# Patent

## Title
Frequency-independent spatial processing for wideband MISO and MIMO systems

## Abstract

Frequency-independent eigensteering in MISO and MIMO systems are described. For principal mode and multi-mode eigensteering, a correlation matrix is computed for a MIMO channel based on channel response matrices and decomposed to obtain NS frequency-independent steering vectors for NS spatial channels of the MIMO channel. ND data symbol streams are transmitted on ND best spatial channels using ND steering vectors, where ND=1 for principal mode eigensteering and ND>1 for multi-mode eigensteering. For main path eigensteering, a data symbol stream is transmitted on the best spatial channel for the main propagation path (e.g., with the highest energy) of the MIMO channel. For receiver eigensteering, a data symbol stream is steered toward a receive antenna based on a steering vector obtained for that receive antenna. For all eigensteering schemes, a matched filter is derived for each receive antenna based on the steering vector(s) and channel response vectors for the receive antenna.

## Background
BACKGROUND
I. Field
The present invention relates generally to data communication, and more specifically to techniques for performing spatial processing for wideband multiple-input single-output (MISO) and multiple-input multiple-output (MIMO) communication systems.
II. Background
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission and is denoted as an (NT, NR) system. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, where NS≦min {NT, NR }. NS spatial channels may be formed by the NS independent channels of the MIMO channel and used for data transmission.
For a time dispersive MIMO channel, a signal sent from a given transmit antenna may reach a given receive antenna via multiple signal paths (i.e., propagation paths). These signal paths may include a line-of-sight path and/or reflected paths, which are created when the transmitted signal reflects off reflection sources (e.g., buildings, obstructions, and so on) and arrives at the receive antenna via different signal paths than the line-of-sight path. The received signal at the receive antenna may thus include multiple instances (i.e., multipath components) of the signal sent from the transmit antenna. The delay spread L of the MIMO channel is the time difference between the earliest and latest arriving multipath components (of some certain minimum energy) for all of the transmit-receive antenna pairs in the MIMO channel.
Time dispersion in the MIMO channel causes frequency selective fading, which is characterized by a frequency response that varies across the system bandwidth (i.e., different channel gains for different frequencies). The multipath components are associated with different complex channel gains and may add constructively or destructively at the receiver. Time dispersion and frequency selective fading are more problematic for a wideband MIMO system with a wide system bandwidth.
Various techniques may be used to combat frequency selectivity in a wideband MIMO channel. For example, a multi-carrier modulation technique such as orthogonal frequency division multiplexing (OFDM) may be used to partition the system bandwidth into multiple (NF) orthogonal frequency subbands. The wideband MIMO channel may then be viewed as being composed of NF flat-fading narrowband MIMO channels, each of which may be decomposed into NS spatial channels. Data may then be transmitted on the NS spatial channels of each of the NF subbands.
For a MIMO system that utilizes OFDM (i.e., a MIMO-OFDM system), the wideband MIMO channel can be characterized with (1) a complex channel gain for each of the NF subbands of each of the NT·NR transmit/receive antenna pairs (i.e., NF·NT·NR channel gains in all) and (2) the noise floor at the receiver. The channel gains and receiver noise floor may then be used to select the data rate(s) for data transmission on the NS spatial channels of each of the NF subbands. The channel gains may also be used for spatial processing at the receiver and possibly the transmitter in order to transmit data on the NS spatial channels of each of the NF subbands. Thus, for the MIMO-OFDM system, frequency selectivity can be combated by treating the wideband MIMO channel as NF flat-fading narrowband MIMO channels and performing spatial processing separately for each of the narrowband MIMO channels. However, this frequency-dependent spatial processing can greatly increase computation complexity at the transmitter and receiver. Moreover, the receiver may need to provide a large amount of feedback information (e.g., the channel gains) to the transmitter to support frequency-dependent spatial processing.
There is therefore a need in the art for techniques to more efficiently perform spatial processing in a wideband MIMO system.
SUMMARY
Techniques for performing frequency-independent eigensteering in MISO and MIMO systems are provided herein. Eigensteering refers to spatial processing performed on a data symbol stream with a steering vector at a transmitter in order to transmit the data symbol stream on a spatial channel of a MISO channel or a MIMO channel. The MISO channel may be characterized by either (1) a sequence of time-domain channel impulse response vectors for a plurality of time delays or (2) a sequence of frequency-domain channel frequency response vectors for the NF subbands. Similarly, the MIMO channel may be characterized by either a sequence of channel impulse response matrices or a sequence of channel frequency response matrices. The eigensteering is frequency-independent in that one steering vector is used for the data symbol stream even if the MISO or MIMO channel is time dispersive and regardless of whether the eigensteering is performed in the time domain or the frequency domain. Eigensteering may be performed on one or multiple data symbol streams with one or multiple steering vectors to transmit the data symbol stream(s) on one or multiple spatial channels. Various frequency-independent eigensteering schemes are described herein including principal mode eigensteering, multi-mode eigensteering, main path eigensteering, and receiver eigensteering.
For principal mode and multi-mode eigensteering, a correlation matrix is computed for the MIMO channel based on the channel (impulse or frequency) response matrices for the MIMO channel, as described below. The correlation matrix is then decomposed (e.g., using eigenvalue decomposition) to obtain NS frequency-independent steering vectors for NS spatial channels of the MIMO channel. For principal mode eigensteering, one data symbol stream is transmitted on the principal or best spatial channel using the steering vector vpm for the best spatial channel. For multi-mode eigensteering, ND data symbol streams are transmitted on the ND best spatial channels using ND steering vectors Vmm for these spatial channels, where NS≧ND>1 in this case.
For main path eigensteering, a data symbol stream is transmitted on the principal spatial channel for the main propagation path of the MIMO channel using a frequency-independent steering vector vmp. For this scheme, the energy of each channel impulse response matrix is first determined. The main path is the time delay of the channel impulse response matrix with the highest energy. A correlation matrix of the channel impulse response matrix with the highest energy is computed and decomposed to obtain the steering vector vmp for the best spatial channel of the main path. The data symbol stream is transmitted on this spatial channel using the steering vector vmp.
For receiver eigensteering, a data symbol stream is steered toward an individual receive antenna based on a frequency-independent steering vector vrx,i obtained for that receive antenna. The MIMO channel may be viewed as being composed of NR MISO channels for the NR receive antennas. A correlation matrix may be computed for each MISO channel based on its sequence of channel (impulse or frequency) response vectors and decomposed to obtain a steering vector for the main spatial channel of that MISO channel. NR frequency-independent steering vectors vrx may be obtained for the NR MISO channels. ND data symbol streams may be transmitted using the NR steering vectors Vrx, where min{NR, NT}≧ND≧1 in this case. Each data symbol stream may be steered to one, multiple, or all receive antennas. For a MISO system with one receive antenna, one steering vector is obtained for the single receive antenna and used to transmit one data symbol stream.
For all of the eigensteering schemes, a matched filter is derived for each receive antenna based on the steering vector(s) used by the transmitter and the sequence of channel (impulse or frequency) response vectors for the receive antenna. The received symbol stream for each receive antenna is filtered with the matched filter for that receive antenna to obtain one or more filtered symbol substreams. The filtered symbol substreams from all NR matched filters for the NR receive antennas are then combined to obtain ND detected symbol streams for the ND data streams sent by the transmitter, where ND≧1 in this case. Equalization and other post-processing may be performed on the ND detected symbol streams to obtain ND recovered symbol streams, which are estimates of the ND data symbol streams sent by the transmitter.
Various aspects and embodiments of the invention are described in further detail below.



BRIEF DESCRIPTION OF THE DRAWINGS
The features and nature of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:
FIG. 1 shows a transmitter and a receiver in a MISO system;
FIG. 2 shows a transmit (TX) data processor in the MISO system;
FIGS. 3A, 3B, and 3C show three embodiments of a TX spatial processor in the MISO system;
FIGS. 4A, 4B, and 4C show three embodiments of a receive (RX) spatial processor in the MISO system;
FIG. 5 shows a block diagram of the receiver in the MISO system;
FIG. 6 shows a transmitter and a receiver in a MIMO system;
FIG. 7 shows a TX data processor in the MIMO system;
FIGS. 8A, 8B, and 8C show three embodiments of a TX spatial processor in the MIMO system;
FIGS. 9A through 9F show six embodiments of an RX spatial processor in the MIMO system;
FIG. 10 shows a block diagram of the receiver in the MIMO system;
FIG. 11 shows a process for performing principal mode eigensteering, multi-mode eigensteering, and main path eigensteering in the MIMO system; and
FIG. 12 shows a process for performing receiver eigensteering in the MISO or MIMO system.



DETAILED DESCRIPTION
The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs.
The eigensteering techniques described herein may be used for various wireless communication systems including single-carrier and multi-carrier MISO and MIMO systems. Multiple carriers may be provided by OFDM or some other multiple-carrier modulation technique or construct. In the description below, the term “MIMO system” generically refers to both single-carrier and multi-carrier MIMO systems.
For clarity, the following notations are used for the description below. Time-domain variables are functions of n and are denoted with cursive texts (e.g., h(n)). Frequency-domain variables are functions of k and are denoted with plain texts (e.g., h(k)). Vectors are denoted with lower-case bold-faced and underlined texts (e.g., h(n) and h(k)). Matrices are denoted with upper-case bold-faced and underlined texts (e.g., H(n) and H(k)). Three-dimension matrices are denoted with upper-case bold-faced and doubly underlined texts (e.g., H(and H).
1. MISO System
A time-dispersive MISO channel with NT transmit antennas and a single receive antenna may be characterized by a time-domain channel impulse response matrix H with dimensions of (L+1)×NT, where L denotes the delay extent of the MISO channel in symbol periods. The delay extent of a channel is the difference between the earliest and latest resolvable propagation paths in the channel. The matrix H is composed of NT channel impulse response vectors hj, for j=1, 2, . . . NT, or equivalently, L+1 row vectors h(n), for n=0, 1, . . . L, which may be expressed as:




  
    
      
        
          
            
              
                H
                _
              
              =
              
                [
                
                  
                    
                      
                        
                          
                            
                              
                                
                                  
                                    
                                      h
                                      _
                                    
                                    ⁡
                                    
                                      (
                                      0
                                      )
                                    
                                  
                                
                              
                              
                                
                                  
                                    
                                      h
                                      _
                                    
                                    ⁡
                                    
                                      (
                                      1
                                      )
                                    
                                  
                                
                              
                            
                          
                        
                        
                          
                            ⋮
                          
                        
                      
                    
                  
                  
                    
                      
                        
                          h
                          _
                        
                        ⁡
                        
                          (
                          L
                          )
                        
                      
                    
                  
                
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              =
              
                [
                
                  
                    
                      
                        
                          h
                          1
                        
                        ⁡
                        
                          (
                          0
                          )
                        
                      
                    
                    
                      
                        
                          h
                          2
                        
                        ⁡
                        
                          (
                          0
                          )
                        
                      
                    
                    
                      ⋯
                    
                    
                      
                        
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                            N
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                        ⁡
                        
                          (
                          0
                          )
                        
                      
                    
                  
                  
                    
                      
                        
                          h
                          1
                        
                        ⁡
                        
                          (
                          1
                          )
                        
                      
                    
                    
                      
                        
                          h
                          2
                        
                        ⁡
                        
                          (
                          1
                          )
                        
                      
                    
                    
                      ⋯
                    
                    
                      
                        
                          h
                          
                            N
                            T
                          
                        
                        ⁡
                        
                          (
                          1
                          )
                        
                      
                    
                  
                  
                    
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                      ⋮
                    
                    
                      ⋰
                    
                    
                      ⋮
                    
                  
                  
                    
                      
                        
                          h
                          1
                        
                        ⁡
                        
                          (
                          L
                          )
                        
                      
                    
                    
                      
                        
                          h
                          2
                        
                        ⁡
                        
                          (
                          L
                          )
                        
                      
                    
                    
                      ⋯
                    
                    
                      
                        
                          h
                          
                            N
                            T
                          
                        
                        ⁡
                        
                          (
                          L
                          )
                        
                      
                    
                  
                
                ]
              
            
          
        
        
          
            
              
                =
                
                  [
                  
                    
                      
                        
                          
                            h
                            _
                          
                          1
                        
                      
                      
                        
                          
                            h
                            _
                          
                          2
                        
                      
                      
                        ⋯
                      
                      
                        
                          
                            h
                            _
                          
                          
                            N
                            T
                          
                        
                      
                    
                  
                  ]
                
              
              ,
            
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          1
          )
        
      
    
  




where entry hj(n), for j=1, 2, . . . NT and n=0, 1, . . . L, is the coupling (i.e., complex gain) between transmit antenna j and the receive antenna for time delay n. Each vector hj, for j=1, 2, . . . NT, includes L+1 complex values for the channel impulse response between transmit antenna j and the receive antenna. Each row vector h(n), for n=0, 1, . . . L, includes NT complex values for the channel gains between the NT transmit antennas and the receive antenna for time delay n.

The channel impulse response may be estimated by the receiver based on pilot symbols sent by the transmitter. The transmitter can “cover” the pilot for each transmit antenna with a unique orthogonal sequence assigned to that antenna. Covering is a process whereby a given modulation symbol p (or a set of W symbols having the same value) to be transmitted is multiplied by all W chips of a W-chip orthogonal sequence to obtain W covered symbols, which are then transmitted. NT orthogonal pilots can be obtained with NT orthogonal sequences for the NT transmit antennas. The covering achieves orthogonality among the NT pilots sent from the NT transmit antennas and allows the receiver to distinguish the individual transmit antennas.
The receiver can “decover” the received pilot symbols with each of the same NT orthogonal sequences to estimate the channel impulse response between each of the NT transmit antennas and the receive antenna. Decovering is a complementary process whereby W received symbols for W covered symbols are multiplied by the W chips of the same W-chip orthogonal sequence to obtain W decovered symbols, which are then accumulated to obtain an estimate of the transmitted symbol p. The decovering is performed at L+1 time delays to obtain the L+1 row vectors h(n), for n=0, 1, . . . L, for the channel impulse response of the MISO channel.
A time dispersive MISO channel may also be characterized by a two-dimensional frequency-domain channel frequency response matrix H with dimensions of NF×NT, where NF is the number of frequency subbands and NF≧(L+1). The matrix H is composed of NT channel frequency response vectors hj, for j=1, 2, . . . NT, i.e., H=[h1 h2 . . . hNT]. Each vector hj includes NF frequency-domain values that may be obtained by performing an NF-point discrete Fourier transform (DFT) on the L+1 time-domain values of the corresponding vector hj of the matrix H. For each transmit-receive antenna pair, there is thus a one-to-one correspondence between the time-domain channel impulse response vector hj and the frequency-domain channel frequency response vector hj. The matrix H is equivalently composed of NF row vectors h(k), for k=1, 2, . . . NF, i.e., H=[hT(1) hT(2) . . . hT(NF)]T, where MT is the transpose of M. Each row vector h(k) includes NT complex gain values for the frequency response between the NT transmit antennas and the receive antenna for subband k. The DFT to obtain h(k) may be expressed as:




  
    
      
        
          
            
              h
              _
            
            ⁡
            
              (
              k
              )
            
          
          =
          
            
              ∑
              
                n
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                  _
                
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          ⁢
          
            
          
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            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          2
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The transmitter may perform eigensteering on a data symbol stream s(n) prior to transmission via the MISO channel in order to achieve higher received signal-to-noise ratio (SNR) for the data symbol stream. The eigensteering may be performed using a frequency-independent steering vector vmiso, which may be derived to maximize the received SNR or based on some other criteria.
In one embodiment, the steering vector vmiso for the transmitter is obtained by first computing an NT×NT correlation matrix Rmiso as follows:




  
    
      
        
          
            
              R
              _
            
            miso
          
          =
          
            
              
                ∑
                
                  n
                  =
                  0
                
                L
              
              ⁢
              
                
                  
                    
                      h
                      _
                    
                    H
                  
                  ⁡
                  
                    (
                    n
                    )
                  
                
                ⁢
                
                  
                    h
                    _
                  
                  ⁡
                  
                    (
                    n
                    )
                  
                
              
            
            =
            
              
                1
                
                  N
                  F
                
              
              ⁢
              
                
                  ∑
                  
                    k
                    =
                    1
                  
                  
                    N
                    F
                  
                
                ⁢
                
                  
                    
                      
                        h
                        _
                      
                      H
                    
                    ⁡
                    
                      (
                      k
                      )
                    
                  
                  ⁢
                  
                    
                      h
                      _
                    
                    ⁡
                    
                      (
                      k
                      )
                    
                  
                
              
            
          
        
        ,
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          3
          )
        
      
    
  




where hH is the conjugate transpose of h. The matrix Rmiso may be viewed as an average of either L+1 individual correlation matrices of h(n) for L+1 time delays or NF individual correlation matrices of h(k) for NF subbands. The individual correlation matrices are given equal weight in equation (3). In another embodiment, the individual correlation matrices may be given unequal weights in the computation of Rmiso. For example, each individual correlation matrix may be weighted by the energy associated with that matrix, which can be computed as described below.

Eigenvalue decomposition of the correlation matrix Rmiso is then performed as follows:

Rmiso=VmisoΛmisoVmisoH,  Eq (4)

    
    
        where Vmiso is an NT×NT unitary matrix whose columns are the eigenvectors of Rmiso; and
        
            Λmiso is an NT×NT diagonal matrix whose diagonal entries are the eigenvalues of Rmiso.

A unitary matrix M is characterized by the property MHM=I, where I is the identity matrix with ones along the diagonal and zeros elsewhere. The NT eigenvectors of the unitary matrix Vmiso, denoted as vj for j=1, 2, . . . NT, are thus orthogonal to one another. Moreover, the length of each eigenvector is equal to one, i.e.,

        
        
    
    






  
    
      
        
        
          
            v
            _
          
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      2
    
    =
    
      
        
          ∑
          
            i
            =
            1
          
          
            N
            T
          
        
        ⁢
        
          
            
            
              v
              
                i
                ,
                j
              
            
            
          
          2
        
      
      =
      1
    
  
  ,




where vj=[ν1,j ν2,j . . . νNT,j]T. The NT eigenvectors are also referred to as steering vectors and may be used for eigensteering by the transmitter and matched filtering by the receiver, as described below.

The matrix Rmiso is of dimension NT×NT and rank Nmiso, where Nmiso≦min {NT, (L+1)}. The diagonal matrix Λmiso thus contains Nmiso positive real values along the diagonal and zeros elsewhere. The largest non-zero entry is referred to as the principal eigenvalue λmiso of the matrix Rmiso and is indicative of the power gain for the spatial channel (or “time-domain eigenmode”) corresponding to that eigenvalue. The frequency-independent steering vector vmiso to use for eigensteering is the “principal” eigenvector of Rmiso, which is the column of Vmiso that corresponds to the principal eigenvalue of Rmiso.
The transmitter performs eigensteering on the data symbol stream s(n) with the steering vector vmiso to obtain NT transmit symbol streams xmiso(n), as follows:

xmiso(n)=s(n)·vmiso.  Eq (5)

With the eigensteering shown in equation (5), the data symbol stream s(n) observes an effective channel that is a single-input single-output (SISO) channel with an effective channel impulse response of h(n)vmiso, for n=0, 1, . . . L. The NT transmit symbol streams xmiso(n) are further processed and transmitted from the NT transmit antennas to the receiver.

The receiver obtains a received symbol stream ymiso(n) from the single receive antenna, which may be expressed as:




  
    
      
        
          
            
              
                
                  
                    y
                    miso
                  
                  ⁡
                  
                    (
                    n
                    )
                  
                
                =
                
                  
                    
                      
                        h
                        _
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                    ⊗
                    
                      
                        
                          x
                          _
                        
                        miso
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                  
                  +
                  
                    
                      n
                      miso
                    
                    ⁡
                    
                      (
                      n
                      )
                    
                  
                
              
              ,
            
          
        
        
          
            
              
                =
                
                  
                    (
                    
                      
                        ∑
                        
                          j
                          =
                          0
                        
                        L
                      
                      ⁢
                      
                        
                          
                            h
                            _
                          
                          ⁡
                          
                            (
                            j
                            )
                          
                        
                        ⁢
                        
                          s
                          ⁡
                          
                            (
                            
                              n
                              -
                              j
                            
                            )
                          
                        
                        ⁢
                        
                          
                            v
                            _
                          
                          miso
                        
                      
                    
                    )
                  
                  +
                  
                    
                      n
                      miso
                    
                    ⁡
                    
                      (
                      n
                      )
                    
                  
                
              
              ,
            
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          6
          )
        
      
    
  




where “{circle around (x)}” denotes a convolution and nmiso(n) is additive white Gaussian noise (AWGN). The received symbol stream ymiso(n) experiences intersymbol interference (ISI), which is a phenomenon whereby each symbol in a received stream acts as distortion to subsequent symbols in the received stream. Intersymbol interference may be mitigated through the use of OFDM in conjunction with a sufficiently long cyclic prefix, as described below. Alternatively, for a single-carrier MISO system, intersymbol interference may be mitigated through the use of appropriate temporal matched filtering in combination with equalization, as also described below.

The receiver can perform matched filtering of the received symbol stream ymiso(n) in either the time domain or the frequency domain. The time-domain matched filtering may be expressed as:




  
    
      
        
          
            
              
                
                  
                    
                      s
                      ~
                    
                    miso
                  
                  ⁡
                  
                    (
                    n
                    )
                  
                
                =
                
                  
                    
                      v
                      _
                    
                    miso
                    H
                  
                  ⁢
                  
                    
                      
                        
                          h
                          _
                        
                        H
                      
                      ⁡
                      
                        (
                        
                          L
                          -
                          n
                        
                        )
                      
                    
                    ⊗
                    
                      
                        y
                        miso
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                  
                
              
              ,
            
          
        
        
          
            
              
                =
                
                  
                    
                      v
                      _
                    
                    miso
                    H
                  
                  ⁢
                  
                    
                      ∑
                      
                        j
                        =
                        0
                      
                      L
                    
                    ⁢
                    
                      
                        
                          
                            h
                            _
                          
                          H
                        
                        ⁡
                        
                          (
                          
                            L
                            -
                            j
                          
                          )
                        
                      
                      ·
                      
                        
                          y
                          miso
                        
                        ⁡
                        
                          (
                          
                            n
                            -
                            j
                          
                          )
                        
                      
                    
                  
                
              
              ,
            
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          7
          )
        
      
    
  




where {tilde over (s)}miso(n) denotes a detected symbol stream, which is an estimate of the data symbol stream s(n) sent by the transmitter. The matched filter mmiso(n)=vmisoHhH(L−n), for n=0, 1, . . . L, maximizes the received SNR.

An equalizer may be used to mitigate intersymbol interference due to time dispersion in the MIMO channel. The equalizer may be a minimum mean square error (MMSE) equalizer, a decision feedback equalizer (DFE), a maximum likelihood sequence estimator (MLSE), or some other type of equalizer. The equalizer may be implemented with an adaptive filter having coefficients that can be updated with pilot and/or data symbols and based on a particular criterion (e.g., minimum mean square error). The equalizer performs equalization on the detected symbol stream {tilde over (s)}miso(n) and provides a recovered symbol stream ŝmiso(n), which is a better estimate of the data symbol stream s(n) sent by the transmitter. In general, the detected symbol stream {tilde over (s)}miso(n) may be provided directly as the recovered symbol stream ŝmiso(n) or may be post-processed (e.g., equalized) to obtain the recovered symbol stream ŝmiso(n).
The frequency-domain matched filtering may be expressed as:

ŝmiso(k)=vmisoHhH(k)ymiso(k), for k=1, 2, . . . NF,  Eq (8)

where ŝmiso(k) is the recovered symbol substream for subband k; and

ymiso(k) is the received symbol substream for subband k.
The NF received symbol substreams ymiso(k), for k=1, 2, . . . NF, may be obtained by performing a fast Fourier transform (FFT) of each set of NF symbols in the received symbol stream ymiso(n). The matched filter mmiso(k)=vmisoHhH(k), for k=1, 2, . . . NF, is a complex-valued scalar that maximizes the received SNR for each subband. The NF recovered symbol substreams for the NF subbands may be multiplexed together to obtain the recovered symbol stream ŝmiso(n).
For both time-domain and frequency-domain matched filtering, the received SNR may be expressed as:




  
    
      
        
          
            SNR
            miso
          
          =
          
            
              
                
                  P
                  total
                
                
                  σ
                  2
                
              
              ⁢
              
                
                  v
                  _
                
                miso
                H
              
              ⁢
              
                
                  R
                  _
                
                miso
              
              ⁢
              
                
                  v
                  _
                
                miso
              
            
            =
            
              
                
                  P
                  total
                
                
                  σ
                  2
                
              
              ⁢
              
                λ
                miso
              
            
          
        
        ,
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          9
          )
        
      
    
  




where Ptotal is the total transmit power used by the transmitter for the data symbol stream;

    
    
        σ2 is the noise floor at the receiver; and
        λmiso is the principal eigenvalue of Rmiso.
    
    


The capacity Cmisofi of the MISO channel with frequency-independent eigensteering may be determined using frequency-domain analysis and assuming that the same steering vector is used for all NF subbands. The capacity Cmisofi may be expressed as:




  
    
      
        
          
            
              
                C
                miso
                fi
              
              =
              
                
                  ∑
                  
                    k
                    =
                    1
                  
                  
                    N
                    F
                  
                
                ⁢
                
                  
                    log
                    2
                  
                  ⁡
                  
                    (
                    
                      1
                      +
                      
                        SNR
                        ⁡
                        
                          (
                          k
                          )
                        
                      
                    
                    )
                  
                
              
            
          
        
        
          
            
              
                =
                
                  
                    ∑
                    
                      k
                      =
                      1
                    
                    
                      N
                      F
                    
                  
                  ⁢
                  
                    
                      log
                      2
                    
                    ⁡
                    
                      (
                      
                        1
                        +
                        
                          
                            ρ
                            ·
                            
                              
                                v
                                _
                              
                              miso
                              H
                            
                          
                          ⁢
                          
                            
                              R
                              _
                            
                            ⁡
                            
                              (
                              k
                              )
                            
                          
                          ⁢
                          
                            
                              v
                              _
                            
                            miso
                          
                        
                      
                      )
                    
                  
                
              
              ,
            
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          10
          )
        
      
    
  




where ρ is the average received SNR as measured at the receive antenna, which is equal to the total receive power divided by the receiver noise σ2. The matrix R(k) is the correlation matrix of h(k), which may be obtained and decomposed as follows:

R(k)=hH(k)h(k)=U(k)Λ(k)UH(k), for k=1, 2, . . . NF,  Eq (11)

where Λ(k) is the diagonal matrix of eigenvalues of R(k), and

    
    
        U(k) is the unitary matrix of eigenvectors of R(k).

The quadratic term in equation (10) may be expressed as:

vmisoHR(k)vmiso=vmisoHU(k)Λ(k)UH(k)vmisco=zH(k)Λ(k)z(k),  Eq (12)

where z(k)=UH(k)vmiso. Since R(k) has only one non-zero eigenvalue, equation (12) may be simplified as follows:

vmisoHR(k)vmiso=|z1(k)|2λ(k),  Eq (13)

where λ(k) is the non-zero eigenvalue of R(k), which is λ(k)=∥h(k)∥2 for a MISO channel, and z1(k) is the element of z(k) corresponding to the eigenvalue λ(k). The capacity Cmisofi of the MISO channel with frequency-independent eigensteering may then be expressed as:

    
    






  
    
      
        
          C
          miso
          fi
        
        =
        
          
            ∑
            
              k
              =
              1
            
            
              N
              F
            
          
          ⁢
          
            
              
                log
                2
              
              ⁡
              
                (
                
                  1
                  +
                  
                    ρ
                    ·
                    
                      
                        
                        
                          
                            z
                            1
                          
                          ⁡
                          
                            (
                            k
                            )
                          
                        
                        
                      
                      2
                    
                    ·
                    
                      λ
                      ⁡
                      
                        (
                        k
                        )
                      
                    
                  
                
                )
              
            
            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          14
          )
        
      
    
  




The capacity Cmison for the MISO channel without eigensteering at the transmitter (or equivalently with a steering vector v=[g g . . . g], where g=√{square root over (1/NT)}) may be expressed as:




  
    
      
        
          C
          miso
          n
        
        =
        
          
            ∑
            
              k
              =
              1
            
            
              N
              F
            
          
          ⁢
          
            
              
                log
                2
              
              ⁡
              
                (
                
                  1
                  +
                  
                    
                      ρ
                      
                        N
                        T
                      
                    
                    ·
                    
                      λ
                      ⁡
                      
                        (
                        k
                        )
                      
                    
                  
                
                )
              
            
            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          15
          )
        
      
    
  




In general, the capacity Cmisofi of the MISO channel with frequency-independent eigensteering is greater than the capacity Cmison of the MISO channel without eigensteering.

An exemplary method has been described above for obtaining the frequency-independent steering vector vmiso for eigensteering at the transmitter in the MISO system. The steering vector may also be obtained in other manners, and this is within the scope of the invention.
The frequency-independent eigensteering may also be used for a MISO system that employs OFDM (i.e., a MISO-OFDM system). The transmitter can perform eigensteering in the time-domain, as shown in equation (5), where s(n) denotes a sequence of time-domain chips for OFDM symbols generated for the data stream by OFDM modulation. OFDM modulation is described below. The transmitter can also perform eigensteering in the frequency-domain on the data symbols for each subband, prior to the OFDM modulation to generate OFDM symbols. The receiver can perform matched filtering in the time-domain, as shown in equation (7), or the frequency-domain, as shown in equation (8).
2. MIMO System
A time-dispersive MIMO channel with NT transmit antennas and NR receive antennas may be characterized by a three-dimensional time-domain channel impulse response matrix H with dimensions of NR×NT×(L+1). The matrix H is composed of L+1 channel impulse response matrices H(n), for n=0, 1, . . . L, i.e., H=[H(0) H(1) . . . H(L)], which may be expressed as:




  
    
      
        
          
            
              H
              _
            
            ⁡
            
              (
              n
              )
            
          
          =
          
            
              [
              
                
                  
                    
                      
                        
                          
                            
                              
                                
                                  
                                    
                                      h
                                      _
                                    
                                    1
                                  
                                  ⁡
                                  
                                    (
                                    n
                                    )
                                  
                                
                              
                            
                            
                              
                                
                                  
                                    
                                      h
                                      _
                                    
                                    2
                                  
                                  ⁡
                                  
                                    (
                                    n
                                    )
                                  
                                
                              
                            
                          
                        
                      
                      
                        
                          ⋮
                        
                      
                    
                  
                
                
                  
                    
                      
                        
                          h
                          _
                        
                        
                          N
                          R
                        
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                  
                
              
              ]
            
            =
            
              [
              
                
                  
                    
                      
                        h
                        
                          1
                          ,
                          1
                        
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                  
                  
                    
                      
                        h
                        
                          1
                          ,
                          2
                        
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                  
                  
                    ⋯
                  
                  
                    
                      
                        h
                        
                          1
                          ,
                          
                            N
                            T
                          
                        
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                  
                
                
                  
                    
                      
                        h
                        
                          2
                          ,
                          1
                        
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                  
                  
                    
                      
                        h
                        
                          2
                          ,
                          2
                        
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                  
                  
                    ⋯
                  
                  
                    
                      
                        h
                        
                          2
                          ,
                          
                            N
                            T
                          
                        
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                  
                
                
                  
                    ⋮
                  
                  
                    ⋮
                  
                  
                    ⋰
                  
                  
                    ⋮
                  
                
                
                  
                    
                      
                        h
                        
                          
                            N
                            R
                          
                          ,
                          1
                        
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                  
                  
                    
                      
                        h
                        
                          
                            N
                            R
                          
                          ,
                          2
                        
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                  
                  
                    ⋯
                  
                  
                    
                      
                        h
                        
                          
                            N
                            R
                          
                          ,
                          
                            N
                            T
                          
                        
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                  
                
              
              ]
            
          
        
        ,
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          16
          )
        
      
    
  




for n=0, 1, . . . L, where entry hi,j(n), for i=1, 2, . . . NR, j=1, 2, . . . NT, and n=0, 1, . . . L, is the coupling (i.e., complex gain) between transmit antenna j and receive antenna i for time delay n. The row vector hi(n), for i=1, 2, . . . NR and n=0, 1, . . . L, includes NT complex values for the channel gains between the NT transmit antennas and receive antenna i for time delay n.

The channel impulse response may be estimated by the receiver based on pilot symbols sent by the transmitter. In one embodiment, the transmitter covers the pilot for each transmit antenna with an orthogonal code assigned to that antenna. The pilots sent from the NT transmit antennas are covered by NT orthogonal codes and may be individually recovered. At the receiver, the received pilot from each receive antenna i is decovered with the NT orthogonal codes at a particular time delay to obtain the channel response between receive antenna i and each of the NT transmit antennas for that time delay, i.e., one row of the matrix H(n). The decovering is performed separately for all NR receive antennas to obtain the NR rows of the matrix H(n). The decovering is also performed at L+1 time delays (i.e., for n=0, 1, . . . L) for each transmit-receive antenna pair to obtain the L+1 time-domain values for the channel impulse response for that transmit-receive antenna pair.
A time dispersive MIMO channel may also be characterized by a corresponding three-dimensional frequency-domain channel frequency response matrix H with dimensions of NR×NT×NF, where NF>L. The matrix H is composed of NF channel frequency response matrices H(k), for k=1, 2, . . . NF, which can be obtained by computing an NF-point discrete Fourier transform on the L+1 channel impulse response matrices H(n), for n=0, 1, . . . L, as follows:




  
    
      
        
          
            
              H
              _
            
            ⁡
            
              (
              k
              )
            
          
          =
          
            
              [
              
                
                  
                    
                      
                        
                          
                            
                              
                                
                                  
                                    
                                      h
                                      _
                                    
                                    1
                                  
                                  ⁡
                                  
                                    (
                                    k
                                    )
                                  
                                
                              
                            
                            
                              
                                
                                  
                                    
                                      h
                                      _
                                    
                                    2
                                  
                                  ⁡
                                  
                                    (
                                    k
                                    )
                                  
                                
                              
                            
                          
                        
                      
                      
                        
                          ⋮
                        
                      
                    
                  
                
                
                  
                    
                      
                        
                          h
                          _
                        
                        
                          N
                          R
                        
                      
                      ⁡
                      
                        (
                        k
                        )
                      
                    
                  
                
              
              ]
            
            =
            
              
                ∑
                
                  n
                  =
                  0
                
                L
              
              ⁢
              
                
                  
                    H
                    _
                  
                  ⁡
                  
                    (
                    n
                    )
                  
                
                ·
                
                  e
                  
                    
                      -
                      
                        j
                        ⁡
                        
                          (
                          
                            2
                            ⁢
                            
                              
                            
                            ⁢
                            
                              π
                              /
                              
                                N
                                F
                              
                            
                          
                          )
                        
                      
                    
                    ⁢
                    
                      (
                      
                        k
                        -
                        1
                      
                      )
                    
                    ⁢
                    n
                  
                
              
            
          
        
        ,
        
          

        
        ⁢
        
          
            for
            ⁢
            
              
            
            ⁢
            k
          
          =
          1
        
        ,
        2
        ,
        
          ...
          ⁢
          
            
          
          ⁢
          
            
              N
              F
            
            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          17
          )
        
      
    
  




Each matrix H(k), for k=1, 2, . . . NF, includes NR row vectors hi(k), for i=1, 2, . . . NR, for the NR receive antennas. Each row vector hi(k) includes NT complex values for the channel gains between the NT transmit antennas and receive antenna i for subband k. Each entry hi,j of the matrix H, for i=1, 2, . . . NR and j=1, 2, . . . NT, includes NF frequency-domain values that may be obtained by taking the DFT of the L+1 time-domain values for a corresponding entry hi,j of the matrix H. For each transmit-receive antenna pair, there is thus a one-to-one correspondence between the channel impulse response hi,j and the channel frequency response hi,j.

Frequency-independent eigensteering may be performed in various manners for a MIMO system. Some exemplary frequency-independent eigensteering schemes are described below.
A. Principal-Mode Eigensteering
For principal-mode eigensteering, a data symbol stream is transmitted on the principal spatial channel of the MIMO channel using a single frequency-independent steering vector vpm. To obtain this steering vector, an NT×NT correlation matrix Rmimo is first computed as follows:




  
    
      
        
          
            
              
                
                  R
                  _
                
                mimo
              
              =
              
                
                  ∑
                  
                    n
                    =
                    0
                  
                  L
                
                ⁢
                
                  
                    
                      
                        H
                        _
                      
                      H
                    
                    ⁡
                    
                      (
                      n
                      )
                    
                  
                  ⁢
                  
                    
                      H
                      _
                    
                    ⁡
                    
                      (
                      n
                      )
                    
                  
                
              
            
          
        
        
          
            
              
                =
                
                  
                    1
                    
                      N
                      F
                    
                  
                  ⁢
                  
                    
                      ∑
                      
                        k
                        =
                        1
                      
                      
                        N
                        F
                      
                    
                    ⁢
                    
                      
                        
                          
                            H
                            _
                          
                          H
                        
                        ⁡
                        
                          (
                          k
                          )
                        
                      
                      ⁢
                      
                        
                          H
                          _
                        
                        ⁡
                        
                          (
                          k
                          )
                        
                      
                    
                  
                
              
              ,
            
          
        
        
          
            
              =
              
                
                  ∑
                  
                    n
                    =
                    0
                  
                  L
                
                ⁢
                
                  
                    ∑
                    
                      j
                      =
                      1
                    
                    
                      N
                      T
                    
                  
                  ⁢
                  
                    
                      
                        
                          h
                          _
                        
                        j
                        H
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                    ⁢
                    
                      
                        
                          h
                          _
                        
                        j
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                  
                
              
            
          
        
        
          
            
              =
              
                
                  1
                  
                    N
                    F
                  
                
                ⁢
                
                  
                    ∑
                    
                      k
                      =
                      1
                    
                    
                      N
                      F
                    
                  
                  ⁢
                  
                    
                      ∑
                      
                        j
                        =
                        1
                      
                      
                        N
                        T
                      
                    
                    ⁢
                    
                      
                        
                          
                            h
                            _
                          
                          H
                        
                        ⁡
                        
                          (
                          k
                          )
                        
                      
                      ⁢
                      
                        
                          
                            h
                            _
                          
                          ⁡
                          
                            (
                            k
                            )
                          
                        
                        .
                      
                    
                  
                
              
            
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          18
          )
        
      
    
  




Eigenvalue decomposition of Rmimo is then performed as follows:

Rmimo=VmimoΛmimoVmimoH,  Eq (19)

where Vmimo is a unitary matrix of eigenvectors of Rmimo and Λmimo is a diagonal matrix whose diagonal entries are the eigenvalues of Rmimo.

A MIMO channel may be decomposed into NS spatial channels, where NS≦min {NT, NR}. The matrix Rmimo is of rank NS and the diagonal matrix Λmimo contains NS non-negative real values along the diagonal. The largest non-zero diagonal entry is referred to as the principal eigenvalue λmimo of the matrix Rmimo. In an embodiment, the steering vector vpm to use for eigensteering is the principal eigenvector of Rmimo, which is the column of Vmimo that corresponds to the principal eigenvalue of Rmimo. The steering vector vpm may be viewed as being for the principal spatial channel of the “averaged” MIMO channel.
The transmitter performs eigensteering on the data symbol stream s(n) with the steering vector vpm to obtain NT transmit symbol streams xpm(n), as follows:

xpm(n)=s(n)·vpm.  Eq (20)

With the eigensteering shown in equation (20), the data symbol stream s(n) observes an effective channel that is a single-input multiple-output (SIMO) channel with an effective channel impulse response of H(n)vpm, for n=0, 1, . . . L. The NT transmit symbol streams xpm(n) are further processed and sent from the NT transmit antennas to the receiver.

The receiver obtains NR received symbol streams ypm(n) from the NR receive antennas, which may be expressed as:

ypm(n)=H(n){circle around (x)}xpm(n)+nmimo(n),  Eq (21)

where nmimo(n) is additive white Gaussian noise with a mean vector of 0 and a covariance matrix of Λn=σ2I, where 0 is a vector of all zeros. The receiver can perform matched filtering of the received symbol streams ypm(n) in either the time domain or the frequency domain.

The time-domain matched filtering may be expressed as:

{tilde over (s)}pm(n)=vpmHHH(L−n){circle around (x)}ypm(n), for n=0, 1, . . . L.  Eq (22)

The receiver matched filter is mpm(n)=vpmH(L−n), for n=0, 1, . . . L, which includes NR individual matched filters for the NR receive antennas. The matched filter mpm,i(n) for each receive antenna has an impulse response of mpm,i(n)=vpmHhiH(L−n), for n=0, 1, . . . L, that maximizes the received SNR for that receive antenna. The outputs of the NR individual matched filters for the NR receive antennas are summed to obtain the detected symbol stream {tilde over (s)}pm(n). Post processing (e.g., equalization) may be performed on the detected symbol stream {tilde over (s)}pm(n) to obtain the recovered symbol stream ŝpm(n).

The frequency-domain matched filtering may be expressed as:

ŝpm(k)=vpmHHH(k)ypm(k), for k=1, 2, . . . NF,  Eq (23)

where ypm(k) is the received symbol substreams for subband k, which can be obtained by performing an FFT of each set of NF symbols in the received symbol streams ypm(n). The receiver matched filter is mpm(k)=vpmHHH(k), for k=1, 2, . . . NF, which includes NR individual matched filters for the NR receive antennas. The matched filter mpm,i(k) for each receive antenna i has a response of mpm,i(k)=vpmHhiH(k), for k=1, 2, . . . NF. The outputs of the NR individual matched filters for the NR receive antennas for each subband k are summed to obtain the recovered symbol substream ŝpm(k) for that subband. The NF recovered symbol substreams for the NF subbands may be multiplexed to obtain the recovered symbol stream ŝpm(n).

For both the time-domain and frequency-domain matched filtering, the received SNR, averaged across the NR receive antennas, may be expressed as:




  
    
      
        
          SNR
          mimi
          pm
        
        =
        
          
            ρ
            
              N
              R
            
          
          ⁢
          
            
              v
              _
            
            pm
            H
          
          ⁢
          
            
              R
              _
            
            mino
          
          ⁢
          
            
              
                v
                _
              
              pm
            
            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          24
          )
        
      
    
  




The matched filter vpmHHH(L−n), for n=0, 1, . . . L, maximizes the received SNR.

The capacity Cmimopm of the MIMO channel with principal-mode eigensteering may be expressed as:




  
    
      
        
          C
          mimo
          pm
        
        =
        
          
            ∑
            
              k
              =
              1
            
            
              N
              F
            
          
          ⁢
          
            
              
                log
                2
              
              ⁡
              
                (
                
                  1
                  +
                  
                    
                      ρ
                      ·
                      
                        
                          v
                          _
                        
                        pm
                        H
                      
                    
                    ⁢
                    
                      
                        
                          H
                          _
                        
                        H
                      
                      ⁡
                      
                        (
                        k
                        )
                      
                    
                    ⁢
                    
                      
                        H
                        _
                      
                      ⁡
                      
                        (
                        k
                        )
                      
                    
                    ⁢
                    
                      
                        v
                        _
                      
                      pm
                    
                  
                
                )
              
            
            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          25
          )
        
      
    
  




The quadratic term in equation (25) may be expressed as:





  
    
      
        
          
            
              
                v
                _
              
              pm
              H
            
            ⁢
            
              
                
                  H
                  _
                
                H
              
              ⁡
              
                (
                k
                )
              
            
            ⁢
            
              
                H
                _
              
              ⁡
              
                (
                k
                )
              
            
            ⁢
            
              
                v
                _
              
              pm
            
          
          =
          
            
              ∑
              
                l
                =
                1
              
              
                N
                T
              
            
            ⁢
            
              
                
                  
                  
                    
                      
                        v
                        _
                      
                      pm
                      H
                    
                    ⁢
                    
                      
                        
                          u
                          _
                        
                        l
                      
                      ⁡
                      
                        (
                        k
                        )
                      
                    
                  
                  
                
                2
              
              ·
              
                
                  λ
                  l
                
                ⁡
                
                  (
                  k
                  )
                
              
            
          
        
        ,
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          26
          )
        
      
    
  




where ul(k) is the eigenvector associated with the l-th eigenvalue λl(k) of the correlation matrix R(k)=HH (k)H(k). The capacity Cmimopm may then be expressed as:





  
    
      
        
          C
          mimo
          pm
        
        =
        
          
            ∑
            
              k
              =
              1
            
            
              N
              F
            
          
          ⁢
          
            
              
                log
                2
              
              ⁡
              
                (
                
                  1
                  +
                  
                    ρ
                    ·
                    
                      
                        ∑
                        
                          l
                          =
                          1
                        
                        
                          N
                          T
                        
                      
                      ⁢
                      
                        
                          
                            
                            
                              
                                
                                  v
                                  _
                                
                                pm
                                H
                              
                              ⁢
                              
                                
                                  
                                    u
                                    _
                                  
                                  l
                                
                                ⁡
                                
                                  (
                                  k
                                  )
                                
                              
                            
                            
                          
                          2
                        
                        ·
                        
                          
                            λ
                            l
                          
                          ⁡
                          
                            (
                            k
                            )
                          
                        
                      
                    
                  
                
                )
              
            
            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          27
          )
        
      
    
  




B. Multi-mode Eigensteering
For multi-mode eigensteering, multiple data symbol streams are transmitted on multiple spatial channels of the MIMO channel using multiple frequency-independent steering vectors in a matrix Vmm, whose columns are the eigenvectors of the correlation matrix Rmimo. Since Rmimo is of rank NS, where NS≦min{NT, NR}, the matrix Vmm may include up to NS eigenvectors vl, for l=1, 2, . . . NS, for up to NS eigenmodes of Rmimo. For clarity, the following description assumes that all NS eigenmodes are used for data transmission.
The transmitter performs eigensteering on NS data symbol streams smm(n) with the steering matrix Vmm to obtain NT transmit symbol streams xmm(n), as follows:

xmm(n)=Vmms(n),  Eq (28)

where smm(n)=[s1(n) s2(n) . . . sNS(n)]T, Vmm=[v1 v2 . . . vNS], and NS≦min {NT, NR} for a full-rank MIMO channel. Each data symbol stream sl(n), for l=1, 2, . . . NS, is steered with a respective steering vector vl in the matrix Vmm. Each data symbol stream sl(n) observes an effective channel that is a SIMO channel with an effective channel impulse response of H(n)vl, for n=0, 1, . . . L. The NT transmit symbol streams xmm(n) are further processed and sent from the NT transmit antennas to the receiver.

The receiver obtains NR received symbol streams ymm(n) from the NR receive antennas, which ymm(n)=H(n){circle around (x)}xmm(n)+nmimo(n). For a time dispersive MIMO channel, the eigensteering with multiple frequency-independent steering vectors in the matrix Vmm does not diagonalize the channel. Thus, when multiple spatial channels are used for data transmission using frequency-independent eigensteering, there will in general be cross-talk between the multiple symbol streams as well as intersymbol interference at the receiver.
The receiver can perform matched filtering of the received symbol streams ymm(n) in either the time domain or the frequency domain. The time-domain matched filtering may be expressed as:

{tilde over (s)}mm(n)=VmmHHH(L−n) {circle around (x)}ymm(n),  Eq (29)

where {tilde over (s)}mm(n) denotes NS detected symbol streams. The receiver matched filter is Mmm(n)=VmmHHH(L−n), for n=0, 1, . . . L, which includes NR individual matched filters for the NR receive antennas. The matched filter mmm,i(n) for each receive antenna has an impulse response of mmm,i(n)=VmmHhiH(L−n), for n=0, 1, . . . L. The output of the matched filter for each receive antenna includes NS filtered symbol substreams corresponding to the NS steering vectors (i.e., NS columns of Vmm). The NR filtered symbol substreams from the NR matched filters for each steering vector are combined to obtain the detected symbol stream {tilde over (s)}l(n) for that steering vector. NS detected symbol streams {tilde over (s)}mm(n) are obtained for the NS data symbols smm(n) sent by the transmitter.

The frequency-domain matched filtering may be expressed as:

{tilde over (s)}mm(k)=VmmHHH(k)ymm(k), for k=1, 2, . . . NF,  Eq (30)

where ymm(k) is the received symbol substream for subband k, which is obtained by performing an FFT of each set of NF symbols in the received symbol stream ymm(n). The receiver matched filter is Mmm(k)=VmmHHH(k), for k=1, 2, . . . NF, which includes NR individual matched filters for the NR receive antennas. The matched filter mmm,i(k) for each receive antenna has a response of mmm,i(k)=VmmHhiH(k), for k=1, 2, . . . NF. For each subband k, the output of the matched filter for each receive antenna includes NS filtered symbol substreams corresponding to the NS transmit steering vectors. For each subband k, the NR filtered symbol substreams from the NR matched filters for each steering vector are combined to obtain the detected symbol substream {tilde over (s)}l(k) for that steering vector. The NF detected symbol substreams for the NF subbands for each steering vector are then multiplexed to obtain the detected symbol stream {tilde over (s)}l(n) for the data symbol stream sl(n) sent with that steering vector. NS detected symbol streams {tilde over (s)}mm(n) are obtained for NS data symbol streams smm(n) sent by the transmitter.

As noted above, if multiple data symbol streams are transmitted simultaneously, then there is cross-talk between these data symbol streams at the receiver for a time dispersive MIMO channel. A space-time or “joint” equalizer may be used to mitigate the cross-talk and intersymbol interference due to time dispersion in the MIMO channel. The space-time equalizer may be a minimum mean square error linear equalizer (MMSE-LE), a decision feedback equalizer (DFE), a maximum likelihood sequence estimator (MLSE), or some other type of equalizer. The space-time equalizer may be designed to operate on the NS detected symbol streams {tilde over (s)}mm(n) in both time and space domains to obtain NS recovered symbol streams ŝmm(n), which are improved estimates of the data symbol streams smm(n) sent by the transmitter. Exemplary designs of the MMSE-LE, DFE, and MLSE are described in commonly assigned U.S. patent application Ser. No. 09/993,087, entitled “Multiple-Access Multiple-Input Multiple-Output (MIMO) Communication System,” filed Nov. 6, 2001.
The space-time equalizer may also implement a successive equalization and interference cancellation receiver processing technique, which successively recovers one data symbol stream at a time. As each data symbol stream is recovered, the interference it causes to the remaining, not yet recovered data symbol streams is estimated and canceled from the detected symbol streams to obtain “modified” symbol streams. The modified symbol streams are then processed to recover the next data symbol stream. The process is repeated until all NS data symbol streams are recovered. By removing the interference due to each recovered data symbol stream, the not yet recovered data symbol streams experience less interference and may achieve higher SNRs. The successive equalization and interference cancellation receiver processing technique is also described in the aforementioned U.S. patent application Ser. No. 09/993,087.
The capacity Cmimomm of the MIMO channel with multi-mode eigensteering may be expressed as:




  
    
      
        
          
            C
            mimo
            mm
          
          =
          
            
              ∑
              
                k
                =
                1
              
              
                N
                F
              
            
            ⁢
            
              
                log
                2
              
              ⁢
              
                
                
                  
                    I
                    _
                  
                  +
                  
                    
                      
                        ρ
                        
                          N
                          S
                        
                      
                      ·
                      
                        
                          V
                          _
                        
                        mm
                        H
                      
                    
                    ⁢
                    
                      
                        R
                        _
                      
                      ⁡
                      
                        (
                        k
                        )
                      
                    
                    ⁢
                    
                      
                        V
                        _
                      
                      mm
                    
                  
                
                
              
            
          
        
        ,
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          31
          )
        
      
    
  




where |M| denotes the determinant of M. A correlation matrix {tilde over (R)}(k) may be defined as {tilde over (R)}(k)=VmmHR(k)Vmm. The eigenvalues of {tilde over (R)}(k) may be computed and denoted as {tilde over (λ)}l(k), for l=1, 2, . . . NS and k=1, 2, . . . NF. The capacity Cmimomm of the MIMO channel with multi-mode eigensteering may then be expressed as:





  
    
      
        
          C
          mimo
          mm
        
        =
        
          
            ∑
            
              k
              =
              1
            
            
              N
              F
            
          
          ⁢
          
            
              ∑
              
                l
                =
                1
              
              
                N
                S
              
            
            ⁢
            
              
                
                  log
                  2
                
                ⁡
                
                  (
                  
                    1
                    +
                    
                      
                        ρ
                        
                          N
                          S
                        
                      
                      ·
                      
                        
                          
                            λ
                            ~
                          
                          l
                        
                        ⁡
                        
                          (
                          k
                          )
                        
                      
                    
                  
                  )
                
              
              .
            
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          32
          )
        
      
    
  




C. Main Path Eigensteering
For main path eigensteering, a data symbol stream is transmitted on the principal spatial channel for the main propagation path of the MIMO channel using a single frequency-independent steering vector vmp. As noted above, a time dispersive MIMO channel can be characterized by L+1 channel impulse response matrices H(n), for n=0, 1, . . . L. In an embodiment, the main path is defined as the propagation path with the most received energy. The energy E(n) of each channel impulse response matrix H(n), for n=0, 1, . . . L, may be computed as follows:




  
    
      
        
          
            E
            ⁡
            
              (
              n
              )
            
          
          =
          
            
              
                
                
                  
                    H
                    _
                  
                  ⁡
                  
                    (
                    n
                    )
                  
                
                
              
              2
            
            =
            
              
                ∑
                
                  j
                  =
                  1
                
                
                  N
                  R
                
              
              ⁢
              
                
                  ∑
                  
                    i
                    =
                    1
                  
                  
                    N
                    T
                  
                
                ⁢
                
                  
                    
                    
                      
                        h
                        
                          i
                          ,
                          j
                        
                      
                      ⁡
                      
                        (
                        n
                        )
                      
                    
                    
                  
                  2
                
              
            
          
        
        ,
        
          
            for
            ⁢
            
              
            
            ⁢
            n
          
          =
          0
        
        ,
        1
        ,
        
          ...
          ⁢
          
            
          
          ⁢
          
            L
            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          33
          )
        
      
    
  




The energy E(n) is also (1) the trace of the correlation matrix R(n)=HH(n)H(n) and (2) the square of the Frobenius norm of the channel impulse response matrix H(n). The largest energy, Emax, for all L+1 time delays is then determined as:





  
    
      
        
          E
          max
        
        =
        
          
            max
            
              n
              =
              
                0
                ⁢
                
                  
                
                ⁢
                ...
                ⁢
                
                  
                
                ⁢
                L
              
            
          
          ⁢
          
            
              {
              
                E
                ⁡
                
                  (
                  n
                  )
                
              
              }
            
            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          34
          )
        
      
    
  




The main path delay nmp is equal to the time delay of the channel impulse response matrix with the highest energy Emax. The channel response matrix Hmp for the main path is then Hmp=H(nmp)

A correlation matrix Rmp of Hmp is computed as Rmp=HmpHHmp. Eigenvalue decomposition of the correlation matrix Rmp may be expressed as:

Rmp=VmpΛmpVmpH,  Eq (35)

where Vmp is a unitary matrix of eigenvectors of Rmp and Λmp is a diagonal matrix of eigenvalues of Rmp.

The matrix Rmp is of rank NS and the diagonal matrix Λmp contains NS non-negative real values along the diagonal. The frequency-independent steering vector vmp to use for eigensteering is the principal eigenvector of Rmp, which is the column of Vmp that corresponds to the largest eigenvalue of Rmp.
The transmitter performs eigensteering on the data symbol stream s(n) with the steering vector vmp to obtain NT transmit symbol streams xmp(n), as follows:

xmp(n)=s(n)·vmp  Eq (36)

The spatial processing shown in equation (36) steers the transmit power in the direction of the principal spatial channel for the strongest propagation path.

The receiver can perform matched filtering of the received symbol streams ymp(n), which is ymp(n)=H(n){circle around (x)}xmp(n)+nmimo(n), in either the time domain or the frequency domain. The time-domain matched filtering may be expressed as:

{tilde over (s)}mp(n)=vmpHHH(L−n){circle around (x)}ymp(n), for n=0, 1,, . . . L.  Eq (37)

The detected symbol stream {tilde over (s)}mp(n) may be post processed (e.g., equalized) to obtain the recovered symbol stream ŝmp(n).

The frequency-domain matched filtering may be expressed as:

ŝmp(k)=vmpHHH(k)ymp(k), for k=1, 2, . . . NF.  Eq (38)

The NF recovered symbol substreams ŝmp(k), for k=1, 2, . . . NF, for the NF subbands may be multiplexed to obtain the recovered symbol stream ŝmp(n).

In general, the receiver processing for main path eigensteering may be performed similar to that described above for principal-mode eigensteering. However, the matched filtering is performed based on the steering vector vmp for the principal spatial channel of the main path instead of the steering vector vpm for the principal spatial channel of the “averaged” MIMO channel.
D. Receiver Eigensteering
For receiver eigensteering, the MIMO channel is viewed as being composed of NR MISO channels for NR receive antennas. NR frequency-independent steering vectors may be obtained for the NR MISO channels in similar manner as that described above for the MISO system.
As shown in equation (16), the matrix H(n) for the MIMO channel is composed of NR channel impulse response vectors hi(n), for i=1, 2, . . . NR. Each row vector hi(n) includes the channel impulse response between the NT transmit antennas and receive antenna i. An NT×NT correlation matrix Ri may be computed for each receive antenna, as follows:




  
    
      
        
          
            
              R
              _
            
            i
          
          =
          
            
              
                ∑
                
                  n
                  =
                  0
                
                L
              
              ⁢
              
                
                  
                    
                      h
                      _
                    
                    i
                    H
                  
                  ⁡
                  
                    (
                    n
                    )
                  
                
                ⁢
                
                  
                    
                      h
                      _
                    
                    i
                  
                  ⁡
                  
                    (
                    n
                    )
                  
                
              
            
            =
            
              
                1
                
                  N
                  F
                
              
              ⁢
              
                
                  ∑
                  
                    k
                    =
                    1
                  
                  
                    N
                    F
                  
                
                ⁢
                
                  
                    
                      
                        h
                        _
                      
                      i
                      H
                    
                    ⁡
                    
                      (
                      k
                      )
                    
                  
                  ⁢
                  
                    
                      
                        h
                        _
                      
                      i
                    
                    ⁡
                    
                      (
                      k
                      )
                    
                  
                
              
            
          
        
        ,
        
          
            for
            ⁢
            
              
            
            ⁢
            i
          
          =
          1
        
        ,
        2
        ,
        
          ...
          ⁢
          
            
          
          ⁢
          
            
              N
              R
            
            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          39
          )
        
      
    
  




Eigenvalue decomposition of the correlation matrix Ri for each receive antenna may be performed as follows:

Ri=ViΛiViH, for i=1, 2, . . . NR,  Eq (40)

where Vi is a unitary matrix whose columns are the eigenvectors of Ri; and

Λi is a diagonal matrix whose diagonal entries are the eigenvalues of Ri.
Since each hi(n) is a row vector for one receive antenna, the correlation matrix Ri has rank of less than or equal to min {(L+1), NT}. For each receive antenna i, the frequency-independent steering vector vrx,i that maximizes the received SNR for that receive antenna is the column of Vi that corresponds to the maximum non-zero eigenvalue of Ri. NR steering vectors vrx,i, for i=1, 2, . . . NR, are obtained for the NR receive antennas and may be represented by an NT×NR matrix Vrx=[vrx,1 vrx,2 . . . vrx,NR].
One or multiple data symbol streams may be transmitted with receiver eigensteering. If one data symbol stream s(n) is transmitted, then the transmitter performs eigensteering on this data symbol stream with each of the NR steering vectors to obtain NT transmit symbol streams xrx(n), as follows:




  
    
      
        
          
            
              κ
              _
            
            rx
          
          ⁡
          
            (
            n
            )
          
        
        =
        
          
            s
            ⁡
            
              (
              n
              )
            
          
          ·
          
            
              ∑
              
                i
                =
                1
              
              
                N
                R
              
            
            ⁢
            
              
                
                  v
                  _
                
                
                  rx
                  ,
                  i
                
              
              .
            
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          41
          )
        
      
    
  




Again, if one data symbol stream is transmitted, then the receiver can perform matched filtering of the NR received symbol streams yrx(n), which is yrx(n)=H(n){circle around (x)}xrx(n)+nmimo(n), in either the time domain or the frequency domain. For the time-domain technique, the matched filtering is first performed for each receive antenna, as follows:

{tilde over (s)}rx,i(n)=vrx,iHhiH(L−n){circle around (x)}ymp(n), for i=1, 2, . . . NR,  Eq (42)

where {tilde over (s)}rx,i(n) is the filtered symbol stream for receive antenna i. The NR filtered symbol streams for all NR receive antennas are then combined to obtain the detected symbol stream {tilde over (s)}rx(n), as follows:





  
    
      
        
          
            
              s
              ~
            
            rx
          
          ⁡
          
            (
            n
            )
          
        
        =
        
          
            ∑
            
              i
              =
              1
            
            
              N
              R
            
          
          ⁢
          
            
              
                
                  s
                  ~
                
                
                  rx
                  ,
                  i
                
              
              ⁡
              
                (
                n
                )
              
            
            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          43
          )
        
      
    
  




The detected symbol stream {tilde over (s)}rx(n) may be post-processed (e.g., equalized) to obtain the recovered symbol stream ŝrx(n), which is an estimate of the transmitted data symbol stream s(n).

For the frequency-domain technique, the matched filtering is first performed for each subband of each receive antenna, as follows:

ŝrx,i(k)=vrx,iHhiH(k)yrx(k), for i=1, 2, . . . NR and k=1, 2, . . . NF,  Eq (44)

where ŝrx,i(k) is the filtered symbol substream for subband k of recieve antenna i. The NR filtered symbol substreams for all NR receive antennas for subband k are then combined to obtain the detected symbol substream ŝrx(k) for subband k, as follows:





  
    
      
        
          
            
              
                s
                ^
              
              rx
            
            ⁡
            
              (
              k
              )
            
          
          =
          
            
              ∑
              
                i
                =
                1
              
              
                N
                R
              
            
            ⁢
            
              
                
                  s
                  ^
                
                
                  rx
                  ,
                  i
                
              
              ⁡
              
                (
                k
                )
              
            
          
        
        ,
        
          
            for
            ⁢
            
              
            
            ⁢
            k
          
          =
          1
        
        ,
        2
        ,
        
          ...
          ⁢
          
            
          
          ⁢
          
            
              N
              F
            
            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          45
          )
        
      
    
  




The NF detected symbol substreams ŝrx(k) for all NF subbands may be multiplexed together to obtain the recovered symbol stream ŝrx(n).

If multiple (ND) data symbol streams are transmitted, where NS≧ND>1, then each data symbol stream may be steered to a respective set of one or more receive antennas. The transmitter performs eigensteering for each data symbol stream sl(n) with a set of Nl steering vectors for a set of Nl receive antennas to which that data symbol stream is steered, where Nl≧1. The eigensteering at the transmitter for each data symbol stream sl(n) may be expressed as:




  
    
      
        
          
            
              
                κ
                _
              
              
                rx
                ,
                l
              
            
            ⁡
            
              (
              n
              )
            
          
          =
          
            
              
                s
                l
              
              ⁡
              
                (
                n
                )
              
            
            ·
            
              
                ∑
                
                  j
                  =
                  1
                
                
                  N
                  l
                
              
              ⁢
              
                
                  v
                  _
                
                
                  rx
                  ,
                  l
                  ,
                  j
                
              
            
          
        
        ,
        
          
            for
            ⁢
            
              
            
            ⁢
            l
          
          =
          1
        
        ,
        2
        ,
        
          ...
          ⁢
          
            
          
          ⁢
          
            
              N
              D
            
            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          46
          )
        
      
    
  




where xrx,l(n) is the NT transmit symbol substreams for data symbol stream sl(n), and

vrx,l,j, for j=1 . . . Nl, are the Nl steering vectors for data symbol stream sl(n). The ND sets of NT transmit symbol substreams for all ND data symbol streams are then combined to obtain the NT transmit symbol streams xrx(n), as follows:




  
    
      
        
          
            
              κ
              _
            
            rx
          
          ⁡
          
            (
            n
            )
          
        
        =
        
          
            ∑
            
              l
              =
              1
            
            
              N
              D
            
          
          ⁢
          
            
              
                
                  κ
                  _
                
                
                  rx
                  ,
                  l
                
              
              ⁡
              
                (
                n
                )
              
            
            .
          
        
      
    
    
      
        Eq
        ⁢
        
          
        
        ⁢
        
          (
          47
          )
        
      
    
  




The receiver can perform matched filtering of the received symbol stream yi(n) for each receive antenna in either the time domain as shown in equation (42) or the frequency domain as shown in equation (44). The receiver can then combine the filtered symbol substreams {tilde over (s)}l,j(n), for j=1 . . . Nl, from all receive antennas used for each data symbol stream sl(n) to obtain the detected symbol stream {tilde over (s)}l(n) for that data symbol stream. A space-time equalizer may be used to equalize the ND detected symbol streams {tilde over (s)}rx(n) to obtain ND recovered symbol streams ŝrx(n).
The frequency-independent eigensteering may also be used for a MIMO-OFDM system. The transmitter can perform eigensteering in the time-domain, as shown in equations (20), (28), (36) and (41), where s(n) and s(n) denote sequence(s) of time-domain chips for OFDM symbols generated for the data stream(s) by OFDM modulation. The transmitter can also perform eigensteering in the frequency-domain on the data symbols for each subband, prior to the OFDM modulation to generate OFDM symbols. The receiver can perform matched filtering in the time-domain, as shown in equations (22), (29), (37), (42) and (43). The receiver can also perform matched filtering in the frequency-domain, as shown in equations (23), (30), (38), (44) and (45).
3. MISO System
FIG. 1 shows a block diagram of a transmitter 110 and a receiver 150 in a MISO system 100. At transmitter 110, a transmit (TX) data processor 120 receives a data stream d(n) from a data source 112, processes (e.g., encodes, interleaves, and modulates) the data stream in accordance with a selected transmission mode, and provides a data symbol stream s(n). The selected transmission mode may be associated with a particular data rate, a particular coding scheme or code rate, and a particular modulation scheme to use for the data stream, which are respectively indicated by the data rate, coding, and modulation controls provided by a controller 140.
A TX spatial processor 130 receives the data symbol stream s(n) and may perform wideband processing such as spectral spreading or multi-carrier modulation, as described below. TX spatial processor 130 further performs eigensteering based on the frequency-independent steering vector vmiso (which is also referred to as a TX steering vector) provided by controller 140. TX spatial processor 130 also multiplexes pilot in with the data and provides NT transmit chip streams cmiso(n) for the NT transmit antennas. The processing by TX data processor 120 and TX spatial processor 130 is described in further detail below.
A transmitter unit (TMTR) 132 receives and conditions (e.g., converts to analog, frequency upconverts, filters, and amplifies) the NT transmit chip streams to obtain NT modulated signals. Each modulated signal is then transmitted from a respective transmit antenna (not shown in FIG. 1) and via the MISO channel to receiver 150. The MISO channel distorts the transmitted signals with a channel impulse response h(n) and further degrades the transmitted signals with additive white Gaussian noise and possibly interference from other transmission sources.
At receiver 150, the NT transmitted signals are received by a single receive antenna (not shown in FIG. 1), and the received signal is provided to a receiver unit (RCVR) 154. Receiver unit 154 conditions and digitizes the received signal to obtain a stream of samples for the transmitted data and pilot. Receiver unit 154 provides a received symbol stream ymiso(n) (for the data) to a receive (RX) spatial processor 160 and received pilot symbols (for the pilot) to a channel estimator 172. RX spatial processor 160 performs matched filtering of the received symbol stream ymiso(n) with a matched filter and provides a recovered symbol stream ŝmiso(n), which is an estimate of the data symbol stream s(n) sent by transmitter 110. An RX data processor 170 then processes (e.g., demodulates, deinterleaves, and decodes) the recovered symbol stream in accordance with the selected transmission mode to obtain a decoded data stream {circumflex over (d)}(n), which is an estimate of the data stream d(n) sent by transmitter 110. RX data processor 170 may further provide the status of each received data packet.
Channel estimator 172 processes the received pilot symbols to obtain channel gain and SNR estimates for the MISO channel. A matrix computation unit 174 then processes the channel gain estimates to obtain the frequency-independent steering vector vmiso for TX spatial processor 130 and the matched filter for RX spatial processor 160. A transmission mode selector 176 receives the SNR estimates from channel estimator 172 and the packet status from RX data processor 170, determines a suitable transmission mode for the data stream, and provides the selected transmission mode to a controller 180.
Controller 180 receives the steering vector vmiso from computation unit 174 and the selected transmission mode from transmission mode selector 176 and assembles feedback information for transmitter 110. The feedback information is sent to transmitter 110 and used to adjust the processing of the data stream d(n) sent to receiver 150. For example, transmitter 110 may use the feedback information to adjust the data rate, the coding scheme, the modulation scheme, the eigensteering, or any combination thereof, for the data stream sent to receiver 150.
Controllers 140 and 180 direct the operation at transmitter 110 and receiver 150, respectively. Memory units 142 and 182 provide storage for program codes and data used by controllers 140 and 180, respectively. Memory units 142 and 182 may be internal to controllers 140 and 180, as shown in FIG. 1, or external to these controllers.
FIG. 2 shows a block diagram of an embodiment of TX data processor 120 in FIG. 1. Within TX data processor 120, an encoder 212 receives and codes the data stream d(n) based on a coding scheme indicated by the coding control and provides code bits. The data stream may carry one or more data packets, and each data packet is typically coded separately to obtain a coded data packet. The coding increases the reliability of the data transmission. The coding scheme may include cyclic redundancy check (CRC) coding, convolutional coding, turbo coding, block coding, and so on, or a combination thereof. A channel interleaver 214 interleaves the code bits based on an interleaving scheme, which may be indicated by an interleaving control if the interleaving is dependent on transmission mode. The interleaving provides time, frequency, and/or spatial diversity for the code bits.
A symbol mapping unit 216 maps the interleaved bits based on a modulation scheme indicated by the modulation control and provides a stream of modulation symbols (or simply, “data symbols”). Unit 216 groups each set of B interleaved bits to form a B-bit binary value, where B≧1, and further maps each B-bit value to a specific modulation symbol based on the modulation scheme (e.g., QPSK, M-PSK, or M-QAM, where M=2B). Each modulation symbol is a complex value in a signal constellation defined by the modulation scheme.
FIG. 3A shows a block diagram of a TX spatial processor 130a, which is an embodiment of TX spatial processor 130 in FIG. 1. TX spatial processor 130a includes an eigensteering unit 330, a TX pilot processor 340, and a multiplexer (MUX) 350.
Eigensteering unit 330 includes NT multipliers 332a through 332t, one multiplier 332 for each of the NT transmit antennas. Each multiplier 332 receives the data symbol stream s(n) and a respective element νmiso,j of the TX steering vector vmiso, multiplies each data symbol with the element νmiso,j, and provides a transmit symbol stream. Multipliers 332a through 332t perform frequency-independent eigensteering as shown in equation (5).
TX pilot processor 340 includes NT multipliers 342a through 342t, one multiplier 342 for each of the NT transmit antennas. Each multiplier 342 receives the pilot symbol and a unique orthogonal sequence wj assigned to its transmit antenna, multiplies the pilot symbol with the orthogonal sequence wj, and provides a sequence of covered pilot symbols. Multipliers 342a through 342t generate NT orthogonal pilots for the NT transmit antennas, which may be used for channel estimation by receiver 150.
Multiplexer 350 includes NT multiplexers 352a through 352t, one multiplexer 352 for each of the NT transmit antennas. Each multiplexer 352 receives and multiplexes the transmit symbols from an associated multiplier 332 with the covered pilot symbols from an associated multiplier 342 and provides a respective stream of transmit chips cj(n). The pilot may be multiplexed with the data using time division multiplexing (TDM), as shown in FIG. 3A, code division multiplexing (CDM), subband multiplexing, or some other multiplexing scheme. In any case, multiplexers 352a through 352t provide NT transmit chip streams cj(n), for j=1, 2, . . . NT, for the NT transmit antennas.
Transmitter unit 132 includes NT transmitters 362a through 362t, one transmitter 362 for each of the NT transmit antennas. Each transmitter 362 receives and conditions a respective transmit chip stream to generate a modulated signal, which is then transmitted from an associated antenna 134.
FIG. 3B shows a block diagram of a TX spatial processor 130b, which is another embodiment of TX spatial processor 130 in FIG. 1. TX spatial processor 130b performs spectral spreading in the time domain and includes a spreader 310, eigensteering unit 330, TX pilot processor 340, and multiplexer 350.
Within TX spatial processor 130b, spreader 310 receives and spectrally spreads the data symbol stream s(n) with a pseudo-random number (PN) sequence and provides a stream of spread data symbols. The spreading is especially applicable for a low rate data symbol stream to spectrally spread the data over the entire system bandwidth. The spreading may be performed in similar manner as for a CDMA system, which is well known in the art. The eigensteering is then performed on the spread data symbol stream (instead of the data symbol stream), as described above for FIG. 3A, to obtain NT transmit chip streams for the NT transmit antennas.
FIG. 3C shows a block diagram of a TX spatial processor 130c, which is yet another embodiment of TX spatial processor 130 in FIG. 1. TX spatial processor 130c performs OFDM modulation and includes an OFDM modulator 320, eigensteering unit 330, TX pilot processor 340, and multiplexer 350.
Within TX spatial processor 130c, an OFDM modulator 320 receives and performs OFDM modulation on the data symbol stream s(n). OFDM effectively partitions the overall system bandwidth into multiple (NF) orthogonal subbands, which are also commonly referred to as tones, bins, and frequency subchannels. With OFDM, each subband is associated with a respective carrier that may be modulated with data. For each OFDM symbol period, one data or pilot symbol may be transmitted on each subband used for transmission, and a signal value of zero is provided for each unused subband. Within OFDM modulator 320, an inverse fast Fourier transform (IFFT) unit receives a set of data/pilot symbols and zeros for the NF subbands for each OFDM symbol period, transforms the set of data/pilot symbols and zeros to the time domain using an inverse fast Fourier transform, and provides a transformed symbol that contains NF time-domain chips. A cyclic prefix generator then repeats a portion of each transformed symbol to obtain an OFDM symbol that contains NF+Ncp chips, where Ncp is the number of chips repeated. The cyclic prefix is used to combat frequency selective fading caused by time dispersion in the channel. OFDM modulator 320 provides a stream of data chips for a stream of OFDM symbols.
Eigensteering is then performed on the data chip stream (instead of the data symbol stream), as described above for FIG. 3A, to obtain NT transmit chip streams for the NT transmit antennas. Alternatively, the data symbol stream may be demultiplexed into NS data symbol substreams and eigensteering may be performed on each data symbol substream. In this case, the same steering vector vmiso is used for all subbands. OFDM modulation may then be performed on the output of the eigensteering for all subbands of each transmit antenna to obtain the transmit chip stream for that transmit antenna. In general, the eigensteering may be performed in either the time domain or the frequency domain. However, eigensteering in the time domain may require fewer multiplications and may thus be simpler to implement.
FIG. 4A shows a block diagram of an RX spatial processor 160a, which is one embodiment of RX spatial processor 160 in FIG. 1 and may be used in conjunction with TX spatial processor 130a in FIG. 3A. An antenna 152 receives the NT transmitted signals from transmitter 110 and provides the received signal. Receiver unit 154 conditions, digitizes, and pre-processes the received signal and provides the received symbol stream ymiso(n). The pre-processing may include filtering, resampling, sample rate conversion, and so on.
Within RX spatial processor 160a, a matched filter 410 performs matched filtering of the received symbol stream ymiso(n) with the matched filter mmiso(n)=vmisoHhH(L−n), as shown in equation (7), and provides the detected symbol stream {tilde over (s)}miso(n). An equalizer 412 then performs equalization on the detected symbol stream and provides the recovered symbol stream ŝmiso(n). Equalizer 412 may implement an MMSE equalizer, a decision feedback equalizer, a maximum likelihood sequence estimator, or some other type of equalizer, all of which are known in the art. The equalization attempts to mitigate intersymbol interference due to frequency selectivity in the MISO channel. The matched filtering and equalization may be integrated together (e.g., matched filter 410 may be embedded in equalizer 412).
FIG. 4B shows a block diagram of an RX spatial processor 160b, which is another embodiment of RX spatial processor 160 in FIG. 1. RX spatial processor 160b performs spectral despreading in the time domain and may be used in conjunction with TX spatial processor 130b in FIG. 3B. Within RX spatial processor 160b, matched filter 410 performs matched filtering of the received symbol stream ymiso(n) with the matched filter mmiso(n)=vmisoHhH(L−n) and provides the detected symbol stream {tilde over (s)}miso(n). A despreader 412 then despreads the detected symbol stream with (the complex conjugate of) the PN sequence used by transmitter 110 and provides the recovered symbol stream ŝmiso(n). The despreading may be performed with a rake receiver in similar manner as for a CDMA system, which is known in the art.
FIG. 4C shows a block diagram of an RX spatial processor 160c, which is yet another embodiment of RX spatial processor 160 in FIG. 1. RX spatial processor 160c performs OFDM demodulation and may be used in conjunction with TX spatial processor 130c in FIG. 3C. RX spatial processor 160c includes an OFDM demodulator 420, NF matched filters 430a through 430f for the NF subbands, and a multiplexer 432.
Within RX spatial processor 160c, OFDM demodulator 420 performs OFDM demodulation on the received symbol stream ymiso(n). OFDM demodulator 420 first removes the cyclic prefix in each received OFDM symbol to obtain a received transformed symbol. OFDM demodulator 420 then transforms each received transformed symbol to the frequency domain using a fast Fourier transform (FFT) to obtain a set of NF received symbols for the NF subbands. OFDM demodulator 420 provides NF received symbol substreams ymiso(k), for k=1, 2, . . . NF, for the NF subbands to NF matched filters 430a through 430f. Each matched filter 430 performs matched filtering of its received symbol substream ymiso(k) with its matched filter mmiso(k)=vmisoHhH(k), which is a complex-valued scalar, and provides a detected symbol substream {tilde over (s)}miso(k). Multiplexer 432 receives and multiplexes the NF detected symbol substreams from all NF matched filters 430a through 430f and provides the recovered symbol stream ŝmiso(n). Fewer than NF subbands may be used for data transmission. In this case, the received symbols for the unused subbands are discarded and the matched filtering is not performed for the unused subbands.
FIG. 5 shows a block diagram of a receiver 150x, which is an embodiment of receiver 150 in FIG. 1. RX spatial processor 160 performs matched filtering and other pre-processing on the received symbol stream ymiso(n) and provides the recovered symbol stream ŝmiso(n) to RX data processor 170.
Within RX data processor 170, a symbol demapping unit 512 demodulates the recovered symbols in accordance with the modulation scheme used for the data stream, as indicated by a demodulation control provided by controller 180. A channel deinterleaver 514 then deinterleaves the demodulated data in a manner complementary to the interleaving performed at transmitter 110. If the interleaving is dependent on transmission mode, then controller 180 provides a deinterleaving control to channel deinterleaver 514. A decoder 516 then decodes the deinterleaved data in a manner complementary to the encoding performed at transmitter 110, as indicated by a decoding control provided by controller 180. For example, a turbo decoder or a Viterbi decoder may be used for decoder 516 if transmitter 110 performs turbo or convolutional coding, respectively. Decoder 516 may also provide the status of each received data packet (e.g., indicating whether the packet was received correctly or in error).
Channel estimator 172 obtains received pilot symbols from receiver unit 154, estimates the MISO channel response and the noise floor at receiver 150×based on the received pilot symbols, and provides the channel impulse response estimate ĥ(n) and the noise floor estimate {circumflex over (σ)}2 to controller 180. Controller 180 performs various functions related to eigensteering, matched filtering, and rate control for data transmission. For example, a matrix computation unit 522 within controller 180 performs computation to derive the frequency-independent steering vector vmiso for transmitter 110 and the matched filter for receiver 150. Unit 522 may also estimate the received SNR of the data stream. A transmission mode selector 524 selects a suitable transmission mode for the data stream d(n) based on the received SNR. Memory unit 182 may store a look-up table (LUT) 526 for all of the transmission modes supported by the MISO system and their required SNRs. Controller 180 provides the selected transmission mode for the data stream, the TX steering vector, acknowledgments (ACKs) and/or negative acknowledgments (NAKs), and so on as feedback information for transmitter 110.
4. MIMO System
For a MIMO system, NS spatial channels are available for data transmission, where NS≦min {NT, NR}. One data stream may be transmitted on each spatial channel. Each data stream may be independently processed in accordance with a transmission mode selected for that data stream.
FIG. 6 shows a block diagram of a transmitter 610 and a receiver 650 in a MIMO system 600. At transmitter 610, a TX data processor 620 receives ND data streams, where NS≧ND≧1. TX data processor 620 codes, interleaves, and modulates each data stream in accordance with its selected transmission mode and provides a corresponding data symbol stream. A TX spatial processor 630 receives ND data symbol streams from TX data processor 620, performs wideband processing (if any) and eigensteering based on a set of ND or NR TX steering vectors provided by controller 640, multiplexes in pilot, and provides NT transmit chip streams for the NT transmit antennas. The processing by TX data processor 620 and TX spatial processor 630 is described in further detail below. A transmitter unit 632 receives and conditions the NT transmit chip streams to obtain NT modulated signals, which are transmitted from NT transmit antennas (not shown in FIG. 6) and via the MIMO channel to receiver 650.
At receiver 650, the NT transmitted signals are received by each of NR receive antennas (not shown in FIG. 6), and the NR received signals from the NR receive antennas are provided to a receiver unit 654. Receiver unit 654 conditions, digitizes, and pre-processes each received signal to obtain a corresponding received symbol stream. Receiver unit 654 provides NR received symbol streams to an RX spatial processor 660 and received pilot symbols to a channel estimator 672. RX spatial processor 660 performs matched filtering of the NR received symbol streams with NR matched filters and provides ND recovered symbol streams, which are estimates of the ND data symbol streams sent by transmitter 610. An RX data processor 670 then processes (e.g., demodulates, deinterleaves, and decodes) each recovered symbol stream in accordance with its transmission mode to obtain a decoded data stream, which is an estimate of the data stream sent by transmitter 610. RX data processor 670 may further provide the status of each received data packet.
Channel estimator 672, matrix computation unit 674, and transmission mode selector 676 perform similar functions as channel estimator 172, matrix computation unit 174, and transmission mode selector 176, respectively, in FIG. 1 to determine the ND or NR TX steering vectors for transmitter 610, the NR matched filters for receiver 650, and the ND transmission modes for the ND data streams. A controller 680 assembles feedback information for transmitter 610, which may include the ND or NR TX steering vectors and the ND transmission modes.
Controllers 640 and 680 direct the operation at transmitter 610 and receiver 650, respectively. Memory units 642 and 682 provide storage for program codes and data used by controllers 640 and 680, respectively. Memory units 642 and 682 may be internal to controllers 640 and 680, as shown in FIG. 6, or external to these controllers.
If ND=1, then the encoding, interleaving, and modulation for the single data stream may be performed as shown in FIG. 2. The eigensteering for the single data stream may be performed as shown in FIG. 3A, 3B, or 3C, depending on whether spectral spreading or OFDM modulation is performed on the data stream. However, the eigensteering is performed with the steering vector vpm for the principal mode or the steering vector vmp for the main path (instead of the steering vector vmiso). The receiver matched filtering may be performed as described below. If ND>1, then the data processing (e.g., encoding, interleaving, and modulation) and the eigensteering may be performed as described below.
FIG. 7 shows a block diagram of an embodiment of TX data processor 620 in FIG. 6. For this embodiment, TX data processor 620 includes one set of encoder 712, channel interleaver 714, and symbol mapping unit 716 for each of the ND data streams. Each set of encoder, channel interleaver, and symbol mapping unit receives and processes a respective data stream dl(n) in similar manner as described above for TX data processor 120 in FIG. 2 to obtain a corresponding data symbol stream s1(n). The coding, interleaving, and modulation for each data stream are performed based on the coding, interleaving, and modulation controls provided by controller 640, which are generated based on the transmission mode selected for that data stream.
FIG. 8A shows a block diagram of a TX spatial processor 630a, which is an embodiment of TX spatial processor 630 in FIG. 6 and may be used for multi-mode eigensteering. For this embodiment, TX spatial processor 630a includes ND eigensteering units 830a through 830d for the ND data streams, a TX pilot processor 840, a combiner 850, and a multiplexer 860.
For multi-mode eigensteering, each eigensteering unit 830 receives a respective data symbol stream sl(n) and a respective frequency-independent steering vector vl in the matrix Vmm. Each eigensteering unit 830 performs eigensteering on its data symbol stream with its steering vector, as described above for FIG. 3A, and provides a respective set of NT transmit symbol substreams xl(n) for the NT transmit antennas. Eigensteering units 830a through 830d provide ND sets of transmit symbol substreams for the ND data streams. Combiner 850 includes NT combiners 852a through 852t, one combiner 852 for each of the NT transmit antennas. Each combiner 852 receives and combines a respective set of ND transmit symbol substreams from eigensteering units 830a through 830d for its transmit antenna and provides a transmit symbol stream. Combiners 852a through 852t provide NT transmit symbol streams xmm(n) for the NT transmit antennas. Eigensteering units 830a through 830d and combiner 850 collectively perform the eigensteering shown in equation (28).
For receiver eigensteering, TX spatial processor 630a would include NR eigensteering units 830, one for each of the NR receive antennas. Each eigensteering unit 830 would receive a respective frequency-independent steering vector vrx,i in the matrix Vrx. If ND=1, then the same data symbol stream s(n) is provided to all NR eigensteering units and steered with the NR steering vectors to obtain NR sets of NT transmit symbol substreams. Each combiner 852 would then receive and combine a respective set of NR transmit symbol substreams from the NR eigensteering units for its transmit antenna and provides a transmit symbol stream. If ND>1, then each data symbol stream may be provided to one or more of the NR eigensteering units for the one or more receive antennas to which the data symbol stream is steered. The eigensteering is then performed in similar manner to obtain the NT transmit symbol streams xrx (n) for the NT transmit antennas.
TX pilot processor 840 receives and covers the pilot symbol with NT orthogonal sequences, as described above for FIG. 3A, and provides NT sequences of covered pilot symbols for the NT transmit antennas. Multiplexer 860 includes NT multiplexers 862a through 862t, one multiplexer 862 for each of the NT transmit antennas. Each multiplexer 862 receives and multiplexes the transmit symbols from an associated combiner 852 with the covered pilot symbols from an associated multiplier 842 and provides a respective stream of transmit chips. Multiplexers 862a through 862t provide NT transmit chip streams cmimo(n)=[c1(n) c2(n) . . . cNT(n)]T for the NT transmit antennas.
FIG. 8B shows a block diagram of a TX spatial processor 630b, which is another embodiment of TX spatial processor 630 in FIG. 6. TX spatial processor 630b performs spectral spreading in the time domain and includes ND spreaders 810a through 810d for ND data streams, ND eigensteering units 830a through 830d, TX pilot processor 840, combiner 850, and multiplexer 860. Each spreader 810 receives and spectrally spreads a respective data symbol stream sl(n) with a PN spreading sequence and provides a corresponding stream of spread data symbols. The same or different PN sequences may be used for the ND data symbol streams. Spreaders 810a through 810d provide ND spread data symbol streams for the ND data symbol streams. Eigensteering is then performed on each of the ND spread data symbol streams (instead of the data symbol streams), in similar manner as described above for FIGS. 3A and 8A, to obtain NT transmit chip streams for the NT transmit antennas.
FIG. 8C shows a block diagram of a TX spatial processor 630c, which is yet another embodiment of TX spatial processor 630 in FIG. 6. TX spatial processor 630c performs OFDM modulation and includes ND OFDM modulators 820a through 820d for ND data streams, ND eigensteering units 830a through 830d, TX pilot processor 840, combiner 850, and multiplexer 860.
Each OFDM modulator 820 performs OFDM modulation on a respective data symbol stream sl(n), in similar manner as described above for FIG. 3C, and provides a stream of data chips. OFDM modulators 820a through 820d provide ND data chip streams for the ND data streams. Eigensteering is then performed on each of the ND data chip streams (instead of the data symbol stream), as described above for FIGS. 3A and 8A, to obtain NT transmit chip streams for the NT transmit antennas. Alternatively, the eigensteering may be performed in the frequency domain on the data symbol substream for each subband. In this case, each eigensteering unit uses the same steering vector vl for all subbands.
FIG. 9A shows a block diagram of an RX spatial processor 660a, which may be used for the case in which a single data stream is transmitted (i.e., ND=1). Each of NR receive antennas 652a through 652r receives the NT transmitted signals from transmitter 610 and provides a received signal to an associated receiver unit 654. Each receiver unit 654 conditions, digitizes, and pre-processes its received signal and provides a received symbol stream yi(n).
RX spatial processor 660a includes NR matched filters 910a through 910r for the NR receive antennas, a combiner 912, and an equalizer 914. Each matched filter 910 performs matched filtering of its received symbol stream yi(n) with a matched filter mi(n)=vmimoHhiH(L−n) and provides a filtered symbol stream. The vector vmimo is equal to the steering vector vpm for principal mode eigensteering, the steering vector vmp for main path eigensteering, or the steering vector vrx,i for receiver eigensteering. For receiver eigensteering, each matched filter 910 is associated with a different steering vector vrx,i for its receive antenna, which is not shown in FIG. 9A. The vector hi(n) is the channel impulse response between the NT transmit antennas and receive antenna i. Combiner 912 receives and combines the NR filtered symbol streams from matched filters 910a through 910r and provides a detected symbol stream {tilde over (s)}mimo(n). Equalizer 914 performs equalization on the detected symbol stream and provides the recovered symbol stream ŝmimo(n). Equalizer 914 may implement an MMSE equalizer, a decision feedback equalizer, a maximum likelihood sequence estimator, or some other type of equalizer.
FIG. 9B shows a block diagram of an RX spatial processor 660b, which may also be used for the case in which a single data stream is transmitted (i.e., ND=1). RX spatial processor 660b performs spectral despreading in the time domain and may be used in conjunction with TX spatial processor 630b in FIG. 8B.
RX spatial processor 660b includes NR matched filters 910a through 910r for the NR receive antennas, combiner 912, and a despreader 916. Each matched filter 910 performs matched filtering of a respective received symbol stream yi(n) with its matched filter mi(n)=vmimoHhiH(L−n) and provides a filtered symbol stream. Combiner 912 receives and combines the NR filtered symbol streams and provides the detected symbol stream {tilde over (s)}mimo(n). Despreader 916 then despreads the detected symbol stream with the PN sequence used by transmitter 610 and provides the recovered symbol stream ŝmimo(n).
FIG. 9C shows a block diagram of an RX spatial processor 660c, which may also be used for the case in which a single data stream is transmitted (i.e., ND=1). RX spatial processor 660c performs OFDM demodulation and may be used in conjunction with TX spatial processor 630c in FIG. 8C.
RX spatial processor 660c includes NR antenna matched filters 920a through 920r for the NR receive antennas, NF combiners 932a through 932f for the NF subbands, and a multiplexer 934. Each antenna matched filter 920 performs matched filtering for one receive antenna and includes an OFDM demodulator 922 and NF matched filters 930a through 930f for the NF subbands.
Within each antenna matched filter 920, OFDM demodulator 922 performs OFDM demodulation on the received symbol stream yi(n) for the associated receive antenna and provides NF received symbol substreams yi(k), for k=1, 2, . . . NF, for the NF subbands to NF matched filters 930a through 930f. Each matched filter 930 performs matched filtering of its received symbol substream yi(k) with its matched filter mi(k)=vmimoHhiH(k) and provides a filtered symbol substream. The vector hi(k) is the channel frequency response for receive antenna i for subband k. Matched filters 930a through 930f for each antenna matched filter 920 provide NF filtered symbol substreams for the NF subbands to NF combiners 932a through 932f. 
Each combiner 932 receives and combines the NR filtered symbol substreams from NR antenna matched filters 920a through 920r for its subband and provides a detected symbol substream for the subband. Multiplexer 934 receives and multiplexes the NF detected symbol substreams from combiners 932a through 932f for the NF subbands and provides the recovered symbol stream ŝmimo(n).
FIG. 9D shows a block diagram of an RX spatial processor 660d, which may be used for multi-mode eigensteering with ND>1. RX spatial processor 660d may be used in conjunction with TX spatial processor 630a in FIG. 8A or TX spatial processor 630b in FIG. 8B.
RX spatial processor 660d includes NR matched filters 940a through 940r for the NR receive antennas, ND combiners 942a through 942d for the ND data streams, a space-time equalizer 944, and ND despreaders 946a through 946d for the ND data streams. Each matched filter 940 performs matched filtering of a respective received symbol stream yi(n) with a matched filter mi(k)=VmmHhiH(L−n) for an associated receive antenna and provides ND filtered symbol substreams for the ND data symbol streams. The matrix Vmm includes ND steering vectors vl, for =1, 2, . . . ND, for the ND data symbol streams. Each matched filter 940 thus performs matched filtering of the received symbol stream yi(n) with ND matched filters mi,l(n)=vlHhiH(L−n), for l=1, 2, . . . ND, where vl is the l-th column of Vmm, to obtain the ND filtered symbol substreams for the associated receive antenna.
Each combiner 942 receives and combines the NR filtered symbol substreams from matched filters 940a through 940r for one data symbol stream and provides the detected symbol stream {tilde over (s)}l(n) for the data stream. Matched filters 940a through 940r and combiners 942a through 942d collectively perform the matched filtering shown in equation (29) and provides ND detected symbol streams {tilde over (s)}(n) for the ND data symbol streams.
If multiple data symbol streams are transmitted, then there is likely to be cross-talk between these data symbol streams at receiver 650. Space-time equalizer 944 performs equalization on the ND detected symbol streams from combiners 942a through 942d and provides ND equalized symbol streams. Space-time equalizer 944 may implement an MMSE linear equalizer, a decision feedback equalizer, a maximum likelihood sequence estimator, or some other type of equalizer that can jointly operate on multiple streams to mitigate cross-talk and/or maximize the received SNR in the presence of cross-talk, intersymbol interference, and noise. Space-time equalizer 944 may also implement the successive equalization and interference cancellation processing technique. Space-time equalizer 944 may also be omitted.
If spreading is not performed at transmitter 610, as shown in FIG. 8A, then the ND equalized symbol streams from space-time equalizer 944 are provided as the ND recovered symbol streams ŝ(n). If spreading is performed at transmitter 610 for each data symbol stream, as shown in FIG. 8B, then each despreader 946 receives and despreads a respective equalized symbol stream with the PN sequence and provides a corresponding recovered symbol stream. Despreaders 946a through 946d would then provide the ND recovered symbol streams ŝ(n).
FIG. 9E shows a block diagram of an RX spatial processor 660e, which may be used for receiver eigensteering with ND>1. RX spatial processor 660e includes NR matched filters 950a through 950r for the NR receive antennas, a combiner 952, a space-time equalizer 954, and ND despreaders 956a through 956d for the ND data symbol streams. Each matched filter 950 performs matched filtering of a respective received symbol stream yi(n) with a matched filter mrx,i(k)=vrx,iHhiH(L−n) for an associated receive antenna and provides a filtered symbol stream. Combiner 952 receives the NR filtered symbol streams from matched filters 950a through 950r, combines the filtered symbol streams for all receive antennas used for each data symbol stream, and provides the detected symbol stream {tilde over (s)}l(n) for that data symbol stream. The combining is dependent on the eigensteering performed at the transmitter (i.e., the specific receive antennas to which each data symbol stream is steered). Combiner 952 provides ND detected symbol streams {tilde over (s)}(n) for the ND data symbol streams. Space-time equalizer 954 and despreaders 956a through 956d operate on the ND detected symbol streams as described above for FIG. 9D and provide the ND recovered symbol streams ŝ(n).
FIG. 9F shows a block diagram of an RX spatial processor 660f, which may also be used for multi-mode eigensteering with ND>1. RX spatial processor 660f performs OFDM demodulation and may be used in conjunction with TX spatial processor 630c in FIG. 8C.
RX spatial processor 660f includes NR antenna matched filters 970a through 970r for the NR receive antennas, NF combiners 982a through 982f for the NF subbands, NF space-time equalizers 984a through 984f for the NF subbands, and a multiplexer 986. Each antenna matched filter 970 performs matched filtering for one receive antenna and includes an OFDM demodulator 972 and NF matched filters 980a through 980f for the NF subbands.
Within each antenna matched filter 970, OFDM demodulator 972 performs OFDM demodulation on the received symbol stream yi(n) for the associated antenna and provides NF received symbol substreams yi(k), for k=1, 2, . . . NF, for the NF subbands to NF matched filters 980a through 980f. Each matched filter 980 performs matched filtering of its received symbol substream yi(k) with its matched filter mi(k)=VmmHhiH(k) and provides ND filtered symbol substreams for the ND data streams for its subband. Matched filters 980a through 980f for each antenna matched filter 970 provide NF sets of ND filtered symbol substreams for the NF subbands to NF combiners 982a through 982f. 
Each combiner 982 receives and combines the NR sets of ND filtered symbol substreams from NR antenna matched filters 970a through 970r for its subband and provides ND detected symbol substreams for its subband. Although not shown in FIG. 9F, each combiner 982 includes ND summers, one summer for each data symbol stream. Each summer receives and sums the NR filtered symbol substreams from antenna matched filters 970a through 970r for its subband and its data symbol stream to obtain the detected symbol substream for its subband.
Each spatial equalizer 984 performs equalization on the ND detected symbol substreams from an associated combiner 982 for its subband and provides ND equalized symbol streams for the subband. Spatial equalizer 984 may implement an MMSE linear equalizer or some other equalizer that jointly operates on multiple symbol streams to mitigate cross-talk and/or maximize the received SNR. Spatial equalizer 984 may also implement the successive equalization and interference cancellation processing technique.
Multiplexer 986 receives NF sets of ND equalized symbol substreams from combiners 984a through 984f for the NF subbands. Multiplexer 986 then multiplexes the NF equalized symbol substreams from combiners 984a through 984f for each data symbol stream and provides the recovered symbol stream ŝl(n) for that data symbol stream.
FIG. 10 shows a block diagram of a receiver 650x, which is an embodiment of receiver 650 in FIG. 6. RX spatial processor 660 performs matched filtering and post-processing on the NR received symbol streams yi(n), for i=1, 2, . . . NR, and provides ND recovered symbol streams ŝl(n), for f=1, 2, . . . ND to RX data processor 670. RX data processor 670 includes one set of symbol demapping unit 1012, deinterleaver 1014, and decoder 1016 for each of the ND recovered symbol streams. Each set of symbol demapping unit, deinterleaver, and decoder processes a respective recovered symbol stream as described above for FIG. 5. RX data processor 670 provides ND decoded data streams {circumflex over (d)}l(n), for l=1, 2, . . . ND.
Channel estimator 672 estimates the channel response and the receiver noise floor based on the received pilot symbols from receiver units 654a through 654r and provides the channel impulse response estimate Ĥ(n) and the noise floor estimate {circumflex over (σ)}2 to controller 680. Controller 680 performs various functions related to eigensteering, matched filtering, and rate control for the data transmission. For example, a matrix computation unit 1022 may perform computation to derive (1) the steering vector vpm for principal mode eigensteering, (2) the steering vector vmp for main path eigensteering, (3) ND steering vectors vl, for l=1, 2, . . . ND, for multi-mode eigensteering, or (4) NR steering vectors vrx,i, for i=1, 2, . . . NR, for receiver eigensteering. Computation unit 1022 also derives NR matched filters for receiver 650 and may further estimate the received SNR of the ND data streams. A transmission mode selector 1024 selects a suitable transmission mode for each data stream based on its received SNR. Memory unit 682 may store a look-up table 1026 for all of the supported transmission modes and their required SNRs. Controller 680 provides the ND TX steering vectors, the ND selected transmission modes for the ND data streams, ACKs and/or NAKs, and so on as feedback information for transmitter 610.
For embodiments described above, the receiver estimates the channel response of the MISO or MIMO channel, derives the steering vector(s) for the transmitter and the matched filter(s) for the receiver, and sends back the steering vector(s) as feedback information. For other embodiments, it may be possible for the transmitter to estimate the channel response and derive the steering vector(s). For example, in a time division duplexed (TDD) system with a shared frequency band, the downlink and uplink channel responses may be assumed to be reciprocal of one another. That is, if H(k) represents a channel frequency response matrix from antenna array A to antenna array B for subband k, then a reciprocal channel implies that the coupling from array B to array A is given by HT(k). For the TDD system, the reciprocal channel characteristics can be exploited to allow the transmitter to estimate the link observed by the receiver based on pilot sent by the receiver on the other link. In general, the channel estimation and the computation of the steering vectors may be performed by the receiver or the transmitter, depending on the system design.
FIG. 11 shows a flow diagram of an embodiment of a process 1100 for performing principal mode eigensteering, multi-mode eigensteering, and main path eigensteering in a MIMO system. Initially, a plurality of channel response matrices are obtained for the channel response of the MIMO channel in the MIMO system (at block 1112). These channel response matrices may be (1) L+1 channel impulse response matrices for L+1 time delays (i.e., H(n) for n=0, 1, . . . L) or (2) NF channel frequency response matrices for NF subbands (i.e., H(k) for k=1, 2, . . . NF).
A single correlation matrix is computed for the MIMO channel based on the channel response matrices (at block 1114). For principal mode eigensteering and multi-mode eigensteering, the single correlation matrix may be obtained by (1) computing a correlation matrix of each of the plurality of channel response matrices and (2) summing the correlation matrices for the channel response matrices to obtain the single correlation matrix, as shown in equation (18). For main path eigensteering, the single correlation matrix may be obtained by (1) determining the energy of each of the channel impulse response matrices, (2) identifying the channel impulse response matrix with the highest energy, (3) computing a correlation matrix of the channel impulse response matrix with the highest energy, and (4) defining the single correlation matrix for the MIMO channel as the correlation matrix of the channel impulse response matrix with the highest energy, as shown in equations (33) and (34) and the related description.
The single correlation matrix is then decomposed (e.g., using eigenvalue decomposition) to obtain ND steering vectors for ND spatial channels of the MIMO channel, where NS≧ND≧1 and NS is the number of eigenmodes of the single correlation matrix (at block 1116). For principal mode eigensteering and main path eigensteering, ND=1 and only one steering vector is obtained. For multi-mode eigensteering, ND>1 and multiple steering vectors are obtained.
The operations shown at blocks 1112, 1114, and 1116 may be performed by receiver 650 in FIG. 6. The operations shown at blocks 1112, 1114, and 1116 may also be performed by transmitter 610 for a time-division duplexed (TDD) system in which the downlink and uplink share the same frequency band. In any case, the ND steering vectors may be used for eigensteering by the transmitter and matched filtering by the receiver.
At the transmitter, each steering vector may be used for frequency-independent eigensteering or spatial processing of a data stream sent on the spatial channel associated with the steering vector (at block 1122). The transmitter performs eigensteering on ND data symbol streams with the ND steering vectors to generate NT transmit symbol streams (at block 1124), which are further processed and transmitted from the NT transmit antennas (at block 1126).
At the receiver, matched filtering of NR received symbol streams for NR receive antennas, where NR≧ND, may be performed in either the time domain or frequency domain. A matched filter may be derived for each receive antenna based on the ND steering vectors and a plurality of channel response vectors for that receive antenna (at block 1132). The channel response vectors for each receive antenna may be obtained from the channel response matrices. The received symbol stream for each receive antenna is filtered with the matched filter for that receive antenna to obtain ND filtered symbol substreams, one substream for each steering vector used by the transmitter (at block 1134). The filtered symbol substreams from all NR matched filters for the NR receive antennas are then combined to obtain ND detected symbol streams for the ND data streams sent by the transmitter (at block 1136). Equalization may be performed on the ND detected symbol streams to obtain ND recovered symbol streams (at block 1138). If ND>1, then space-time equalization (e.g., with an MMSE-LE, a DFE, or an MLSE) may be performed on the multiple detected symbol streams to obtain multiple recovered symbol streams.
FIG. 12 shows a flow diagram of an embodiment of a process 1200 for performing receiver eigensteering in a MISO or MIMO system with NT transmit antennas and NR receive antennas, where NT>1 and NR ≧1 in this case. Initially, NR sets of channel response vectors are obtained for the NR receive antennas, one set for each receive antenna (at block 1212). Each set of channel response vectors is indicative of the channel frequency response or the channel impulse response between the NT transmit antennas and one receive antenna.
A single correlation matrix is computed for each receive antenna based on the set of channel response vectors for that receive antenna (at block 1214). This may be achieved by (1) computing a correlation matrix of each of the channel response vectors for the receive antenna and (2) summing correlation matrices for the channel response vectors for the receive antenna to obtain the single correlation matrix for the receive antenna. The single correlation matrix for each receive antenna is then decomposed (e.g., using eigenvalue decomposition) to obtain a steering vector for the receive antenna (at block 1216). The operations shown at blocks 1212, 1214, and 1216 may be performed by receiver 150 in FIG. 1 or receiver 650 in FIG. 6. The operations shown at blocks 1212, 1214, and 1216 may also be performed by transmitter 110 in FIG. 1 or transmitter 610 in FIG. 6 for a TDD system. In any case, NR steering vectors are obtained for the NR receive antennas and may be used for spatial processing by the transmitter and matched filtering by the receiver.
At the transmitter, the NR steering vectors may be used for frequency-independent eigensteering or spatial processing of ND data streams, where NR≧ND≧1 and NT≧ND (at block 1222). For a MISO system with only one receive antenna (NR=1), one data stream is sent using one steering vector obtained for the one receive antenna (at blocks 1224 and 1226). For a MIMO system with a plurality of receive antennas (NR>1), one or multiple data streams may be sent using the NR steering vectors obtained for the NR receive antennas. Each data stream may be steered toward one or multiple receive antennas.
At the receiver, matched filtering of NR received symbol streams for the NR receive antennas may be performed in either the time domain or the frequency domain. A matched filter is derived for each receive antenna based on the steering vector and the set of channel response vectors for that receive antenna (at block 1232). The received symbol stream for each receive antenna is filtered with the matched filter for that receive antenna to obtain a filtered symbol stream for the receive antenna (at block 1234). The NR filtered symbol streams from the NR matched filters for the NR receive antennas are then combined to obtain ND detected symbol streams for the ND data streams sent by the transmitter (at block 1236). Equalization may be performed on the ND detected symbol streams to obtain ND recovered symbol streams for the ND data streams sent by the transmitter (at block 1238).
5. Rate Selection
For both MISO system 100 and MIMO system 600, the receiver may estimate the received SNR for each spatial channel. The SNR computation may be dependent on the eigensteering scheme used for data transmission, as described above. The receiver may then compute an operating SNR, γop(l), for each spatial channel based on the received SNR, γrx(l), and an SNR offset, γos(l) for the spatial channel (e.g., γop(l)=γrx(l)+γos(l), where the units are in dB). The SNR offset may be used to account for estimation error, variability in the channel, and other factors. The receiver may select a suitable transmission mode for each spatial channel based on the operating SNR for that spatial channel.
The system may be designed to support a set of transmission modes. One of the supported transmission modes may be for a null rate (i.e., a data rate of zero). Each of the remaining transmission modes is associated with a particular non-zero data rate, a particular coding scheme or code rate, a particular modulation scheme, and a particular minimum SNR required to achieve the desired level of performance (e.g., 1% packet error rate (PER)) for a non-fading AWGN channel. For each supported transmission mode with a non-zero data rate, the required SNR is obtained based on the specific system design (i.e., the particular code rate, interleaving scheme, modulation scheme, and so on, used by the system for that transmission mode) and for an AWGN channel. The required SNR may be obtained by computer simulation, empirical measurements, and so on, as is known in the art. The set of supported transmission modes and their required SNRs may be stored in a look-up table.
The operating SNR, γop(l), for each spatial channel may be provided to the look-up table, which then provides the transmission mode q(l) for that spatial channel. This transmission mode q(l) is the supported transmission mode with the highest data rate and a required SNR, γreq(l), that is less than or equal to the operating SNR (i.e., γreq(l)≦γop(l)). The receiver thus selects the highest possible data rate for each spatial channel based on the operating SNR for that spatial channel.
For clarity, specific embodiments of various eigensteering schemes have been described above. Other variants of these eigensteering schemes may also be devised, and this is within the scope of the invention. For example, the single correlation matrix for the MIMO channel may be computed in other manners than that described above for the principal mode and multi-mode eigensteering schemes. As another example, multiple data symbol streams may be transmitted on multiple spatial channels of the main path. As yet another example, ND data symbol streams may be transmitted on the ND best spatial channels based on the energy of the spatial channels. Other eigensteering schemes may also be devised based on the teachings provided herein, and this is within the scope of the invention.
The eigensteering techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the processing at the transmitter for eigensteering and other pertinent functions may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. The processing at the receiver for matched filtering and other pertinent functions may also be implemented within one or more ASICs, DSPs, and so on.
For a software implementation, the eigensteering techniques may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in memory units (e.g., memory units 142 and 182 in FIG. 1 or memory units 642 and 682 in FIG. 6) and executed by a processor (e.g., controllers 140 and 180 in FIG. 1 or controller 640 and 680 in FIG. 6). The memory unit may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various means as is known in the art.
Headings are included herein for reference and to aid in locating certain sections. These headings are not intended to limit the scope of the concepts described therein under, and these concepts may have applicability in other sections throughout the entire specification.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

## Claims

1. A method of performing spatial processing in a wireless multiple-input multiple output (MIMO) communication system, comprising:
obtaining a plurality of channel response matrices for a channel response of a MIMO channel in the MIMO system;
computing a correlation matrix for the MIMO channel based on the plurality of channel response matrices; and
decomposing the correlation matrix to obtain at least one steering vector for at least one spatial channel of the MIMO channel, wherein the at least one steering vector is used by a transmitting entity for frequency-independent spatial processing of a data stream sent on the at least one spatial channel associated with the at least one steering vector.

2. The method of claim 1, wherein the plurality of channel response matrices comprise a plurality of channel impulse response matrices for a plurality of time delays of a channel impulse response of the MIMO channel.

3. The method of claim 1, wherein the plurality of channel response matrices comprise a plurality of channel frequency response matrices for a channel frequency response for a plurality of subbands of the MIMO channel.

4. The method of claim 1, wherein the computing the correlation matrix for the MIMO channel includes:
computing a correlation matrix of each of the plurality of channel response matrices to obtain a plurality of correlation matrices for the plurality of channel response matrices, and
summing the plurality of correlation matrices for the plurality of channel response matrices to obtain the correlation matrix for the MIMO channel.

5. The method of claim 2, wherein the computing the correlation matrix fir the MIMO channel includes:
determining energy of each of the plurality of channel impulse response matrices, identifying a channel impulse response matrix with highest energy among the plurality of channel impulse response matrices, and
computing a correlation matrix of the channel impulse response matrix with the highest energy to generate the correlation matrix for the MIMO channel.

6. The method of claim 1, wherein eigenvalue decomposition of the correlation matrix is performed to obtain the at least one steering vector for the at least one spatial channel of the MIMO channel.

7. The method of claim 1, further comprising:
sending the at least one steering vector as feedback information to the transmitting entity.

8. The method of claim 1, wherein the at least one steering vector is used by the transmitting entity to generate a plurality of transmit chip streams for at least one data stream sent on the at least one spatial channel of the MIMO channel, and wherein the plurality of transmit chip streams are transmitted from a plurality of transmit antennas at the transmitting entity.

9. The method of claim 1, wherein the frequency-independent spatial processing is performed by the transmitting entity in the time-domain on a stream of time-domain chips generated for the data stream by OFDM modulation.

10. The method of claim 1, wherein the frequency-independent spatial processing is performed by the transmitting entity in the frequency-domain for each of a plurality of subbands on data symbols generated for the data steam.

11. The method of claim 1, further comprising:
obtaining, from the plurality of channel response matrices, a plurality of channel response vectors for each of a plurality of receive antennas at a receiving entity; and
deriving a matched filter for each of the plurality of receive antennas based on the at least one steering vector and the plurality of channel response vectors for the respective receive antenna.

12. The method of claim 11, wherein the matched filter for each of the plurality of receive antennas is used to maximize received signal-to-noise ratio (SNR) for the respective receive antenna.

13. The method of claim 11, further comprising: filtering a plurality of received symbol streams forte plurality of receive antennas with the plurality of matched filters.

14. The method of claim 13, wherein the plurality of channel response matrices comprise a plurality of channel impulse response matrices for a plurality of time delays of a channel impulse response of the MIMO channel, and wherein the filtering is performed in the time domain with a plurality of time-domain matched filters derived for the plurality of receive antennas based on the at least one steering vector and the plurality of channel impulse response matrices.

15. The method of claim 13, wherein the plurality of channel response matrices comprise a plurality of channel frequency response matrices for a channel frequency response for a plurality of subbands of the MIMO channel, and wherein the filtering is preformed in the frequency domain with a plurality of frequency-domain matched filters derived for the plurality of receive antennas based on the at least one steering vector and the plurality of channel frequency response matrices.

16. The method of claim 1, wherein one steering vector is obtained and used by the transmitting entity for frequency-independent spatial processing of one data stream.

17. The method of claim 16, further comprising:
deriving a matched filter for each of a plurality of receive antennas at a receiving entity based on the one steering vector and a plurality of channel response vectors for the receive antenna, wherein the plurality of channel response vectors for each receive antennas are obtained from the plurality of channel response matrices,
filtering a plurality of received symbol streams for the plurality of receive antennas with the plurality of matched filters to obtain a plurality of filtered symbol streams; and
combining the plurality of filtered symbol streams to obtain a detected symbol stream for the one data strewn sent by the transmitting entity.

18. The method of claim 17, further comprising;
performing equalization on the detected symbol stream to obtain a recovered symbol stream for the one data steam.

19. The method of claim 1, wherein a plurality of steering vectors are obtained and used by the transmitting entity for frequency-independent spatial processing of a plurality of data streams sent on a plurality of spatial channels associated with the plurality of steering vectors.

20. The method of claim 19, further comprising:
deriving a matched filter for each of a plurality of receive antennas at a receiving entity based on the plurality of steering vectors and a plurality of channel response vectors for the receive antenna, wherein the plurality of channel response vectors for each receive antenna are obtained from the plurality of channel response matrices,
filtering a plurality of received symbol streams for the plurality of receive antennas with the plurality of matched filters to obtain a plurality of filtered symbol substreams; and
combining the plurality of filtered symbol substreams to obtain a plurality of detected symbol steams for the plurality of data streams sent by the transmitting entity.

21. The method of claim 20, further comprising:
performing space-time equalization for the plurality of detected symbol streams to obtain a plurality of recovered symbol streams for the plurality of data streams.

22. The method of claim 21, wherein the space-time equalization is preformed with a minimum mean square error linear equalizer (MMSE-LE), a decision feedback equalizer (DFE), or a maximum likelihood sequence estimator (MLSE).

23. An apparatus in a wireless multiple-input multiple-output (MIMO) communication system, comprising:
a channel estimator to obtain a plurality of channel response matrices for a channel response of a MIMO channel in a MIMO system; and
a controller to compute a correlation matrix for the MIMO channel based on the plurality of channel response matrices and to decompose the correlation matrix to obtain at least one steering vector for at least one spatial channel of the MIMO channel, wherein the at least one steering vector is used by a transmitting entity for frequency-independent spatial processing of a data stream sent on the at least spatial channel associated with the at least one steering vector.

24. The apparatus of claim 23, wherein the controller computes a correlation matrix of each of the plurality of channel response matrices to obtain a plurality of correlation matrices for the plurality of channel response matrices, and to sum the plurality of correlation matrices to obtain the correlation matrix for the MIMO channel.

25. The apparatus of claim 23, wherein the plurality of channel response matrices comprise a plurality of channel impulse response matrices for a plurality of time delays of a channel impulse response of the MIMO channel, and wherein the controller determines energy of each of the plurality of channel impulse response matrices and computes a correlation matrix of a channel impulse response matrix with highest energy among the plurality of channel impulse response matrices to obtain.

26. The apparatus of claim 23, further comprising:
a plurality of matched filters for a plurality of receive antennas, one matched filter for each receive antenna, each matched filter is used to filter a received symbol stream for an associated receive antenna to obtain a filtered symbol stream, wherein the matched filter for each receive antenna is derived based on the at least one steering vector and a plurality of channel response vectors for the receive antenna, and wherein the plurality of channel response vectors for each receive antenna are obtained from the plurality of channel response matrices; and
a combiner to combine a plurality of filtered symbol streams from the plurality of matched filters to obtain at least one detected symbol stream for at least one data steam sent by the transmitting entity.

27. An apparatus in a wireless multiple-input multiple-output (MIMO) communication system, comprising:
means for obtaining a plurality of channel response matrices for a channel response of a MIMO channel in the MIMO system;
means for computing a correlation matrix for the MIMO channel based on the plurality of channel response matrices; and
means for decomposing the correlation matrix to obtain at least one steering vector for at least one spatial channel of the MIMO channel, wherein the at least one steering vector is used by a transmitting entity for frequency-independent spatial processing of a data stream sent on the at least one spatial channel associated with the at least one steering vector.

28. The apparatus of claim 27, wherein the means for computing the correlation matrix includes:
means for computing a correlation matrix of each of the plurality of channel response matrices to obtain a plurality of correlation matrices for the plurality of channel response matrices, and
means for summing the plurality of correlation matrices to obtain the correlation matrix for the MIMO channel.

29. The apparatus of claim 27, wherein the plurality of channel response matrices comprise a plurality of channel impulse response matrices for a plurality of time delays of a channel impulse response of the MIMO channel.

30. The apparatus of claim 29, wherein the means for computing the correlation matrix includes:
means for determining energy of each of the plurality of channel impulse response matrices, and
means for computing a correlation matrix of a channel impulse response matrix with highest energy among the plurality of channel impulse response matrices to obtain the correlation matrix for the MIMO channel.

31. A processor readable media for storing instructions operable to:
receive a plurality of channel response matrices for a channel response of a multiple-input multiple-output (MIMO) channel in a MIMO system;
compute a correlation matrix for the MIMO channel based on the plurality of channel response matrices; and
decompose the correlation matrix to obtain at least one steering vector for at least one spatial channel of the MIMO channel, wherein the at least one steering vector is used by a transmitting entity for frequency-independent spatial processing of a data stream sent on the at least one spatial channel associated with the at least one steering vector.

32. The processor readable media of claim 31 and father storing instructions operable to:
compute a correlation matrix of each of the plurality of channel response matrices to obtain a plurality of correlation matrices for the plurality of channel response matrices; and
sum the plurality of correlation matrices to obtain the correlation matrix for the MIMO channel.

33. The processor readable media of claim 31, wherein the plurality of channel response matrices comprise a plurality of channel impulse response matrices for a plurality of time delays of a channel impulse response of the MIMO channel.

34. The processor readable media of claim 33, and further storing instructions operable to:
compute energy of each of the plurality of channel impulse response matrices; and
compute a correlation matrix of a channel impulse response matrix with highest energy among the plurality of channel impulse response matrices to obtain the correlation matrix for the MIMO channel.

35. A method of performing spatial processing in a multiple-input multiple-output (MIMO) communication system, comprising:
obtaining a plurality of channel impulse response matrices for a MIMO channel in the MIMO system, wherein the plurality of channel impulse response matrices comprise a plurality of time delays of a channel impulse response of the MIMO channel;
computing energy of each of the plurality of channel impulse response matrices;
identifying a channel impulse response matrix with highest energy among the plurality of channel impulse response matrices as a channel impulse response matrix for a main, path of the MIMO channel;
computing a correlation matrix of the channel impulse response matrix for the main path; and
decomposing the correlation matrix to obtain a steering vector for a spatial channel of the main path, wherein the steering vector is used by a transmitting entity for frequency-independent spatial processing of a data stream sent via the MIMO channel.

36. The method of claim 35, wherein eigenvalue decomposition of the correlation matrix for the main path is performed to obtain the steering vector for the spatial channel of the main path.

37. The method of claim 35, further comprising:
deriving a matched filter for each of a plurality of receive antennas at a receiving entity based on the steering vector and a plurality of channel impulse response vectors for the receive antenna, wherein the plurality of channel impulse response vectors for each receive antenna are obtained from the plurality of channel impulse response matrices; and
filtering a plurality of received symbol streams for the plurality of receive antennas with the plurality of matched filters.

38. A method of performing spatial processing in a wireless communication system with a plurality of transmit antennas at a transmitting entity and a plurality of receive antennas at a receiving entity, the method comprising:
obtaining a plurality of sets of channel response vectors for the plurality of receive antennas, one set for each receive antenna, wherein each set of channel response vectors is indicative of a channel response between the plurality of transmit antennas and one of the plurality of receive antennas;
computing a correlation matrix for each of the plurality of receive antennas based on the set of channel response vectors for the receive antenna; and
decomposing the correlation matrix for each receive antenna to obtain a steering vector for the receive antenna, wherein a plurality of steering vectors are obtained for the plurality of receive antennas and the plurality of steering vectors are used by the transmitting entity for frequency-independent spatial processing of at least one data stream sent to the receiving entity.

39. The method of claim 38, wherein the computing the correlation matrix for each receive antenna includes:
computing a correlation matrix of each of the plurality of channel response vectors for the receive antenna to obtain a plurality of correlation matrices for the plurality of channel response vectors for the receive antenna, and
summing the plurality of correlation matrices for the plurality of channel response vectors for the receive antenna to obtain the correlation matrix for the receive antenna.

40. The method of claim 38, further comprising:
deriving a matched filter for each of the plurality of receive antennas based on the steering vector and the set of channel response vectors for the receive antenna,
filtering a received symbol stream for each of the plurality of receive antennas with the matched filter for the receive antenna to obtain a filtered symbol stream for the receive antenna; and
combining a plurality of filtered symbol streams forte plurality of receive antennas to obtain at least one detected symbol stream for the at least one data stream sent by the transmitting entity.

41. The method of claim 38, wherein one data stream is sent by the transmitting entity to the plurality of receive antennas using the plurality of steering vectors.

42. The method of claim 38, wherein a plurality of data streams are sent by the transmitting entity to the plurality of receive antennas using the plurality of steering vectors.

43. The method of claim 42, further comprising:
deriving a matched filter for each of the plurality of receive antennas based on the steering vector and the plurality of channel response vectors for the receive antenna, wherein a plurality of matched filters are derived for the plurality of receive antennas;
filtering a plurality of received symbol streams for the plurality of receive antennas with the plurality of matched filters to obtain a plurality of filtered symbol streams; and
combining the plurality of filtered symbol streams to obtain a plurality of detected symbol streams for the plurality of data streams sent by the transmitting entity.

44. The method of claim 43, further comprising:
performing space-time equalization on the plurality of detected symbol streams to obtain a plurality of recovered symbol streams for the plurality of data streams.

45. An apparatus in a wireless communication system with a plurality of transmit antennas at a transmitting entity and a plurality of receive antennas at a receiving entity, the apparatus comprising:
a channel estimator to obtain a plurality of sets of channel response vectors for the plurality of receive antennas, one set for each receive antenna, wherein each set of channel response vectors is indicative of a channel response between the plurality of transmit antennas and one of the plurality of receive antennas; and
a controller to compute a correlation matrix for each of the plurality of receive antennas based on the set of channel response vectors for the receive antenna and to decompose the single correlation matrix for each receive antenna to obtain a steering vector for the receive antenna, wherein a plurality of steering vectors are obtained for the plurality of receive antennas and the plurality of steering vectors are used by the transmitting entity for frequency-independent spatial processing of at least one data stream sent to the receiving entity.

46. The apparatus of claim 45, wherein the controller computes a correlation matrix of each of the plurality of channel response vectors for each receive antenna to obtain a plurality of correlation matrices for the plurality of channel response vectors for the receive antenna and to sum the plurality of correlation matrices for the plurality of channel response vectors for the receive antenna to obtain the correlation matrix for the respective receive antenna.

47. The apparatus of claim 45, wherein the controller derives a matched filter for each of the plurality of receive antennas based on the steering vector and the set of channel response vectors for the respective receive antenna.

48. The apparatus of claim 47, further comprising:
a plurality of matched filters for the plurality of receive antennas, one matched filter for each receive antenna, each matched filter is used to filter a received symbol stream for the associated receive antenna to obtain a filtered symbol stream; and
a combiner to combine a plurality of filtered symbol streams from the plurality of matched filters to obtain at least one detected symbol stream for the at least one data stream sent by the transmitting entity.

49. An apparatus in a wireless communications system, comprising:
means for obtaining a plurality of sets of channel response vectors for a plurality of receive antennas, one set for each receive antenna, wherein each set of channel response vectors is indicative of a channel response between a plurality of transmit antennas and one of the plurality of receive antennas;
means for computing a correlation matrix for each of the plurality of receive antennas based on the set of channel response vectors for the respective receive antenna; and
means for decomposing the single correlation matrix for each receive antenna to obtain a steering vector for the respective receive antenna, wherein a plurality of steering vectors are obtained for the plurality of receive antennas and are used by a transmitting entity for frequency-independent spatial processing of at least one data stream sent to a receiving entity.

50. The apparatus of claim 49, further comprising:
means for computing a correlation matrix of each of the plurality of channel response vectors for each receive antenna to obtain a plurality of correlation matrices for the plurality of channel response vectors for the receive antenna, and
means for summing the plurality of correlation matrices for the plurality of channel response vectors for each receive antenna to obtain the correlation matrix for the respective receive antenna.

51. The apparatus of claim 49, further comprising:
means for deriving a matched filter for each of the plurality of receive antennas based on the steering vector and the set of channel response vectors for the respective receive antenna;
means for filtering a received symbol steam for each of the plurality of receive antennas with the matched fiber for the receive antenna to obtain a filtered symbol stream for the respective receive antenna; and
means for combining a plurality of filtered symbol streams for the plurality of receive antennas to obtain at least one detected symbol stream for the at least one data stream sent by the transmitting entity.

52. A computer-readable media for storing instructions operable to:
receive a plurality of sets of channel response vectors for a plurality of receive antennas, one set for each receive antenna, wherein each set of channel response vectors is indicative of a channel response between a plurality of transmit antennas and one of the plurality of receive antennas;
compute a correlation matrix for each of the plurality of receive antennas based on the set of channel response vectors for the respective receive antenna; and
decompose the correlation matrix for each receive antenna to obtain a steering vector for the respective receive antenna, wherein a plurality of steering vectors are obtained for the plurality of receive antenna and are used by a transmitting entity for frequency-independent spatial processing of at least one data stream sent to a receiving entity.

53. The processor readable media of claim 52 and further storing instructions operable to:
compute a correlation matrix of each of the plurality of channel response vectors for each receive antenna to obtain a plurality of correlation matrices for the plurality of channel response vectors for the respective receive antenna; and
sum the plurality of correlation matrices for the plurality of channel response vectors for each receive, antenna to obtain the correlation matrix for the respective receive antenna.

54. The processor readable media of claim 52 and further storing instructions operable to:
derive a matched filter for each of the plurality of receive antennas based on the steering vector and the set of channel response vectors for the respective receive antenna;
filter a received symbol stream for each of the plurality of receive antennas with the matched filter for the receive antenna to obtain a filtered symbol stream for the respective receive antenna; and
combine a plurality of filtered symbol streams for the plurality of receive antennas to obtain at least one detected symbol stream for the at least one data stream sent by the transmitting entity.

55. A method of performing spatial processing in a multiple-input single-output (MISO) system utilizing orthogonal frequency division multiplexing (OFDM), the method comprising:
obtaining a set of channel response vectors indicative of a channel response between a plurality of transmit antennas at a transmitting entity and a receive antenna at a receiving entity in the MISO system;
computing a correlation matrix based on the set of channel response vectors; and
decomposing the correlation matrix to obtain a steering vector used by the transmitting entity for frequency-independent spatial processing of a data stream sent to the receiving entity.

56. The method of claim 55, wherein the frequency-independent spatial processing is performed by the transmitting entity in the dine-domain on a stream of time-domain chips generated for the data stream by OFDM modulation.

57. The method of claim 55, wherein the frequency-independent spatial processing is performed by the transmitting entity in the frequency-domain for each of a plurality of subbands on data symbols generated for the data stream.

58. The method of claim 55, further comprising:
deriving a matched filter based on the steering vector and the set of channel response vectors; and
filtering a received symbol stream with the matched filter to obtain a detected symbol stream.