Abstract:
A method of joint processing of data in a radio access network (RAN) that includes a plurality of radio nodes each associated with a cell and a services node operatively coupled to the radio nodes is provided. The services node provides connectivity to a core network. The method includes determining that a plurality of first UEs (User Equipment) each being serviced by a selected set of the cells is to operate in accordance with a hybrid joint processing scheme. Information is transferred between the plurality of first UEs and the radio nodes in accordance with the hybrid joint processing scheme by performing L1 layer processing on the radio nodes and L2 layer processing at the services node.

Description:
BACKGROUND 
     Operators of mobile systems such as Universal Mobile Telecommunications Systems (UMTS) are increasingly relying on wireless small cell radio access networks (RANs) in order to deploy indoor voice and data services to enterprises and other customers. Such small cell RANs typically utilize multiple-access technologies capable of supporting communications with multiple users using radio frequency (RF) signals and sharing available system resources such as bandwidth and transmit power. While such small cell RANs operate satisfactorily in many applications, there exists a need for further improvements in small cell RAN technologies. 
     For example, one problem with small cell RANs is that inter-cell interference prevents these systems from coming close to their theoretical data rates for multi-cell networks. Such interference arises because the same spectral resources are used in different cells, leading to interference for terminals (e.g., User Equipment) located at the edge between cells. Release 11 of the 3GPP Long Term Evolution (LTE) specifications proposes the use of Coordinated Multi-Point (CoMP) technology to mitigate problems caused by inter-cell interference. One aspect of CoMP technology involves the scheduling of time-frequency resources by cells in a cooperative manner. The use of CoMP technology generally comes at the cost of increased demand on the backhaul (high capacity and low latency), higher complexity, increased synchronization requirements and so on. 
     This Background is provided to introduce a brief context for the Summary and Detailed Description that follow. This Background is not intended to be an aid in determining the scope of the claimed subject matter nor be viewed as limiting the claimed subject matter to implementations that solve any or all of the disadvantages or problems presented above. 
     SUMMARY 
     A radio access network, such as an LTE Enterprise Radio Access Network (E-RAN), employs a hierarchical architecture that includes a services node that provides connectivity between the radio nodes in the RAN and a core network. The RAN may employ a modification to a joint processing scheme in which downlink data for a given UE is available in a given time-frequency resource (e.g., a resource block) from more than one transmitting cell while multiple cells receive the same uplink data from a single UE on a given time-frequency resource. As modified, a hybrid joint processing scheme is employed in which data protocol processing is split between the radio nodes and the services node. 
     In one embodiment, hybrid joint processing is used in connection with cell-edge UEs but not cell-interior UEs. In this embodiment, the L2 protocol stack (PDCP/RLC/MAC) may be hosted on the services node for cell-edge UEs while the L1 processing (PHY) remains on the radio nodes. Such data protocol processing can be dynamically moved between the services node and radio nodes as a UE moves from the interior of a cell to the edge of a cell, and vice versa. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows an illustrative mobile telecommunications environment in which the present invention be practiced. 
         FIG. 2  shows details of an EPC (Evolved Packet Core) and E-UTRAN (Evolved UMTS Terrestrial Radio Access Network, where UMTS is an acronym for Universal Mobile Telecommunications System) arranged under LTE (Long Term Evolution) with which a small cell network may interoperate. 
         FIGS. 3 and 4  shows one example of a protocol stack that may be employed for the control plane and the user plane, respectively, used in the system of  FIGS. 1 and 2  in terms of the OSI (Open Systems Interconnection) model of logical layers. 
         FIG. 5  shows an example of a downlink transmission in a system with joint transmission and a system without joint transmission. 
         FIG. 6  shows some of the functionality performed by the L2 portion of the OSI protocol stack along with the PHY layer. 
         FIG. 7  shows the location of portions of the protocol stack performed on the services node and the radio nodes when hybrid joint processing is employed (e.g. for cell-edge UEs) and when hybrid joint processing is not employed (e.g., for cell-interior UEs). 
         FIG. 8  shows the location of portions of the protocol stack performed on the services node and the radio nodes when hybrid joint processing is employed for UEs with different data radio bearers. 
         FIG. 9  shows a simplified functional block diagram of illustrative hardware infrastructure for a radio node that may be utilized to implement the present hybrid joint processing scheme. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an illustrative mobile telecommunications environment  100  in which the present invention be practiced. The mobile telecommunications environment  100 , in this illustrative example, is arranged as an LTE (Long Term Evolution) system as described by the Third Generation Partnership Project (3GPP) as an evolution of the GSM/UMTS standards (Global System for Mobile communication/Universal Mobile Telecommunications System). It is emphasized, however, that the present principles described herein may also be applicable to other network types and protocols. The environment  100  includes an enterprise  105  in which a small cell RAN  110  is implemented. The small cell RAN  110  includes a plurality of radio nodes (RNs)  1151  . . . N. Each radio node  115  has a radio coverage area (graphically depicted in the drawings as a hexagonal shape) that is commonly termed a small cell. A small cell may also be referred to as a femtocell, or using terminology defined by 3GPP as a Home Evolved Node B (HeNB). In the description that follows, the term “cell” typically means the combination of a radio node and its radio coverage area unless otherwise indicated. A representative cell is indicated by reference numeral  120  in  FIG. 1 . 
     The size of the enterprise  105  and the number of cells deployed in the small cell RAN  110  may vary. In typical implementations, the enterprise  105  can be from 50,000 to 500,000 square feet and encompass multiple floors and the small cell RAN  110  may support hundreds to thousands of users using mobile communication platforms such as mobile phones, smartphones, tablet computing devices, and the like (referred to as “user equipment” (UE) and indicated by reference numerals  1251 -N in  FIG. 1 ). However, the foregoing is intended to be illustrative and the solutions described herein can be typically expected to be readily scalable either upwards or downwards as the needs of a particular usage scenario demand. 
     In this particular illustrative example, the small cell RAN  110  includes one or more services nodes (represented as a single services node  130  in  FIG. 1 ) that manage and control the radio nodes  115 . In alternative implementations, the management and control functionality may be incorporated into a radio node, distributed among nodes, or implemented remotely (i.e., using infrastructure external to the small cell RAN  110 ). The radio nodes  115  are coupled to the services node  130  over a direct or local area network (LAN) connection (not shown in  FIG. 1 ) typically using secure IPsec tunnels. The services node  130  aggregates voice and data traffic from the radio nodes  115  and provides connectivity over an IPsec tunnel to a security gateway SeGW  135  in an Evolved Packet Core (EPC)  140  network of a mobile operator. The EPC  140  is typically configured to communicate with a public switched telephone network (PSTN)  145  to carry circuit-switched traffic, as well as for communicating with an external packet-switched network such as the Internet  150 . 
     The environment  100  also generally includes Evolved Node B (eNB) base stations, or “macrocells”, as representatively indicated by reference numeral  155  in  FIG. 1 . The radio coverage area of the macrocell  155  is typically much larger than that of a small cell where the extent of coverage often depends on the base station configuration and surrounding geography. Thus, a given UE  125  may achieve connectivity to the network  140  through either a macrocell or small cell in the environment  100 . 
     Along with macrocells  155 , the small cell RAN  110  forms an access network, i.e., an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) under 3GPP as represented by reference numeral  205  in  FIG. 2 . As shown, there is no centralized controller in the E-UTRAN  205 , hence an LTE network architecture is commonly said to be “flat.” The macrocells  155  are typically interconnected using an X2 interface and to the EPC  140  by means of an S1 interface. More particularly, the macrocells are connected to the MME (Mobility Management Entity)  210  in the EPC  140  using an S1-MME interface and to the S-GW (Serving Gateway)  215  using an S1-U interface. An S5 interface couples the S-GW  215  to a P-GW (Packet Data Network Gateway)  220  in the EPC  140  to provide the UE  125  with connectivity to the Internet  150 . A UE  125  connects to the radio nodes  115  over an LTE-Uu interface. 
     The SeGW  135  is also connected to the MME  210  and S-GW  215  in the EPC  140  using the appropriate S1 connections. Accordingly, as each of radio nodes  115  in the small cell RAN  110  is operatively coupled to the services node  130  (as representatively shown by lines  225 ), the connections from the radio nodes  115  to the EPC  140  are aggregated to the EPC  140 . Such aggregation preserves the flat characteristics of the LTE network while reducing the number of S1 connections that would otherwise be presented to the EPC  140 . The small cell RAN  110  thus essentially appears as a single eNB  230  to the EPC  140 , as shown. 
     The services node includes a central scheduler  235  as shown in  FIG. 2 . The radio nodes  115  may also be configured to support individual schedulers (representatively indicated by reference numeral  240  in  FIG. 2 ). The operation of the schedulers is discussed below in connection with  FIG. 7 . 
       FIGS. 3 and 4  shows one example of a protocol stack that may be employed for the control plane and the user plane, respectively, used in the system of  FIGS. 1 and 2  in terms of the OSI (Open Systems Interconnection) model of logical layers. As is known in the art, the Non-Access Stratum (NAS) layer protocol is responsible for signaling and traffic between UE and the network for control purposes such as network attach, authentication, setting up of bearers, and mobility management. The Radio Resource Control (RRC) protocol is responsible for control plane signaling between a UE and the network, i.e. such tasks as broadcast of system information; establishment, maintenance and release of RRC connection; establishment, configuration, maintenance and release of signaling and data radio bearers; security functions including key management; mobility functions such as control of UE cell selection/reselection; paging; UE measurement configuration, processing and reporting; handover; quality of service (QoS) management functions; UE measurement reporting and control of the reporting, but not exclusively. The Packet Data Control Protocol (PDCP) layer is responsible for (de-) compressing the headers of user plane IP packets. The Radio Link Control (RLC) layer is used to format and transport traffic and a Medium Access Control (MAC) layer provides addressing and channel access control mechanisms. The physical (PHY) layer, translates logical communication requests into hardware-specific operations such as modulation, bit synchronization, multiplexing, equalization, forward error correction etc. 
     Protocol layers that may be employed between the services node  130  and the radio nodes  115  and between the services node  130  and the S-GW  215  may include an Internet Protocol (IP) layer, an Internet Protocol Security Encapsulating Security Payload (IPSec ESP) layer for signing and encrypting packets, a user datagram protocol (UDP) and a GPRS Tunneling Protocol-User plane (GTP-U) for creating IP-based tunnels. 
     As previously mentioned, a major challenge in a multi-cell deployment is cell-edge performance. For instance, a UE at the cell-edge experiences significant interference from the downlink (DL) transmissions of adjacent radio nodes in decoding the DL transmissions from its serving cell. Likewise, uplink transmissions from a cell-edge user can cause significant interference to adjacent radio nodes. As a result the cell-edge spectral efficiency is significantly poorer than that in the interior of the cell. Several coordination mechanisms have been introduced into the LTE standard to improve cell-edge performance. 
     One such coordination mechanism is Coordinated Multi-Point or CoMP technology, which has been introduced to improve cell-edge spectral efficiency. As previously mentioned, CoMP is included in Release 11 of the 3GPP Long Term Evolution (LTE) specification. These schemes involve coordination among multiple sectors/cells co-located at a single radio node or coordination among geographically separated radio nodes. 
     The CoMP transmission schemes that have been proposed may be divided into two primary categories: Joint Processing (JP) and Coordinated Scheduling/Beamforming (CS/CB). Joint processing includes joint transmission, in which the data for a UE is available in a given time-frequency resource (e.g., a resource block) from more than one transmitting cell. Likewise, joint processing also includes joint reception, in which multiple cells receive the same transmission from a single UE on a given time-frequency resource. The physical layer information received by each cell is sent up to a common point, and the data is soft combined as part of the decoding process to result in effective signal-to-noise (SNR) improvements. 
       FIG. 5  shows an example of a downlink transmission in a system  300  without joint transmission and a system  400  with joint transmission. In system  300  a UE  304  is located at the edge of three cells  301 ,  302  and  303 . As the solid arrow indicates, only cell  301  is involved in data transmission to the UE  304 . However, as the dashed arrows indicate, UE  304  receives interference from cells  302  and  303 . In system  400 , the UE  404  is located at the edge of three cells  401 ,  402  and  403 . In this case however, the UE  404  receives joint transmission from all three cells  401 ,  402  and  403 . System  400  can achieve increased gain and a signal-to-noise ratio as a result of the diversity gain provided by the three cells  401 ,  402  and  403 . 
     The presence of the services node  130  enables use of a hybrid joint processing technique. Hybrid joint processing may be implemented as a modification of Joint Processing as described in 3GPP Release 11. As described in more detail below, the modification entails splitting processing between the small cell radio nodes  115  and the services node  130  shown in  FIG. 1 . 
     In one implementation the use of hybrid joint processing may be limited to cell-edge UEs and the joint transmission and joint reception endpoints are located at the services node. For example, in the downlink, for cell-edge UEs, the L2 stack (PDCP/RLC/MAC) may be hosted on the services node  130  while the L1 processing (PHY) remains on the radio nodes  115  (see the user plane protocol stack in  FIG. 4 ). Downlink data (i.e., PDSCH—Physical Downlink Shared Channel) may be scheduled for simultaneous transmission from multiple cells using UE-specific reference signals. These scheduling decisions can be made by the centralized scheduler  235  disposed at the services node  130  and the decisions may then be communicated to the MAC schedulers in the individual radio nodes  115 . Note that the PDCCH (Physical Downlink Control Channel) would be sent from a single radio node (e.g., the serving radio node) only since the channel estimates for PDCCH decoding are derived from cell-specific reference signals and these are different for different radio nodes. 
     In another implementation, the use of hybrid joint processing may be limited to QoS-sensitive low data rate services like VoIP. In this case the L2 protocol stack for VoIP users could always be hosted at the services node whether or not the user is in the interior of a cell or at the edge of a cell. When at a cell edge, the macro diversity benefits of joint processing involving multiple cells can be exploited. 
       FIG. 6  shows some of the functionality performed by the L2 stack along with the PHY layer. As shown, the MAC layer performs such functions as scheduling, segmentation/reassembly and HARQ processing. The PHY layer performs symbol processing and sample processing. 
     With the MAC layer residing on the services node  130 , the latency between the radio nodes  115  and the services node  130  may be larger than the fastest HARQ (Hybrid Automatic Repeat Request) turn-around time of 8 ms. Hybrid Automatic Repeat reQuest (HARQ) is an error control method for data transmission which uses acknowledgments and timeouts to achieve reliable data transmission. By using HARQ, the user data can be transmitted multiple times. For each transmission or retransmission either the same (Chase combining) or potentially a different redundancy version (incremental redundancy) is sent. When a corrupted packet is received, the receiver saves the soft information, requests a retransmission by sending a negative acknowledgement and later combines the already received soft information with the soft information conveyed in the retransmissions to recover the error-free packet as efficiently as possible. By doing so it essentially accumulates the energy of all transmissions and retransmissions. Typically, after a few HARQ retransmissions the data is successfully received. 
     Because the latency can extend beyond the HARQ turn-around time, hosting the L2 stack on the services node would be feasible for lower data rate users that can tolerate larger HARQ delays (It is noted that DL HARQ is asynchronous and thus retransmissions may be delayed) or those that can operate without HARQ retransmissions (e.g., VoIP users configured to have very low first-transmission block error rates (BLER)). 
     In the uplink, for cell-edge UEs, the L2 stack may again be hosted on the services node  130  while the L1 processing remains on the radio nodes  115 . The centralized scheduler  235  may instruct multiple radio nodes  115  to simultaneously decode a UE&#39;s transmission and these scheduling decisions are communicated to the individual schedulers  240  in the radio nodes. The decoded MAC PDUs (Protocol Data Unit) may be delivered separately from the individual radio nodes to the MAC layer in the L2 stack at the services node. 
     The MAC layer on the services node  130  will typically need to be enhanced to perform de-duplication of MAC PDUs before HARQ retransmissions. The benefit of macro-diversity is that a HARQ retransmission may be avoided even if only one among the multiple radio nodes successfully decodes the UL transmission from the UE. It is noted that this is unlike HSUPA (High Speed Uplink Packet Access), where the de-duplication and reordering operations are performed in the MAC-es layer in the RNC (Radio Network Controller) after HARQ retransmissions. 
     With the MAC layer residing on the services node, the latency between the radio nodes and the services node may be larger than the fastest HARQ turn-around time of 8 ms. While retransmissions on the uplink are synchronous (e.g., 8 ms periodicity), the UE can be instructed by the eNodeB to not retransmit by signaling an ACK on the PHICH channel but not sending any UL control information using DCI format 0 over PDCCH. The retransmission could thus be delayed from 8 ms to a higher value in increments of 8 ms i.e., 16 ms, 24 ms etc. Similar to downlink, since the HARQ delays are larger, such a hybrid joint processing scheme would benefit delay-tolerant users with lower data rates or those that could operate without HARQ retransmissions (e.g., VoIP users configured to have very low first-transmission BLER) 
     In one illustrative example, hybrid joint processing techniques may be implemented using the protocol stacks shown in  FIG. 7 . Here, hybrid joint processing may be used with cell-interior UEs  710  handled through an L2 stack in the radio nodes  115  of the individual cells and cell-edge UEs  720  may be handled through the centralized L2 stack in the services node  130 . 
     In another illustrative example, for UEs with different data radio bearers (RBs), the PDCP and RLC entities associated with a QoS-sensitive radio bearer (e.g., VoIP) can be handled at the services node, while these entities corresponding to QoS-insensitive radio bearers can be handled at the radio node.  FIG. 8  show the protocol stack for such a UE  400 . When the services node  130  schedules data from the QoS-sensitive RB in a TTI and communicates that to the radio node  115 , this data is prioritized and no data from other RBs corresponding to this user would be scheduled during this TTI by the radio node. Data from different radio bearers can thus be time-multiplexed to provide better QoS through hybrid joint processing only for some RBs of a given user. 
     As  FIG. 7  illustrates, in hybrid joint processing the data protocol processing can be dynamically moved between the services node and radio nodes. Such a dynamic process may be used, for example, for a cell-edge UE that migrates to a cell-interior, or vice versa. 
     The system can determine the relative position of each active UE based on measurement reports received from the UE. If a UE is determined to be in an area of poor signal quality, and if it is determined that it is necessary to put the UE into a joint processing mode of operation in order to provide the required QoS characteristics, the RAN can start processing the UE&#39;s data at the services node. In this case the services node  130 , using its central scheduler  235 , can allocate scheduling resources on each radio node a few milliseconds ahead of on-air transmission/reception. This allocation of scheduling resources is then signaled to the radio node. On receiving the scheduling allocation for cell-edge UEs, each individual scheduler  240  can be responsible for resource allocation of all other UEs connected to it while avoiding the resources allocated by the central scheduler  235  to cell-edge UEs. That is, in some implementations the central scheduler  235  may be given priority in scheduling radio resources for UEs and the individual radio node schedulers  240  would be required to schedule radio resources in a way that does not conflict with the scheduling of resources by the central scheduler  235  in the services node  130 . 
     The services node  130  also constructs the scheduling downlink control information (DCI), which provides the UE with information used to properly receive and decode the downlink data transmissions. The scheduling DCI, along with an associated timestamp specifying when the DCI is to be transmitted, is sent to the serving cell by the services node  130 . In turn, the radio node  115  associated with the serving cell  130  transmits the scheduling DCI to the UE at the time specified by the timestamp. 
       FIG. 9  shows a simplified functional block diagram  450  of illustrative hardware infrastructure for a radio node that may be utilized to implement the present hybrid joint processing scheme. A controller/processor  405  typically handles high level processing. The controller/processor  405  may include one or more sub-processors  410  or cores that are configured to handle specific tasks or functions. An RF processor  415  implements various signal processing functions including the lower level L1 processing. The RF processor  415  may include one or more sub-processors  420  or cores that are configured to handle specific tasks or functions. A memory  425  stores computer-readable code  430  that is executable by one or more processors in the controller/processor  405  and/or the RF processor  415 . The memory  425  may also include various data sources and data sinks (collectively represented by element  435 ) that may provide additional functionalities. The code  430  in typical deployments is arranged to be executed by the one or more processors to implement the hybrid joint processing scheme by modifying the MAC layer in order to distribute the scheduling functionality between the radio node and the services node. 
     The hardware infrastructure may also include various interfaces (I/Fs) including a communication I/F  440  which may be used, for example, to implement a link to the services node  130  ( FIG. 1 ), LAN, or to an external processor, control, or data source. In some cases, a user I/F  445  may be utilized to provide various indications such as power status or to enable some local control of features or settings. It is noted that the block diagram  400  may also be substantially applicable to a services node that may be utilized to implement the present hybrid joint processing scheme. More particularly, the RF processor  415  may be eliminated in some applications and any functionality that it provides that is needed to implement the services node may be provided by the controller/processor  405 . 
     Several aspects of telecommunication systems will now be presented with reference to various apparatus and methods described in the foregoing detailed description and illustrated in the accompanying drawing by various blocks, modules, components, circuits, steps, processes, algorithms, etc. (collectively referred to as “elements”). These elements may be implemented using electronic hardware, computer software, or any combination thereof. Whether such elements are implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. By way of example, an element, or any portion of an element, or any combination of elements may be implemented with a “processing system” that includes one or more processors. Examples of processors include microprocessors, microcontrollers, digital signal processors (DSPs), field programmable gate arrays (FPGAs), programmable logic devices (PLDs), state machines, gated logic, discrete hardware circuits, and other suitable hardware configured to perform the various functionalities described throughout this disclosure. One or more processors in the processing system may execute software. Software shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software modules, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. The software may reside on a computer-readable media. Computer-readable media may include, by way of example, a magnetic storage device (e.g., hard disk, floppy disk, magnetic strip), an optical disk (e.g., compact disk (CD), digital versatile disk (DVD)), a smart card, a flash memory device (e.g., card, stick, key drive), random access memory (RAM), read only memory (ROM), programmable ROM (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM), a register, a removable disk, and any other suitable media for storing or transmitting software. The computer-readable media may be resident in the processing system, external to the processing system, or distributed across multiple entities including the processing system. Computer-readable media may be embodied in a computer-program product. By way of example, a computer-program product may include one or more computer-readable media in packaging materials. Those skilled in the art will recognize how best to implement the described functionality presented throughout this disclosure depending on the particular application and the overall design constraints imposed on the overall system. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.