Abstract:
Disclosed is a method and apparatus of transmit diversity that is backward compatible and does not significantly degrade performance in additive white guassan noise (AWGN) conditions using a transmission architecture that incorporates a form of phase sweep transmit diversity (PSTD) referred to herein as biased PSTD. Biased PSTD involves transmitting a signal and a frequency swept version of the same signal over diversity antennas at different power levels. By transmitting the two signals at different power levels, the depths of nulls normally seen in AWGN conditions when PSTD is utilized is reduced and performance degradation in AWGN conditions is mitigated.

Description:
RELATED APPLICATION  
         [0001]    Related subject matter is disclosed in the following applications filed concurrently and assigned to the same assignee hereof: U.S. patent application Ser. No. ______ entitled, “Space Time Spreading and Phase Sweep Transmit Diversity,” inventors Roger Benning, R. Michael Buehrer, Robert Atmaram Soni and Paul A Polakos; U.S. patent application Ser. No. ______ entitled, “Symmetric Sweep Phase Sweep Transmit Diversity,” inventors Roger Benning, R. Michael Buehrer, Paul A Polakos and Mark Kraml; U.S. patent application Ser. No. ______ entitled, “Split Shift Phase Sweep Transmit Diversity,” inventors Roger Benning, R. Michael Buehrer, Robert Atmaram Soni and Paul A Polakos.  
         BACKGROUND OF THE RELATED ART  
         [0002]    Performance of wireless communication systems is directly related to signal strength statistics of received signals. Third generation wireless communication systems utilize transmit diversity techniques for downlink transmissions (i.e., communication link from a base station to a mobile-station) in order to improve received signal strength statistics and, thus, performance. Two such transmit diversity techniques are space time spreading (STS) and phase sweep transmit diversity (PSTD).  
           [0003]    [0003]FIG. 1 depicts a wireless communication system  10  employing STS. Wireless communication system  10  comprises at least one base station  12  having two antenna elements  14 - 1  and  14 - 2 , wherein antenna elements  14 - 1  and  14 - 2  are spaced far apart for achieving transmit diversity. Base station  12  receives a signal S for transmitting to mobile-station  16 . Signal S is alternately divided into signals s e  and s o , wherein signal s e  comprises even data bits and signal s o  comprises odd data bits. Signals s e  and s o  are processed to produce signals S 14-1  and S 14-2 . Specifically, s e  is multiplied with Walsh code w 1  to produce signal s e w 1 ; a conjugate of signal s o  is multiplied with Walsh code w 2  to produce signal s o *w 2 ; signal s o  is multiplied with Walsh code w 1  to produce s o w 1 ; and a conjugate of signal s e  is multiplied with Walsh code w 2  to produce s e *w 2 . Signal s e w 1  is added to signal s o *w 2  to produce signal S 14-1  (i.e., S 14-1 =s e w 1 +s o *w 2 ) and signal s e *w 2  is subtracted from signal s o w 1  to produce signal S 14-2  (i.e., S 14-2 =s o w 1 −s e *w 2 ). Signals S 14-1  and S 14-2  are transmitted at substantially equal or identical power levels over antenna elements  14 - 1  and  14 - 2 , respectively. For purposes of this application, power levels are “substantially equal” or “identical” when the power levels are within 1% of each other.  
           [0004]    Mobile-station  16  receives signal R comprising γ 1 (S 14-2 )+γ 2 (S 14-2 ), wherein γ 1  and γ 2  are distortion factor coefficients associated with the transmission of signals S 14-1  and S 14-2  from antenna elements  14 - 1  and  14 - 2  to mobile-station  16 , respectively. Distortion factor coefficients γ 1  and γ 2  can be estimated using pilot signals, as is well-known in the art. Mobile-station  16  decodes signal R with Walsh codes w 1  and w 2  to respectively produce outputs: 
             W   1 =γ 1   s   e +γ 2   s   o   equation 1 
             W   2 =γ 1   s   o *−γ 2   s   e *  equation 1a 
           [0005]    Using the following equations, estimates of signals s e  and s o , i.e., Ŝ e  and Ŝ o , may be obtained: 
             Ŝ   e =γ 1   *W   1 −γ 2   W*   2   =s   e (|γ 1 | 2 +|γ 2 | 2 )+noise  equation 2 
             Ŝ   o =γ 2   *W   1 +γ 1   W   2   *=s   o (|γ 1 | 2 +|γ 2 | 2 )+noise  equation 2a 
           [0006]    However, STS is a transmit diversity technique that is not backward compatible from the perspective of the mobile-station. That is, mobile-station  16  is required to have the necessary hardware and/or software to decode signal R. Mobile-stations without such hardware and/or software, such as pre-third generation mobile-stations, would be incapable of decoding signal R.  
           [0007]    By contrast, phase sweep transmit diversity (PSTD) is backward compatible from the perspective of the mobile-station. FIG. 2 depicts a wireless communication system  20  employing PSTD. Wireless communication system  20  comprises at least one base station  22  having two antenna elements  24 - 1  and  24 - 2 , wherein antenna elements  24 - 1  and  24 - 2  are spaced far apart for achieving transmit diversity. Base station  22  receives a signal S for transmitting to mobile-station  26 . Signal S is evenly power split into signals s 1  and s 2  and processed to produce signals S 24-1  and S 24-2 , where s 1 =s 2 . Specifically, signal s 1  is multiplied by Walsh code w k  to produce S 24-1 =s 1 w k , where k represents a particular user or mobile-station. Signal s 2  is multiplied by Walsh code w k  and a phase sweep frequency signal e J2 πf   t  to produce S 24-2 , i.e., S 24-2 =s 2 w k e J2 πf     s     t =s 1 w k e J2 πf     s      t =S 24-1 e J2 πf     s     t , where f s  is a phase sweep frequency and t is time. Signals S 24-1  and S 24-2  are transmitted at substantially equal power levels over antenna elements  24 - 1  and  24 - 2 , respectively. Note that the phase sweep signal e J2 πf     s     t  is being represented in complex baseband notation, i.e., e J2 πf     s     t =cos(2 πf s t)+j sin(2 πf s t). It should be understood that the phase sweep signal may also be applied at an intermediate frequency or a radio frequency.  
           [0008]    Mobile-station  26  receives signal R comprising γ 1 S 24-1 +γ 2 S 24-2 . Simplifying the equation for R results in 
             R=γ   1   S   24-1 +γ 2   S   24-1   e   J2 πf     s     t   equation 3 
             R=S   24-1 {γ 1 +γ 2   e   J2 πf     s     t }  equation 3a 
             R=S   24-1 γ eq   equation 3b 
           [0009]    where γ eq  is an equivalent channel seen by mobile-station  26 . Distortion factor coefficient γ eq  can be estimated using pilot signals and used, along with equation 3b, to obtain estimates of signal s 1  and/or s 2 .  
           [0010]    In slow fading channel conditions, PSTD improves performance (relative to when no transmit diversity technique is used) by making the received signal strength statistics associated with a slow fading channel at the receiver look like those associated with a fast fading channel. However, in additive white gaussan noise (AWGN) conditions, PSTD can significantly degrade performance. Accordingly, there exists a need for a transmit diversity technique that is backward compatible without significantly degrading performance in AGWN conditions.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention is a method and apparatus of tansmit diversity that is backward compatible and does not significantly degrade performance in additive white guassan noise (AWGN) conditions using a transmission architecture that incorporates a form of phase sweep transmit diversity (PSTD) referred to herein as biased PSTD. Biased PSTD involves transmitting a signal and a frequency swept version of the same signal over diversity antennas at different power levels. By transmitting the two signals at different power levels, the depths of nulls normally seen in AWGN conditions when PSTD is utilized is reduced and performance degradation in AWGN conditions is mitigated. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where  
         [0013]    [0013]FIG. 1 depicts a wireless communication system employing space time spreading techniques in accordance with the prior art;  
         [0014]    [0014]FIG. 2 depicts a wireless communication system employing phase sweep transmit diversity in accordance with the prior art; and  
         [0015]    [0015]FIG. 3 depicts a base station employing code division multiple access (CDMA) and a form of phase sweep transmit diversity (PSTD) referred to herein as biased PSTD in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]    [0016]FIG. 3 depicts a base station  30  employing code division multiple access (CDMA) and a form of phase sweep transmit diversity (PSTD) referred to herein as biased PSTD in accordance with the present invention. Biased PSTD involves transmitting a signal and a frequency swept version of the same signal over diversity antennas at different power levels to reduce the depths of nulls. Advantageously, biased PSTD is backwards compatible from the perspective of mobile-stations and does not degrade performance as much as PSTD in additive white gaussan noise (AWGN) conditions. CDMA is well-known in the art.  
         [0017]    Base station  30  provides wireless communication services to mobile-stations, not shown, in its associated geographical coverage area or cell, wherein the cell is divided into three sectors α, β, γ. Base station  30  includes a transmission architecture that biased PSTD, as will be described herein.  
         [0018]    Base station  30  comprises a processor  32 , a splitter  34 , multipliers  36 ,  38 , amplifiers  44 ,  46 , and a pair of diversity antennas  48 ,  50 . Note that base station  30  also includes configurations of splitters, multipliers, amplifiers and antennas for sectors β, γ that are identical to those for sector a. For simplicity sake, the configurations for sectors β, γ are not shown. Additionally, for discussion purposes, it is assumed that signals S k  are intended for mobile-stations k located in sector α and, thus, the present invention will be described with reference to signals S k  being processed for transmission over sector α.  
         [0019]    Processor  32  includes software for processing signals S k  in accordance with well-known CDMA techniques to produce an output signal S k−1 . Note that, in another embodiment, processor  32  is operable to process signals S k  in accordance with a multiple access technique other than CDMA, such as time or frequency division multiple access.  
         [0020]    Signal S k−1  is split by splitter  34  into signals S k−1 (a), S k−1 (b) and processed along paths A and B, respectively, by multipliers  36 ,  38 , and amplifiers  44 ,  46  in accordance with bias PSTD techniques, wherein signal S k−1 (a) is identical to signal S k−1 (b) in terms of data. In one embodiment, signal S k , is unevenly power split by splitter  34  such that the power level of signal S k−1 (a) is higher than the power level of signal S k−1 (b). For example, signal S k−1  is power split such that signal S k−1 (a) gets ⅝ of signal S k−1 &#39;s power and signal S k−1 (b) gets ⅜ of signal S k−1 &#39;s power, i.e., S k−1 (a)={square root}{square root over (⅝)}(S k−1 ) and S k−1 (b)={square root}{square root over (⅜)}(S k−1 ). In another example, signal S k−1  is power split such that signal S k−1 (a) gets ⅔ of signal S k−1 &#39;s power and signal S k−1 (b) gets ⅓ of signal S k−1 &#39;s power. In one embodiment, signal S k−1  is unevenly power split by splitter  34  such that the power level of signal S k−1 (b) is higher than the power level of signal S k−1 (a), or signal S k−1  is evenly power split into signals S k−1 (a), S k−1 (b). Signal S k−1 (a) and carrier signal e J2 πf     c     t  are provided as inputs into multiplier  36  to produce signal S 36 , where S 36 =S k−1 (a)e J2 πf     c     t , e J2 πf     c     t =cos(2 πf c T)+j sin(2 πf c t), f c  represents a carrier frequency and t represents time.  
         [0021]    Signal S k−1 (b), phase sweep frequency signal e JΘ     s     (t)  and carrier signal e J2 πf     c     t  are provided as inputs into multiplier  38  where signal S k−1 (b) is frequency phase swept with signal e jΘ     s     (t)  and modulated onto carrier signal e J2 πf     c     t  to produce signal S 38 =S k−1 (b)e J2 πf     c     t e JΘ     s     (t) , wherein Θ s =2 πf s t, e JΘ     s     (t) =cos(2 πf s t)+j sin(2 πf s t) and f s  represents a phase sweep frequency.  
         [0022]    Signals S 36 , S 38  are amplified by amplifiers  44 ,  46  to produce signals S 44  and S 46  for transmission over antennas  48 ,  50 , respectively, where signal S 44 =A 44 S k−1 (a)e J2 πf     c     t , S 46 =A 46 S k−1 (b)e J2 πf     c     t e JΘ     s     (t) , A 44  represents the amount of gain associated with amplifier  44  and A 46  represents the amount of gain associated with amplifier  46 .  
         [0023]    In one embodiment, the amounts of gain A 44 , A 46  are equal. In this embodiment, signal S k−1  is split by splitter  34  such that the power level of signal S k−1 (a) is higher than the power level of signal S k−1 (b), or vice-versa, so that differences in power level between signals S 44  and S 46  are not as large compared to an even power split of signal S k−1 .  
         [0024]    In another embodiment, the amounts of gain A 44 , A 46  are different and related to how splitter  34  power splits signal S k−1 . For example, the amount of gain A 44 , A 46  applied to signals S 36 , S 38  should be an amount that would cause the power levels of signals S 44  and S 46  to be approximately equal. For purposes of this application, power levels are “approximately equal” when the power levels are within 10% of each other. In another example, the signal, e.g., S 36  or S 38 , associated with a greater power level is amplified more than the other signal.  
         [0025]    In the case where signal s α−1  and/or signals S 36 , S 40  are not biased or unevenly split or amplified, STS performance will degrade because signal S 44  will be transmitted at approximately ⅓ of the power at which signal S 46  will be transmitted. Advantageously, biasing or unevenly splitting signal s α−1  and/or biasing or unevenly amplifying signals S 36 , S 40  mitigates this degradation to STS performance relative to the case where neither signal s α−1  nor signals S 36 , S 40  are biased or unevenly split or amplified.  
         [0026]    Although the present invention has been described in considerable detail with reference to certain embodiments, other versions are possible. Therefore, the spirit and scope of the present invention should not be limited to the description of the embodiments contained herein.