Abstract:
A method includes broadcasting, at a transmitter, messages comprising antenna configuration, antenna spacing and a number of antenna of the transmitter and reference signals; generating, at a receiver, a codebook comprising a plurality of antenna beams based on the broadcasted messages; receiving, at the receiver, the broadcasted reference signals; selecting, at the receiver, an antenna beam among the plurality of antenna beams within the codebook in dependence upon a predetermined performance criteria of a data communication system and in dependence upon the broadcasted reference signals; feedbacking to the transmitter, at the receiver, information comprising the antenna beam selected by the receiver; optimizing, at the transmitter, a beamforming process by utilizing the feedback information from the receiver; transmitting, at the transmitter, data signals by utilizing the optimized beamforming process; and receiving and processing, at the receiver, the data signals in dependence upon the selected antenna beams within the codebook.

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from applications earlier filed in the U.S. Patent &amp; Trademark Office on 9 Aug. 2007 and there duly assigned Ser. No. 60/935,384, and on 10 Aug. 2007 and there duly assigned Ser. No. 60/935,416, respectively. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a single-user closed-loop transmit beamforming (SU-CLTB) scheme of a Multiple Input Multiple Output (MIMO) system, and more particularly, to a single-user closed-loop transmit beamforming (SU-CLTB) scheme of Multiple Input Multiple Output (MIMO) system with the transmit beamforming scheme employing a codebook. 
     2. Description of the Related Art 
     OFDM (Orthogonal Frequency Division Multiplexing) is a technology of multiplexing data in a frequency domain. Modulation symbols are carried by multiple frequency sub-carriers. The total bandwidth in an OFDM system is divided into narrowband frequency units called subcarriers. The number of subcarriers is equal to the FFT/IFFT size N used in the system. In general, the number of subcarriers used for data is less than N because some subcarriers at the edge of the frequency spectrum are reserved as guard subcarriers. In general, no information is transmitted on guard subcarriers. The advantage of OFDM (Orthogonal Frequency Division Multiplexing) over other transmission schemes is the robustness to multipath fading. The multipath fading in time domain translates into frequency selective fading in frequency domain. With the cyclic prefix or zero prefix added, the inter-symbol-interference between adjacent OFDM symbols are avoided or largely alleviated. Moreover, because each modulation symbol is carried over a narrow bandwith, the modulation symbol experiences a single path fading. Simple equalization scheme may be applied to combat frequency selection fading. 
     Multiple Input Multiple Output (MIMO) schemes use multiple transmitting antennas and multiple receiving antennas to improve the capacity and reliability of a wireless communication channel. A MIMO system capacity increases a function of K where K is the minimum of number of transmit antennas (M) at transmitter and receive antennas (N) at receiver, i.e. K=min(M,N). The transmitted signals are received at the four receive antennas. Spatial signal processing is performed on the received signals in order to recover the four data streams. An example of spatial signal processing is V-BLAST which uses the successive interference cancellation principle to recover the transmitted data streams. Other variants of MIMO schemes include schemes that perform some kind of space-time coding across the transmit antennas (e.g. D-BLAST) and also beamforming schemes such as SDMA (Spatial Division multiple Access). In addition, MIMO may be implemented with transmit/receive diversity scheme and transmit/receive beamforming scheme in order to improve the link reliability or system capacity in wireless communication systems. The MIMO channel estimation consists of estimating the channel gain and phase information for links from each of the transmit antennas to each of the receive antennas. Therefore, the channel for M×N MIMO system consists of an N×M matrix: 
             H   =     [           a   11           a   12         …         a     1   ⁢   M                 a   21           a   22         …         a     2   ⁢   M               ⋮       ⋮       …       ⋮             a     N   ⁢           ⁢   1             a     M   ⁢           ⁢   2           …         a   NM           ]           
where matrix H is the MIMO channel matrix and a ij  represents the channel gain from transmit antenna j to receive antenna i. In order to enable the estimations of the elements of the MIMO channel matrix, separate pilots are transmitted from each of the transmit antennas.
 
     A contemporary transmit beamforming in wireless systems may be done with either closed-loop or open-loop manners. Beamforming is a technique of signal processing performed with transmitters arrays or receivers arrays and may control the transceiving direction and sensitivity of a transceived signal. During transmitting a signal, transmit beamforming may increase a power in the direction along which the signal is to be transmitted. A transmission gain may be achieved by the transmit beamforming process comparing to an omnidirectional transmission. 
     Open-loop system is typically well suited for TDD (Time Division Duplexing) system. Open-loop system does not require any feedback of channel information. Therefore, less overhead is introduced in open-loop system. The disadvantage of open-loop system however is that an open-loop system needs to constantly conduct phase calibration in order to compensate the phase difference between transmission and reception RF (radio frequency) chains among multiple transmit antennas. Another disadvantage of the open-loop system is that the open-loop system requires a constant uplink phase reference such as an uplink pilot, this requirement may induce an excessive feedback overhead. The process of phase calibration is generally costly, and sensitive to radio channel environment. 
     Closed-loop, on the other hand, does not require phase calibration process. The closed-loop system however requires the channel feedback to the transmitters. Therefore, overhead is significantly increased in closed-loop system comparing with the open-loop system. Additionally, the closed-loop system may be sensitive to the feedback channel error due to either feedback delay or fast channel variation. Typically, FDD (Frequency Division Duplexing) employs closed-loop transmit beamforming scheme. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the present invention to provide an improved transmit beamforming scheme for wireless communication system to overcome the above stated disadvantages. 
     It is another object of the present invention to provide a single-user closed-loop transmit beamforming (SU-CLTB) scheme in MIMO system, with the transmit beamforming scheme employing a codebook. The codebook includes of a set of predetermined antenna beams known to mobile stations. And the set of predetermined antennas beams is formed based on the antenna array response vectors of a serving base station. 
     It is an embodiment of the present invention that OFDM (Orthogonal Frequency Division Multiplexing) radio signals are employed in the communication between a base station and a mobile station. 
     It is another embodiment of the present invention that the antenna array response vector is cell-specific, and may be carried through the broadcasting channel (BCH) in a real cellular system. The mobile station may select the best antenna beam within the codebook and feed back the best antenna beam to the mobile station&#39;s serving base station in order to improve the throughput of the system. 
     It is still another embodiment that the best antenna beam information is selected from the set of predetermined antenna beams in the codebook based on certain performance criteria such as maximizing Signal-to-Noise ratio (SNR). 
     It is still another embodiment that the performance enhancement of the proposed SU-CLTB for wireless systems may be achieved by two methods. One method is boosting the energy of the transceived signals according to the beamforming gain which results in SNR gain. The other method is reducing a radiated energy distributed to other base stations according to a narrower radiation beam pattern, which results in SIR (signal-to-interference) gain in a wireless system. The overall system enhancement of CLTB is the combination of SNR and SIR gains, which depend on the operating load of systems. 
     It is still another embodiment that a transmitter of the proposed codebook-based SU-CLTB system at a base station includes a transmitter processing stage and a transmit beamforming stage. The transmitter includes a CRC (cyclic redundancy check) inserter inserting a CRC to an single information block, a turbo coder or LDPC (low density parity check) coder, channel interleaver, modulator, a transmit beamforming (TB) generating the codebook, and a contemporary OFDM transmission stage. 
     It is still another embodiment that the codebook is adaptable in the sense that codebook design is generated based the antenna configuration, antenna spacing, and the number of antenna of its serving base station. The proposed codebook is a set of transmit beamforming vectors, C j  with {j=1,2, . . . J}, which are used to form a set of predetermined antenna beams. J is the size of codebook. A mobile station may then select the best antenna beam and feedback the best antenna beam to the serving base station in the cell in order to improve system throughput. 
     It is still another embodiment that, for a uniform linear array (ULA) configuration, beamforming codebook is given by: 
               c   j     =       [             w   1     ⁡     (     θ   j     )                   w   2     ⁡     (     θ   j     )               ⋮               w   p     ⁡     (     θ   j     )               ⋮               w   p     ⁡     (     θ   j     )             ]     =     [         1             ⅇ       -   j2π     ⁢     D   λ     ⁢   si   ⁢           ⁢     n   (     θ   j     )                 ⋮             ⅇ       -   j2π     ⁢         (     p   -   1     )     ⁢   D     λ     ⁢   s   ⁢           ⁢   i   ⁢           ⁢     n   (     θ   j     )                 ⋮             ⅇ       -   j2π     ⁢         (     p   -   1     )     ⁢   D     λ     ⁢   si   ⁢           ⁢     n   (     θ   j     )               ]             
where j=1, . . . , J, J is size of the codebook, p is index of transmitting antennas, i.e., p=1, . . . , P, P is number of transmitting antennas, D is the space between the transmitting antennas, λ=c/f_c is wavelength of a carrier where c is speed of light and f_c is frequency of the carrier, θ j  is an main angle of a direction of departure of a j&#39;th transmit antenna beam. The set of θ j  s, where j=1, . . . , J, is specified and known to the serving base station and all of the mobile stations within the cell.
 
     It is still another embodiment that the set of θ j  s , j=1, . . . , J, is a set where all antenna beams have uniform angular spacing. In particular, in a three-sector system where each sector has 120 degrees angular spacing, the set θ j  s , j=1, . . . , J, is given by 
                 θ   j     =       (     j   +     1   2       )     ×     120   J     ⁢           ⁢     (   degrees   )         ,         
when the reference angle, i.e, zero-degree corresponds to the section edge; or
 
                 θ   j     =         (     j   +     1   2       )     ×     120   J       -     60   ⁢           ⁢     (   degrees   )           ,         
when the reference angle, i.e., zero-degree corresponds to the center of the sector.
 
     It is still another embodiment that, for a uniform linear array (ULA), the set of θ j  s, j=1, . . . , J, is a set where the antenna beams do not have uniform equal angular spacing. 
     It is still another embodiment that, for a uniform circular array (UCA), the beamforming codebook is given by: 
               c   j     =       [             w   1     ⁡     (     θ   j     )                   w   2     ⁡     (     θ   j     )               ⋮               w   p     ⁡     (     θ   j     )               ⋮               w   p     ⁡     (     θ   j     )             ]     =     [           ⅇ       -   j2π     ⁢     R   λ     ⁢   si   ⁢           ⁢     n   ⁡     (   ζ   )       ⁢   co   ⁢           ⁢     s   ⁡     (       θ   j     -     φ   1       )                     ⅇ       -   j2π     ⁢     R   λ     ⁢   si   ⁢           ⁢     n   ⁡     (   ζ   )       ⁢   co   ⁢           ⁢     s   ⁡     (       θ   j     -     φ   2       )                   ⋮             ⅇ       -   j2π     ⁢     R   λ     ⁢   si   ⁢           ⁢     n   ⁡     (   ζ   )       ⁢   co   ⁢           ⁢     s   ⁡     (       θ   j     -     φ   p       )                   ⋮             ⅇ       -   j2π     ⁢     R   λ     ⁢   si   ⁢           ⁢     n   ⁡     (   ζ   )       ⁢   co   ⁢           ⁢     s   ⁡     (       θ   j     -     φ   p       )                 ]             
for j=1, . . . , J. Here R is the circular radius of the antenna array, ζ is the elevation angle, θ j  is the main angle of the direction of departure of the j&#39;th transmit antenna beam at a base station. For simplicity, only azimuth angles are considered in the propagation geometry (i.e., ζ=90 degrees) but the results may be generalized to three dimensions.
 
     It is still another embodiment that, the set of θ j  s, j=1, . . . , J, is a set where all of the antenna beams have a uniform angular spacing. In particular, in a three-sector system where each sector has 120 degrees angular spacing, the set θ j  s, j=1, . . . , J, is given by 
                 θ   j     =       (     j   +     1   2       )     ×     120   J     ⁢           ⁢     (   degrees   )         ,         
when a reference angle, i.e., zero-degree direction, corresponds to the edge of a sector; or
 
                 θ   j     =         (     j   +     1   2       )     ×     120   J       -     60   ⁢           ⁢     (   degrees   )           ,         
when the reference angle, i.e., zero-degree direction, corresponds to the center of the sector. Here, three-sector system is a system having 120-degree angular spacing per sector, and the 120-degree angular spacing is angular coverage of a base station.
 
     It is still another embodiment that the uniform linear array may be deployed in sectorized cell, while the uniform circular array may be probably used in an omni-directional cell. 
     It is still another embodiment that the codebook employs a space between antennas equal to half of the wavelength. 
     It is still another embodiment that, for a uniform circular array (UCA), the set of θ j  s, j=1, . . . , J, is a set where the beams do not uniform equal angular spacing. 
     It is still another embodiment that, for a uniform linear array (ULA) of single polarization antenna, a first set of common reference signals are sequentially mapped to a set of antennas and the set of antennas are neighboring to each other (i.e., mapping type A); or the first set of common reference signals are mapped to discrete antennas with at least one antenna of a free state located in between except for the 1st antenna and the last antenna in the linear array (i.e., mapping type B). 
     It is still another embodiment that, for a uniform circular array (UCA) of single polarization antenna, the first set of common reference signals are mapped to a set of discrete antennas disposed either perpendicular to or overlapped with the diameter of the circle (i.e., mapping type A); the first set of common reference signals are mapped to a set of discrete antennas positioned with a 45 degrees angle against the diameter of the uniform circular array (i.e., mapping type B). 
     It is still another embodiment that, for a uniform linear array (ULA) of dual slat ±45degree polarization antenna, the first set of common reference signals are mapped to the antennas positioned with 45 degrees against a virtual vertical line in a clockwise direction (i.e., mapping type A); or the first set of common reference signals are mapped to the antennas positioned with 45 degrees against the virtual vertical line in a counter clockwise direction (i.e., mapping type B). 
     It is still another embodiment that, for a uniform linear array (ULA) of dual vertical/horizontal polarization antenna, the first set of common reference signals are mapped to the antennas positioned aligned with the virtual vertical line (i.e., mapping type A); or the first set of common reference signals are mapped to the antennas positioned perpendicularly to the virtual vertical line (i.e., mapping type B). 
     It is still another embodiment that, for a uniform circular array (UCA) of dual slat ±45 degree polarization, the first set of common reference signals are mapped to the antennas positioned with 45 degrees against the virtual vertical line in a clockwise direction (i.e., mapping type A); or the first set of common reference signals are mapped to the antennas positioned with 45 degrees against the virtual vertical line in a counter clockwise direction (i.e., mapping type B). 
     It is still another embodiment that, for a uniform circular array (UCA) of dual vertical/horizontal polarization, the first set of common reference signals are mapped to the antennas positioned aligned with the virtual vertical line (i.e., mapping type A); or the first set of common reference signals are mapped to the antennas positioned perpendicularly to the virtual vertical line (i.e., mapping type B). 
     It is still another embodiment that a second set of common reference signals sent at a much lower frequency comparing to the first set of common reference signals and are mapped to the transmitting antennas which are not associated with the first set of common reference signals. 
     It is still another embodiment that, for the uniform linear array (ULA) of single polarization antenna, the first set of common reference signals are sequentially mapped to a set of antennas and the set of antennas are neighboring to each other, and the second set of common reference signals are sequentially mapped to another set of antennas and the another antennas are neighboring to each other (i.e., mapping type A); or, the first set of common reference signals are mapped to discrete antennas with at least one antenna mapped to one of the second set of common reference signals in between except for the first antenna and last antenna in the line, and the second set of the common reference signals are mapped to discrete antennas with at least one antenna mapped with one of the first set of the common reference signals in between except for the first antenna and the last antenna in the linear array(i.e., mapping type B);. 
     It is still another embodiment that, for the UCA of single polarization antenna, the first set of the common reference signals are sequentially mapped to a set of neighboring antennas and the second set of the common reference signals are mapped to another set of neighboring antennas (i.e., mapping type A); or, one of the first set of the common reference signals and one of the second set of the common reference signals are alternately mapped to individual antenna arrange at the periphery of UCA (i.e., mapping type B). 
     It is still another embodiment that, for the uniform circular array (UCA) of dual vertical/horizontal polarization, the first set of the common reference signals are mapped to neighboring sets of antennas, and the second set of the common reference signals are mapped to other neighboring sets of antennas with each set of antennas having two antennas across each other (i.e., mapping type A); or, one of the first set of the common reference signals and one of the second set of the common reference signals are mapped to two antennas across each other respectively (i.e., mapping type B). 
     It is still another embodiment that, for the uniform linear array (ULA) of dual slat ±45 degree polarization, the first set of the common reference signals are mapped to neighboring sets of antennas, and the second set of the common reference signals are mapped to other neighboring sets of antennas (i.e., mapping type A); or, one of the first set of the common reference signals and one of the second set of the common reference signals are mapped to two antennas across each other respectively (i.e., mapping type B). 
     It is still another embodiment that the best antenna beam information is selected from the set of pre-determined antenna beam based on certain performance criteria maximum signal-to-noise ration (MSNR) or Minimum Mean Square Error (MMSE). In the case of noise-dominant environment, MSNR is used; and in the presence of interference dominant environment, MMSE is employed. 
     It is still another embodiment that, when MSNR is employed, the transmit beamforming vector of the best antenna beam for the k&#39;th subcarrier, W k , may be selected by, for mapping type A of mapping the common reference signals: 
               W   k     =     C     j   ,     m   ⁢           ⁢   ax                   where                 C     j   ,     m   ⁢           ⁢   ax         =     arg   ⁢               ⁢   max     j     ⁢     {       ∑   k     ⁢       ∑     m   =   1     M     ⁢       ∑     p   =   1       P   2       ⁢         w   p     ⁡     (     θ   j     )       ⁢       H   ^       p   ,   m   ,   k               }         ,         
where Ĥ p,m,k  denotes the channel estimate for transmit antenna p, receive antenna m in the k&#39;th subcarrier, and w p (θ j )is vectors of codeword of the codebook and is selected according to the configuration of transmitting antennas.
 
     It is still another embodiment that, when MSNR is employed, the transmit beamforming vector of the best antenna beam for the k&#39;th subcarrier, W k , may be selected by, for mapping type B of mapping the common reference signals: 
               W   k     =     C     j   ,     m   ⁢           ⁢   ax                   where                 C     j   ,     m   ⁢           ⁢   ax         =     arg   ⁢               ⁢   max     j     ⁢     {       ∑   k     ⁢       ∑     m   =   1     M     ⁢       ∑     p   =   1       P   2       ⁢         w       2   ⁢   p     -   1       ⁡     (     θ   j     )       ⁢       H   ^       p   ,   m   ,   k               }         ,         
where Ĥ p,m,k  denotes the channel estimate for transmit antenna p, receive antenna m in the k&#39;th subcarrier, and w p (θ j )is vectors of codeword of the codebook and is selected according to the configuration of transmitting antennas.
 
     It is still another embodiment that, the mobile station feedbacks only one choice of beamforming codeword of the codebook for the entire bandwidth. 
     It is still another embodiment that, signals of layers higher than a Physical layer are employed to transmit the feedback information of the choice of beamforming codeword of the codebook. 
     It is still another embodiment that, a difference between CQI (channel quality indication) calculated by dedicated signals and CQI calculated by common reference signals is reported to the base station by the mobile station. In addition, the rate of feedback to the base station for two types of C QI reporting is different. The CQI calculated by dedicated signals is typically faster than the CQI calculated by the common reference signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
         FIG. 1  is a diagram showing an example of communication system constructed according to the principles of the present invention; 
         FIG. 2  is a diagram showing an example of data transmission and reception by using OFDM (Orthogonal Frequency Division Multiplexing); 
         FIG. 3  is a two dimensional diagram illustrating an Orthogonal Frequency Division Multiplexing (OFDM) symbol in frequency domain; 
         FIG. 4   a  is a diagram showing a transmitted OFDM symbol in time domain; 
         FIG. 4   b  is a diagram showing a received OFDM symbols in time domain; 
         FIG. 5  is a diagram showing a simplified example of a 4×4 MIMO (multiple input multiple output) system; 
         FIG. 6  is a diagram showing an example of system level description for the codebook-based SU-CLTB constructed according to the principles of the present invention; 
         FIG. 7  is a diagram showing an example of transmitter processing of a single-codeword OFDM transmission scheme constructed according to the principles of the present invention; 
         FIG. 8  is an illustration showing a codebook-based transmit beamforming constructed according to the principles of the present invention; 
         FIG. 9  is a diagram showing common reference signals (RS) being distributed in a two dimensional space formed by OFDM symbol index and sub-carrier index; 
         FIG. 10  is an illustration showing an example of common reference signals (RS) mapping for an eight-antenna uniform linear array (ULA) with single antenna polarization constructed according to the principles of the present invention; 
         FIG. 11  is an illustration showing an example of common reference signals (RS) mapping for an eight-antenna uniform circular array (UCA) with single antenna polarization constructed according to the principles of the present invention; 
         FIG. 12  is an illustration showing an example of common reference signals (RS) mapping for an eight-antenna uniform linear array with dual slat ±45 degree polarization constructed according to the principles of the present invention; 
         FIG. 13  is an illustration showing an example of common reference signals (RS) mapping for an eight-antenna uniform linear array with dual vertical/horizontal polarization constructed according to the principles of the present invention; 
         FIG. 14  is an illustration showing an example of common reference signals (RS) mapping for an eight-antenna uniform circular array with dual slat ±45 degree polarization constructed according to the principles of the present invention; 
         FIG. 15  is an illustration showing an example of common reference signals (RS) mapping for an eight-antenna uniform circular array with dual vertical/horizontal polarization constructed according to the principles of the present invention; 
         FIG. 16  is an illustration showing an example of common reference signals (RS) mapping for an eight-antenna single polarization ULA systems with additional common reference signals constructed according to the principles of the present invention; 
         FIG. 17  is an illustration showing an example of common reference signals (RS) mapping for an eight-antenna single polarization UCA systems with additional common reference signals constructed according to the principles of the present invention; 
         FIG. 18  is an illustration showing an example of common reference signals (RS) mapping for an eight-antenna dual vertical/horizontal polarization ULA systems with additional common reference signals constructed according to the principles of the present invention; 
         FIG. 19  is an illustration showing an example of common reference signals (RS) mapping for an eight-antenna dual slat ±45 degree polarization ULA systems with additional common reference signals constructed according to the principles of the present invention; 
         FIG. 20  is a diagram showing an example of CQI difference reporting for the SU-CLTB scheme constructed according to the principles of the present invention; 
         FIG. 21  is a histogram showing beamforming gain of the SU-CLTB over the baseline system with uniform linear array antennas configuration constructed according to the principles of the present invention; 
         FIG. 22  is a histogram showing beamforming gain of the SU-CLTB over the baseline system with uniform circular array antennas configuration constructed according to the principles of the present invention; 
         FIG. 23  is a two dimensional graph showing a spectral efficiency (SE) improvement of the SU-CLTB over the baseline system under various feedback bandwidths constructed according to the principles of the present invention; 
         FIG. 24  is a two dimensional graph showing the spectral efficiency (SE) improvement of the SU-CLTB over the baseline system under various feedback rates constructed according to the principles of the present invention; 
         FIG. 25  is a two dimensional graph showing the spectral efficiency (SE) improvement of the SU-CLTB over the baseline system under various radio channel environments constructed according to the principles of the present invention; 
         FIG. 26  is a two dimensional graph showing the spectral efficiency (SE) improvement of the SU-CLTB over the baseline system under realistic channel estimation (CE) constructed according to the principles of the present invention; 
         FIG. 27  is a two dimensional graph showing a CDF (Cumulative Distribution Function) of capacity gain of the SU-CLTB over the baseline system with uniform linear array (ULA) constructed according to the principles of the present invention; 
         FIG. 28  is a histogram showing the SIR gain of the SU-CLTB over the baseline system with uniform circular array (UCA) constructed according to the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A contemporary transmit beamforming in wireless systems may be done with either closed-loop or open-loop manners. Beamforming is a technique of signal processing performed with arrays of transmitters or receivers and may control the transceiving direction and sensitivity of a transceived signal. During transmitting a signal, transmit beamforming may increase a power in the direction along which the signal is to be transmitted. A transmission gain may be achieved by the transmit beamforming process comparing to an omnidirectional transmission. 
     Open-loop system is typically well suited for TDD (Time Division Duplexing) system. Open-loop system does not require any feedback of channel information. Therefore, less overhead is introduced in open-loop system. The disadvantage of open-loop system however is that an open-loop system needs to constantly conduct phase calibration in order to compensate the phase difference between transmission and reception RF (radio frequency) chains among multiple transmit antennas. Another disadvantage of the open-loop system is that the open-loop system requires a constant uplink phase reference such as an uplink pilot, this requirement may induce an excessive feedback overhead. The process of phase calibration is generally costly, and sensitive to radio channel environment. 
     Closed-loop, on the other hand, does not require phase calibration process. The closed-loop system however requires the channel feedback to the transmitters. Therefore, overhead is significantly increased in closed-loop system comparing with the open-loop system. Additionally, the closed-loop system may be sensitive to the feedback channel error due to either feedback delay or fast channel variation. Typically, FDD (Frequency Division Duplexing) employs closed-loop transmit beamforming scheme. 
     Therefore, it is necessary to provide an improved wireless communication system to overcome the above stated disadvantages. 
       FIG. 1  is a diagram showing an example of communication system constructed according to the principles of the present invention. A base station  100  is communicated with multiple of mobile stations (i.e., mobile station # 1   101 , mobile station # 2   102 , mobile station # 3   103 ) in a wireless communication, and this wireless communication system is known as downlink communication. Base station  100  and mobile stations  101 ,  102  and  103  employ multiple antennas for both of transmission and reception of radio wave signals. The radio wave signal may be Orthogonal Frequency Division Multiplexing (OFDM) signals. The mobile stations may be PDAs, laptops, and/or other portable devices. 
     Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO) will be described in details as follows. 
     OFDM (Orthogonal Frequency Division Multiplexing) 
     OFDM is a technology of multiplexing data in a frequency domain. Modulation symbols are carried by frequency sub-carriers.  FIG. 2  shows a simpilifed example of data transmission and reception by using OFDM (Orthogonal Frequency Division Multiplexing). The data to be transmitted is modulated by a quadrature amplitude modulation (QAM) modulator  111 . The QAM modulated symbols are serial-to-parallel converted by a serial-to-parallel convertor  113  and input to an inverse fast Fourier transform (IFFT) unit  115 . The serial-to-parallel converted modulated symbols are precoded by a precoder encoder  114 . At the output of IFFT unit  115 , N time-domain samples are obtained. Here N refers to the sampling number of IFFT/FFT used by the OFDM system. The signal from IFFT unit  115  is parallel-to-serial converted by a parallel-to-serial convertor  117  and a cyclic prefix (CP)  119  is added to the signal sequence. The resulting sequence of samples is referred to as OFDM symbol. At the receiver, the cyclic prefix  121  is first removed and the signal is serial-to-parallel converted by parallel-to-serial convertor  123  before feeding the converted parallel signal into fast Fourier transform (FFT) transformer  125 . The precoded modulated symbols are recovered by a precoder decoder  126 . Output of precoder decoder  126  is parallel-to-serial converted by parallel-to-serial convertor  127  and the resulting output is input to the QAM demodulator  129 . At the output of IFFT  115 , N time-domain samples are obtained. Here N refers to the IFFT/FFT size used by the OFDM system. CP is added to each OFDM symbol to avoid or mitigate the impact due to multipath fading at stage  119 . The resulting sequence of samples is referred to as OFDM symbol. At the receiver side, assuming perfect time and frequency synchronization are achieved, the receiver first removes the CP and the signal is serial-to-parallel converted before feeding it into FFT  125 . The output of FFT  125  is parallel-to-serial converted and the resulting QAM modulation symbols are input to QAM demodulator  129 . 
     The total bandwidth in an OFDM system is divided into narrowband frequency units called subcarriers. The number of subcarriers is equal to the FFT/IFFT size N used in the system. In general, the number of subcarriers used for data is less than N because some subcarriers at the edge of the frequency spectrum are reserved as guard subcarriers. In general, no information is transmitted on guard subcarriers. 
     Because each OFDM symbol has finite duration in time domain, the sub-carriers overlap with each other in frequency domain. The orthogonality however is maintained at the sampling frequency assuming the transmitter and receiver has perfect frequency synchronization, as shown in  FIG. 3 .  FIG. 3  is an illustration of an Orthogonal Frequency Division Multiplexing (OFDM) symbol in frequency domain. Sub-carrier 0   10 , sub-carrier 1   11  and sub-carrier 2   12  overlap with each other in frequency domain. sub-carrier 0   10 , sub-carrier 1   11  and sub-carrier 2   12  have almost indentical or similar wave shapes. These three sub-carriers are mathematically perpendicular to each other, in other words, the inner products of any two of the sub-carriers are zero. In the case of frequency offset due to imperfect frequency synchronization or high mobility, the orthogonality of the sub-carriers at sampling frequencies is destroyed, resulting in inter-carrier-interference (ICI). 
     A time domain illustration of the transmitted and received OFDM symbol is shown in  FIG. 4   a  and  4   b .  FIG. 4   a  is an illustration of a transmitted OFDM symbol in time domain, and  FIG. 4   b  is an illustration of the received OFDM symbols in time domain. Transmit signal  20  has continously transmitted OFDM symbols (i.e. OFDM Symbol  1 , OFDM Symbol  2 , . . . ), and cylic prefix (CP) portions (i.e. CP 1  and CP 2 ) are located between any of two OFDM Symbols. After transmitted through multipath fading channel  122 , receive signal  27  has continously CP inserted OFDM symbols (i.e. Rx OFDM Symbol 1   28 , Rx OFDM Symbol 2   29 , . . . ). Rx OFDM Symbol 1   28  and Rx OFDM Symbol 2   29  are corrupted by their own CP, respectively. For example, CP 3  corrupts into Rx OFDM Symbol 1   28 . Because of multipath fading between the transmitter and receiver, the CP portion of the received signal is often corrupted by the previous OFDM symbol. As long as the CP is sufficiently long, the received OFDM symbol without CP should only contain its own signal convoluted by the multipath fading channel. In general, a Fast Fourier Transform (FFT) is taken at the receiver side to allow further processing frequency domain. The advantage of OFDM over other transmission schemes is the robustness to multipath fading. The multipath fading in time domain translates into frequency selective fading in frequency domain. With the cyclic prefix or zero prefix added, the inter-symbol-interference between adjacent OFDM symbols are avoided or largely alleviated. Moreover, because each modulation symbol is carried over a narrow bandwith, the modulation symbol experiences a single path fading. Simple equalization scheme may be applied to combat frequency selection fading. 
     MIMO (Multiple Input Multiple Output) 
     Multiple Input Multiple Output (MIMO) schemes use multiple transmit antennas and multiple receive antennas to improve the capacity and reliability of a wireless communication channel. A MIMO system capacity increases a function of K where K is the minimum of number of transmit antennas (M) at transmitter and receive antennas (N) at receiver, i.e. K=min(M,N). A simplified example of a 4×4 MIMO system is shown in  FIG. 5 . In this example, four different data streams Data Streams  1  to  4  are transmitted separately from the four transmit antennas Ant 1   T  to Ant 4   T . The transmitted signals are received at the four receive antennas Ant 1   R  to Ant 4   R . Spatial signal processing is performed on the received signals in order to recover the four data streams. An example of spatial signal processing is V-BLAST which uses the successive interference cancellation principle to recover the transmitted data streams. Other variants of MIMO schemes include schemes that perform some kind of space-time coding across the transmit antennas (e.g. D-BLAST) and also beamforming schemes such as SDMA (Spatial Division multiple Access). In addition, MIMO may be implemented with transmit/receive diversity scheme and transmit/receive beamforming scheme in order to improve the link reliability or system capacity in wireless communication systems. 
     The MIMO channel estimation consists of estimating the channel gain and phase information for links from each of the transmit antennas to each of the receive antennas. Therefore, the channel for M×N MIMO system consists of an N×M matrix: 
                   H   =     [           a   11           a   12         …         a     1   ⁢   M                 a   21           a   22         …         a     2   ⁢   M               ⋮       ⋮       …       ⋮             a     N   ⁢           ⁢   1             a     M   ⁢           ⁢   2           …         a   NM           ]             (   1   )               
where H is the MIMO channel matrix and arepresents the channel gain from transmit antenna j to receive antenna i. In order to enable the estimations of the elements of the MIMO channel matrix, separate pilots are transmitted from each of the transmit antennas.
 
     The different embodiments of the present invention will be described in details as follow. 
     Proposed Single-User Closed-Loop Transmit Beamforming (SU-CLTB) 
     The contemporary principle of OFDM waveform and MIMO system in wireless communication has been previously described. The following specification will concern the principle of the proposed single-user closed-loop transmit beamforming (SU-CLTB) scheme of MIMO system. Specifically, the case where a base station employs transmit beamforming and is communicated with a single mobile station at a time through the usage of OFDM radio signal will be considered. The proposed SU-CLTB scheme employs a codebook, and the codebook consists of a set of predetermined antenna beams known to mobile stations. The set of predetermined antennas beams is formed based on the antenna array response vectors of a serving base station, which is a function of antenna spacing, angle of arrival, and antenna configuration (for example, uniform linear array or uniform circular array). The base station and all mobile stations are included in a cell. The antenna array response vector is cell-specific, and may be carried through the broadcasting channel (BCH) in a real cellular system. A mobile station may then select the best antenna beam within the codebook and feed back the best antenna beam to the mobile station&#39;s serving base station in order to improve the throughput of the system. The best antenna beam information is selected from the set of predetermined antenna beams in the codebook based on certain performance criteria such as maximizing Signal-to-Noise ratio (SNR). The performance enhancement of the proposed SU-CLTB for wireless systems may be achieved by two methods. One method is boosting the energy of the transceived signals according to the beamforming gain, which results in SNR gain. The other method is reducing a radiated energy distributed to other base stations according to a narrower radiation beam pattern, which results in SIR (signal-to-interference) gain in a wireless system. The overall system enhancement of CLTB is the combination of SNR and SIR gains, which depend on the operating load of systems. For instance, in a lightly loaded system (or coverage-limited system), SNR gain is dominant; while in a heavy-loaded system (or interference-limited system) SIR gain is dominant. 
     An example of system level description for the proposed codebook-based SU-CLTB is shown in  FIG. 6 . As shown in  FIG. 6 , data stream is input into transmitter at base station, is processed by the transmitter and then is transmitted by transmitting antennas Ant 1   T -Ant 8   T . a The base station employs transmit beamforming stage  203 . In the present invention, a codebook containing a set of predetermined antenna beams known to mobile stations is employed in the transmit beamforming stage. Receiving antennas Ant 1   R  and Ant 2   R  receive the signals transmitted from the transmitter. The receiver of the proposed codebook-based SU-CLTB system at a mobile station has an antenna beam selection processing stage  201 , which is used to determine best antenna beam  202  among the predetermined antenna beams in the codebook, based on certain performance such as maximum signal-to-noise ration (MSNR) or Minimum Mean Square Error (MMSE). Best antenna beam information  202  is then feed backed to a serving base station for transmit beamforming stage  203 . The detail of code-book design, codebook generation, antenna beam selection algorithm, signaling scheme, and reference signal (RS) mapping for various antenna configurations is described as follows. 
     As shown in  FIG. 6 , the transmitter of the proposed codebook-based SU-CLTB system at a base station includes a transmitter processing stage and a transmit beamforming stage. An example of transmitter processing is single-codeword OFDM transmission scheme as shown in  FIG. 7 . A CRC inserter  210  adds a CRC (cyclic redundancy check) to an single information block and then either turbo coding or LDPC (low density parity check) coding stage  211 , channel inter-leaving stage  213 , and modulation stage  214  are performed sequentially. A turbo encoder is formed by parallel concatenation of two recursive systematic convolutional (RSC) encoders separated by an interleaver. After transmit beamforming (TB) stage  215 , only one predetermined antenna beam T is generated in this example. A contemporary OFDM transmission as show in  FIG. 2  is implemented after the transmit beamforming stage  215 . 
     A detailed codebook-based transmit beamforming processing  215  is shown in  FIG. 8 . Beamforming signal T is the sum of the signals weighted by W 1 , W 2 , . . . , W 8  for Ant 1   T    1 , Ant 2   T , . . . and Ant 8   T , respectively. W 1 , W 2 , . . . , W 8  are called beamforming weights, which are derived from codebook. T is a transmit signal after beamforming (i.e., beamformed transmit signal) and [W 1 , W 2 , . . . , W 8 ] is a codeword of the codebook. W 1 -W 8 ] are vectors of each codeword W of the codebook. The details of codebook design and code generation will be described in next section. 
     Codebook Design and Codebook Generation for SU-CLTB 
     In this section, a codebook design is proposed. The proposed codebook is not fixed, but adaptable in the sense that codebook design is optimized for each cell including the base station and its corresponding mobile stations. That is, the codebook is cell-specific, and is generated based the antenna configuration, antenna spacing, and the number of antenna of its serving base station. The proposed codebook is a set of transmit beamforming vectors, C j  with {j=1,2, . . . J}, which are used to form a set of predetermined antenna beams. J is the size of codebook or the number of transmit beam vectors. A mobile station may then select the best antenna beam and feedback the best antenna beam to the serving base station in the cell in order to improve system throughput. C j  is formed by the antenna array response vector of a serving base station, which is function of antenna spacing, angle of arrival, antenna configuration (uniform linear array, uniform circular array), and antenna polarization. 
     In one embodiment of the invention, for a uniform linear array (ULA), the proposed beamforming codebook is given by: 
                     c   j     =       [             w   1     ⁡     (     θ   j     )                   w   2     ⁡     (     θ   j     )               ⋮               w   p     ⁡     (     θ   j     )               ⋮               w   p     ⁡     (     θ   j     )             ]     =     [         1             ⅇ       -   j2π     ⁢     D   λ     ⁢   si   ⁢           ⁢     n   (     θ   j     )                 ⋮             ⅇ       -   j2π     ⁢         (     p   -   1     )     ⁢   D     λ     ⁢   si   ⁢           ⁢     n   (     θ   j     )                 ⋮             ⅇ       -   j2π     ⁢         (     p   -   1     )     ⁢   D     λ     ⁢   si   ⁢           ⁢     n   (     θ   j     )               ]               (   2   )               
where j=1, . . . , J, J is size of the codebook, p is index of transmitting antennas, i.e., p=1, . . . , P, P is number of transmitting antennas, D is the space between the transmitting antennas, λ=c/f_c is wavelength of a carrier where c is speed of light and f_c is frequency of the carrier, θ j  is the main angle of the direction of departure of a j&#39;th transmit antenna beam. The set of θ j  s, where j=1, . . . , J, is specified and known to the serving base station and all of the mobile stations within the cell. Each codeword can form an antenna beam with multiple antennas for a given carrier. A codebook is a collection of the codewords. That is, a codeword is a vector, in which each element is a weight that applies to one of antennas in the antenna array.
 
     One example of the set of θ j  s , j=1, . . . , J, is a set where all antenna beams have uniform angular spacing. In particular, in a three-sector system where each sector has 120 degrees angular spacing, the set θ j  s, j=1, . . . , J, is given by 
                     θ   j     =       (     j   +     1   2       )     ×     120   J     ⁢           ⁢     (   degrees   )               (   3   )               
when the reference angle, i.e, zero-degree corresponds to the edge of a sector, or
 
                     θ   j     =         (     j   +     1   2       )     ×     120   J       -     60   ⁢           ⁢     (   degrees   )                 (   4   )               
when the reference angle, i.e., zero-degree corresponds to the center of the sector.
 
     Another example of the set of θ j  s, j=1, . . . , J, is a set where the antenna beams do not have uniform equal angular spacing. This is useful when the base station has prior knowledge of the geographical locations of the mobile stations, and may add more beam granularity in directions where a large concentration of mobile stations exists, while reducing beam granularity in directions where less amount of mobile stations exists. 
     In another embodiment of the present invention, for a uniform circular array (UCA), the proposed beamforming codebook is given by: 
                       C   j     =       [             w   1     ⁡     (     θ   j     )                   w   2     ⁡     (     θ   j     )               ⋮               w   p     ⁡     (     θ   j     )               ⋮               w   P     ⁡     (     θ   j     )             ]     =     [           ⅇ       -   j2π     ⁢     R   λ     ⁢   si   ⁢           ⁢     n   ⁡     (   ζ   )       ⁢   c   ⁢           ⁢     os   ⁡     (       θ   j     -     φ   1       )                     ⅇ       -   j2π     ⁢     R   λ     ⁢   si   ⁢           ⁢     n   ⁡     (   ζ   )       ⁢   c   ⁢           ⁢     os   ⁡     (       θ   j     -     φ   2       )                   ⋮             ⅇ       -   j2π     ⁢           ⁢     R   λ     ⁢   si   ⁢           ⁢     n   ⁡     (   ζ   )       ⁢   c   ⁢           ⁢     os   ⁡     (       θ   j     -     φ   p       )                   ⋮             ⅇ       -   j2π     ⁢           ⁢     R   λ     ⁢   si   ⁢           ⁢     n   ⁡     (   ζ   )       ⁢   co   ⁢           ⁢     s   ⁡     (       θ   j     -     φ   P       )                 ]         ,           (   5   )               
where j=1, . . . , J, R is a radius of uniform circular array, ζ is an elevation angle which is an angle of z-axis perpendicular to x-y plane ranging from −90 degree to +90 degrees, θ j  is the main angle of the direction of departure of the j&#39;th transmitting antenna beam, φ is an angle of x-y plane, ranging from 0 degree to 360 degrees, and w p (θ j )is vectors of the codeword of the codebook. For simplicity, only azimuth angles are considered in the propagation geometry (i.e., ζ=90 degrees) but the results may be generalized to three dimensions.
 
     It is noted that the antenna array response vector C j _{j=1,2, . . . J} is cell-specific, which may be carried out through the broadcasting channel (BCH) in the implementation of a real cellular wireless system. “C j _{j=1,2, . . . J}” here is a codeword index for jth transmit antenna beam. The proposed codebook-base design assures that the codebook design is optimized for each cell-site since in a real deployment of the antenna configuration among adjacent cell-sites maybe quite different. In practical application, uniform linear array is likely to be deployed in sectorized cell, while uniform circular array is probably used in an omni-directional cell. To mitigate the effect of antenna sidelobes due large antenna spacing, the proposed codebook C j  uses a closed antenna spacing equal to half of the wavelength. Similar to the ULA case, here θ j  is main angle of direction of departure of the j&#39;th transmit antenna beam at a base station. The set of θ j s, j=1, . . . , J, is specified and known at both the serving base station and all mobile stations in the cell. One example of the set of θ j  s , j=1, . . . , J, is a set where all beams have uniform angular spacing. In particular, in a three-sector system where each sector has 120 degrees angular spacing, the set θ j  s, j=1, . . . , J, is given by 
                     θ   j     =       (     j   +     1   2       )     ×     120   J     ⁢           ⁢     (   degrees   )               (   6   )               
when the reference angle, i.e., zero-degree direction, corresponds to the section edge, or
 
                     θ   j     =         (     j   +     1   2       )     ×     120   J       -     60   ⁢           ⁢     (   degrees   )                 (   7   )               
if the reference angle, i.e., zero-degree direction, corresponds to the center of the sector.
 
     Another example of the set of θ j  s, j=1, . . . , J, is a set where the beams do not uniform equal angular spacing. This is useful if the base station has the prior knowledge of the geographical locations of the mobile stations, and may add more beam granularity in directions where there are a large concentration of mobile stations, while reducing beam granularity in directions where there are less amount of mobile stations. 
     RS Mapping Antenna Configuration 
     Two types of reference signals for transmit beamforming systems are typically needed in wireless communication systems: common reference signals and dedicated reference signals. Common reference signals are shared by multiple mobile stations. It is used for many purposes such as channel estimation, cell search, and so on. Dedicated reference signal is used for data demodulation for a specific mobile station. In this section, the common reference signals (RS) mapping to antenna ports for various antenna configurations will be discussed. Example of RS 1 , RS 2 , RS 3 , and RS 4  structure is shown in  FIG. 9 . Note that RS 1 , RS 2 , RS 3 , and RS 4  are common RS signals for antenna  1 , antenna  2 , antenna  3  and antenna  4 , respectively. The four kinds of common RS are distributed in a predetermined order in the two dimensional space of OFDM symbol index and sub-carrier index. 
       FIGS. 10-15  show examples of RS mapping for an eight-antenna system and each figure contains examples of two alternative RS mapping, i.e., type A and type B. All of the antennas indicated in  FIGS. 10-15  refer to transmitting antennas. Each RS is mapped to a corresponding antenna with a predetermined rule. 
     For single polarization antenna, examples are shown in  FIGS. 10 and 11 . 
       FIG. 10  shows the example of RS mapping for eight-antenna uniform linear array. Antennas have one common direction in a uniform linear array and are allocated linearly. For type A RS mapping, RS 1  is mapped to ANT 1 (antenna  1 ), RS 2  is mapped to ANT 2 , RS 3  is mapped to ANT 3 , RS 4  is mapped to ANT 4 . For type B RS mapping, RS 1  is mapped to ANT 1 (antenna  1 ), RS  2  is mapped to ANT 3 , RS 3  is mapped to ANT 5 , RS  4  is mapped to ANT 7 . An example of type A RS mapping may be represented as follows: 
     RS 1             ANT 1 ;
     RS 2             ANT 2 ;
     RS 3             ANT 3 ; and
     RS 4             ANT 4 .
     An example of type B RS mapping may be represented as follows: 
     RS 1             ANT 1 ;
     RS 2             ANT 3 ;
     RS 3             ANT 5 ; and
     RS 4             ANT 7 .
     The mapping type A represents a case where RSs are sequentially mapped to a set of antennas and the set of antennas are neighboring to each other; mapping type B represents a case where RSs are mapped to discrete antennas with at least one antenna of a free state located in between except for the 1st antenna and the last antenna. 
       FIG. 11  shows the example of RS mapping for eight-antenna uniform circular array. Antennas have one common direction in a uniform linear array and are evenly located at the periphery of a circle having a predetermined radius. Two of the antennas are overlapped with the diameter of the circle and two of the antennas are perpendicular to the diameter of the circle. Each of the rest four antennas is positioned with a 45 degrees angle against the diameter of the circle. In  FIG. 11 , RSs are mapped to discrete antennas with at least one antenna of a free state located in between for both of mapping type A and type B. The mapping type A represents a case where RSs are mapped to a set of discrete antennas disposed either perpendicular to or overlapped with the diameter of the circle; mapping type B represents a case where RSs are mapped to a set of discrete antennas positioned with a 45 degrees angle against the diameter of the circle. 
     For dual polarization antenna, two types of antennas are considered: dual slat ±45 degree polarization and dual vertical/horizontal polarization. The eight antennas are arranged into four sets with each set having two antennas crossing each other. Examples are shown in  FIGS. 12-15 . 
       FIG. 12  shows the example of RS mapping for eight-antenna uniform linear array with dual slat ±45 degree polarization. Two antennas across each other in one set are positioned with 45 degrees against a virtual vertical line in a clockwise direction and with −45 degrees against the virtual vertical line in a counter-clockwise direction respectively. The mapping type A maps RSs to the antennas positioned with 45 degrees against a virtual vertical line in a clockwise direction while the mapping type B maps RSs to the antennas positioned with 45 degrees against a virtual vertical line in a counter clockwise direction. 
       FIG. 13  shows the example of RS mapping for eight-antenna uniform linear array with dual vertical/horizontal polarization. Antennas across each other in one set are positioned perpendicular to a virtual vertical line in a clockwise direction and aligned with the virtual vertical line respectively. The mapping type A maps RSs to the antennas positioned aligned with the virtual vertical line while the mapping type B maps RSs to the antennas positioned perpendicularly to the virtual vertical line. 
       FIG. 14  shows the example of RS mapping for eight-antenna uniform circular array with dual slat ±45 degree polarization. Four sets of the antennas are evenly located at the periphery of a circle. Similar to  FIG. 12 , the mapping type A maps RSs to the antennas positioned with 45 degrees against a virtual vertical line in a clockwise direction while the mapping type B maps RSs to the antennas positioned with 45 degrees against a virtual vertical line in a counter clockwise direction. 
       FIG. 15  shows the example of RS mapping for eight-antenna uniform circular array with dual vertical/horizontal polarization. Four sets of the antennas are evenly located at the periphery of a circle. Similar to  FIG. 13 , the mapping type A maps RSs to the antennas positioned aligned with the virtual vertical line while the mapping type B maps RSs to the antennas positioned perpendicularly to the virtual vertical line. 
     Antenna Beam Selection Algorithm for SU-CLTB 
     In this section, the antenna beam selection algorithm used in the proposed codebook-based SU-CLTB scheme will be discussed. The antenna selection processing at the receiver is based on the common pilot signal transmitted from a base station. The best antenna beam information is selected from the set of pre-determined antenna beam based on certain performance criteria maximum signal-to-noise ration (MSNR) or Minimum Mean Square Error (MMSE). In the case of noise-dominant environment, MSNR is used while in the presence of interference dominant environment, MMSE is employed. 
     For example, when MSNR is used, the transmit beamforming vector of the best antenna beam for the k&#39;th subcarrier, W k , may be selected, for type A RS mapping: 
                     W   k     =     C     j   ,     ma   ⁢           ⁢   x                 (   8   )             where                           C     j   ,     m   ⁢           ⁢   ax         =     arg   ⁢           ⁢       max   j     ⁢     {       ∑   k     ⁢       ∑     m   =   1     M     ⁢       ∑     p   =   1       P   2       ⁢         w   p     ⁡     (     θ   j     )       ⁢       H   ^       p   ,   m   ,   k               }                 (   9   )               
where Ĥ p,m,k  denotes the channel estimate for transmit antenna p, receive antenna m in the k&#39;th subcarrier, P is and w p (θ j ) is w p (θ j )is vectors of codeword of the codebook as shown in either equation (2) or (5) in dependence upon the RS mapping antenna configuration.
 
     For type-B RS mapping, the beam selection algorithm should be modified to 
                     W   k     =     C     j   ,     m   ⁢           ⁢   ax                 (   10   )             where                           C     j   ,     m   ⁢           ⁢   ax         =     arg   ⁢           ⁢       max   j     ⁢     {       ∑   k     ⁢       ∑     m   =   1     M     ⁢       ∑     p   =   1       P   2       ⁢         w       2   ⁢   p     -   1       ⁡     (     θ   j     )       ⁢       H   ^       p   ,   m   ,   k               }                 (   11   )               
where Ĥ p,m,k  denotes the channel estimate for transmit antenna p, receive antenna m in the k&#39;th subcarrier, P is total number of transmit antenna, and P is and w p (θ j ) is vectors of codeword of the codebook as shown in either equation (2) or (5) in dependence upon the RS mapping antenna configuration.
 
Additional Common Pilot
 
     In one embodiment of the present invention, another four common reference signals (RS 5 , RS 6 , RS 7 , RS 8 ) are added, in addition to the current common pilots RS 1 , RS 2 , RS 3 , RS 4 . These four common pilots are used for the four transmit antennas that are currently not associated with existing RS 1 , RS 2 , RS 3 , RS 4 . In addition, these additional common pilots may be sent in the downlink at a much lower frequency, for example, every 10 seconds, instead of the frequency of sending the existing common reference signals (i.e., the frequency may be every slot, in order of milliseconds, for example 1 ms-10 ms). 
       FIG. 16  shows the RS mapping for eight-antenna single polarization ULA systems, with additional common reference signals (RS 5 , RS 6 , RS 7 , RS 8 ). An example of type A RS mapping may be represented as follows: 
     RS 1             ANT 1 ;
     RS 2             ANT 2 ;
     RS 3             ANT 3 ;
     RS 4             ANT 4 ;
     RS 5             ANT 5 ;
     RS 6             ANT 6 ;
     RS 7             ANT 7 ; and
     RS 8             ANT 8 .
     An example of type B RS mapping may be represented as follows: 
     RS 1             ANT 1 ;
     RS 5             ANT 2 ;
     RS 2             ANT 3 ;
     RS 6             ANT 4 ;
     RS 3             ANT 5 ;
     RS 7             ANT 6 ;
     RS 4             ANT 7 ; and
     RS 8             ANT 8 .
     The mapping type A represents a case where RSs 1 - 4  are sequentially mapped to a set of antennas and the set of antennas are neighboring to each other, and RSs 5 - 8  are sequentially mapped to another set of antennas and the another antennas are neighboring to each other; mapping type B represents a case where RSs  1 - 4  are mapped to discrete antennas with at least one antenna mapped with one of RSs  5 - 8  in between except for the  1 st antenna and last antenna in the line, and RSs  5 - 8  are mapped to discrete antennas with at least one antenna mapped with one of RSs 1 - 4  in between except for the 1 st  antenna and the last antenna in the line. 
       FIG. 17  shows the RS mapping for eight-antenna single polarization for UCA systems, with additional common reference signals (RS 5 , RS 6 , RS 7 , RS 8 ). Antennas are evenly positioned at the periphery of a circle and all of the antennas have an identical direction. Mapping type A sequentially maps RSs  1 - 4  to a set of neighboring antennas and maps RSs 5 - 8  to another set of neighboring antennas. Mapping type B alternately maps one of RSs  1 - 4  and one of RSs  5 - 8  to the antennas. In other words, the neighboring antennas of one antenna mapped with RSs from RSs 1 - 4  are mapped with RSs from RSs  5 - 8 , and the neighboring antennas of one antenna mapped with RSs from RSs 5 - 8  are mapped with RSs from RSs  1 - 4 . 
       FIG. 18  shows the RS mapping for eight-antenna dual vertical polarization for ULA with additional common pilots (RS 5 , RS 6 , RS 7 , RS 8 ). Eight antennas are arranged into four sets with each set having two antennas across each other. Mapping type A maps RSs  1 - 4  to two neighboring sets of antennas, and maps RSs  5 - 8  to another two neighboring sets of antennas. Mapping type B selects one of RSs  1 - 4  and one of RSs 5 - 8  and maps them to antennas of each set respectively. 
       FIG. 19  shows the RS mapping for eight-antenna slat±45 degree polarization for ULA with additional common reference signals (RS 5 , RS 6 , RS 7 , RS 8 ). Similar to  FIG. 18 , Mapping type A maps RSs  1 - 4  to two neighboring sets of antennas, and maps RSs  5 - 8  to another two neighboring sets of antennas. Mapping type B selects one of RSs  1 - 4  and one of RSs  5 - 8  and maps them to antennas of each set respectively. 
     Signaling Scheme for SU-CLTB 
     Based on the performance observed in the Annex section (to be discussed), the proposed SU-CLTB not only provides significant system gain over the baseline system (without transmit beamforming), but provides signaling overhead reduction. There are two method of reducing signaling overhead with the proposed SU-CLTB: feedback rate and feedback bandwidth. The feedback rate indicates the frequency of sending the feedback channel information to a base station. Generally speaking, the faster the feedback rate is, the larger the signaling overhead is. The feedback bandwidth is referred to how wide the bandwidth is required to feedback to a base station. Generally speaking, the larger the feedback bandwidth is, the smaller is the feedback overhead. As shown in the reference section, the feedback rate of the proposed SU-CLTB may be signaling at the order of seconds, instead of milliseconds in a typical system. This results in significant overhead reduction. With this feedback rate, the signaling can be done at higher Layer (slower) signaling. Additionally, the feedback bandwidth of the proposed SU-CLTB is equal to the whole system bandwidth, instead of bandwidth of sub-band or sub-carrier. Note that sub-carrier is the smallest bandwidth unit in OFDM systems, and sub-band is referred as a group of sub-carriers. Sub-band is regarded as partial system bandwidth. As compared to the prior art such as conventional open-loop transmit beamforming, the proposed SU-CLTB does not require phase calibration processing, which is generally costly and sensitive to radio channel variation. As compared to contemporary closed-loop beamforming, the proposed SU-CLTB provides a significant signaling overhead reduction and the codebook design of the SU-CLTB is less sensitive to radio channel variation. 
     In one embodiment of the present invention, the mobile station feedbacks only one choice of beamforming codeword of the codebook for the entire bandwidth, instead of every sub-band or every subcarrier. This results in significant saving of feedback bandwidth. 
     In another embodiment of the present invention, we propose to use higher layer signaling to transmit the feedback information of the choice of beamforming codeword of the codebook. The resulting feedback rate may be much smaller than the contemporary codebook based feedback scheme, where the feedback is carried on Physical layer signals. 
     CQI Reporting for SU-CLTB 
     As previously mentioned, two types of reference signals for transmit beamforming systems are typically needed in wireless communication system: common reference signals and dedicated reference signals. Common reference signals are for CQI (channel quality indication) reporting and codeword choice reporting, while dedicated reference signals are specific for data demodulation and detection when transmit beamforming is employed. In this section, CQI (channel quality indication) reporting is employed by the proposed SU-CLTB. Specifically, the number of common reference signal is less the number of transmit antennas. In this case, a δCQI is needed for CQI reporting due to the fact there is a CQI difference between CQI calculation based on dedicated signal and CQI calculation based on common reference signal. A sending rate of the CQI calculated by the dedicated signals is faster than a sending rate of the CQI calculated by the reference signals. 
     An example of δ CQI reporting for proposed SU-CLTB scheme is shown in  FIG. 20 . Based station  401  transmits to mobile station  402  common reference signals on four of transmitting antennas and transmits dedicated reference signals on eight of the transmitting antennas. The common reference signals are for CQI (channel quality indication) reporting and codeword choice reporting, while dedicated reference signals are specific for data demodulation and detection when transmit beamforming is employed. Mobile station  402  transmits back to base station  401  the normal CQI signal reporting based on common reference signals and δCQI which is the difference between CQI calculation based on dedicated signals and CQI calculation based on common reference signals. 
     In one embodiment of the current invention, a δ CQI reporting is proposed to report the channel estimation different between dedicated pilot and common pilot. This δ CQI reporting is in addition to the existing normal CQI reporting. Furthermore, the reporting frequency of the δ CQI may be different from the reporting frequency of the existing normal CQI. 
     Annex: Performance Result for SU-CLTB 
     In this section, system performances of the proposed SU-CLTB systems are presented by  FIGS. 21-29  in order to show the improvement of the proposed SU-CLTB compare to the contemporary system. 
       FIG. 21  and  FIG. 22  show the histogram of beamforming gain of the proposed SU-CLTB over the baseline system with uniform linear array (ULA) and uniform circular array (UCA), respectively. As shown, the proposed SU-CLTB significantly outperforms the baseline systems. It is noted that the baseline system is a single transmit antenna system. The baseline in the present invention refers to a contemporary system. The vertical axis of  FIGS. 21 and 22  is probability density. That is, it basically shows the possibility of the occurrence of the gain in horizontal axis in the experiments. The results are based on 500,000 computer experiments, and thus the number in vertical axis refers to the times the gain in horizontal axis occurs. The gain at 0 db point represents the contemporary industry standard system (i.e. baseline system), which is a single transmit antenna system. It is clearly shown that the proposed SU-CLTB has better performance of SNR gain over the baseline system. 
       FIG. 23  shows the spectral efficiency (SE) improvement of the proposed SU-CLTB over the baseline system under various feedback bandwidths. As shown, the proposed SU-CLTB provides substantial SE improvement over the baseline systems. For example, with same signal-noise-ratio 5 dB, the spectral efficiency of the baseline system is for example almost half of the proposed SU-CLTB. The curves representing the proposed SU-CLTB with different feedback bandwidths almost overlap with each other. Therefore, it indicates that there is almost no performance difference for various feedback bandwidths for the proposed SU-CLTB. As previously stated, the large feedback bandwidth is, the smaller signaling overhead is. This result suggests that the proposed SU-CLTB may feedback the best antenna beam per whole system bandwidth without sacrifice of performance of the system. 
       FIG. 24  shows the spectral efficiency (SE) improvement of the proposed SU-CLTB over the baseline system under various feedback rates. For example, at signal to noise ratio is 5 dB, the proposed SU-CLTB provides almost double spectral efficiency over the baseline systems. It is also shown that there is almost no performance difference for various feedback rates. As previously stated, the faster feedback bandwidth is, the larger signaling overhead is. This result suggests that the proposed SU-CLTB may feedback the best antenna beam at very slow feedback rate without sacrifice of performance of the system. 
       FIG. 25  shows the spectral efficiency (SE) improvement of the proposed SU-CLTB over the baseline system under various radio channel environments such as urban macro cell channel, urban micro cell channel, and suburban channel. As shown, the proposed SU-CLTB provides large SE improvement over the baseline systems. It is also shown that there is not much performance difference for various radio channels. The result suggests that the proposed SU-CLTB is not sensitive to various radio environments. 
       FIG. 26  shows the spectral efficiency (SE) improvement of the proposed SU-CLTB over the baseline system under realistic channel estimation (CE). As shown, the proposed SU-CLTB provides large SE improvement over the baseline systems in both of ideal and realistic channel estimations. The result suggests that the proposed SU-CLTB operation is not sensitive to realistic channel estimation error. 
       FIG. 27  show the CDF (Cumulative Distribution Function) of capacity gain of the proposed SU-CLTB over the baseline system with uniform linear array (ULA) and uniform circular array (UCA), respectively. With same probability, the ratio of spectral efficiency (SE) of the proposed SU-CLTB over the baseline system is greater than one. The proposed SU-CLTB provides significant capacity gain over the baseline systems. And this means that spectral efficiency (SE) of the proposed SU-CLTB is significantly improved comparing to the baseline system. It is also noted that the capacity gain increases when signal-to-noise ratio (SNR) decreases. The result suggests that the proposed SU-CLTB may significantly improve system performance when SNR is low, which is typically occurs at the edge of the cell. 
       FIG. 28  show the SIR gain of the proposed SU-CLTB over the baseline system with uniform circular array (UCA). As shown, the proposed SU-CLTB provides significant SIR gain over the baseline systems. The vertical axis of  FIG. 29  is probability density. That is, it basically shows the possibility of the occurrence of the gain in horizontal axis in the experiments. The results are based on 500,000 computer experiments, and thus the number in vertical axis refers to the times the gain in horizontal axis occurs. The gain at 0 db point represents the contemporary industry standard system (i.e. baseline system), which is a single transmit antenna system. It is clearly shown that the proposed SU-CLTB has better performance of SIR gain over the baseline system.