# Patent

## Title
Subspace tracking in full dimension MIMO

## Abstract

Disclosed in some examples are methods, systems, devices, and machine readable mediums which reduce the amount of bandwidth consumed by the reference signals. In some examples, this is achieved by finding the optimal subspace containing all the active UEs in the cell and transmitting reference signals to that subspace. In some examples, second order statistics may be utilized to calculate a projected channel to the optimal subspace at the UE and then feeding this back to the eNodeB. The projected channel to optimal subspace may be utilized at the UE and the eNodeB to transform the codebook and align the codewords with the channel direction.

## Background
PRIORITY CLAIM
This patent application claims the benefit of priority, under 35 U.S.C. Section 119 to U.S. Provisional Patent Application Ser. No. 61/841,230, entitled “ADVANCED WIRELESS COMMUNICATION SYSTEMS AND TECHNIQUES,” filed on Jun. 28, 2013, which is hereby incorporated by reference herein in its entirety.
COPYRIGHT NOTICE
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. The following notice applies to the software and data as described below and in the drawings that form a part of this document: Copyright Intel, Inc., All Rights Reserved.
TECHNICAL FIELD
Embodiments pertain to wireless communication systems. Some embodiments relate to systems, methods, devices, and machine readable mediums for Multiple Input, Multiple Output (MIMO) wireless communication systems.
BACKGROUND
Modern cellular telecommunications systems utilize a base station which communicates with a mobile device such as a cellular telephone. In some examples, the air interface between the base station and the mobile device may utilize a method to encode data. Example methods include an Orthogonal Frequency Division Multiplexing (OFDM) method a Wideband Code Division Multiple Access (W-CDMA), and the like.
Advanced wireless systems may utilize Multiple Input, Multiple Output (MIMO) techniques in which multiple transmit and multiple receive antennas are used to create separate spatial streams which take advantage of a multipath environment.



BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document.
FIG. 1 shows a flowchart of a method of processing beamformed channel state information at the UE.
FIG. 2 is a diagram of codewords before a transform according to some examples of the present disclosure.
FIG. 3 is a diagram of codewords after a transform according to some examples of the present disclosure.
FIG. 4 shows a flowchart of signals transmitted to and from the UE and eNodeB according to some examples of the present disclosure.
FIG. 5, shows a flowchart of a method of beamforming reference signals to a UE is shown according to some examples of the present disclosure.
FIG. 6, shows a block schematic of an eNodeB and UE according to some examples of the present disclosure.
FIG. 7 is a block diagram illustrating an example of a machine upon which one or more embodiments may be implemented.



DETAILED DESCRIPTION
One example OFDM MIMO wireless network is a Long Term Evolution (LTE) wireless network. The specification for LTE is maintained by the Third Generation Partnership Project (3GPP). In LTE the base station is called an enhanced Node B (eNodeB) and the mobile is called a User Equipment (UE). One of the items for study in LTE Release 12 is the utilization of Full dimension MIMO (FD-MIMO). In FD-MIMO systems, the eNodeB, has a large number of antennas (e.g., greater than or equal to eight) for beam-forming separate spatial streams to UEs. In Frequency Division Duplex (FDD) mode in which the uplink and downlink channels of the air interface between the eNodeB and the UE are separated by frequency, downlink channel information is required at the eNodeB in order to enable beam-forming downlink data to the UE. This channel information may be obtained at the eNodeB by transmitting reference signals from the eNodeB to the UEs. The UE's may then utilize the reference signals to measure the downlink channel. Based upon these measurements, the UE may quantize both the channel direction and amplitude and send these measurements back to the eNodeB.
In current LTE systems, the overhead for these reference signals increases linearly with the number of antennas. With systems that have a large number of antennas, such as FD-MIMO, this overhead becomes prohibitive.
Disclosed in some examples are methods, systems, devices, and machine readable mediums which reduce the amount of overhead associated with these reference signals. In some examples, this is achieved by finding the optimal subspace containing all the active UEs in the cell and transmitting reference signals to only that subspace. In some examples, the UE may utilize second order statistics to calculate a projected channel to the optimal subspace. The UE may then feed this back to the eNodeB. The projected channel to optimal subspace may be utilized at the UE and the eNodeB to transform the codebook and align the codewords with the channel direction. This reduces the quantization error in channel direction quantization and consequently, improves system performance.
The dimension of the full space can go up to the number of antennas at the eNodeB, which can be very large. In reality, within this full space, the signals sent by the eNodeB can only reach a handful of active UEs via a handful of beamforming directions while other beamforming directions are useless and cause interference. Given that the useful beamforming directions are limited, the channel training resources—e.g., the Channel State Information-Reference Signal (CSI-RS) may be deployed only around those directions for lower overhead and higher efficiency. This can be done by beamforming the channel training signals to the signal subspace spanned by all the active UEs or part of the active UEs. As the dimension of the signal subspace is mainly determined by the number of active UEs, the required number of training resources, e.g., antenna ports, is small.
For beamforming the channel training signals (e.g., the CSI-RS) to the UEs, the beamforming directions need to be known. These directions can be estimated from the previous UE channel state feedback. For example, the signal subspace can be computed from the singular value decomposition of the matrix formed by the fed back beam-forming vectors. Note that as the mobile channel varies over time, the UE may move out from the initial beam-forming direction. To address this, in some examples the eNodeB may regularly track channel variations.
During an initial training phase, the downlink (DL) channels of the UEs in one cell are estimated at each UE. The estimated DL channels for the active UEs are then fed back to the eNodeB where they are collected. Next, the optimal subspace containing almost all the energy of those channels is calculated. This information is then used to optimize future DL training and reduce signal overhead by scanning part of that optimal subspace constantly and the rest of the subspace occasionally (to capture sudden changes in the DL channel of the UEs).
System Model
The channel from the eNodeB to UEk in OFDM symbol t and subcarrier n can be expressed as:




  
    
      H
      k
    
    ⁡
    
      [
      
        n
        ,
        t
      
      ]
    
  
  =
  
    
      
        ∑
        
          m
          =
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        M
      
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              k
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              n
              ,
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          m
        
      
    
    =
    
      
        ∑
        
          m
          =
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            g
            
              k
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          ⁡
          
            [
            
              n
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          w
          m
        
      
    
  




where em=(0,0, . . . , 0,1,0,0, . . . , 0)T with 1 in the m-th position and zeroes elsewhere and wm vectors are unit-norm and orthogonal. In order to compress the channel and reduce dimensionality from M to D<M, one has to find the optimal basis w1, . . . , wM such that, gk,m[n]≈0 when m>D. Then, for d=1,2, . . . , D, coefficients gk,m[n,t] are given by gk,d[n,t]=wdHHk[n,t].

Beamforming Reference Signal at the eNodeB

From the above, it is clear that, instead of utilizing M reference signals to estimate the channel in the original M-dimensional space and feeding back that information, the eNodeB can only employ D reference signals. By beamforming these reference signals with w1, . . . , wD, each UE can estimate the coefficient gk,d[n] directly and send that information to the eNodeB. By utilizing this approach, the amount of overhead is reduced by a factor of D/M. Moreover, as UEs don't need to know the beamforming vectors, this process is totally transparent to the UEs. In order to obtain the optimal subspace for each cell, the channel covariance matrix is estimated at the end of the q-th training phase as:




  
    
      R
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Where T is the length of the training period for sample covariance matrix calculation at the eNodeB in terms of OFDM symbols, Uq is an M×M unitary matrix and Λq is a diagonal matrix of eigenvalues λq,1, . . . , λq,D. Assume that λq,1≧ . . . ≧λq,D≧λD+1≈ . . . ≈λM≈0. Then, the basis of the optimal D dimension subspace will be {uq,1, . . . , uq,D} where uq,d is the d-th column of Uq, i.e., the eigenvector corresponding to the d-th strongest eigenvalue.

As discussed previously, vectors {uq,1, . . . , uq,D} can be used to beam-form reference signals. However, as mentioned before, in order to track sudden changes in the channel and the cases that the channel may be outside this subspace, the eNodeB may occasionally scan other directions orthogonal to the D-dimension optimal subspace, i.e., uq,D+1, . . . , uq,M. Therefore, during reference signal transmission, the eNodeB may always beam-form D−r reference signals with uq,1, . . . , uq,D−r and the remaining r reference signals are beam-formed with r vectors selected out of uq,D−r+1, . . . , uq,M in order to cover the rest of the M-dimension space.
Channel State Information at the UE
Turning now to FIG. 1 a flowchart of a method 1000 of processing beamformed channel state information at the UE is shown according to some examples. The UE receives the beamformed reference signals at operation 1010.
After receiving beam-formed reference signals, each UE estimates projected channel coefficients in the new space at operation 1020 as:

ĝk,d[n,t]=uq,dHĤk[n,t],d=1,2, . . . ,D

Then, the projected channel direction is obtained as:




  
    
      
        g
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At operation 1030, in order to quantize this direction, each UE transforms its codebook (3GPP LTE codebook or any other codebook) by the covariance matrix of the “projected” channel given by:




  
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    ′
  
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The new codewords are hence given as:




  
    c
    
      k
      ,
      i
    
    ′
  
  =
  
    
      
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        k
        ′
      
      ⁢
      
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      
      
        
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          ′
        
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          c
          
            k
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Where cis and ci's are codewords before and after transform respectively. FIG. 2 illustrates codewords before the transform and FIG. 3 shows code words after the transform. Arrows 2010 and 3010 show the direction of projected channel and arrows 2020 and 3020 represent the quantized projected channel direction. Each UE calculates its transform codebook at operation 1030 and uses that to quantize the projected channel direction at operation 1040. This can be done for example by maximizing the inner product as




  
    max
    
      all
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    〉
  




In other examples, this may be done based on maximizing capacity and the like. The index of the optimal codeword is sent to the eNodeB (precoding matrix indicator (PMI)) along with a Channel Quality Indicator (CQI—representing SINR) to enable eNB to reconstruct the channel at operation 1050.
Note that, as covariance matrix Rk′ is UE-dependent and unknown at the eNodeB, each UE has to quantize this matrix. This can be done in different ways for example by scalar quantization of its elements. After quantization, each UE feeds back its own covariance matrix to the eNodeB at operation 1060. This introduces some additional overhead to the system. However, this overhead can be reduced by exploiting the structure of covariance matrix for example by only quantizing and feeding back upper or lower triangular elements of the matrix. Moreover, the second order statistics of the channel vary in a much slower rate comparing to the instantaneous channel itself. Therefore, this additional feedback can be sent much less frequently than compared to PMI and CQI.
FIG. 4 shows a flowchart of signals transmitted to and from the UE and eNodeB 4000 according to some examples of the present disclosure. At operation 4010, the eNodeB transmits a beamformed reference signal. The mobile device calculates the PMI and CQI. At operation 4020, the UE sends the PMI and CQI to the eNodeB. At operation 4030, the UE sends the quantized covariance matrix to the eNodeB to reconstruct the channel. In some examples, operation 4030 may happen at various times (e.g., not necessary directly after the PMI and CQI are calculated and transmitted).
Channel Reconstruction at the eNodeB
The eNodeB reconstructs the channel in the original M dimension space as:




  
    
      
        H
        ^
      
      k
    
    ⁡
    
      [
      
        t
        ,
        n
      
      ]
    
  
  ≈
  
    
      
        
          SINR
          k
        
        ⁡
        
          [
          
            t
            ,
            n
          
          ]
        
      
    
    ⁢
    
      
        ∑
        
          d
          =
          1
        
        
          D
          -
          1
        
      
      ⁢
      
        
          
            
              g
              ^
            
            
              k
              ,
              d
            
          
          ⁡
          
            [
            
              n
              ,
              t
            
            ]
          
        
        ⁢
        
          u
          
            q
            ,
            d
          
        
      
    
  




Turning now to FIG. 5, a flowchart of a method 5000 of beamforming reference signals to a UE is shown according to some examples of the present disclosure. At operation 5010 the optimal signal subspace is computed. This may be based upon previous reference signals sent by UEs. At operation 5020, the reference signals are beam-formed to the optimal signal subspace. At operation 5030, projected channels to the optimal subspace is feedback from the UEs. At operation 5040, this fed-back information is used by the eNodeB to transform the codebook and align the codewords with the channel directions of the received projected channels. At operation 5050, the codebook and codewords are utilized to transmit to the UEs.
Turning now to FIG. 6, a block schematic of an eNodeB 6010 and UE 6060 according to some examples of the present disclosure is shown. eNodeB 6010 includes a signal subspace calculation module 6020 to determine an optimal signal subspace containing active UEs in the cell. eNodeB 6010 includes a reference signal output module 6030 configured to transmit beam formed reference signals to the optimal signal subspace. eNodeB 6010 includes an uplink reception module 6040 configured to receive a projected channel to the optimal signal subspace from one or more of the active UEs in the cell. eNodeB also includes a transformation module 6050 configured to transform a codebook and align codewords with a channel direction of the received projected channel.
UE 6060 may include a reference signal reception module 6070 configured to receive a beam formed reference signal sent from an eNodeB. UE 6060 may include an estimation module 6080 configured to estimate projected channel coefficients and direction. UE 6060 may also include a transformation module 6090 configured to transform a codebook by a covariance matrix of the projected channel coefficients and direction. UE 6060 may also include a quantization module 6100 configured to quantize the projected channel direction. UE 6060 may also include a feedback module 6110 configured to calculate a precoding matrix indicator and a channel quality indicator based upon the covariance matrix as well as transmit the precoding matrix indicator, and a quantized covariance matrix calculated from the covariance matrix to the eNodeB.
UE 6060 and eNodeB 6010 may contain other modules not shown to provide additional functionality. Furthermore, the modules shown in FIG. 6 are exemplary and may provide additional functionality. Additionally, one of ordinary skill in the art with the benefit of Applicants' disclosure will appreciate that the functionality of the modules may be organized differently than shown and described with respect to FIG. 6.
While examples herein were illustrated in practice with an LTE system, one of ordinary skill in the art with the benefit of Applicants' disclosure will note the applicability of the techniques to other advanced wireless systems that utilized channel state feedback with MIMO. Other example systems may include, the Institute for Electronics and Electrical Engineers (IEEE) 802.11 standard (such as 802.11n-2009 or 802.11ac), IEEE 802.16 (Wi-Max), Bluetooth, or the like.
Machine Examples
FIG. 7 illustrates a block diagram of an example machine 7000 upon which any one or more of the techniques (e.g., methodologies) discussed herein may be performed. For example, the machine may be configured to implement the modules of FIG. 6 or the methods shown in FIGS. 1 and 5. In alternative embodiments, the machine 7000 may operate as a standalone device or may be connected (e.g., networked) to other machines. In a networked deployment, the machine 7000 may operate in the capacity of a server machine, a client machine, or both in server-client network environments. In an example, the machine 7000 may act as a peer machine in peer-to-peer (P2P) (or other distributed) network environment. The machine 7000 may be a personal computer (PC), a tablet PC, a set-top box (STB), a personal digital assistant (PDA), a mobile telephone (e.g., a UE), a web appliance, a network router, switch or bridge, an eNodeB, or any machine capable of executing instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein, such as cloud computing, software as a service (SaaS), other computer cluster configurations.
Examples, as described herein, may include, or may operate on, logic or a number of components, modules, or mechanisms. Modules are tangible entities (e.g., hardware) capable of performing specified operations and may be configured or arranged in a certain manner. In an example, circuits may be arranged (e.g., internally or with respect to external entities such as other circuits) in a specified manner as a module. In an example, the whole or part of one or more computer systems (e.g., a standalone, client or server computer system) or one or more hardware processors may be configured by firmware or software (e.g., instructions, an application portion, or an application) as a module that operates to perform specified operations. In an example, the software may reside on a machine readable medium. In an example, the software, when executed by the underlying hardware of the module, causes the hardware to perform the specified operations.
Accordingly, the term “module” is understood to encompass a tangible entity, be that an entity that is physically constructed, specifically configured (e.g., hardwired), or temporarily (e.g., transitorily) configured (e.g., programmed) to operate in a specified manner or to perform part or all of any operation described herein. Considering examples in which modules are temporarily configured, each of the modules need not be instantiated at any one moment in time. For example, where the modules comprise a general-purpose hardware processor configured using software, the general-purpose hardware processor may be configured as respective different modules at different times. Software may accordingly configure a hardware processor, for example, to constitute a particular module at one instance of time and to constitute a different module at a different instance of time.
Machine (e.g., computer system) 7000 may include a hardware processor 7002 (e.g., a central processing unit (CPU), a graphics processing unit (GPU), a hardware processor core, or any combination thereof), a main memory 7004 and a static memory 7006, some or all of which may communicate with each other via an interlink (e.g., bus) 7008. The machine 7000 may further include a display unit 7010, an alphanumeric input device 7012 (e.g., a keyboard), and a user interface (UI) navigation device 7014 (e.g., a mouse). In an example, the display unit 7010, input device 7012 and UI navigation device 7014 may be a touch screen display. The machine 7000 may additionally include a storage device (e.g., drive unit) 7016, a signal generation device 7018 (e.g., a speaker), a network interface device 7020, and one or more sensors 7021, such as a global positioning system (GPS) sensor, compass, accelerometer, or other sensor. The machine 7000 may include an output controller 7028, such as a serial (e.g., universal serial bus (USB), parallel, or other wired or wireless (e.g., infrared (IR), near field communication (NFC), etc.) connection to communicate or control one or more peripheral devices (e.g., a printer, card reader, etc.).
The storage device 7016 may include a machine readable medium 7022 on which is stored one or more sets of data structures or instructions 7024 (e.g., software) embodying or utilized by any one or more of the techniques or functions described herein. The instructions 7024 may also reside, completely or at least partially, within the main memory 7004, within static memory 7006, or within the hardware processor 7002 during execution thereof by the machine 7000. In an example, one or any combination of the hardware processor 7002, the main memory 7004, the static memory 7006, or the storage device 7016 may constitute machine readable media.
While the machine readable medium 7022 is illustrated as a single medium, the term “machine readable medium” may include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) configured to store the one or more instructions 7024.
The term “machine readable medium” may include any medium that is capable of storing, encoding, or carrying instructions for execution by the machine 7000 and that cause the machine 7000 to perform any one or more of the techniques of the present disclosure, or that is capable of storing, encoding or carrying data structures used by or associated with such instructions. Non-limiting machine readable medium examples may include solid-state memories, and optical and magnetic media. Specific examples of machine readable media may include: non-volatile memory, such as semiconductor memory devices (e.g., Electrically Programmable Read-Only Memory (EPROM), Electrically Erasable Programmable Read-Only Memory (EEPROM)) and flash memory devices; magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; Random Access Memory (RAM); and CD-ROM and DVD-ROM disks. In some examples, machine readable media may include non-transitory machine readable media. In some examples, machine readable media may include machine readable media that is not a transitory propagating signal.
The instructions 7024 may further be transmitted or received over a communications network 7026 using a transmission medium via the network interface device 7020 utilizing any one of a number of transfer protocols (e.g., frame relay, internet protocol (IP), transmission control protocol (TCP), user datagram protocol (UDP), hypertext transfer protocol (HTTP), etc.). Example communication networks may include a local area network (LAN), a wide area network (WAN), a packet data network (e.g., the Internet), mobile telephone networks (e.g., cellular networks), Plain Old Telephone (POTS) networks, and wireless data networks (e.g., Institute of Electrical and Electronics Engineers (IEEE) 802.11 family of standards known as Wi-Fi®, IEEE 802.16 family of standards known as WiMax®), IEEE 802.15.4 family of standards, peer-to-peer (P2P) networks, among others. In an example, the network interface device 7020 may include one or more physical jacks (e.g., Ethernet, coaxial, or phone jacks) or one or more antennas to connect to the communications network 7026. In an example, the network interface device 7020 may include a plurality of antennas to wirelessly communicate using at least one of single-input multiple-output (SIMO), multiple-input multiple-output (MIMO), or multiple-input single-output (MISO) techniques. The term “transmission medium” shall be taken to include any intangible medium that is capable of storing, encoding or carrying instructions for execution by the machine 7000, and includes digital or analog communications signals or other intangible medium to facilitate communication of such software.
EXAMPLES
The following are additional examples.
Example 1 includes subject matter (such as a method, means for performing acts, machine readable medium including instructions) for reducing channel estimation overhead, comprising: at an eNodeB which provides a cell: determining an optimal signal subspace containing active UEs in the cell; transmitting beam formed reference signals to the optimal signal subspace; receiving a projected channel to the optimal signal subspace from at least one of the active UEs in the cell; and transforming a codebook and aligning codewords with a channel direction of the received projected channel.
In Example 2, the subject matter of Example 1 may optionally include wherein transmitting reference signals includes transmitting Channel State Information Reference Signals.
In Example 3, the subject matter of any one or more of examples 1-2 may optionally include wherein transmitting reference signals and receiving the projected channel is done using an Orthogonal Frequency Division Multiplexing protocol.
In Example 4, the subject matter of any one or more of examples 1-3 may optionally include wherein the optimal signal subspace has a dimension that is less than a full signal subspace.
In Example 5, the subject matter of any one or more of examples 1-4 may optionally include wherein transmitting reference signals comprises transmitting the reference signals to the active UEs in the cell.
In Example 6, the subject matter of any one or more of examples 1-5 may optionally include determining a change in the optimal signal subspace by scanning directions orthogonal to the optimal signal subspace.
In Example 7, the subject matter of any one or more of examples 1-6 may optionally include wherein determining the optimal signal subspace comprises estimating a channel covariance matrix at the end of a training phase.
Example 8 includes or may optionally be combined with the subject matter of any one of examples 1-7 to include subject matter (such as a device, apparatus, or machine such as an eNodeB) comprising: a signal subspace calculation module configured to: determine an optimal signal subspace containing active UEs in the cell; a reference signal output module configured to: transmit beam formed reference signals to the optimal signal subspace; an uplink reception module configured to: receive a projected channel to the optimal signal subspace from at least one of the active UEs in the cell; a transformation module configured to: transform a codebook and align codewords with a channel direction of the received projected channel.
In Example 9, the subject matter of any one or more of examples 1-8 may optionally include wherein the reference signal output module is configured to transmit reference signals by at least being configured to transmit Channel State Information Reference Signals.
In Example 10, the subject matter of any one or more of examples 1-9 may optionally include wherein the reference signal output module is configured to transmit the reference signals using at least an Orthogonal Frequency Division Multiplexing protocol.
In Example 11, the subject matter of any one or more of examples 1-10 may optionally include wherein the optimal signal subspace has a dimension that is less than a full signal subspace.
In Example 12, the subject matter of any one or more of examples 1-11 may optionally include wherein the reference signal output module is configured to transmit the reference signals transmitting reference signals by at least being configured to transmit the reference signals to the active UEs in the cell.
In Example 13, the subject matter of any one or more of examples 1-12 may optionally include wherein the optimal subspace calculation module is configured to determine a change in the optimal signal subspace by scanning directions orthogonal to the optimal signal subspace.
In Example 14, the subject matter of any one or more of examples 1-13 may optionally include wherein the optimal subspace calculation module is configured to determine the optimal signal subspace by at least being configured to estimate a channel covariance matrix at the end of a training phase.
Example 15 includes or may optionally be combined with the subject matter of any one of examples 1-14 to include subject matter (such as a device, apparatus, or machine such as a UE) comprising: a reference signal reception module configured to receive a beam formed reference signal sent from an eNodeB; an estimation module configured to estimate projected channel coefficients and direction; a transformation module configured to transform a codebook by a covariance matrix of the projected channel coefficients and direction; a quantization module configured to quantize the projected channel direction; a feedback module configured to: calculate a precoding matrix indicator and a channel quality indicator based upon the covariance matrix; and transmit the precoding matrix indicator, and a quantized covariance matrix calculated from the covariance matrix to the eNodeB.
In Example 16, the subject matter of any one or more of examples 1-15 may optionally include wherein the feedback module is configured to calculate the quantized covariance matrix by at least being configured to: calculate the quantized covariance matrix based upon the covariance matrix by scalar quantization of the elements of the covariance matrix.
In Example 17, the subject matter of any one or more of examples 1-16 may optionally include wherein the feedback module is configured to calculate the quantized covariance matrix based upon the covariance matrix by at least being configured to only quantize one of: the upper and the lower triangular elements of the matrix.
Example 18 includes or may optionally be combined with the subject matter of any one of examples 1-17 to include subject matter (such as a method, means for performing acts, machine readable medium including instructions) comprising: receiving a beam formed reference signal sent from an eNodeB; estimating projected channel coefficients and direction; transforming a codebook by a covariance matrix of the projected channel coefficients and direction; quantizing the projected channel direction; calculating a precoding matrix indicator and a channel quality indicator based upon the covariance matrix; and transmitting the precoding matrix indicator, and a quantized covariance matrix calculated from the covariance matrix to the eNodeB.
In Example 19, the subject matter of any one or more of examples 1-18 may optionally include receiving a beam formed reference signal sent from an eNodeB; estimating projected channel coefficients and direction; transforming a codebook by a covariance matrix of the projected channel coefficients and direction; quantizing the projected channel direction; calculating a precoding matrix indicator and a channel quality indicator based upon the covariance matrix; and transmitting the precoding matrix indicator, and a quantized covariance matrix calculated from the covariance matrix to the eNodeB.

## Claims

1. A method for reducing channel estimation overhead, the method comprising:
at an eNodeB that provides a cell:
calculating a beamforming vector matrix using previously received channel state feedback for a plurality of active User Equipment(UE)s in the cell;

determining an optimal signal subspace containing the plurality of active UEs in the cell using the beamforming vector matrix;
transmitting a beam formed reference signal to the determined optimal signal subspace;
determining that a UE of the plurality of active UEs in the cell has moved out of the optimal signal subspace;
recalculating the beamforming vector matrix for the plurality of active UEs including the UE of the plurality of active UEs in the cell;
transmitting a second beam formed reference signal to the UE of the plurality of active UEs in the cell using the recalculated beamforming reference matrix, the second beam formed reference signal being outside the optimal signal subspace;
receiving a projected channel from the UE of the plurality of active UEs in the cell, the projected channel calculated by the UE of the plurality of active UEs in the cell based upon the second beam formed reference signal; and
transforming a codebook and aligning codewords with a channel direction of the received projected channel.

2. The method of claim 1, wherein transmitting reference signals includes transmitting Channel State Information Reference Signals.

3. The method of claim 1, wherein transmitting reference signals and receiving the projected channel are done using an Orthogonal Frequency Division Multiplexing standard.

4. The method of claim 1, wherein the optimal signal subspace has a dimension that is less than a full signal subspace.

5. The method of claim 4, wherein transmitting reference signals comprises transmitting the reference signals to the plurality of active UEs in the cell.

6. The method of claim 1, comprising determining a change in the optimal signal subspace by scanning directions orthogonal to the optimal signal subspace.

7. The method of claim 1, wherein determining the optimal signal subspace comprises estimating a channel covariance matrix at the end of a training phase.

8. An eNodeB comprising:
a signal subspace calculation circuitry configured to:
calculate a beamforming vector matrix using previously received channel state feedback for a plurality of active User Equipment(UE)s in a cell;

determine an optimal signal subspace containing the plurality of active UEs in the cell using the beamforming vector matrix;
in response to determining that a UE of the plurality of active UEs in the cell has moved out of the optimal signal subspace, recalculating the beamforming vector matrix for the plurality of active UEs including the UE of the plurality of active UEs in the cell;
a reference signal output circuitry configured to:
transmit a beam formed reference signal to the optimal signal subspace;

transmit a second beam formed reference signal to the UE of the plurality of active UEs in the cell using the recalculated beamforming reference matrix, the second beam formed reference signal being outside the optimal signal subspace;
an uplink reception circuitry configured to:
receive a projected channel from the UE of the plurality of active UEs in the cell, the projected channel calculated by the UE of the plurality of active UEs in the cell based upon the second beam formed reference signal; and

a transformation circuitry configured to:
transform a codebook and align codewords with a channel direction of the received projected channel.

9. The eNodeB of claim 8, wherein the reference signal output circuitry is configured to transmit reference signals by at least being configured to transmit Channel State Information Reference Signals.

10. The eNodeB of claim 8, wherein the reference signal output circuitry is configured to transmit the reference signals using at least an Orthogonal Frequency Division Multiplexing standard.

11. The eNodeB of claim 8, wherein the optimal signal subspace has a dimension that is less than a full signal subspace.

12. The eNodeB of claim 11, wherein the reference signal output circuitry is configured to transmit the reference signals by at least being configured to transmit the reference signals to the plurality of active UEs in the cell.

13. The eNodeB of claim 8, wherein the signal subspace calculation circuitry is configured to determine a change in the optimal signal subspace by scanning directions orthogonal to the optimal signal subspace.

14. The eNodeB of claim 8, wherein the signal subspace calculation circuitry is configured to determine the optimal signal subspace by at least being configured to estimate a channel covariance matrix at the end of a training phase.

15. Anon-transitory machine readable medium that stores instructions which when performed by a machine, cause the machine to perform operations comprising:
calculating a beamforming vector matrix using previously received channel state feedback for a plurality of active User Equipment(UE)s in the cell;
determining an optimal signal subspace containing the plurality of active UEs in the cell using the beamforming vector matrix;
transmitting a beam formed reference signal to the optimal signal subspace;
determining that a UE of the plurality of active UEs in the cell has moved out of the optimal signal subspace;
recalculating the beamforming vector matrix for the plurality of active UEs including the UE of the plurality of active UEs in the cell;
transmitting a second beam formed reference signal to the UE of the plurality of active UEs in the cell using the recalculated beamforming reference matrix, the second beam formed reference signal being outside the optimal signal subspace;
receiving a projected channel from the UE of the plurality of active UEs in the cell, the projected channel calculated by the UE of the plurality of active UEs the cell based upon the second beam formed reference signal; and
transforming a codebook and align codewords with a channel direction of the received projected channel.

16. A User Equipment (UE) comprising:
a reference signal reception circuitry configured to receive a beam formed reference signal sent from an eNodeB, the beam formed reference signal using a beamforming vector matrix, and the UE that is a member of a plurality of active UEs, the UE having moved outside of an optimal signal space of the eNodeB;
an estimation circuitry configured to estimate projected channel coefficients and direction based upon he received beam formed reference signal sent from the eNodeB;
a transformation circuitry configured to transform a codebook by a covariance matrix of the projected channel coefficients and direction;
a quantization circuitry configured to quantize the projected channel direction using the transformed codebook;
a feedback circuitry configured to:
calculate a precoding matrix indicator based upon the quantized projected channel direction; and

transmit the precoding matrix indicator, and a quantized covariance matrix calculated from the covariance matrix to the eNodeB.

17. The UE of claim 16, wherein the feedback circuitry is configured to calculate the quantized covariance matrix by at least being configured to:
calculate the quantized covariance matrix based upon the covariance matrix by scalar quantization of elements of the covariance matrix.

18. The UE of claim 17, wherein the feedback circuitry is configured to calculate the quantized covariance matrix based upon the covariance matrix by at least being configured to only quantize one of:
upper and the lower triangular elements of the matrix.

19. A method comprising:
receiving a beam formed reference signal sent from an eNodeB, the beam formed reference signal using a beamforming vector matrix, and a User Equipment (UE) that is a member of a plurality of active UEs in a cell, the UE having moved outside of an optimal signal space of the eNodeB;
estimating projected channel coefficients and direction based upon the beam formed reference signal sent from the eNodeB;
transforming a codebook by a covariance matrix of the projected channel coefficients and direction;
quantizing the projected channel direction using the transformed codebook;
calculating a precoding matrix indicator based upon the quantized projected channel direction; and
transmitting the precoding matrix indicator, and a quantized covariance matrix calculated from the covariance matrix to the eNodeB.

20. A non-transitory machine readable medium that stores instructions which when performed by a machine, cause the machine to perform operations comprising:
receiving a beam formed reference signal sent from an eNodeB, the beam formed reference signal using a beamforming vector matrix, and a User Equipment (UE) that is a member of a plurality of active UEs in a cell, the UE having moved outside of an optimal signal space of the eNodeB;
estimating projected channel coefficients and direction based upon the received beam formed reference signal sent from the eNodeB;
transforming a codebook by a covariance matrix of the projected channel coefficients and direction;
quantizing the projected channel direction using the transformed codebook;
calculating a precoding matrix indicator based upon the quantized projected channel direction; and
transmitting the precoding matrix indicator, and a quantized covariance matrix calculated from the covariance matrix to the eNodeB.