# Patent

## Title
Generalized co-phasing for multiple transmitting and receiving antennas

## Abstract

A method for determining a phase of each of a plurality of transmitting antennas in a multiple input and multiple output (MIMO) communication system includes: calculating, for first and second ones of the plurality of transmitting antennas, a value based on first and second groups of channel gains, the first group including channel gains between the first transmitting antenna and each of a plurality of receiving antennas, the second group including channel gains between the second transmitting antenna and each of the plurality of receiving antennas; and determining the phase of each of the plurality of transmitting antennas based on at least the value.

## Background
RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from Provisional Application No. 60/910,706, filed Apr. 9, 2007, and Provisional Application No. 60/992,882, filed Dec. 6, 2007, the entire contents of which are incorporated herein by reference.


FIELD OF THE INVENTION
This invention pertains in general to communication systems and methods and, more particularly, to communication systems and methods using multiple transmitting and receiving antennas.
BACKGROUND
Recently, there has been great interest in multiple input and multiple output (MIMO) systems, which use multiple antennas on both a transmitter side and a receiver side of a communication system to improve communication performance. The MIMO technique has been employed in a variety of communication systems and is included in IEEE standards 802.11 and 802.16. There is, therefore, a need for increasing the performance of MIMO systems.
Improving spatial diversity gain may provide improved communication performance in a MIMO system. For example, when data signals including the same data are respectively transmitted by the multiple transmitting antennas of the MIMO system, spatial diversity results. With the data signals transmitted over multiple spatially separated communications channels, loss of signal due to fade or inference may be reduced, and data signals respectively received by the multiple receiving antennas of the MIMO system may be constructively combined to retrieve the data. In other words, the MIMO system has spatial diversity gain.
Because there are multiple transmitting antennas and multiple receiving antennas in the MIMO system, a communication channel is established between each of the transmitting antennas and each of the receiving antennas. A channel matrix H may be used to represent the communication channels between the transmitting antennas and the receiving antennas. Each element hi,j in the channel matrix H denotes a channel gain of a communication channel between a jth one of the transmitting antennas and an ith one of the receiving antennas. Typically, the channel gain hi,j is a complex number having a magnitude and a phase. For example, for a signal at a certain frequency propagating through the communication channel having the channel gain hi,j, the magnitude of the channel gain hi,j indicates how much the signal would be amplified or attenuated, and the phase of the channel gain hi,j indicates how much a phase of the signal would be changed.
An example method to increase the spatial diversity gain of the MIMO system is to determine a phase of each of the multiple transmitting antennas of the MIMO system, based on singular value decomposition (SVD) of the channel matrix H, where the SVD is a matrix factorization method in the art of linear algebra. However, in reality, each of the multiple transmitting antennas of the MIMO system may have a transmitting power constraint. In other words, the MIMO system may have per-antenna transmitting power constraints. When the SVD-based method is used to determine a phase of each of the transmitting antennas of the MIMO system that has per-antenna transmitting power constraints, the determined phase of each of the transmitting antennas may degrade performance of the MIMO system.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided a method for determining a phase of each of a plurality of transmitting antennas in a MIMO communication system, comprising: calculating, for first and second ones of the plurality of transmitting antennas, a value based on first and second groups of channel gains, the first group including channel gains between the first transmitting antenna and each of a plurality of receiving antennas, the second group including channel gains between the second transmitting antenna and each of the plurality of receiving antennas; and determining the phase of each of the plurality of transmitting antennas based on at least the value.
Also in accordance with the invention, there is provided a method for determining an adjustment phase for each of a plurality of data signals to be respectively transmitted by a plurality of transmitting antennas in a MIMO communication system, comprising: calculating, for first and second ones of the plurality of transmitting antennas, a value based on first and second groups of channel gains, the first group including channel gains between the first transmitting antenna and each of a plurality of receiving antennas, the second group including channel gains between the second transmitting antenna and each of the plurality of receiving antennas; and determining the adjustment phase for each of the plurality of data signals to be transmitted based on at least the value.
Further in accordance with the invention, there is provided an apparatus for determining a phase of each of a plurality of transmitting antennas in a MIMO communication system, comprising: means for calculating, for first and second ones of the plurality of transmitting antennas, a value based on first and second groups of channel gains, the first group including channel gains between the first transmitting antenna and each of a plurality of receiving antennas, the second group including channel gains between the second transmitting antenna and each of the plurality of receiving antennas; and means for determining the phase of each of the plurality of transmitting antennas based on at least the value.
Additionally in accordance with the invention, there is provided a communication system comprising: a plurality of transmitting antennas each configured to transmit a data signal, and to adjust a phase of the data signal by an adjustment phase; and a plurality of receiving antennas each configured to receive data signals respectively transmitted by the transmitting antennas; wherein the adjustment phase of each of the transmitting antennas is determined based on at least a value calculated for first and second ones of the plurality of transmitting antennas, the value being calculated based on first and second groups of channel gains, the first group including channel gains between the first transmitting antenna and each of the plurality of receiving antennas, the second group including channels gain between the second transmitting antenna and each of the plurality of receiving antennas.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.



BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
FIG. 1 illustrates a schematic block diagram of a MIMO system according to some embodiments of the present invention.
FIG. 2 illustrates a flowchart of a global co-phasing method to determine a phase of each of a plurality of transmitting antennas in a MIMO system according to some embodiments of the present invention.
FIG. 3 illustrates a flowchart of an ordering-based pairwise co-phasing method to determine a phase of each of a plurality of transmitting antennas in a MIMO system according to some embodiments of the present invention.
FIG. 4 illustrates a flowchart of an ordering-based cumulative co-phasing method to determine a phase of each of a plurality of transmitting antennas in a MIMO system according to some embodiments of the present invention.
FIG. 5 illustrates a flowchart of an iterative co-phasing method to determine a phase of each of a plurality of transmitting antennas in a MIMO system according to some embodiments of the present invention.



DESCRIPTION OF THE EMBODIMENTS
Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments consistent with the present invention do not represent all implementations consistent with the invention. Instead, they are merely examples of systems and methods consistent with aspects related to the invention as recited in the appended claims.
FIG. 1 illustrates a schematic block diagram of a MIMO system 100 according to some embodiments of the present invention. MIMO system 100 includes a plurality of transmitters 102-1, 102-2, . . . , 102-NT each having a transmitting antenna 104-1, 104-2, . . . , 104-NT (NT is the total number of the transmitters or transmitting antennas), and a plurality of receivers 106-1, 106-2, . . . , 106-NR each having a receiving antenna 108-1, 108-2, . . . , 108-NR (NR is the total number of the receivers or receiving antennas). The transmitting antennas 104-1, 104-2, . . . , 104-NT are configured to respectively have phases θ1, θ2, . . . , θNT which will be fully described below. An antenna having a phase may adjust a phase of a signal to be transmitted by the antenna based on the phase of the antenna. In other words, the phase of the antenna may be an adjustment phase for the signal to be transmitted by the antenna.
In some embodiments of the present invention, a communication channel is established between each of the transmitting antennas 104-1, 104-2, . . . , 104-NT and each of the receiving antennas 108-1, 108-2, . . . , 108-NR. The established communication channels may be flat fading channels or frequency selective channels. When a signal is transmitted through a flat fading channel, all frequency components of the signal in a particular operating band are attenuated equally. When the signal is transmitted through a frequency selective channel, components of the signal at certain frequencies are passed. For example, when orthogonal frequency division multiplexing (OFDM) techniques are used in the MIMO system 100, the established communication channels may be frequency selective channels, although each sub-carrier may experience flat fading.
Solely for the purpose of explaining the exemplary embodiments, it is assumed that the established communication channels are flat fading channels. Each of the communication channels has a channel gain hi,j(i=1, 2, . . . , NR; j=1, 2, . . . , NT). For example, the communication channel with the channel gain h1,1 is established between the transmitting antenna 104-1 and the receiving antenna 108-1. Also, for example, the communication channel with the channel gain h2,NT is established between the transmitting antenna 104-NT and the receiving antenna 108-2. Typically, the channel gain hi,j is a complex number having a magnitude and a phase. For example, for a signal at a certain frequency propagating through the communication channel having the channel gain hi,j, the magnitude of the channel gain hi,j may indicate how much the signal would be amplified or attenuated, and the phase of the channel gain hi,j may indicate how much a phase of the signal would be changed.
A channel matrix H can be used to represent the channel gains between the transmitting antennas 104-1, 104-2, . . . , 104-NT and the receiving antennas 108-1, 108-2, . . . , 108-NR as follows:




  
    
      
        H
        =
        
          
            [
            
              
                
                  
                    h
                    
                      1
                      ,
                      1
                    
                  
                
                
                  ...
                
                
                  
                    h
                    
                      1
                      ,
                      
                        N
                        T
                      
                    
                  
                
              
              
                
                  ⋮
                
                
                  ⋱
                
                
                  ⋮
                
              
              
                
                  
                    h
                    
                      N
                      
                        R
                        ,
                        1
                      
                    
                  
                
                
                  ...
                
                
                  
                    h
                    
                      
                        N
                        R
                      
                      ,
                      
                        N
                        T
                      
                    
                  
                
              
            
            ]
          
          .
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          1
          )
        
      
    
  




For convenience of illustration, the channel matrix H can be expressed as:

H=[h1 . . . hNT],  Equation (2)

where ht(t=1, 2, . . . , NT) is a vector corresponding to the tth column of the channel matrix H.

In some embodiments of the present invention, each of the transmitting antennas 104-1, 104-2, . . . , 104-NT has a transmitting power constraint. For example, each of the transmitting antennas 104-1, 104-2, . . . , 104-NT has a maximum transmitting power P. Therefore, the transmitting antennas 104-1, 104-2, . . . , 104-NT may not amplify, but may make phase adjustment to data signals to be transmitted thereby. After the phase adjustment, the transmitting antennas 104-1, 104-2, . . . , 104-NT respectively transmit data signals x1, x2, . . . , xNT including the same data u. The transmitted data signals x1, x2, . . . , xNT can be denoted by a vector x as follows:

x=[x1 . . . xNT]T,  Equation (3)

where “T” denotes matrix transposition. For convenience of illustration, the data u and signal components in the transmitted data signals x1, x2, . . . , xNT that may affect spatial diversity gain of the MIMO system 100 are used to represent the transmitted data signals x1, x2, . . . , xN, and Equation (3) is further expressed as follows:





  
    
      
        
          x
          =
          
            
              
                [
                
                  
                    e
                    
                      jθ
                      1
                    
                  
                  ⁢
                  ...
                  ⁢
                  
                    
                  
                  ⁢
                  
                    e
                    
                      jθ
                      
                        N
                        T
                      
                    
                  
                
                ]
              
              T
            
            ⁢
            u
          
        
        ,
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          4
          )
        
      
    
  




where j is an imaginary unit, and θ1, θ2, . . . , θNT are the phases of the transmitting antennas 104-1, 104-2, . . . , 104-NT, respectively.

For example, the signal x1 transmitted by the transmitting antenna 104-1 propagates through the communication channels with the channel gains h1,1, h2,1, . . . , hNR,1, and is then received by each of the receiving antennas 108-1, 108-2, . . . , 108-NR. Also, for example, the signal xNT transmitted by the transmitting antenna 104-NT propagates through the communication channels with the channel gains h1,NT, h2,NT, . . . , hNR,NT, and is then received by each of the receiving antennas 108-1, 108-2, . . . , 108-NR.
A vector y may be used to denote signals y1, y2, . . . , yNR respectively received by the receiving antennas 108-1, 108-2, . . . , 108-NR as follows:

y=[y1 . . . yNR]T.  Equation (5)

The vector y containing the signals y1, y2, . . . , yNR can be further calculated as follows:

y=Hx+z,  Equation (6)

where H is the channel matrix, x is the vector containing the data signals x1, x2, . . . , xNT respectively transmitted by the transmitting antennas 104-1, 104-2, . . . , 104-NT, and z is a vector containing noise signals z1, z2, . . . , zNR respectively generated in the receiving antennas 108-1, 108-2, . . . , 108-NR, or

z=[z1 . . . zNR]T.  Equation (7)

Solely for the purpose of explaining the exemplary embodiments, it is assumed that each of the noise signals z1, z2, . . . , zNR is a Gaussian noise signal. A Gaussian noise signal has a probability density function of a normal distribution.

Based on Equations (2), (4), and (6), the vector y containing the signals y1, y2, . . . , yNR may be further calculated as follows:




  
    
      
        y
        =
        
          
            
              ∑
              
                t
                =
                1
              
              
                N
                T
              
            
            ⁢
            
              
            
            ⁢
            
              
                e
                
                  jθ
                  t
                
              
              ⁢
              
                h
                t
              
              ⁢
              u
            
          
          +
          
            z
            .
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          8
          )
        
      
    
  




In some embodiments of the present invention, an equivalent channel gain He is defined as follows:





  
    
      
        
          H
          e
        
        =
        
          
            ∑
            
              t
              =
              1
            
            
              N
              T
            
          
          ⁢
          
            
          
          ⁢
          
            
              e
              
                jθ
                t
              
            
            ⁢
            
              
                h
                t
              
              .
            
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          9
          )
        
      
    
  




The vector y containing the signals y1, y2, . . . , yNR may then be expressed as follows:

y=Heu+z.  Equation (10)

The receiving antennas 108-1, 108-2, . . . , 108-NR respectively receive the signals y1, y2, . . . , yNR. The signals y1, y2, . . . , yNR are further processed based on, e.g., maximum ratio combining (MRC) techniques, to obtain a signal r for retrieving the data u as follows:

r=H*ey=∥He∥2u+{tilde over (z)},  Equation (11)

where “*” denotes matrix conjugate transposition, “∥He∥2” denotes a norm of the equivalent channel gain He and is equal to H*eHe, and

{tilde over (z)}=H*ez.  Equation (12)

Therefore a signal to noise ratio (SNR) of the obtained signal r can be calculated as follows:




  
    
      
        
          SNR
          =
          
            
              
                
                
              
              ⁢
              
                H
                e
              
              ⁢
              
                
                  
                  
                
                2
              
              ⁢
              P
            
            
              σ
              z
              2
            
          
        
        ,
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          13
          )
        
      
    
  




where σz2 is a variance of each individual noise signal z1, z2, . . . , zNR, and P is the maximum transmitting power of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT.

In some embodiments of the present invention, the spatial diversity gain of the MIMO system 100 and, hence, the SNR of the obtained signal r may be increased by using co-phasing methods. The co-phasing methods may determine the phase of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT, or the adjustment phase for the data signal to be transmitted by each of the transmitting antennas 104-1, 104-2, . . . , 104-NT, such that data signals received by the receiving antennas 108-1, 108-2, . . . , 108-NR may be constructively combined. Examples of co-phasing methods are fully described below.
A. Global Co-Phasing
Referring back to Equation (13), the SNR of the obtained signal r may be improved by maximizing the norm of the equivalent channel gain He. Based on Equation (9), the norm of the equivalent channel gain He may be further expressed as follows:




  
    
      
        
          
            
              
              
            
            ⁢
            
              H
              e
            
            ⁢
            
              
                
                
              
              2
            
          
          =
          
            
              
                ∑
                
                  t
                  =
                  1
                
                
                  N
                  T
                
              
              ⁢
              
                
              
              ⁢
              
                
                  
                  
                
                ⁢
                
                  h
                  t
                
                ⁢
                
                  
                    
                    
                  
                  2
                
              
            
            +
            
              2
              ⁢
              
                
                  ∑
                  
                    t
                    =
                    1
                  
                  
                    N
                    T
                  
                
                ⁢
                
                  
                
                ⁢
                
                  
                    ∑
                    
                      k
                      =
                      
                        t
                        +
                        1
                      
                    
                    
                      N
                      T
                    
                  
                  ⁢
                  
                    
                  
                  ⁢
                  
                    R
                    ⁢
                    
                      {
                      
                        
                          e
                          
                            -
                            
                              j
                              ⁡
                              
                                (
                                
                                  
                                    θ
                                    t
                                  
                                  -
                                  
                                    θ
                                    k
                                  
                                
                                )
                              
                            
                          
                        
                        ⁡
                        
                          (
                          
                            
                              h
                              t
                              *
                            
                            ⁢
                            
                              h
                              k
                            
                          
                          )
                        
                      
                      }
                    
                  
                
              
            
          
        
        ,
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          14
          )
        
      
    
  




where R{e−j(θt−θk)(h*thk)} represents a real part of e−j(θt−θk)(h*thk).

The first term




  
    ∑
    
      t
      =
      1
    
    
      N
      T
    
  
  ⁢
  
    
  
  ⁢
  
    
      
      
    
    ⁢
    
      h
      t
    
    ⁢
    
      
        
        
      
      2
    
  




on the right side of Equation (14) relates to the communication channel between each of the transmitting antennas 104-1, 104-2, . . . , 104-NT and each of the receiving antennas 108-1, 108-2, . . . , 108-NR, and has a determined value once locations of the transmitting antennas 104-1, 104-2, . . . , 104-NT and the receiving antennas 108-1, 108-2, . . . , 108-NR are determined. The second term





  2
  ⁢
  
    
      ∑
      
        t
        =
        1
      
      
        N
        T
      
    
    ⁢
    
      
    
    ⁢
    
      
        ∑
        
          k
          =
          
            t
            +
            1
          
        
        
          N
          T
        
      
      ⁢
      
        
      
      ⁢
      
        R
        ⁢
        
          {
          
            
              e
              
                -
                
                  j
                  ⁡
                  
                    (
                    
                      
                        θ
                        t
                      
                      -
                      
                        θ
                        k
                      
                    
                    )
                  
                
              
            
            ⁡
            
              (
              
                
                  h
                  t
                  *
                
                ⁢
                
                  h
                  k
                
              
              )
            
          
          }
        
      
    
  




on the right side of Equation (14) relates to the communication channel between each of the transmitting antennas 104-1, 104-2, . . . , 104-NT and each of the receiving antennas 108-1, 108-2, . . . , 108-NR, and the phase θ1, θ2, . . . , θNT of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT. In particular, the term





  2
  ⁢
  
    
      ∑
      
        t
        =
        1
      
      
        N
        T
      
    
    ⁢
    
      
    
    ⁢
    
      
        ∑
        
          k
          =
          
            t
            +
            1
          
        
        
          N
          T
        
      
      ⁢
      
        
      
      ⁢
      
        R
        ⁢
        
          {
          
            
              e
              
                -
                
                  j
                  ⁡
                  
                    (
                    
                      
                        θ
                        t
                      
                      -
                      
                        θ
                        k
                      
                    
                    )
                  
                
              
            
            ⁡
            
              (
              
                
                  h
                  t
                  *
                
                ⁢
                
                  h
                  k
                
              
              )
            
          
          }
        
      
    
  




relates to a phase difference between each combination of any two of the transmitting antennas 104-1, 104-2, . . . , 104-NT.

In some embodiments of the present invention, a function f(θ1, θ2, . . . , θNT) is defined for determining the phases θ1, θ2, . . . , θNT of the transmitting antennas 104-1, 104-2, . . . , 104-NT. The function f(θ1, θ2, . . . , θNT) corresponds to the second term




  2
  ⁢
  
    
      ∑
      
        t
        =
        1
      
      
        N
        T
      
    
    ⁢
    
      
    
    ⁢
    
      
        ∑
        
          k
          =
          
            t
            +
            1
          
        
        
          N
          T
        
      
      ⁢
      
        
      
      ⁢
      
        R
        ⁢
        
          {
          
            
              e
              
                -
                
                  j
                  ⁡
                  
                    (
                    
                      
                        θ
                        t
                      
                      -
                      
                        θ
                        k
                      
                    
                    )
                  
                
              
            
            ⁡
            
              (
              
                
                  h
                  t
                  *
                
                ⁢
                
                  h
                  k
                
              
              )
            
          
          }
        
      
    
  




on the right side of Equation (14) and may be expressed as:





  
    
      
        
          
            
              f
              ⁡
              
                (
                
                  
                    θ
                    1
                  
                  ,
                  
                    θ
                    2
                  
                  ,
                  ...
                  ⁢
                  
                    
                  
                  ,
                  
                    θ
                    
                      N
                      T
                    
                  
                
                )
              
            
            ⁢
            2
          
          =
          
            
              ∑
              
                t
                =
                1
              
              
                N
                T
              
            
            ⁢
            
              
            
            ⁢
            
              
                ∑
                
                  k
                  =
                  
                    t
                    +
                    1
                  
                
                
                  N
                  T
                
              
              ⁢
              
                
              
              ⁢
              
                R
                ⁢
                
                  {
                  
                    
                      e
                      
                        -
                        
                          j
                          ⁡
                          
                            (
                            
                              
                                θ
                                t
                              
                              -
                              
                                θ
                                k
                              
                            
                            )
                          
                        
                      
                    
                    ⁡
                    
                      (
                      
                        
                          h
                          t
                          *
                        
                        ⁢
                        
                          h
                          k
                        
                      
                      )
                    
                  
                  }
                
              
            
          
        
        ,
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          15
          )
        
      
    
  




where 0≦θi<2π for i=1, . . . ,NT. The SNR of the obtained signal r may be improved by determining the phases θ1, θ2, . . . , θNT such that the function f(θ1, θ2, . . . , θNT) may be maximized. In particular, it is possible to maximize the function f(θ1, θ2, . . . , θNT) if the following expressions, or constraints, are satisfied:

θt−θk=∠(h*thk) for t=1, . . . , NT−1, and k=t+1, . . . , NT,  Equation (16)

where the value h*thk is typically a complex number.

For example, the value h*thk , as the complex number, can be expressed as follows:

h*thk=At,k·ejφt,k,  Equation (17)

where At,k is a magnitude of h*thk (At,k=|h*thk), and φt,k is a phase of h*thk (φt,k=∠(h*thk)). For a given t and k,

R{e−j(θt−θk)(h*thk)}≦|e−j(θt−θk)(h*thk)|=|e−j(θt−θk)(At,k·ejφt,k)|≦At,k.  Equation (18)

Therefore, R{e−j(θt−θk)(h*thk)} and, hence, the function f(θ1, θ2, . . . , θNT) may be maximized if φt,k=θt−θk, i.e., Equation (16), is satisfied.

Each expression in Equation (16) corresponds to one value h*thk, which may be used to determine a phase difference between the transmitting antennas 104-t and 104-k. In addition, Equation (16) includes the value h*thk for each combination of any two of the transmitting antennas 104-t and 104-k (t=1, . . . , NT−1; k=t+1, . . . , NT). The value h*thk can be calculated based on first and second groups of channel gains respectively included in the vectors ht and hk. The vector ht includes channel gains between the transmitting antenna 104-t and each of the receiving antennas 108-1, 108-2, . . . , 108-NR, and the vector hk includes channel gains between the transmitting antenna 104-k and each of the receiving antennas 108-1, 108-2, . . . , 108-NR.
There are




  
    
      N
      T
    
    ⁡
    
      (
      
        
          N
          T
        
        -
        1
      
      )
    
  
  2




expressions in Equation (16). If the MIMO system 100 includes a total of two transmitting antennas 104-1 and 104-2, i.e., NT=2, there is only one expression in Equation (16), which determines a phase difference between the two transmitting antennas 104-1 and 104-2. The phases of the transmitting antennas 104-1 and 104-2 may be determined by assigning a phase value to the phase of one of the transmitting antennas 104-1 and 104-2, and solving Equation (16) for the phase of the other one of the transmitting antennas 104-1 and 104-2.

Generally, all the expressions in Equation (16) may not be satisfied simultaneously. For example, for the MIMO system 100 having a total of four transmitting antennas 104-1, 104-2, 104-3, and 104-4, Equation (16) may be expressed as follows:

θ1−θ2=∠(h*1h2),

θ1−θ3=∠(h*1h3),

θ1−θ4=∠(h*1h4),

θ2−θ3=∠(h*2h3),

θ2−θ4=∠(h*2h4),

θ3−θ4=∠(h*3h4).  Equation (19)

Based on the first and second expressions in Equation (19), it follows that

θ2−θ3=(θ1−θ3)−(θ1−θ2)=∠(h*1h3)−∠(h*1h2).  Equation (20)

However, Equation (19) also shows θ2−θ3=∠(h*2h3). Since ∠(h*1h3)−∠(h*1h2) might not be equal to ∠(h*2h3), all the expressions in Equation (19) may not be satisfied simultaneously. Therefore, approximate methods may be used to determine the phase θ1, θ2, . . . , θNT of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT based on Equation (16).

FIG. 2 illustrates a flowchart of a global co-phasing method to determine the phase θ1, θ2, . . . , θNT of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT of the MIMO system 100, according to some embodiments of the present invention. For convenience of illustration, let (tp,kp) denote the indices (t,k) in a pth expression in Equation (16). The expressions in Equation (16) are ordered such that the expressions start from the lowest number of t and then start from the lowest number of k for the same t. For example, for the MIMO system 100 having the four transmitting antennas 104-1, 104-2, 104-3, and 104-4, expressions in Equation (19) are ordered such that (t1,k1)=(1,2), (t2,k2)=(1,3), (t3,k3)=(1,4), (t4,k4)=(2,3), (t5,k5)=(2,4), and (t6,k6)=(3,4).
In accordance with a weighted least squares method, Equation (16) can be modified to be:

wt,k(θt−θk−∠(h*thk))≈0, for t=1, . . . , NT−1, and k=t+1, . . . , NT,  Equation (21)

where wt,k is a weight applied to the one of the expressions in Equation (16) having the indices (t,k) and the weight wt,k is associated with the value h*thk. Based on the weighted least squares method, an expression with a relatively large weight attached to it would be satisfied more accurately.

In addition, a phase value may be assigned to the phase of one of the transmitting antennas 104-1, 104-2, . . . , 104-NT to provide an additional expression or constraint, such as θT=0. Together with this constraint, Equation (21) may be formulated in matrix form as follows:




  
    
      
        
          
            WA
            ⁡
            
              [
              
                
                  
                    
                      θ
                      1
                    
                  
                
                
                  
                    
                      θ
                      2
                    
                  
                
                
                  
                    ⋮
                  
                
                
                  
                    
                      θ
                      
                        N
                        T
                      
                    
                  
                
              
              ]
            
          
          =
          
            W
            ⁡
            
              [
              
                
                  
                    
                      ∠
                      ⁡
                      
                        (
                        
                          
                            h
                            
                              t
                              1
                            
                            *
                          
                          ⁢
                          
                            h
                            
                              k
                              1
                            
                          
                        
                        )
                      
                    
                  
                
                
                  
                    ⋮
                  
                
                
                  
                    
                      ∠
                      ⁡
                      
                        (
                        
                          
                            h
                            
                              t
                              
                                g
                                ⁡
                                
                                  (
                                  
                                    N
                                    T
                                  
                                  )
                                
                              
                            
                            *
                          
                          ⁢
                          
                            h
                            
                              k
                              
                                g
                                ⁡
                                
                                  (
                                  
                                    N
                                    T
                                  
                                  )
                                
                              
                            
                          
                        
                        )
                      
                    
                  
                
                
                  
                    0
                  
                
              
              ]
            
          
        
        ,
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          22
          )
        
      
    
  




where A is a coefficient matrix and W is a weight matrix, and





  
    g
    ⁡
    
      (
      
        N
        T
      
      )
    
  
  =
  
    
      
        
          N
          T
        
        ⁡
        
          (
          
            
              N
              T
            
            -
            1
          
          )
        
      
      2
    
    .
  




For example, the coefficient matrix A may have a size of (g(NT)+1)×NT. An element ap,q in the pth row and the qth column of the coefficient matrix A is:




  
    
      
        
          a
          
            p
            ,
            q
          
        
        =
        
          {
          
            
              
                
                  1
                
                
                  
                    
                      for
                      ⁢
                      
                        
                      
                      ⁢
                      q
                    
                    =
                    
                      
                        
                          
                            t
                            p
                          
                          ⁢
                          
                            
                          
                          ⁢
                          and
                          ⁢
                          
                            
                          
                          ⁢
                          p
                        
                        ⁢
                        
                          <
                          _
                        
                        ⁢
                        
                          
                            g
                            ⁡
                            
                              (
                              
                                N
                                T
                              
                              )
                            
                          
                          ⁢
                          
                            
                          
                          ⁢
                          or
                          ⁢
                          
                            
                          
                          ⁢
                          q
                        
                      
                      =
                      
                        
                          
                            N
                            T
                          
                          ⁢
                          
                            
                          
                          ⁢
                          and
                          ⁢
                          
                            
                          
                          ⁢
                          p
                        
                        =
                        
                          
                            g
                            ⁡
                            
                              (
                              
                                N
                                T
                              
                              )
                            
                          
                          +
                          1
                        
                      
                    
                  
                
              
              
                
                  
                    -
                    1
                  
                
                
                  
                    
                      for
                      ⁢
                      
                        
                      
                      ⁢
                      q
                    
                    =
                    
                      
                        
                          k
                          p
                        
                        ⁢
                        
                          
                        
                        ⁢
                        and
                        ⁢
                        
                          
                        
                        ⁢
                        p
                      
                      ⁢
                      
                        <
                        _
                      
                      ⁢
                      
                        g
                        ⁡
                        
                          (
                          
                            N
                            T
                          
                          )
                        
                      
                    
                  
                
              
              
                
                  0
                
                
                  otherwise
                
              
            
            .
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          23
          )
        
      
    
  




The weight matrix W may have a size of (g(NT)+1)×(g(NT)+1) and is a diagonal matrix including the weights wt,k (t=1, . . . , NT−1; k=t+1, . . . , NT), respectively, associated with the values h*thk or the expressions in Equation (16).

Referring to FIG. 2, a phase value is assigned to the phase of one of the transmitting antennas 104-1, 104-2, . . . , 104-NT, as noted above (step 202). The assigned phase value may be equal to or greater than zero, and smaller than 2π. For example, in Equation (22) the phase θNT of the transmitting antenna 104-NT is assigned a phase value zero. In addition, assigning the phase value to the phase of one of the transmitting antennas 104-1, 104-2, . . . , 104-NT may make the coefficient matrix A a full-rank matrix.
In step 204, the value h*thk is also calculated for each combination of any two of the transmitting antennas 104-1, 104-2, . . . , 104-NT. For example, the value h*thk is calculated based on the first and second groups of channel gains respectively included in the vectors ht and hk, as explained above. Typically, the calculated value h*thk is a complex number. The phase ∠h*thk of the calculated value h*thk is further calculated to determine the phase θ1, θ2, . . . , θNT of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT, as shown in Equation (22). In the illustrated embodiment shown in the FIG. 2, step 204 is performed after step 202. Alternatively, step 204 may be performed before step 202.
The weight wt,k associated with the value h*thk calculated for each combination of any two of the transmitting antennas 104-1, 104-2, . . . , 104-NT is further determined (step 206). For example, the weight wt,k associated with the value h*thk calculated for a first combination of two of the transmitting antennas 104-1, 104-2, . . . , 104-NT may be equal to that calculated for a second combination of two of the transmitting antennas 104-1, 104-2, . . . , 104-NT. If the weight wt,k is so determined, step 206 can be performed before or after any of the steps 202 and 204. Also, for example, the weight wt,k associated with the value h*thk calculated for the two transmitting antennas 104-t and 104-k can be a magnitude or a squared magnitude of the value h*thk. Further, for example, the weight wt,k associated with the value h*thk calculated for the two transmitting antennas 104-t and 104-k can be an exponential function of the squared magnitude of the value h*thk.
When the weight wt,k associated with the value h*thk calculated for each combination of any two of the transmitting antennas 104-1, 104-2, . . . , 104-NT is determined, the weight matrix W may be determined. For example, if the weight wt,k associated with the value h*thk is a magnitude of the value h*thk, i.e., |h*thk|, the weight matrix W in Equation (22) may be determined as follows:




  
    
      
        
          W
          =
          
            [
            
              
                
                  
                    
                    
                      
                        h
                        
                          t
                          1
                        
                        *
                      
                      ⁢
                      
                        h
                        
                          k
                          1
                        
                      
                    
                    
                  
                
                
                  0
                
                
                  0
                
                
                  0
                
              
              
                
                  0
                
                
                  ⋱
                
                
                  0
                
                
                  0
                
              
              
                
                  0
                
                
                  0
                
                
                  
                    
                    
                      
                        h
                        
                          t
                          
                            g
                            ⁡
                            
                              (
                              
                                N
                                T
                              
                              )
                            
                          
                        
                        *
                      
                      ⁢
                      
                        h
                        
                          k
                          
                            g
                            ⁡
                            
                              (
                              
                                N
                                T
                              
                              )
                            
                          
                        
                      
                    
                    
                  
                
                
                  0
                
              
              
                
                  0
                
                
                  0
                
                
                  0
                
                
                  
                    w
                    0
                  
                
              
            
            ]
          
        
        ,
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          24
          )
        
      
    
  




where w0 is a weight associated with the expression θT=0 and may be any nonzero real value.

After the weight matrix W is determined, a solution to Equation (22) can be expressed as follows:




  
    
      
        
          
            [
            
              
                
                  
                    θ
                    1
                  
                
              
              
                
                  
                    θ
                    2
                  
                
              
              
                
                  ⋮
                
              
              
                
                  
                    θ
                    
                      N
                      T
                    
                  
                
              
            
            ]
          
          =
          
            
              
                (
                WA
                )
              
              †
            
            ⁢
            
              W
              ⁡
              
                [
                
                  
                    
                      
                        ∠
                        ⁡
                        
                          (
                          
                            
                              h
                              
                                t
                                1
                              
                              *
                            
                            ⁢
                            
                              h
                              
                                k
                                1
                              
                            
                          
                          )
                        
                      
                    
                  
                  
                    
                      ⋮
                    
                  
                  
                    
                      
                        ∠
                        ⁡
                        
                          (
                          
                            
                              h
                              
                                t
                                
                                  g
                                  ⁡
                                  
                                    (
                                    
                                      N
                                      T
                                    
                                    )
                                  
                                
                              
                              *
                            
                            ⁢
                            
                              h
                              
                                k
                                
                                  g
                                  ⁡
                                  
                                    (
                                    
                                      N
                                      T
                                    
                                    )
                                  
                                
                              
                            
                          
                          )
                        
                      
                    
                  
                  
                    
                      0
                    
                  
                
                ]
              
            
          
        
        ,
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          25
          )
        
      
    
  




where (WA)+ is a pseudo-inverse of WA, or

(WA)+=(ATWTWA)−1ATWT,  Equation (26)

where (ATWTWA)−1 denotes an inverse of ATWTWA. Thus, in step 208, the phase θ1, θ2, . . . , θNT of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT can be determined based on the weight matrix W.

B. Pairwise Co-Phasing
In some embodiments of the present invention, a pairwise co-phasing method may be used to determine the phase θ1, θ2, . . . , θNT of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT. As noted above, there are




  
    
      N
      T
    
    ⁡
    
      (
      
        
          N
          T
        
        -
        1
      
      )
    
  
  2




expressions in Equation (16), and each expression in Equation (16) corresponds to a value h*thk, which may be used to determine a phase difference between the transmitting antennas 104-t and 104-k. Generally, all the expressions in Equation (16) may not be satisfied simultaneously. Therefore, based on the pairwise co-phasing method, NT−1 of the





  
    
      N
      T
    
    ⁡
    
      (
      
        
          N
          T
        
        -
        1
      
      )
    
  
  2




expressions in Equation (16), respectively corresponding to NT−1 of the





  
    
      N
      T
    
    ⁡
    
      (
      
        
          N
          T
        
        -
        1
      
      )
    
  
  2




values h*thk (t=1, . . . , NT−1; k=t+1, . . . , NT), may be selected to determine the phase θ1, θ2, . . . , θNT of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT.

For example, the NT−1 expressions in Equation (16) including the phase θT of the transmitting antenna 104-NT, respectively corresponding to the NT−1 values h*thNT (t=1, . . . , NT−1), may be selected to determine the phase θ1, θ2, . . . , θNT of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT. In other words, the transmitting antennas 104-1, 104-2, . . . , 104-NT−1 each are co-phased with the transmitting antenna 104-NT. In some embodiments of the present invention, a phase value zero is assigned to the phase θNT of the transmitting antenna 104-NT. Accordingly, the phase θ1, θ2, . . . , θNT of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT may be determined as follows:




  
    
      
        
          θ
          t
        
        =
        
          {
          
            
              
                
                  0
                
                
                  
                    
                      for
                      ⁢
                      
                        
                      
                      ⁢
                      t
                    
                    =
                    
                      N
                      T
                    
                  
                
              
              
                
                  
                    ∠
                    (
                    
                      
                        h
                        t
                        *
                      
                      ⁢
                      
                        h
                        
                          N
                          T
                        
                      
                    
                  
                
                
                  
                    
                      
                        for
                        ⁢
                        
                          
                        
                        ⁢
                        t
                      
                      =
                      1
                    
                    ,
                    ...
                    ⁢
                    
                      
                    
                    ,
                    
                      
                        N
                        T
                      
                      -
                      1
                    
                  
                
              
            
            .
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          27
          )
        
      
    
  




In some embodiments of the present invention, an ordering-based pairwise co-phasing method may be used to determine the phase θ1, θ2, . . . , θNT of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT. Based on the ordering-based pairwise co-phasing method, the expressions in Equation (16) corresponding to the values h*thk that have a relatively large magnitude may be used to determine the phase θ1, θ2, . . . , θNT of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT.
FIG. 3 illustrates a flowchart of the ordering-based pairwise co-phasing method according to some embodiments of the present invention. In step 302, for each combination of any two (104-t and 104-k) of the transmitting antennas 104-1, 104-2, . . . , 104-NT, the value h*thk is calculated. In step 304, the values h*thk (t=1, . . . , NT−1; k=t+1, . . . , NT) are sorted in a decreasing order of magnitude. In step 306, one of the sorted values h*tmaxhkmax that has the largest magnitude is selected to determine the phases of the two transmitting antennas 104-tmax and 104-kmax corresponding to the selected value.
In step 308, a judgment is made whether the phases of all of the transmitting antennas 104-1, 104-2, . . . , 104-NT have been determined. A next one of the sorted values h*tnexthknext is selected in step 310 if the phases of all of the transmitting antennas 104-1, 104-2, . . . , 104-NT have not been determined. Based on the selected next value h*tnexthknext, in step 312 the phase of a first one of the two transmitting antennas 104-tnext and 104-knext corresponding to the selected next value h*tnexthknext may be determined if the phase of the first one of the two transmitting antennas 104-tnext and 104-knext has not been determined. Also, based on the selected next value h*tnexthknext, in step 314 the phase of a second one of the two transmitting antennas 104-tnext and 104-knext corresponding to the selected next value h*tnexthknext may be determined if the phase of the second one of the two transmitting antennas 104-tnext and 104-knext has not been determined. Steps 308 to 314 are repeated until the phases of all of the transmitting antennas 104-1, 104-2, . . . , 104-NT have been determined.
For example, for the MIMO system 100 having a total of three transmitting antennas 104-1, 104-2, and 104-3, Equation (16) can be expressed as:

θ1−θ2=∠(h*1h2),

θ1−θ3=∠(h*1h3),

θ2−θ3=∠(h*2h3).  Equation (28)

The values h*1h2, h*1h3, and h*2h3 are calculated and sorted in a decreasing order of magnitude. In some embodiments of the present invention, the magnitude of h*1h2 is smaller than the magnitude of h*1h3, which is further smaller than the magnitude of h*2h3. Therefore, after sorting, the values h*1h2, h*1h3, and h*2h3 would be presented as h*2h3, h*1h3, and h*1h2. Accordingly, the expressions in Equation (28) can be reordered as follows:

θ2−θ3=∠(h*2h3),

θ1−θ3=∠(h*1h3),

θ1−θ2=∠(h*1h2).  Equation (29)

The value h*2h3 having the largest magnitude is selected to determine the phases of the transmitting antennas 104-2 and 104-3 corresponding to the value h*2h3. For example, the phase θ3 of the transmitting antenna 104-3 may be determined by assigning a phase value zero to the phase θ3. The phase θ2 of the transmitting antenna 104-2 may be determined based on the determined phase θ3 and the value h*2h3 using Equation (29), i.e., θ2=0+∠(h*2h3)=∠(h*2h3).
Next, a judgment is made whether the phases of all of the transmitting antennas 104-1, 104-2, and 104-3 have been determined. Here, the phase θ1 of the transmitting antenna 104-1 has not been determined, and therefore a next one of the sorted values h*1h3, corresponding to the transmitting antennas 104-1 and 104-3, is selected. Because the phase θ3 of the transmitting antenna 104-3 has been determined but the phase θ1 of the transmitting antenna 104-1 has not been determined, the phase θ1 may be determined based on the determined phase θ3 and the value h*1h3 using Equation (29), i.e., θ1=θ2+∠(h*1h3)=∠(h*2h3)+∠(h*1h3). Thus, the phases of all of the transmitting antennas 104-1, 104-2, and 104-3 have been determined.
C. Cumulative Co-Phasing
In some embodiments of the present invention, a cumulative co-phasing method may be used to determine the phase θ1, θ2, . . . , θNT of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT. Referring back to Equation (15), Equation (15) can also be written as follows:




  
    
      
        
          f
          ⁡
          
            (
            
              
                θ
                1
              
              ,
              
                θ
                2
              
              ,
              ...
              ⁢
              
                
              
              ,
              
                θ
                
                  N
                  T
                
              
            
            )
          
        
        =
        
          
            ∑
            
              t
              =
              1
            
            
              N
              T
            
          
          ⁢
          
            
          
          ⁢
          
            R
            ⁢
            
              
                {
                
                  
                    e
                    
                      -
                      
                        jθ
                        t
                      
                    
                  
                  ⁢
                  
                    
                      ∑
                      
                        k
                        =
                        
                          t
                          +
                          1
                        
                      
                      
                        N
                        T
                      
                    
                    ⁢
                    
                      
                    
                    ⁢
                    
                      
                        e
                        
                          jθ
                          k
                        
                      
                      ⁡
                      
                        (
                        
                          
                            h
                            t
                            *
                          
                          ⁢
                          
                            h
                            k
                          
                        
                        )
                      
                    
                  
                
                }
              
              .
            
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          30
          )
        
      
    
  




As noted above, the SNR of the obtained signal r may be improved by determining the phases θ1, θ2, . . . , θNT such that the function f(θ1, θ2, . . . , θNT) may be maximized. Here, based on Equation (30), it is possible to maximize the function f(θ1, θ2, . . . , θNT) if the phase θt (t=1, . . . , NT) of the transmitting antenna 104-t is determined as follows:





  
    
      
        
          θ
          t
        
        =
        
          
            ∠
            ⁡
            
              (
              
                
                  ∑
                  
                    k
                    =
                    
                      t
                      +
                      1
                    
                  
                  
                    N
                    T
                  
                
                ⁢
                
                  
                
                ⁢
                
                  
                    e
                    
                      jθ
                      k
                    
                  
                  ⁡
                  
                    (
                    
                      
                        h
                        t
                        *
                      
                      ⁢
                      
                        h
                        k
                      
                    
                    )
                  
                
              
              )
            
          
          =
          
            
              ∠
              ⁡
              
                (
                
                  
                    h
                    t
                    *
                  
                  ⁢
                  
                    
                      ∑
                      
                        k
                        =
                        
                          t
                          +
                          1
                        
                      
                      
                        N
                        T
                      
                    
                    ⁢
                    
                      
                    
                    ⁢
                    
                      
                        e
                        
                          jθ
                          k
                        
                      
                      ⁢
                      
                        h
                        k
                      
                    
                  
                
                )
              
            
            .
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          31
          )
        
      
    
  




For example, the phase θNT of the transmitting antenna 104-NT may be determined by assigning a phase value, e.g., zero, to the phase θNT. Based on Equation (31), the phase θNT−1 of the transmitting antenna 104-NT−1 may be determined. The determined phases θNT−1 of the transmitting antenna 104-NT−1 and θNT of the transmitting antenna 104-NT may be further used to determine the phase θNT−2 of the transmitting antenna 104-NT−2, again, based on Equation (31). Similarly, based on Equation (31), the phase θt (t=1, . . . , NT−3) of the transmitting antenna 104-t may be determined by the phases θt+1, θt+2, . . . , θNT of the transmitting antennas 104-t+1, 104-t+2, . . . , 104-NT. In other words, the transmitting antenna 104-t (t=1, . . . , NT−1) may be approximately co-phased with the transmitting antenna(s) 104-t+1, 104-t+2, . . . , 104-NT.
FIG. 4 illustrates a flowchart of an ordering-based cumulative co-phasing method, according to some embodiments of the present invention. In step 402 of FIG. 4, first and second ones (e.g., 104-t1 and 104-t2) of the transmitting antennas 104-1, 104-2, . . . , 104-NT are selected such that the magnitude of the value h*thk (t=1, . . . , NT−1; k=t+1, . . . , NT) may be maximized when t=t2 and k=t1. For example, for each combination of any two of the transmitting antennas 104-t and 104-k (t=1, . . . , NT−1; k=t+1, . . . , NT), the value h*thk may be calculated. The transmitting antennas 104-t1 and 104-t2 corresponding to the value h*t2ht1 that has a largest magnitude among the values h*thk (t=1, . . . , NT−1; k=t+1, . . . , NT) are selected.
Based on the value h*t2ht1, in step 404 the phases θt1 and θt2 of the selected transmitting antennas 104-t1 and 104-t2 are determined. For example, the phase θt1 of the transmitting antennas 104-t1 may be determined by assigning a phase value zero to θt1. The phase θt2 of the transmitting antennas 104-t2 may then be determined based on the determined phase θt1 of the transmitting antennas 104-t1, the value h*t2ht1, and Equation (31).
Next, in step 406 a judgment is made whether the phases θ1, θ2, . . . , θNT of all of the transmitting antennas 104-1, 104-2, . . . , 104-NT have been determined. If the phases θ1, θ2, . . . , θNT of all of the transmitting antennas 104-1, 104-2, . . . , 104-NT have not been determined, a next transmitting antenna 104-tn (n−1 is the number of the transmitting antennas whose phases have been determined) is selected from ones of the transmitting antennas 104-1, 104-2, . . . , 104-NT whose phases have not been determined in step 408. That selection is made such that the magnitude of the value




  
    h
    t
    *
  
  ⁡
  
    (
    
      
        ∑
        
          k
          =
          1
        
        
          n
          -
          1
        
      
      ⁢
      
        
      
      ⁢
      
        
          e
          
            jθ
            
              t
              k
            
          
        
        ⁢
        
          h
          
            t
            k
          
        
      
    
    )
  




may be maximized when t=tn, where t is the index of one of the transmitting antennas 104-1, 104-2, . . . , 104-NT whose phases have not been determined, and tk (k=1, 2, . . . , n−1) is the indexes of all of the transmitting antennas 104-1, 104-2, . . . , 104-NT whose phases have been determined. In step 410, the phase of the selected next transmitting antenna 104-tn is determined. For example, for each of the ones of the transmitting antennas 104-1, 104-2, . . . , 104-NT whose phases have not been determined, the value





  
    h
    t
    *
  
  ⁡
  
    (
    
      
        ∑
        
          k
          =
          1
        
        
          n
          -
          1
        
      
      ⁢
      
        
      
      ⁢
      
        
          e
          
            jθ
            
              t
              k
            
          
        
        ⁢
        
          h
          
            t
            k
          
        
      
    
    )
  




may be calculated. The transmitting antenna 104-tn corresponding to the value





  
    h
    
      t
      n
    
    *
  
  ⁡
  
    (
    
      
        ∑
        
          k
          =
          1
        
        
          n
          -
          1
        
      
      ⁢
      
        
      
      ⁢
      
        
          e
          
            jθ
            
              t
              k
            
          
        
        ⁢
        
          h
          
            t
            k
          
        
      
    
    )
  




that has a largest magnitude among the values





  
    h
    t
    *
  
  ⁡
  
    (
    
      
        ∑
        
          k
          =
          1
        
        
          n
          -
          1
        
      
      ⁢
      
        
      
      ⁢
      
        
          e
          
            jθ
            
              t
              k
            
          
        
        ⁢
        
          h
          
            t
            k
          
        
      
    
    )
  




is selected. The phase θtn of the selected transmitting antennas 104-tn is substantively equal to the phase of the value





  
    
      h
      
        t
        n
      
      *
    
    ⁡
    
      (
      
        
          ∑
          
            k
            =
            1
          
          
            n
            -
            1
          
        
        ⁢
        
          
        
        ⁢
        
          
            e
            
              jθ
              
                t
                k
              
            
          
          ⁢
          
            h
            
              t
              k
            
          
        
      
      )
    
  
  ,




as shown in Equation (31). Steps 406 to 410 are repeated until the phases of all of the transmitting antennas 104-1, 104-2, . . . , 104-NT have been determined.

D. Iterative Co-Phasing
In some embodiments of the present invention, an iterative co-phasing method may be used to determine the phase θ1, θ2, . . . , θNT of each of the transmitting antennas 104-1, 104-2, . . . , 104-NT. Referring back to Equation (15), Equation (15) can also be written as follows:




  
    
      
        
          f
          ⁡
          
            (
            
              
                θ
                1
              
              ,
              
                θ
                2
              
              ,
              ...
              ⁢
              
                
              
              ,
              
                θ
                
                  N
                  T
                
              
            
            )
          
        
        =
        
          
            R
            ⁢
            
              {
              
                
                  e
                  
                    -
                    
                      jθ
                      1
                    
                  
                
                ⁢
                
                  
                    ∑
                    
                      k
                      =
                      2
                    
                    
                      N
                      T
                    
                  
                  ⁢
                  
                    
                  
                  ⁢
                  
                    
                      e
                      
                        jθ
                        k
                      
                    
                    ⁡
                    
                      (
                      
                        
                          h
                          1
                          *
                        
                        ⁢
                        
                          h
                          k
                        
                      
                      )
                    
                  
                
              
              }
            
          
          +
          
            
              ∑
              
                t
                =
                2
              
              
                N
                T
              
            
            ⁢
            
              
            
            ⁢
            
              R
              ⁢
              
                
                  {
                  
                    
                      e
                      
                        -
                        
                          jθ
                          t
                        
                      
                    
                    ⁢
                    
                      
                        ∑
                        
                          k
                          =
                          
                            t
                            +
                            1
                          
                        
                        
                          N
                          T
                        
                      
                      ⁢
                      
                        
                      
                      ⁢
                      
                        
                          e
                          
                            jθ
                            k
                          
                        
                        ⁡
                        
                          (
                          
                            
                              h
                              t
                              *
                            
                            ⁢
                            
                              h
                              k
                            
                          
                          )
                        
                      
                    
                  
                  }
                
                .
              
            
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          32
          )
        
      
    
  




The second term





  
    ∑
    
      t
      =
      2
    
    
      N
      T
    
  
  ⁢
  
    
  
  ⁢
  
    R
    ⁢
    
      {
      
        
          e
          
            -
            
              jθ
              t
            
          
        
        ⁢
        
          
            ∑
            
              k
              =
              
                t
                +
                1
              
            
            
              N
              T
            
          
          ⁢
          
            
          
          ⁢
          
            
              e
              
                jθ
                k
              
            
            ⁢
            
              (
              
                
                  h
                  t
                  *
                
                ⁢
                
                  h
                  k
                
              
              )
            
          
        
      
      }
    
  




in Equation (32) does not depend on θ1. In general, Equation (15) can further be written as follows:





  
    
      
        
          
            f
            ⁢
            
              (
              
                
                  θ
                  1
                
                ,
                
                  θ
                  2
                
                ,
                ...
                ⁢
                
                  
                
                ,
                
                  θ
                  
                    N
                    T
                  
                
              
              )
            
          
          =
          
            
              R
              ⁢
              
                {
                
                  
                    e
                    
                      -
                      
                        jθ
                        t
                      
                    
                  
                  ⁢
                  
                    
                      ∑
                      
                        
                          k
                          =
                          1
                        
                        ,
                        
                          k
                          ≠
                          t
                        
                      
                      
                        N
                        T
                      
                    
                    ⁢
                    
                      
                    
                    ⁢
                    
                      
                        e
                        
                          jθ
                          k
                        
                      
                      ⁢
                      
                        h
                        t
                        *
                      
                      ⁢
                      
                        h
                        k
                      
                    
                  
                
                }
              
            
            +
            
              c
              t
            
          
        
        ,
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          33
          )
        
      
    
  




where ct does not depend on θt. As noted above, the SNR of the obtained signal r may be improved by determining the phases θ1, θ2, . . . , θNT such that the function f(θ1, θ2, . . . , θNT) may be maximized. Here, for given NT−1 of the NT phases θ1, θ2, . . . , θNT excluding the phase θt, it is possible to maximize the function f(θ1, θ2, . . . , θNT) by determining the phase θt such that the first term





  R
  ⁢
  
    {
    
      
        e
        
          -
          
            jθ
            t
          
        
      
      ⁢
      
        
          ∑
          
            
              k
              =
              1
            
            ,
            
              k
              ≠
              t
            
          
          
            N
            T
          
        
        ⁢
        
          
        
        ⁢
        
          
            e
            
              jθ
              k
            
          
          ⁢
          
            h
            t
            *
          
          ⁢
          
            h
            k
          
        
      
    
    }
  




on the right side of Equation (33) may be maximized. Accordingly, the phase θt may be determined as follows:





  
    
      
        
          θ
          t
        
        =
        
          
            ∠
            ⁡
            
              (
              
                
                  ∑
                  
                    
                      k
                      =
                      1
                    
                    ,
                    
                      k
                      ≠
                      t
                    
                  
                  
                    N
                    T
                  
                
                ⁢
                
                  
                
                ⁢
                
                  
                    e
                    
                      jθ
                      k
                    
                  
                  ⁢
                  
                    h
                    t
                    *
                  
                  ⁢
                  
                    h
                    k
                  
                
              
              )
            
          
          .
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        
          (
          34
          )
        
      
    
  




In some embodiments of the present invention, in step 502 of FIG. 5, the phases θ1, θ2, . . . , θNT each are assigned an initial value, e.g., zero. Therefore each of the phases θ1, θ2, . . . , θNT has a current value zero. Then starting from the phase θ1, in step 504 the phase θt (t=1, . . . , NT) may be sequentially calculated based on current values of the phases θ1, θ2, . . . , θNT excluding θt, using Equation (34). Each time the phase θt (t=1, . . . , NT) is calculated using Equation (34), the current value of the phase θt is updated, which constitutes one iteration. In step 506, a judgment is then made whether a predetermined exit condition has been satisfied. For example, the predetermined exit condition may be whether the calculated phase θt (t=1, . . . , NT) has converged. Also, for example, the predetermined exit condition may be whether steps 504 and 506 have been repeated for a prescribed number of iterations. Steps 504 and 506 are repeated until the predetermined exit condition has been satisfied. When the predetermined exit condition has been satisfied, the phases θ1, θ2, . . . , θNT of the transmitting antennas 104-1, 104-2, . . . , 104-NT have been determined.
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed here. This application is intended to cover any variations, uses, or adaptations of the invention following the general principles thereof and including such departures from the present disclosure as come within known or customary practice in the art. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
It will be appreciated that the present invention is not limited to the exact construction that has been described above and illustrated in the accompanying drawings, and that various modifications and changes can be made without departing from the scope thereof. It is intended that the scope of the invention only be limited by the appended claims.

## Claims

1. A method for determining a phase of each of a plurality of transmitting antennas in a multiple input and multiple output (MIMO) communication system, comprising:
calculating, for first and second ones of the plurality of transmitting antennas, a value based on first and second groups of channel gains, the first group including channel gains between the first transmitting antenna and each of a plurality of receiving antennas, the second group including channel gains between the second transmitting antenna and each of the plurality of receiving antennas; and
determining the phase of each of the plurality of transmitting antennas based on at least the value, wherein the phase of each of the plurality of transmitting antennas is determined independently from a transmitting power of a respective one of the plurality of transmitting antennas.

2. The method of claim 1, wherein calculating the value comprises:
calculating a plurality of values by multiplying conjugates of channel gains between the first transmitting antenna and each of the plurality of receiving antennas with channel gains between the second transmitting antenna and each of the plurality of receiving antennas, respectively; and
adding the plurality of calculated values to generate the value for the first and second transmitting antennas.

3. The method of claim 1, wherein determining the phase of each of the transmitting antennas comprises determining a phase difference between the first and second transmitting antennas based on a phase of the calculated value.

4. The method of claim 3, wherein determining the phase of each of the transmitting antennas comprises assigning a phase value to the phase of one of the transmitting antennas.

5. The method of claim 1, wherein calculating the value comprises calculating, for each combination of the first transmitting antenna and any one of remaining ones of the transmitting antennas, a value based on the respective first and second groups of channel gains.

6. The method of claim 5, wherein determining the phase of each of the transmitting antennas comprises determining a phase difference between the first transmitting antenna and one of the remaining transmitting antennas, based on a phase of the value calculated for the first transmitting antenna and the one of the remaining transmitting antennas.

7. The method of claim 6, wherein determining the phase of each of the transmitting antennas comprises assigning a phase value to the phase of one of the transmitting antennas.

8. The method of claim 1, wherein determining the phase of each of the transmitting antennas comprising:
determining the phases of the first and second transmitting antennas based on the calculated value;
judging whether the phases of all the transmitting antennas have been determined;
selecting one of the transmitting antennas if the phases of all the transmitting antennas have not been determined, the phase of the selected transmitting antenna having not been determined; and
determining the phase of the selected transmitting antenna based on channel gains between the selected transmitting antenna and each of the receiving antennas.

9. The method of claim 1, wherein calculating the value comprises calculating, for each combination of any two of the transmitting antennas, a value based on the respective first and second groups of channel gains, to generate a plurality of calculated values each corresponding to a combination of two of the transmitting antennas.

10. The method of claim 9, wherein determining the phase of each of the transmitting antennas comprises using a weighted least squares method to calculate the phase of each of the transmitting antennas.

11. The method of claim 10, wherein determining the phase of each of the transmitting antennas comprises assigning a phase value to the phase of one of the transmitting antennas.

12. The method of claim 10, wherein determining the phase of each of the transmitting antennas comprises determining a weight associated with the value calculated for each combination of any two of the transmitting antennas.

13. The method of claim 12, wherein the weight associated with the value calculated for a first combination of two of the transmitting antennas is set equal to the weight associated with the value calculated for a second combination of two of the transmitting antennas.

14. The method of claim 12, wherein the weight associated with the value calculated for a combination of two of the transmitting antennas is one of a magnitude, a squared magnitude, and an exponential function of the squared magnitude of the value calculated for the combination of the two transmitting antennas.

15. The method of claim 9, wherein determining the phase of each of the transmitting antennas comprises:
sorting the generated values based on magnitudes of the generated values, such that a first one of the sorted values has the largest magnitude and a last one of the sorted values has the smallest magnitude;
selecting the first one of the sorted values;
determining, based on the selected first value, the phases of the two transmitting antennas corresponding to the selected first value;
judging whether the phases of all of the transmitting antennas have been determined;
selecting a next one of the sorted values if the phases of all the transmitting antennas have not been determined;
determining, based on the selected next value, the phase of a first one of the two transmitting antennas corresponding to the selected next value, if the phase of the first one of the two transmitting antennas has not been determined; and
determining, based on the selected next value, the phase of a second one of the two transmitting antennas corresponding to the selected next value, if the phase of the second one of the two transmitting antennas has not been determined.

16. The method of claim 9, wherein determining the phase of each of the transmitting antennas comprises:
selecting first and second ones of the transmitting antennas, the selected first and second transmitting antennas corresponding to one of the generated values that has a largest magnitude;
determining the phases of the selected first and second transmitting antennas based on the one of the generated values;
judging whether the phases of all the transmitting antennas have been determined;
selecting a next one of the transmitting antennas if the phases of all the transmitting antennas have not been determined, the phase of the selected next transmitting antenna having not been determined; and
determining the phase of the selected next transmitting antenna based on channel gains between the selected next transmitting antenna and each of the receiving antennas.

17. The method of claim 16, wherein selecting the next one of the transmitting antennas comprises:
calculating, for each one of the transmitting antennas whose phases have not been determined, a value based on channel gains between the one of the transmitting antennas and each of the receiving antennas; and
selecting the next one of the transmitting antennas, the selected next transmitting antenna corresponding to the calculated value having a largest magnitude.

18. The method of claim 1, wherein determining the phase of each of the transmitting antennas comprises:
assigning a current value to the phase of each of the transmitting antennas;
sequentially selecting one of the transmitting antennas and updating the current value of the phase of the selected transmitting antenna based on the current values of the phases of the transmitting antennas excluding the one being selected;
judging whether a predetermined condition has been satisfied; and
repeating the selecting and updating if the predetermined condition has not been satisfied.

19. The method of claim 18, wherein updating the current value comprises:
calculating, for each one of the transmitting antennas excluding the one being selected, a value based on channel gains between the one of the transmitting antennas and each of the receiving antennas;
adding the values calculated for the transmitting antennas excluding the one being selected to generate a result; and
updating with the result the current value of the phase of the transmitting antenna being selected.

20. A method for determining an adjustment phase for each of a plurality of data signals to be respectively transmitted by a plurality of transmitting antennas in a multiple input and multiple output (MIMO) communication system, comprising:
calculating, for first and second ones of the plurality of transmitting antennas, a value based on first and second groups of channel gains, the first group including channel gains between the first transmitting antenna and each of a plurality of receiving antennas, the second group including channel gains between the second transmitting antenna and each of the plurality of receiving antennas; and
determining the adjustment phase for each of the plurality of data signals to be transmitted based on at least the value, wherein the adjustment phase for each of the plurality of data signals is determined independently from a transmitting power of a respective one of the plurality of transmitting antennas.

21. The method of claim 20, wherein calculating the value comprises:
calculating a plurality of values by multiplying conjugates of channel gains between the first transmitting antenna and each of the plurality of receiving antennas with channel gains between the second transmitting antenna and each of the plurality of receiving antennas, respectively; and
adding the plurality of calculated values to generate the value for the first and second transmitting antennas.

22. The method of claim 20, wherein determining the adjustment phase for each of the data signals comprises determining a difference between the adjustment phases for first and second ones of the data signals respectively to be transmitted by the first and second transmitting antennas, based on a phase of the calculated value.

23. The method of claim 22, wherein determining the adjustment phase for each of the data signals comprises assigning an adjustment phase value to the adjustment phase for one of the data signals.

24. The method of claim 20, wherein calculating the value comprises calculating, for each combination of the first transmitting antenna and any one of remaining ones of the transmitting antennas, a value based on the respective first and second groups of channel gains.

25. The method of claim 24, wherein determining the adjustment phase for each of the data signals comprises determining a difference between the adjustment phases for first and second ones of the data signals to be respectively transmitted by the first transmitting antenna and one of the remaining transmitting antennas, based on a phase of the value calculated for the first transmitting antenna and the one of the remaining transmitting antennas.

26. The method of claim 25, wherein determining the adjustment phase for each of the data signals comprises assigning an adjustment phase value to the adjustment phase of one of the data signals.

27. The method of claim 20, wherein determining the adjustment phase for each of the data signals comprising:
determining the adjustment phases for first and second ones of the data signals to be respectively transmitted by the first and second transmitting antennas based on the calculated value;
judging whether the adjustment phases of all the data signals have been determined;
selecting one of the data signals if the adjustment phases of all the data signals have not been determined, the adjustment phase for the selected data signal having not been determined; and
determining the adjustment phase for the selected data signal based on channel gains between the transmitting antenna to transmit the selected data signal and each of the receiving antennas.

28. The method of claim 20, wherein calculating the value comprises calculating, for each combination of any two of the transmitting antennas, a value based on the respective first and second groups of channel gains, to generate a plurality of calculated values each corresponding to a combination of two of the transmitting antennas.

29. The method of claim 28, wherein determining the adjustment phase for each of the data signals comprises using a weighted least squares method to calculate the adjustment phase for each of the date signals.

30. The method of claim 29, wherein determining the adjustment phase for each of the data signals comprises assigning an adjustment phase value to the adjustment phase of one of the data signals.

31. The method of claim 29, wherein determining the adjustment phase for each of the data signals comprises determining a weight associated with the value calculated for each combination of any two of the transmitting antennas.

32. The method of claim 31, wherein the weight associated with the value calculated for a first combination of two of the transmitting antennas is set equal to the weight associated with the value calculated for a second combination of two of the transmitting antennas.

33. The method of claim 31, wherein the weight associated with the value calculated for a combination of two of the transmitting antennas is one of a magnitude, a squared magnitude, and an exponential function of the squared magnitude of the value calculated for the combination of the two transmitting antennas.

34. The method of claim 28, wherein determining the adjustment phase for each of the data signals comprises:
sorting the generated values based on magnitudes of the generated values, such that a first one of the sorted values has the largest magnitude and a last one of the sorted values has the smallest magnitude;
selecting the first one of the sorted values;
determining, based on the selected first value, the adjustment phases for two of the data signals to be respectively transmitted by the two transmitting antennas corresponding to the selected first value;
judging whether the adjustment phases for all the data signals have been determined;
selecting a next one of the sorted values if the adjustment phases for all the data signals have not been determined;
determining, based on the selected next value, the adjustment phase for one of the data signals to be transmitted by a first one of the two transmitting antennas corresponding to the selected next value, if that adjustment phase has not been determined; and
determining, based on the selected next value, the adjustment phase for one of the data signals to be transmitted by a second one of the two transmitting antennas corresponding to the selected next value, if that adjustment phase has not been determined.

35. The method of claim 28, wherein determining the adjustment phase for each of the data signals comprises:
selecting first and second ones of the data signals, the selected first and second data signals to be respectively transmitted by first and second ones of the transmitting antennas corresponding to one of the generated values that has a largest magnitude;
determining the adjustment phases for the first and second data signals based on the one of the generated values;
judging whether the adjustment phases for all the data signals have been determined;
selecting a next one of the data signals if the adjustment phases for all the data signals have not been determined, the adjustment phase for the selected next data signal having not been determined; and
determining the adjustment phase for the selected next data signal based on channel gains between the transmitting antenna to transmit the selected next data signal and each of the receiving antennas.

36. The method of claim 35, wherein selecting the next one of the data signals comprises:
calculating, for each one of the data signals whose adjustment phases have not been determined, a value based on channel gains between the transmitting antenna to transmit the one of the data signals and each of the receiving antennas; and
selecting the next one of the data signals, the selected next data signal corresponding to the calculated value having a largest magnitude.

37. The method of claim 28, wherein determining the adjustment phase for each of the data signals comprises:
assigning a current value to the adjustment phase for each of the data signals;
sequentially selecting one of the data signals and updating the current value of the adjustment phase for the selected data signal, based on current values of the adjustment phases for the data signals excluding the one being selected;
judging whether a predetermined condition has been satisfied; and
repeating the selecting and updating if the predetermined condition has not been satisfied.

38. The method of claim 37, wherein updating the current value comprises:
calculating, for each one of the data signals excluding the one being selected, a value based on channel gains between the transmitting antenna to transmit the one of the data signals and each of the receiving antennas;
adding the values calculated for the data signals excluding the one being selected to generate a result; and
updating with the result the current value of the adjustment phase for the data signal being selected.

39. An apparatus for determining a phase of each of a plurality of transmitting antennas in a multiple input and multiple output (MIMO) communication system, comprising:
means for calculating, for first and second ones of the plurality of transmitting antennas, a value based on first and second groups of channel gains, the first group including channel gains between the first transmitting antenna and each of a plurality of receiving antennas, the second group including channel gains between the second transmitting antenna and each of the plurality of receiving antennas; and
means for determining the phase of each of the plurality of transmitting antennas based on at least the value, wherein the phase of each of the plurality of transmitting antennas is determined independently from a transmitting power of a respective one of the plurality of transmitting antennas.

40. A communication system comprising:
a plurality of transmitting antennas, each of the transmitting antennas being configured to transmit a data signal, and to adjust a phase of the data signal by an adjustment phase; and
a plurality of receiving antennas each configured to receive data signals respectively transmitted by the transmitting antennas; wherein
the adjustment phase of each of the transmitting antennas is determined based on at least a value calculated for first and second ones of the plurality of transmitting antennas, the value being calculated based on first and second groups of channel gains, the first group including channel gains between the first transmitting antenna and each of the plurality of receiving antennas, the second group including channel gains between the second transmitting antenna and each of the plurality of receiving antennas, and
the adjustment phase of each of the transmitting antennas is determined independently from a transmitting power of respective one of the transmitting antennas.