSCH mapping type, activation of the frequency hopping scheme, and the number of additional DM-RS symbols. Up to 4 DM-RS symbols (i.e., up to 3 additional positions) in a slot are possible for high mobility scenarios. Exemplary DM-RS allocations with additional positions are presented in  FIG.  10 E . The position(s) of the DM-RS symbols (l̃), the time-domain index (l′), and the supported antenna ports p̃ j  are provided by Tables 6.4.1.1.3-3 through 6.4.1.1.3-6 in TS 38.211 
     In the following step, the intermediate quantity,  
     
       
         
           
             
               
                 a 
                 ˜ 
               
               
                 k 
                 , 
                 l 
               
               
                 
                   
                     
                       
                         p 
                         ˜ 
                       
                       j 
                     
                     , 
                     μ 
                   
                 
               
             
           
         
       
     
     , is precoded with W, multiplied with the amplitude scaling factor  
     
       
         
           
             
               β 
               
                 P 
                 U 
                 S 
                 C 
                 H 
               
               
                 D 
                 M 
                 R 
                 S 
               
             
           
         
       
     
     and mapped to the physical resources as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           a 
                           
                             k 
                             , 
                             l 
                           
                           
                             
                               
                                 0 
                                 , 
                                 μ 
                               
                             
                           
                         
                         
                           m 
                         
                       
                     
                   
                   
                     
                       ⋮ 
                     
                   
                   
                     
                       
                         
                           a 
                           
                             k 
                             , 
                             l 
                           
                           
                             
                               
                                 
                                   p 
                                   
                                     v 
                                     − 
                                     1 
                                   
                                 
                                 , 
                                 μ 
                               
                             
                           
                         
                         
                           m 
                         
                       
                     
                   
                 
               
             
             = 
             
               β 
               
                 P 
                 U 
                 S 
                 C 
                 H 
               
               
                 D 
                 M 
                 R 
                 S 
               
             
             W 
             
               
                 
                   
                     
                       
                         
                           
                             a 
                             ˜ 
                           
                           
                             k 
                             , 
                             l 
                           
                           
                             
                               
                                 0 
                                 , 
                                 μ 
                               
                             
                           
                         
                         
                           m 
                         
                       
                     
                   
                   
                     
                       ⋮ 
                     
                   
                   
                     
                       
                         
                           
                             a 
                             ˜ 
                           
                           
                             k 
                             , 
                             l 
                           
                           
                             
                               
                                 
                                   
                                     p 
                                     ˜ 
                                   
                                   
                                     v 
                                     − 
                                     1 
                                   
                                 
                                 , 
                                 μ 
                               
                             
                           
                         
                         
                           m 
                         
                       
                     
                   
                 
               
             
           
         
       
     
      where and β DMRS  is given by Table 6.2.2-1 in TS 38.214. The required β DMRS  value is signaled with the number of DM-RS CDM groups without data parameter. Also, W is given by Table 6.3.1.5-1 to Table 6.3.1.5.-7 in TS 38.211. The required precoding matrix is signaled through the TPMI index parameter. 
     The DM-RS sequence can either be sent from O-DU to O-RU or the DM-RS configuration parameters, which are required to re-generate the DM-RS sequence in the O-RU, can be transmitted from O-DU to O-RU. 
     Embodiment 7: Channel Estimation 
     In an OFDM system with N subcarriers, the received signal in the frequency domain can be expressed as follows: 
     
       
         
           
             Y 
             = 
             X 
             H 
             + 
             Z 
           
         
       
     
      where Y is received signal vector, X is a diagonal matrix where each diagonal element represents a reference signal (such as DM-RS) in subcarrier k, H is the channel vector, and Z is the noise vector. 
       FIG.  11    E is flow chart showing 4 exemplary implementations for DM-RS channel estimation techniques, described in more detail with respect to  FIGS.  11 A- 11 D . Examples of DM-RS channel estimation techniques that O-RU can use include the following: 
     Frequency-Domain Channel Estimation Techniques: 
     1) Least-Squares (LS) Channel Estimation 
       FIG.  11 A  illustrates an LS channel estimation. After extracting the DM-RS symbols of the received signal in the frequency domain, the LS channel estimation can be expressed as follows: 
     
       
         
           
             
               H 
               
                 L 
                 S 
               
             
             
               
                 
                   k 
                   i 
                 
                 , 
                 
                   l 
                   j 
                 
                 , 
                 
                   r 
                   m 
                 
                 , 
                 
                   t 
                   n 
                 
               
             
             = 
             
               
                 
                   Y 
                   
                     R 
                     S 
                   
                 
                 
                   
                     
                       k 
                       i 
                     
                     , 
                     
                       l 
                       j 
                     
                     , 
                     
                       r 
                       m 
                     
                     , 
                     
                       t 
                       n 
                     
                   
                 
               
               
                 
                   X 
                   
                     D 
                     M 
                     R 
                     S 
                   
                 
                 
                   
                     
                       k 
                       i 
                     
                     , 
                     
                       l 
                       j 
                     
                     , 
                     
                       r 
                       m 
                     
                     , 
                     
                       t 
                       n 
                     
                   
                 
               
             
           
         
       
     
      where k i  represents the subcarrier index, l j  represents the symbol index, r m  is the receiver antenna index, and t n  is the DM-RS transmission port index. If the DM-RS belongs to code-division multiplexing (CDM) group of size L with an orthogonal cover code (OCC), the channel estimate can be de-spreaded as follows: 
     
       
         
           
             
               
                 
                   H 
                   ^ 
                 
                 ′ 
               
               
                 L 
                 S 
               
             
             
               
                 D 
                 M 
                 R 
                 
                   S 
                   g 
                 
                 , 
                 
                   r 
                   m 
                 
                 , 
                 
                   t 
                   n 
                 
               
             
             = 
             
               
                 ∑ 
                 
                   i 
                   , 
                   j 
                   = 
                   1 
                 
                 2 
               
               
                 
                   
                     
                       H 
                       
                         L 
                         S 
                       
                     
                     
                       
                         
                           k 
                           i 
                         
                         , 
                         
                           l 
                           j 
                         
                         , 
                         
                           r 
                           m 
                         
                         , 
                         
                           t 
                           n 
                         
                       
                     
                     
                       c 
                       
                         i 
                         j 
                       
                     
                     
                       
                         
                           t 
                           n 
                         
                       
                     
                   
                   L 
                 
               
             
           
         
       
     
      where c ij (t n ) represents the code in symbol i, subcarrier j, and DM-RS transmission port n. Also, L is the size of the RE group where the 2-length OCC is applied. When single DM-RS is used, L is equal to 2, and when double DM-RS is used, L is equal to 4. 
     The channel estimates, Ĥ′ LS (DMRS g ,r m ,t n ), in a given RB can be interpolated in the time and frequency domains to obtain channel estimates over data symbol. Also, these channel estimates can be averaged for an initial channel estimate as follows: 
     
       
         
           
             
               
                 
                   H 
                   ^ 
                 
                 ′ 
               
               
                 L 
                 S 
               
             
             
               
                 
                   n 
                   
                     R 
                     B 
                   
                 
                 , 
                 
                   n 
                   s 
                 
                 , 
                 
                   r 
                   m 
                 
                 , 
                 
                   t 
                   n 
                 
               
             
             = 
             
               
                 ∑ 
                 
                   g 
                   = 
                   1 
                 
                 G 
               
               
                 
                   
                     
                       
                         
                           H 
                           ^ 
                         
                         ′ 
                       
                       
                         L 
                         S 
                       
                     
                     
                       
                         D 
                         M 
                         R 
                         
                           S 
                           g 
                         
                         , 
                         
                           r 
                           m 
                         
                         , 
                         
                           t 
                           n 
                         
                       
                     
                   
                   G 
                 
               
             
           
         
       
     
      where n RB  represents the RB index, n s  shows the slot index, DMRS g  denotes the DM-RS RE group index, and G is the total number of DM-RS RE groups in a given RB. Depending on the complexity and channel conditions, the channel estimates belong to multiple RBs can be averaged as well. 
     2) Minimum Mean-Square Error (MMSE) Channel Estimation 
       FIG.  11 B  illustrates an MMSE channel estimation. Considering, a channel estimate of Ĥ, the MMSE finds a better linear estimate (i.e., WĤ) that minimizes the minimum square error (MSE), which is calculated as E{||H — Ĥ|| 2 }. Without loss of generality, the LS channel estimate (Ĥ LS ) can be used as an initial estimate, and the MMSE channel estimate can be expressed as follows: 
     
       
         
           
             
               
                 H 
                 ^ 
               
               
                 M 
                 M 
                 S 
                 E 
               
             
             = 
             W 
             
               
                 H 
                 ^ 
               
               
                 L 
                 S 
               
             
             = 
             
               R 
               
                 H 
                 
                   
                     H 
                     ^ 
                   
                   
                     L 
                     S 
                   
                 
               
             
             
               
                 
                   
                     
                       R 
                       
                         H 
                         H 
                       
                     
                     + 
                     
                       
                         
                           σ 
                           Z 
                           2 
                         
                       
                       
                         
                           σ 
                           x 
                           2 
                         
                       
                     
                     I 
                   
                 
               
               
                 − 
                 1 
               
             
             
               
                 H 
                 ^ 
               
               
                 L 
                 S 
               
             
           
         
       
     
      where  
     
       
         
           
             
               
                 
                   σ 
                   Z 
                   2 
                 
               
               
                 
                   σ 
                   x 
                   2 
                 
               
             
           
         
       
     
     denotes the  
     
       
         
           
             
               1 
               
                 S 
                 N 
                 R 
               
             
           
         
       
     
     and R is the cross-correlation matrix. The expression inside the parenthesis (i.e., R HH  +  
     
       
         
           
             
               
                 
                   σ 
                   Z 
                   2 
                 
               
               
                 
                   σ 
                   x 
                   2 
                 
               
             
           
         
       
     
     I) is called as the MMSE matrix and can be denoted with Ø. The elements of R HĤ LS    and R HH  are given as follows: 
     
       
         
           
             E 
             
               
                 
                   h 
                   
                     K 
                     , 
                     l 
                   
                 
                 
                   h 
                   ^ 
                 
                 
                   * 
                   
                     k 
                     ′ 
                     , 
                     l 
                     ′ 
                   
                 
               
             
             = 
             E 
             
               
                 
                   h 
                   
                     k 
                     , 
                     l 
                   
                 
                 h 
                 
                   * 
                   
                     k 
                     ′ 
                     , 
                     l 
                     ′ 
                   
                 
               
             
             = 
             
               r 
               f 
             
             
               
                 k 
                 − 
                 k 
                 ′ 
               
             
             
               r 
               t 
             
             
               
                 l 
                 − 
                 l 
                 ′ 
               
             
           
         
       
     
     Assuming an exponentially decaying multipath power delay profile, the frequency-domain correlation can be stated as follows: 
     
       
         
           
             
               r 
               f 
             
             
               k 
             
             = 
             
               1 
               
                 1 
                 + 
                 j 
                 2 
                 π 
                 
                   τ 
                   
                     R 
                     M 
                     S 
                   
                 
                 k 
                 Δ 
                 f 
               
             
           
         
       
     
      where τ RMS  denotes the RMS delay spread of the channel, and Δf is the subcarrier spacing. Also, assuming a fading channel with Jake’s spectrum, the time-domain correlation can be expressed as follows: 
     
       
         
           
             
               r 
               t 
             
             
               l 
             
             = 
             
               J 
               0 
             
             
               
                 2 
                 π 
                 
                   f 
                   
                     
                       D 
                       
                         m 
                         a 
                         x 
                       
                     
                   
                 
                 l 
                   
                 
                   T 
                   s 
                 
               
             
           
         
       
     
      where f D max    is the maximum Doppler spread of the channel, T s  is the OFDM symbol duration, and J 0  is the first kind of the 0 th -order Bessel function. Alternatively, a sinc function can be adapted to represent the frequency and time domain correlations as well, and the elements of R HĤ LS    and R HH  can also be formulated as follows: 
     
       
         
           
             
               r 
               f 
             
             
               
                 k 
                 − 
                 
                   k 
                   ′ 
                 
               
             
             
               r 
               t 
             
             
               
                 l 
                 − 
                 
                   l 
                   ′ 
                 
               
             
             = 
             s 
             i 
             n 
             c 
             
               
                 2 
                 
                   τ 
                   
                     R 
                     M 
                     S 
                   
                 
                 Δ 
                 f 
                 
                   
                     k 
                     − 
                     
                       k 
                       ′ 
                     
                   
                 
               
             
             s 
             i 
             n 
             c 
             
               
                 2 
                 π 
                 
                   f 
                   
                     
                       D 
                       
                         m 
                         a 
                         x 
                       
                     
                   
                 
                 
                   T 
                   s 
                 
                 
                   
                     l 
                     − 
                     
                       l 
                       ′ 
                     
                   
                 
               
             
           
         
       
     
     After calculating W, the MMSE channel estimation can be performed as below: 
     
       
         
           
             
               
                 
                   H 
                   ^ 
                 
                 ′ 
               
               
                 M 
                 M 
                 S 
                 E 
               
             
             
               
                 k 
                 , 
                 l 
                 , 
                 r 
                 , 
                 t 
               
             
             = 
             W 
             
               
                 k 
                 , 
                 l 
                 ; 
                 
                   k 
                   ′ 
                 
                 , 
                 
                   l 
                   ′ 
                 
               
             
             
               
                 H 
                 ^ 
               
               
                 L 
                 S 
               
             
             
               
                 
                   k 
                   ′ 
                 
                 , 
                 
                   l 
                   ′ 
                 
                 , 
                 r 
                 , 
                 t 
               
             
           
         
       
     
     Considering the DM-RS structure, the MMSE channel estimation in the vicinity of a group of REs, where a 2-length OCC is applied, can be calculated as follows: 
     
       
         
           
             
               
                 
                   
                     
                       H 
                       ^ 
                     
                     ′ 
                   
                   
                     M 
                     M 
                     S 
                     E 
                   
                 
                 
                   
                     D 
                     M 
                     R 
                     
                       S 
                       g 
                     
                     
                       
                         
                           k 
                           i 
                         
                         , 
                         
                           l 
                           j 
                         
                       
                     
                     , 
                     
                       r 
                       m 
                     
                     , 
                     
                       t 
                       n 
                     
                   
                 
                 = 
               
             
             
               
                 W 
                 
                   
                     
                       k 
                       i 
                     
                     , 
                     
                       l 
                       j 
                     
                     ; 
                     
                       k 
                       
                         i 
                         ′ 
                       
                     
                     , 
                     
                       l 
                       
                         j 
                         ′ 
                       
                     
                   
                 
                 
                   
                     
                       H 
                       ^ 
                     
                     ′ 
                   
                   
                     L 
                     S 
                   
                 
                 
                   
                     D 
                     M 
                     R 
                     
                       S 
                       g 
                     
                     
                       
                         
                           k 
                           
                             i 
                             ′ 
                           
                         
                         , 
                         
                           l 
                           
                             j 
                             ′ 
                           
                         
                       
                     
                     , 
                     
                       r 
                       m 
                     
                     , 
                     
                       t 
                       n 
                     
                   
                 
               
             
           
         
       
     
      where k i , and l j , show the subcarrier and symbol indices of the DM-RS RE group. Also, these channel estimates can be averaged for an initial channel estimate as follows: 
     
       
         
           
             
               
                 H 
                 ^ 
               
               
                 M 
                 M 
                 S 
                 E 
               
             
             
               
                 
                   n 
                   
                     R 
                     B 
                   
                 
                 , 
                 
                   n 
                   s 
                 
                 , 
                 
                   r 
                   m 
                 
                 , 
                 
                   t 
                   n 
                 
               
             
             = 
             
               
                 ∑ 
                 
                   i 
                   , 
                   j 
                   , 
                   g 
                   = 
                   1 
                 
                 
                   I 
                   , 
                   J 
                   , 
                   G 
                 
               
               
                 
                   
                     
                       
                         
                           H 
                           ^ 
                         
                         ′ 
                       
                       
                         M 
                         M 
                         S 
                         E 
                       
                     
                     
                       
                         D 
                         M 
                         R 
                         
                           S 
                           g 
                         
                         
                           
                             
                               k 
                               i 
                             
                             , 
                             
                               l 
                               j 
                             
                           
                         
                         , 
                         
                           r 
                           m 
                         
                         , 
                         
                           t 
                           n 
                         
                       
                     
                   
                   
                     I 
                     × 
                     J 
                     × 
                     G 
                   
                 
               
             
           
         
       
     
      where n RB  represents the RB index, n s  shows the slot index, I and J represents the total number of subcarriers and symbols that are used in this averaging operation. Depending on the complexity and channel conditions, the channel estimates belong to multiple RBs can be averaged as well. 
     Accordingly, the noise power can be estimated as follows: 
     
       
         
           
             
               
                 
                   Z 
                   ^ 
                 
                 
                   
                     
                       k 
                       i 
                     
                     , 
                       
                     
                       l 
                       j 
                     
                     , 
                       
                     
                       r 
                       m 
                     
                   
                 
                 = 
                 
                   Y 
                   
                     R 
                     S 
                   
                 
                 
                   
                     
                       k 
                       i 
                     
                     , 
                       
                     
                       l 
                       j 
                     
                     , 
                       
                     
                       r 
                       m 
                     
                   
                 
                 − 
                 
                   
                     ∑ 
                     n 
                   
                   
                     
                       X 
                       
                         D 
                         M 
                         R 
                         S 
                       
                     
                     
                       
                         
                           k 
                           i 
                         
                         , 
                           
                         
                           l 
                           j 
                         
                         , 
                           
                         
                           t 
                           n 
                         
                       
                     
                       
                     
                       c 
                       
                         i 
                         j 
                       
                     
                     
                       
                         
                           t 
                           n 
                         
                       
                     
                   
                 
               
             
             
               
                 
                   
                     
                       H 
                       ^ 
                     
                     ′ 
                   
                   
                       
                     M 
                     M 
                     S 
                     E 
                   
                 
                 
                   
                     D 
                     M 
                     R 
                     
                       S 
                       g 
                     
                     
                       
                         
                           k 
                           i 
                         
                         , 
                           
                         
                           l 
                           j 
                         
                       
                     
                     , 
                       
                     
                       r 
                       m 
                     
                     , 
                       
                     
                       t 
                       n 
                     
                   
                 
               
             
             
               
                 
                   
                     σ 
                     ^ 
                   
                   Z 
                   2 
                 
                 = 
                 m 
                 e 
                 a 
                 n 
                 
                   
                     
                       Z 
                       ^ 
                     
                     
                       
                         Z 
                         ^ 
                       
                       H 
                     
                   
                 
               
             
           
         
       
     
      where the mean operation is defined over all receiver antennas and DM-RS REs. 
     Time-Domain Channel Estimation: 
     1) Discrete Fourier Transform (DFT) Based Channel Estimation 
     A DFT-based channel estimation is illustrated in  FIG.  11 C . The performance of channel estimation (LS or MMSE) can be improved with a DFT-based channel estimation technique by filtering the noise outside the maximum delay of the channel. Considering the complexity of the system, the LS channel estimate (Ĥ LS ) can be used as an initial estimate, and it can be converted to the time domain with an IDFT operation as follows: 
     
       
         
           
             
               
                 h 
                 ^ 
               
               
                 L 
                 S 
               
               
                 D 
                 F 
                 T 
               
             
             
               
                 
                   n 
                   i 
                 
                 , 
                   
                 
                   l 
                   j 
                 
                 , 
                   
                 
                   r 
                   m 
                 
                 , 
                   
                 
                   t 
                   n 
                 
               
             
             = 
             I 
             D 
             F 
             T 
             
               
                 
                   
                     
                       H 
                       ^ 
                     
                     ′ 
                   
                   
                       
                     L 
                     S 
                   
                 
                 
                   
                     
                       k 
                       i 
                     
                     , 
                       
                     
                       l 
                       j 
                     
                     , 
                       
                     
                       r 
                       m 
                     
                     , 
                       
                     
                       t 
                       n 
                     
                   
                 
               
             
           
         
       
     
     Afterward, a window (e.g., rectangular) can be applied to filter the noise outside the maximum delay of the channel as follows: 
     
       
         
           
             
               
                 h 
                 ^ 
               
               
                 f 
                 i 
                 l 
                 t 
                 e 
                 r 
                 e 
                 d 
               
               
                 D 
                 F 
                 T 
               
             
             
               
                 
                   n 
                   i 
                 
                 , 
                   
                 
                   l 
                   j 
                 
                 , 
                   
                 
                   r 
                   m 
                 
                 , 
                   
                 
                   t 
                   n 
                 
               
             
             = 
             
               
                 
                   
                     
                       
                         
                           
                             h 
                             ^ 
                           
                           
                             L 
                             S 
                           
                           
                             D 
                             F 
                             T 
                           
                         
                         
                           
                             
                               n 
                               i 
                             
                             , 
                               
                             
                               l 
                               j 
                             
                             , 
                               
                             
                               r 
                               m 
                             
                             , 
                               
                             
                               t 
                               n 
                             
                           
                         
                       
                     
                     
                       
                         0 
                         ≤ 
                         
                           n 
                           i 
                         
                         ≤ 
                         w 
                       
                     
                   
                   
                     
                       0 
                     
                     
                       
                         o 
                         t 
                         h 
                         e 
                         r 
                         w 
                         i 
                         s 
                         e 
                       
                     
                   
                 
               
             
           
         
       
     
      where w denotes the window length. To improve the performance further, the channel taps that are below a certain threshold value are set to 0. This threshold value can be calculated by the average energy of the channel taps outside the window. The thresholding operation is performed as follows: 
     
       
         
           
             
               
                 h 
                 ^ 
               
               
                 t 
                 h 
                 r 
                 e 
                 s 
                 h 
                 o 
                 l 
                 d 
                 e 
                 d 
               
               
                 D 
                 F 
                 T 
               
             
             
               
                 
                   n 
                   i 
                 
                 , 
                   
                 
                   l 
                   j 
                 
                 , 
                   
                 
                   r 
                   m 
                 
                 , 
                   
                 
                   t 
                   n 
                 
               
             
             = 
             
               
                 
                   
                     
                       
                         
                           
                             h 
                             ^ 
                           
                           
                             L 
                             S 
                           
                           
                             D 
                             F 
                             T 
                           
                         
                         
                           
                             
                               n 
                               i 
                             
                             , 
                               
                             
                               l 
                               j 
                             
                             , 
                               
                             
                               r 
                               m 
                             
                             , 
                               
                             
                               t 
                               n 
                             
                           
                         
                       
                     
                     
                       
                         
                           P 
                           
                             
                               n 
                               i 
                             
                           
                         
                         ≥ 
                         T 
                         h 
                         r 
                         e 
                         s 
                         h 
                         o 
                         l 
                         d 
                       
                     
                   
                   
                     
                       0 
                     
                     
                       
                         o 
                         t 
                         h 
                         e 
                         r 
                         w 
                         i 
                         s 
                         e 
                       
                     
                   
                 
               
             
           
         
       
     
      where P n i    represents the tap power at n i . Also, the noise power can be estimated (i.e.,  
     
       
         
           
             
               
                 
                   
                     σ 
                     ^ 
                   
                   Z 
                   2 
                 
               
             
           
         
       
     
     by taking the average power of all taps that are set to 0. Following this operation, the time-domain estimate is transformed to the frequency domain by the DFT operation. 
     
       
         
           
             
               
                 
                   H 
                   ^ 
                 
                 ′ 
               
               
                 D 
                 F 
                 T 
               
             
             
               
                 
                   k 
                   i 
                 
                 , 
                 
                   l 
                   j 
                 
                 , 
                 
                   r 
                   m 
                 
                 , 
                 
                   t 
                   n 
                 
               
             
             = 
             D 
             F 
             T 
             
               
                 
                   
                     h 
                     ^ 
                   
                   
                     t 
                     h 
                     r 
                     e 
                     s 
                     h 
                     o 
                     l 
                     d 
                     e 
                     d 
                   
                   
                     D 
                     F 
                     T 
                   
                 
                 
                   
                     
                       n 
                       j 
                     
                     , 
                     
                       l 
                       j 
                     
                     , 
                     
                       r 
                       m 
                     
                     , 
                     
                       t 
                       n 
                     
                   
                 
               
             
           
         
       
     
     These channel estimates can be averaged for an initial channel estimate as follows: 
     
       
         
           
             
               
                 H 
                 ^ 
               
               
                 D 
                 F 
                 T 
               
             
             
               
                 
                   n 
                   
                     R 
                     B 
                   
                 
                 , 
                 
                   n 
                   s 
                 
                 , 
                 
                   r 
                   m 
                 
                 , 
                 
                   t 
                   n 
                 
               
             
             = 
             
               ∑ 
               
                 
                     
                   
                     i 
                     , 
                     j 
                   
                   
                     I 
                     , 
                     J 
                   
                 
                 
                   
                     
                       
                         
                           H 
                           ^ 
                         
                         ′ 
                       
                       
                         D 
                         F 
                         T 
                       
                     
                     
                       
                         
                           k 
                           i 
                         
                         , 
                         
                           l 
                           j 
                         
                         , 
                         
                           r 
                           m 
                         
                         , 
                         
                           t 
                           n 
                         
                       
                     
                   
                   
                     I 
                     × 
                     J 
                   
                 
               
             
           
         
       
     
     Depending on the complexity and channel conditions, the channel estimates belong to multiple RBs can be averaged as well. 
     2) Discrete Cosine Transform (DCT) Based Channel Estimation 
     A DCT based channel estimation is illustrated in  FIG.  11 D . 
     The performance of channel estimation (LS or MMSE) can be improved with a DCT-based channel estimation technique by filtering the noise outside the maximum delay of the channel, similar to the DFT-based estimation. However, the DCT-based estimation provides better power concentration and lower aliasing error than the DFT-based interpolation. Considering the complexity of the system, the LS channel estimate (Ĥ LS ) can be used as an initial estimate, and it can be converted to the transform domain with a DCT operation as follows: 
     
       
         
           
             
               
                 h 
                 ^ 
               
               
                 L 
                 S 
               
               
                 D 
                 C 
                 T 
               
             
             
               
                 
                   n 
                   i 
                 
                 , 
                 
                   l 
                   j 
                 
                 , 
                 
                   r 
                   m 
                 
                 , 
                 
                   t 
                   n 
                 
               
             
             = 
             D 
             C 
             T 
             
               
                 
                   
                     
                       H 
                       ^ 
                     
                     ′ 
                   
                   
                     L 
                     S 
                   
                 
                 
                   
                     
                       k 
                       i 
                     
                     , 
                     
                       l 
                       j 
                     
                     , 
                     
                       r 
                       m 
                     
                     , 
                     
                       t 
                       n 
                     
                   
                 
               
             
           
         
       
     
     Afterward, a window (e.g., rectangular) can be applied to filter the noise outside the maximum delay of the channel as follows: 
     
       
         
           
             
               
                 h 
                 ^ 
               
               
                 f 
                 i 
                 l 
                 t 
                 e 
                 r 
                 e 
                 d 
               
               
                 D 
                 C 
                 T 
               
             
             
               
                 
                   n 
                   i 
                 
                 , 
                 
                   l 
                   j 
                 
                 , 
                 
                   r 
                   m 
                 
                 , 
                 
                   t 
                   n 
                 
               
             
             = 
             
               
                 
                   
                     
                         
                       
                         
                           
                             h 
                             ^ 
                           
                           
                             L 
                             S 
                           
                           
                             D 
                             C 
                             T 
                           
                         
                       
                     
                   
                   
                       
                     0 
                   
                   
                     
                       
                         
                           n 
                           i 
                         
                         , 
                         
                           l 
                           j 
                         
                         , 
                         
                           r 
                           m 
                         
                         , 
                         
                           t 
                           n 
                         
                       
                     
                   
                 
                 
                     
                   
                     o 
                     t 
                     h 
                     e 
                     r 
                     w 
                     i 
                     s 
                     e 
                   
                   
                     0 
                     ≤ 
                       
                       
                     
                       n 
                       j 
                     
                       
                       
                     ≤ 
                     w 
                   
                 
               
             
           
         
       
     
      where w denotes the window length. To improve the performance further, the channel taps that are below a certain threshold value are set to 0. This threshold value can be calculated by the average energy of the channel taps outside the window. The thresholding operation is performed as follows: 
     
       
         
           
             
               
                 h 
                 ^ 
               
               
                 t 
                 h 
                 r 
                 e 
                 s 
                 h 
                 o 
                 l 
                 d 
                 e 
                 d 
               
               
                 D 
                 C 
                 T 
               
             
             
               
                 
                   n 
                   i 
                 
                 , 
                 
                   l 
                   j 
                 
                 , 
                 
                   r 
                   m 
                 
                 , 
                 
                   t 
                   n 
                 
               
             
             = 
             
               
                 
                   
                     
                         
                       
                         
                           
                             h 
                             ^ 
                           
                           
                             L 
                             S 
                           
                           
                             D 
                             C 
                             T 
                           
                         
                       
                     
                   
                   
                       
                     0 
                   
                   
                     
                       
                         
                           n 
                           i 
                         
                         , 
                         
                           l 
                           j 
                         
                         , 
                         
                           r 
                           m 
                         
                         , 
                         
                           t 
                           n 
                         
                       
                     
                   
                 
                 
                     
                   
                       
                     o 
                     t 
                     h 
                     e 
                     r 
                     w 
                     i 
                     s 
                     e 
                   
                   
                     
                       P 
                       
                         n 
                         j 
                       
                     
                     ≥ 
                     T 
                     h 
                     r 
                     e 
                     s 
                     h 
                     o 
                     l 
                     d 
                   
                 
               
             
           
         
       
     
      where P n i    represents the tap power at n i . Following this operation, it is transformed to the frequency domain by the IDCT operation. 
     
       
         
           
             
               
                 
                   H 
                   ^ 
                 
                 ′ 
               
               
                 D 
                 C 
                 T 
               
             
             
               
                 
                   k 
                   i 
                 
                 , 
                 
                   l 
                   j 
                 
                 , 
                 
                   r 
                   m 
                 
                 , 
                 
                   t 
                   n 
                 
               
             
             = 
             I 
             D 
             C 
             T 
             
               
                 
                   
                     h 
                     ^ 
                   
                   
                     t 
                     h 
                     r 
                     e 
                     s 
                     h 
                     o 
                     l 
                     d 
                     e 
                     d 
                   
                   
                     D 
                     C 
                     T 
                   
                 
                 
                   
                     
                       n 
                       i 
                     
                     , 
                     
                       l 
                       j 
                     
                     , 
                     
                       r 
                       m 
                     
                     , 
                     
                       t 
                       n 
                     
                   
                 
               
             
           
         
       
     
      These channel estimates can be averaged for an initial channel estimate as follows: 
     
       
         
           
             
               
                 H 
                 ^ 
               
               
                 D 
                 C 
                 T 
               
             
             
               
                 
                   n 
                   
                     R 
                     B 
                     , 
                   
                 
                 
                   n 
                   s 
                 
                 , 
                 
                   r 
                   m 
                 
                 , 
                 
                   t 
                   n 
                 
               
             
             = 
             
               
                 ∑ 
                 
                   i 
                   , 
                   j 
                 
                 
                   I 
                   , 
                   J 
                 
               
               
                 
                   
                     
                       
                         
                           H 
                           ^ 
                         
                         ′ 
                       
                       
                         D 
                         C 
                         T 
                       
                     
                     
                       
                         
                           k 
                           i 
                         
                         , 
                         
                           l 
                           j 
                         
                         , 
                         
                           r 
                           m 
                         
                         , 
                         
                           t 
                           n 
                         
                       
                     
                   
                   
                     I 
                     × 
                     J 
                   
                 
               
             
           
         
       
     
     Depending on the complexity and channel conditions, the channel estimates belong to multiple RBs can be averaged as well, similar to the other channel estimation techniques that are described previously. 
     Embodiment 8: Channel Equalization 
     Examples of channel equalization techniques that O-RU can use include the following: 
     A. Zero-Forcing (ZF) Equalizer 
     In a MIMO OFDM system, the received data symbols in the frequency domain can be expressed as follows: 
     
       
         
           
             
               Y 
               
                 D 
                 S 
               
             
             
               
                 k 
                 , 
                 r 
               
             
             = 
             X 
             
               
                 k 
                 , 
                 t 
               
             
             H 
             
               
                 k 
                 , 
                 r 
                 , 
                 t 
               
             
             + 
             Z 
             
               
                 k 
                 , 
                 r 
               
             
           
         
       
     
     
       
         
           
             
               
                 H 
                 
                   
                     k 
                     , 
                     r 
                     , 
                     t 
                   
                 
                 = 
               
             
             
               
                 
                   
                     
                       
                         
                           
                             H 
                             
                               
                                 k 
                                 , 
                                 0 
                                 , 
                                 0 
                               
                             
                           
                         
                         
                           
                             H 
                             
                               
                                 k 
                                 , 
                                 0 
                                 , 
                                 1 
                               
                             
                           
                         
                         
                           ⋯ 
                         
                         
                           
                             H 
                             
                               
                                 k 
                                 , 
                                 0 
                                 , 
                                 
                                   N 
                                   
                                     t 
                                     x 
                                   
                                 
                                 − 
                                 1 
                               
                             
                           
                         
                       
                       
                         
                           
                             H 
                             
                               
                                 k 
                                 , 
                                 1 
                                 , 
                                 0 
                               
                             
                           
                         
                         
                           
                             H 
                             
                               
                                 k 
                                 , 
                                 1 
                                 , 
                                 1 
                               
                             
                           
                         
                         
                           ⋯ 
                         
                         
                           
                             H 
                             
                               
                                 k 
                                 , 
                                 1 
                                 , 
                                 
                                   N 
                                   
                                     t 
                                     x 
                                   
                                 
                                 − 
                                 1 
                               
                             
                           
                         
                       
                       
                         
                           ⋮ 
                         
                         
                           ⋮ 
                         
                         
                           ⋱ 
                         
                         
                           ⋮ 
                         
                       
                       
                         
                           
                             H 
                             
                               
                                 k 
                                 , 
                                 
                                   N 
                                   
                                     r 
                                     x 
                                   
                                 
                                 − 
                                 1 
                                 , 
                                 0 
                               
                             
                           
                         
                         
                           
                             H 
                             
                               
                                 k 
                                 , 
                                 
                                   N 
                                   
                                     r 
                                     x 
                                   
                                 
                                 − 
                                 1 
                                 , 
                                 1 
                               
                             
                           
                         
                         
                           ⋯ 
                         
                         
                           
                             H 
                             
                               
                                 k 
                                 , 
                                 
                                   N 
                                   
                                     r 
                                     x 
                                   
                                 
                                 − 
                                 1 
                                 , 
                                 
                                   N 
                                   
                                     t 
                                     x 
                                   
                                 
                                 − 
                                 1 
                               
                             
                           
                         
                       
                     
                   
                 
               
             
           
         
       
     
      where Y DS  represents the frequency domain data symbols, X shows the transmitted signal, H is the estimated channel response, Z is the noise, k denotes the subcarrier index, r shows the receiver antenna index, t represents the DM-RS transmission port index, N tx  is the number of DM-RS transmission ports and N rx  is the number of receiver antennas. 
     The MIMO ZF equalization matrix W is calculated as follows: 
     
       
         
           
             W 
             = 
             
               
                 
                   
                     
                       H 
                       H 
                     
                     H 
                   
                 
               
               
                 − 
                 1 
               
             
             
               H 
               H 
             
           
         
       
     
     Correspondingly, the equalized data symbol of a user (Ŷ DS ) can be expressed as follows: 
     
       
         
           
             
               
                 Y 
                 ^ 
               
               
                 D 
                 S 
               
             
             = 
             W 
             
               Y 
               
                 D 
                 S 
               
             
             
               
                 k 
                 , 
                 r 
               
             
             = 
             
               
                 
                   
                     
                       H 
                       H 
                     
                     H 
                   
                 
               
               
                 − 
                 1 
               
             
             
               H 
               H 
             
             
               Y 
               
                 D 
                 S 
               
             
             
               
                 k 
                 , 
                 r 
               
             
           
         
       
     
     B. Minimum Mean-Square Error (MMSE) Equalizer 
     Considering the same system model in part a, the MIMO MMSE equalization matrix W is calculated as follows: 
     
       
         
           
             W 
             = 
             
               
                 
                   
                     
                       H 
                       H 
                     
                     H 
                     + 
                     
                       σ 
                       Z 
                       2 
                     
                     
                       I 
                       
                         
                           N 
                           t 
                         
                       
                     
                   
                 
               
               
                 − 
                 1 
               
             
             
               H 
               H 
             
           
         
       
     
      where  
     
       
         
           
             
               σ 
               Z 
               2 
             
           
         
       
     
     represents the estimated noise power. Correspondingly, the equalized data symbol of a user (Ŷ DS ) can be expressed as follows: 
     
       
         
           
             
               
                 Y 
                 ^ 
               
               
                 D 
                 S 
               
             
             = 
             W 
             
               Y 
               
                 D 
                 S 
               
             
             
               
                 k 
                 , 
                 r 
               
             
             = 
             
               
                 
                   
                     
                       H 
                       H 
                     
                     H 
                     + 
                     
                       σ 
                       Z 
                       2 
                     
                     
                       I 
                       
                         
                           N 
                           t 
                         
                       
                     
                   
                 
               
               
                 − 
                 1 
               
             
             
               H 
               H 
             
             
               Y 
               
                 D 
                 S 
               
             
             
               
                 k 
                 , 
                 r 
               
             
           
         
       
     
     C. MMSE-Interference Rejection Combiner (MMSE-IRC) Equalizer 
     Considering an interference component in the system model that is described in part a, the received data symbols in the frequency domain can be expressed as follows: 
     
       
         
           
             
               Y 
               
                 D 
                 S 
               
             
             
               
                 k 
                 , 
                 r 
               
             
             = 
             X 
             
               
                 k 
                 , 
                 t 
               
             
             H 
             
               
                 k 
                 , 
                 r 
                 , 
                 t 
               
             
             + 
             Z 
             
               
                 k 
                 , 
                 r 
               
             
             + 
             Q 
             
               
                 k 
                 , 
                 r 
               
             
           
         
       
     
      where Q denotes the sum of interference from all interference users. The MIMO MMSE-IRC equalization matrix W is calculated as follows: 
     
       
         
           
             W 
             = 
             
               H 
               H 
             
             
               
                 
                   
                     H 
                     
                       H 
                       H 
                     
                     + 
                     
                       R 
                       
                         n 
                         n 
                       
                     
                   
                 
               
               
                 − 
                 1 
               
             
           
         
       
     
     
       
         
           
             
               R 
               
                 n 
                 n 
               
             
             = 
             
               1 
               
                 
                   N 
                   
                     D 
                     M 
                     R 
                     S 
                   
                 
               
             
             
               ∑ 
               
                 
                   R 
                   
                     I 
                     p 
                     N 
                   
                 
               
             
           
         
       
     
     
       
         
           
             
               R 
               
                 I 
                 p 
                 N 
               
             
             = 
             
               
                 
                   Y 
                   
                     D 
                     S 
                   
                 
                 − 
                 H 
                 
                   X 
                   
                     D 
                     M 
                     R 
                     S 
                   
                 
               
             
             
               
                 
                   
                     
                       Y 
                       
                         D 
                         S 
                       
                     
                     − 
                     H 
                     
                       X 
                       
                         D 
                         M 
                         R 
                         S 
                       
                     
                   
                 
               
               H 
             
           
         
       
     
      where R IpN  represents the covariance matrix of interference plus noise and N DMRS  denotes the number of DM-RS subcarriers in an RB. An interpolation operation in the time/frequency domains is required to calculate the covariance matrix in the scheduled subcarriers. Correspondingly, the equalized data symbol of a user (Ŷ DS ) can be expressed as follows: 
     
       
         
           
             
               
                 Y 
                 ^ 
               
               
                 D 
                 S 
               
             
             = 
             W 
             
               Y 
               
                 D 
                 S 
               
             
             
               
                 k 
                 , 
                 r 
               
             
             = 
             
               H 
               H 
             
             
               
                 
                   
                     H 
                     
                       H 
                       H 
                     
                     + 
                     
                       R 
                       
                         n 
                         n 
                       
                     
                   
                 
               
               
                 − 
                 1 
               
             
             
               Y 
               
                 D 
                 S 
               
             
             
               
                 k 
                 , 
                 r 
               
             
           
         
       
     
     The demodulation procedure requires “additional channel equalization information” during the log-likelihood ratio (LLR) calculation, and it can be provided in several ways. A universal solution, which is independent of the equalization method, can be providing one of the following options: 
     a) Equalization Gain and Post-processed Noise plus Interference Power (i.e., after equalization operation)   b) Post-processed Noise plus Interference Power divided by Equalization Gain   c) Equalization Gain divided by Post-processed Noise plus Interference Power   

     The equalization gain can be calculated as follows: 
     
       
         
           
             γ 
             = 
             R 
             e 
             a 
             l 
             
               
                 d 
                 i 
                 a 
                 g 
                 
                   
                     W 
                     H 
                   
                 
               
             
           
         
       
     
      where y is the equalization gain, W denotes the equalization matrix, and H represents the estimated channel response. The post-processed noise plus interference power refers to the noise plus interference power that is obtained after the equalization procedure. Also, it should be noted that in the case of options (b) and (c), the equalized signal should be normalized by either equalization gain or post-processed noise plus interference power, respectively. 
     When the MMSE equalizer is used, only “equalization gain” can be sufficient as the “additional channel equalization information”, whereas only “post-processed noise plus interference power” is required when ZF equalizer is used. Furthermore, these values, which are used during the LLR calculation operation, can be calculated from the channel estimates and estimated DM-RS noise plus interference power as well. 
     Embodiment 9: Combining/Digital Beamforming Matrix Calculation 
     In the following, we provide a brief description of example combining/digital beamforming matrix calculation methods which can be used in any of the embodiments in this disclosure. In all of these me, the UL channel between the UEs and BS is denoted as H, which is a matrix of size N RX  × N UE  where N RX  is the number of RX antennas at BS and N UE  is the number of UEs or layers being simultaneously transmitted over the same resources on the PUSCH slot. It is noted that H is an estimation of the UL channel, for example, based on SRS or DM-RS or a combination thereof or a linearly predicted UL channel based on SRS channel estimation.
     a) Conjugate combining: In this method, the combining/digital beamforming matrix is calculated using H H .   b) Zero forcing: In this method, the combining/digital beamforming matrix is calculated using (H H  H) -1 H H .   c) MMSE: In this method, the combining/digital beamforming matrix can be calculated in two alternative forms:                      H   H     H   +     σ   Z   2       I       N     U   E                   −   1         H   H             or H H  (HH H  +                      σ   Z   2       I       N     R   X                   −   1               where              σ   Z   2             is the estimated noise power.   d) MMSE-IRC: In this method, the combining/digital beamforming matrix is calculated using H H  (HH H  + R nn ) -1  where R nn  the covariance matrix of interference plus noise.   e) Null-space dimension increase based: In this method, the singular-value decomposition (SVD) of the channel subspace of the desired UEs and the intra-site interfering UEs, if any, is first calculated, and its null-space is characterized. A sufficient number N of singular vector(s) are selected in the null-space so that N + N UE  = L where L is the total number of streams to be transferred from RU to DU. Then, the combining/digital beamforming matrix is constructed as a vertical concatenation of H H  and the selected singular vectors.   f) DFT compression: In this method, the energy of each row of W × H is calculated and sorted in decreasing order of energy, where W is the two-dimensional Discrete-Fourier Transform (DFT) matrix. Then, the combining/digital beamforming matrix is constructed as the L rows of W which correspond to the first L rows in the sorted list, where L is the total number of streams to be transferred from RU to DU.   

     Acronyms 
     
         
         3GPP: Third generation partnership project 
         BS: Base Station 
         CA: Carrier Aggregation 
         CAPEX: Capital Expenditure 
         CBRS: Citizens Broadband Radio Services 
         CC: Component carrier 
         COTS: Commercial off-the-shelf 
         CP: cyclic prefix 
         C-plane: Control plane 
         C-RAN: cloud radio access network 
         CU: Central unit 
         DCI: downlink control indicator 
         DL: downlink 
         DU: Distribution unit 
         eAxC ID: Extended Antenna-Carrier identifier: a data flow for a single antenna (or spatial stream) for a single carrier in a single sector. 
         eNB: Evolved Node B (applies to LTE) 
         FDD: Frequency-division duplex 
         FEC: forward error correction 
         FH: Fronthaul 
         FFT: Fast Fourier Transform 
         gNB: g NodeB (applies to NR) 
         iFFT: inverse Fast Fourier Transform 
         HARQ: hybrid automatic repeat request 
         LTE: long term evolution 
         LTE-A: LTE Advanced 
         M-plane: Management plane 
         MCS: modulation and coding scheme 
         MIMO: multiple input, multiple output 
         MMSE-IRC: Minimum mean square error - interference rejection combining 
         MMSE-MRC: Minimum mean square error - maximum-ratio combining 
         mmWave: millimeter wave 
         MNO: Mobile network operator 
         NR: New radio 
         OAM: Operation and management 
         O-DU: O-RAN Distributed Unit 
         O-RU: O-RAN Radio Unit 
         O-RAN: Open RAN (Basic O-RAN specifications are prepared by the O-RAN alliance) 
         OPEX: Operating Expense 
         PBCH: Physical Broadcast Channel 
         PCFICH: Physical Control Format Indicator Channel 
         PDCCH: Physical downlink Control Channel 
         PDCP: Packet Data Convergence Protocol 
         PDSCH: physical downlink shared channel 
         PHICH: Physical Hybrid ARQ Indicator Channel 
         PHY: physical layer 
         LPHY: lower physical layer 
         UPHY: upper physical layer 
         PUCCH: Physical Uplink Control Channel 
         PUSCH: Physical Uplink Shared Channel 
         QAM: quadrature amplitude modulation 
         QPSK: Quadrature Phase Shift Keying 
         RACH: random access channel 
         PRACH: physical random access channel 
         RF: radio frequency interface 
         RLC: Radio Link Control 
         RRC: Radio Resource Control 
         RRM: Radio resource management 
         RRU: Remote radio unit 
         RU: Radio Unit 
         RS: reference signal 
         RSSI: received signal strength indicator 
         RPC: Remote procedure call 
         SMO: Service Management and Orchestration 
         S-plane: Synchronization plane 
         SCell: Secondary cell 
         SIMO: single input, multiple output 
         SINR: signal-to-interference-plus-noise ratio 
         SRS: Sounding reference signal 
         SSS: Secondary Synchronization Signal 
         TB: transport block 
         TTI: Transmission Time Interval 
         TDD: Time division duplex 
         U-plane: User plane 
         UCI: Uplink Control Information 
         UE: user equipment 
         UL: uplink 
         UL DMRS: uplink demodulation reference signal 
         ULSCH: Uplink Shared Channel 
         vBBU: Virtualized baseband unit 
         VNF: Virtual Network Function 
       
    
     Definitions 
     Channel: the contiguous frequency range between lower and upper frequency limits. 
     C-plane: Control Plane: refers specifically to real-time control between O-DU and O-RU, and should not be confused with the UE’s control plane 
     DL: DownLink: data flow towards the radiating antenna (generally on the LLS interface) 
     LLS: Lower Layer Split: logical interface between O-DU and O-RU when using a lower layer (intra-PHY based) functional split. 
     M-Plane: Management Plane: refers to non-real-time management operations between the O-DU and the O-RU 
     O-CU: O-RAN Control Unit - a logical node hosting PDCP, RRC, SDAP and other control functions 
     O-DU: O-RAN Distributed Unit: a logical node hosting RLC/MAC/High-PHY layers based on a lower layer functional split. 
     O-RU: O-RAN Radio Unit: a logical node hosting Low-PHY layer and RF processing based on a lower layer functional split. This is similar to 3GPP’s “TRP” or “RRH” but more specific in including the Low-PHY layer (FFT/iFFT, PRACH extraction). 
     OTA: Over the Air 
     S-Plane: Synchronization Plane: refers to traffic between the O-RU or O-DU to a synchronization controller which is generally an IEEE 1588 Grand Master (however, Grand Master functionality may be embedded in the O-DU). 
     U-Plane: User Plane: refers to IQ sample data transferred between O-DU and O-RU 
     UL: UpLink: data flow away from the radiating antenna (generally on the LLS interface) 
     References 
     ORAN-WG4.MP.0-v06.00: O-RAN Fronthaul Working Group Management Plane Specification, Release 06.00. 
     ORAN-WG4.CUS-plane.0-v06.00: O-RAN Fronthaul Working Group Control, User, and Synchronization Plane Specification, Release 06.00.