Abstract:
In a wireless communication system including a plurality of sub-carriers and a plurality of eigenbeams, a method for tuning a beamformed signal. The method includes adjusting a total gain of each of the plurality of sub-carriers and eigenbeams, and applying the adjusted total gain to each of the sub-carriers and each of the eigenbeams.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional application No. 60/782,459, filed Mar. 15, 2006 which is incorporated by reference as if fully set forth. 
    
    
     FIELD OF INVENTION 
     The present invention relates generally to the field of wireless communications systems. More specifically, the present invention relates to a method and apparatus for beamforming in a multiple-in/multiple-out (MIMO) orthogonal frequency division multiplexing (OFDM) wireless communication system. 
     BACKGROUND 
     In multiple in-multiple out (MIMO) orthogonal frequency division multiplexing (OFDM) wireless communication systems, transmit beamforming (TxBF) typically will improve signal-to-noise ratio (SNR) at a receiver. Transmit beamforming may provide a higher throughput and in turn allow for higher data rates as compared to, for example, direct mapping or spatial spreading. 
     Channel state information (CSI) typically must be available at the transmitter in order to employ TxBF techniques. A transmitter may estimate CSI by assuming channel reciprocity, or the transmitter may determine CSI from a receiver by way of signaling. It should be noted that channel reciprocity requires radio calibration which could be achieved by exchanging sounding packets. The transmitter may then perform beamforming based on the estimation of received CSI and select a proper modulation and coding scheme (MCS) based on the MCS index recommended by the receiver through signaling. 
     Prior art wireless communication receivers utilize MCS indexes in order to match code rate and modulation as close as possible to channel conditions. However, since the number of MCS indexes is limited, MCS indexes selected by the receiver may not closely match the existing channel conditions. Prior art receivers select from a limited set of MCS indexes to match rate and modulations as closely as possible to channel conditions. 
     Therefore, it would be desirable to have a method and apparatus for redistributing power for all sub-carriers and eigenbeams within the set of selected MCS indexes. This would provide a fine adjustment to the MCS indexes in order to closely match current channel conditions. 
     SUMMARY 
     The present invention is a method and apparatus for beamforming in MIMO-OFDM wireless communication systems. In a preferred embodiment, power loading is used to redistribute power for all sub-carriers and eigenbeams, thus providing modulation and coding schemes (MCS) indexes with fine adjustments in order to more closely match the current channel conditions. Performance is thereby increased in terms of decreased packet error rates (PER) and higher throughput. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more detailed understanding of the invention may be had from the following description of a preferred embodiment, given by way of example and to be understood in conjunction with the accompanying drawing(s) wherein: 
         FIG. 1  shows an exemplary wireless system, including an access point (AP) and a plurality of wireless transmit/receive units (WTRUs), configured in accordance with the present invention; 
         FIG. 2  is a functional block diagram of an AP and a WTRU of the wireless communication system of  FIG. 11 ; and 
         FIG. 3  is a flow diagram of a method of power loading in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone (without the other features and elements of the preferred embodiments) or in various combinations with or without other features and elements of the present invention. 
     When referred to hereafter, the terminology “wireless transmit/receive unit (WTRU)” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a computer, or any other type of user device capable of operating in a wireless environment. When referred to hereafter, the terminology “base station” includes but is not limited to a Node-B, a site controller, an access point (Base station), or any other type of interfacing device capable of operating in a wireless environment. 
     Turning now to  FIG. 1 , there is shown an exemplary wireless communication system  100  configured in accordance with the present invention. The wireless communication system  100  includes a plurality of wireless communication devices, such as an AP  110  and a plurality of WTRUs  120 , capable of wirelessly communicating with one another. Although the wireless communication devices depicted in the wireless communication system  100  are shown as APs and WTRUs, it should be understood that any combination of wireless devices may comprise the wireless communication system  100 . That is, the wireless communication system  100  may comprise any combination of APs, WTRUs, stations (STAs), and the like. 
     For example, the wireless communication system  100  may include an AP and client device operating in an infrastructure mode, WTRUs operating in ad-hoc mode, nodes acting as wireless bridges, or any combination thereof. Additionally, in a preferred embodiment of the present invention, the wireless communication system  1100  is a wireless local area network (WLAN). However, the wireless communication system  100  may be any other type of wireless communication system. 
       FIG. 2  is a functional block diagram of an AP  210  and a WTRU  120  of the wireless communication system  100  of  FIG. 1 . As shown in  FIG. 2 , the AP  110  and the WTRU  120  are in wireless communication with one another. In addition to the components that may be found in a typical AP, the AP  110  includes a processor  215 , a receiver  216 , a transmitter  217 , and an antenna  218 . The processor  215  is configured to generate, transmit, and receive data packets in accordance with the present invention. The receiver  216  and the transmitter  217  are in communication with the processor  215 . The antenna  218  is in communication with both the receiver  216  and the transmitter  217  to facilitate the transmission and reception of wireless data. 
     Similarly, in addition to the components that may be found in a typical WTRU, the WTRU  120  includes a processor  225 , a receiver  226 , a transmitter  227 , and an antenna  228 . The processor  225  is configured to generate, transmit, and receive data packets in accordance with the present invention. The receiver  236  and the transmitter  227  are in communication with the processor  225 . The antenna  228  is in communication with both the receiver  226  and the transmitter  227  to facilitate the transmission and reception of wireless data. 
     The present invention may be implemented in a WTRU or base station. The present invention is applicable to both the physical layer (PHY) and the digital baseband. The present invention may be implemented in wireless communication systems employing the following air interfaces: wideband code division multiple access (WCDMA), time division duplex (TDD), including HCR, LCR, and TDS-CDMA, frequency division duplex (FDD), and IEEE 802.11n air interfaces. 
     In a currently preferred embodiment of the invention, power loading in accordance with the present invention is applied to the eigen beamforming mode of a MIMO-OFDM wireless communication system. Preferably, power loading is only applied while closed loop power control is in operation, and when accurate and recent CSI is available for use in preceding for eigen beamforming. 
       FIG. 3  is a flow diagram of a power loading method  300  in accordance with one embodiment of the present invention. The method  300  begins by ranking the eigenvalues of the channel correlation matrix using a channel estimation matrix per subcarrier as shown at step  302  and equation (1).
 
(λ 1 ( k )&gt;λ 2 ( k )&gt; . . . &gt;λ nT ( k ));  Equation (1)
 
At step  304 , create eigenbeams (E 1 , E 2 , . . . , E nT ) by grouping the ranked eigenvalues for all subcarriers according to equation (2).
 
 E   i ={λ i (1), λ i (2), . . . , λ i ( K )} for  i= 1, 2, . . . ,  nT   Equation (2)
 
In equation (2), K is the number of sub-carriers, nT is the number of eigenbeams/data streams, and λ i (j) is the i th  eigenvalue of the j th  subcarrier.
 
     In step  306 , the average of the eigenvalues per eigenbeam is computed according to equation (3). 
     
       
         
           
             
               
                 
                   
                     
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     In step  308 , a gain a(i,j) is computed such that 
                 a   ⁡     (     i   ,   j     )       =             λ   i   av         λ   i     ⁡     (   j   )           ⁢           ⁢   for   ⁢           ⁢   i     =   1       ,   2   ,         
. . . , nT and j=1, 2, . . . , K. At step  310 , the gain a(i,j) is compared to a threshold. If the gain a(i,j) is greater than a threshold, TH a , at step  312 , a(i,j) is set equal to TH a . This puts a limit on the gain and limits the power loading to very poor sub-carriers.
 
     At step  314 , a gain b(i) is computed such that 
                 b   ⁡     (   i   )       =       G   mod     ⁢     G   code     ⁢         λ   1   av       λ   i   av             ,         
where G mod  is a relative modulation order of i th  eigenbeam to the first/strongest eigenbeam and G code  is a relative channel coding gain of i th  eigenbeam to the first/strongest eigenbeam. By way of example, M-QAM modulation requires, approximately, an additional 5 dB by adding one more bit to a symbol. If the first/strongest eigenbeam uses 256-QAM and the second eigenbeam uses 64-QAM, G mod  is approximately 10 5(N     1       64QAM     −N     2       256QAM     )/20 =1/√{square root over (10)}, where N 1   64QAM =log 2  64 and N 1   256QAM =log 2  256. Likewise, G code  is computed based on code gain between two eigenbeams.
 
     Since the total power with the new gains (a and b) must be the same as the original power with unit gain, at step  316 , the equation 
             c   =       k   ⁢       ∑     i   =   1     nT     ⁢     λ   i   av             ∑     i   =   1     nT     ⁢       ∑     j   =   1     K     ⁢         b   2     ⁡     (   i   )       ⁢       a   2     ⁡     (     i   ,   j     )       ⁢       λ   i     ⁡     (   j   )                     
is solved, resulting in a value for a variable c. At step  318 , gain g(i,j) is computed for all sub-carriers and eigenbeams such that g(i, j)=√{square root over (c)}b(i)a(i, j) for i=1, 2, . . . , nT and j=1, 2, . . . , K. At step  320 , the gain g(i,j) is applied to all sub-carriers and eigenbeams of long training fields (LTFs) and data OFDM symbols.
 
     Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention. The methods or flow charts provided in the present invention may be implemented in a computer program, software, or firmware tangibly embodied in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of computer-readable storage mediums include a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs). 
     Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any other type of integrated circuit (IC), and/or a state machine. 
     A processor in association with software may be used to implement a radio frequency transceiver for use in a wireless transmit receive unit (WTRU), user equipment (UE), terminal, base station, radio network controller (RNC), or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software, such as a camera, a video camera module, a videophone, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a hands free headset, a keyboard, a Bluetooth® module, a frequency modulated (FM) radio unit, a liquid crystal display (LCD) display unit, an organic light-emitting diode (OLED) display unit, a digital music player, a media player, a video game player module, an Internet browser, and/or any wireless local area network (WLAN) module