Abstract:
Disclosed is a method and apparatus of transmit diversity that is backward compatible and does not degrade performance using a transmission architecture that incorporates a form of phase sweep transmit diversity (PSTD) referred to herein as split shift PSTD. Split shift PSTD involves transmitting at least two phase swept versions of a signal over diversity antennas, wherein the two phase swept versions of the signal have a different phase. The phase sweep frequency signals may have a fixed or varying phase shifting rate, may have an identical or different phase shifting rate, may be offset from each other and/or may be phase shifting in the same or opposite direction.

Description:
RELATED APPLICATION 
   Related subject matter is disclosed in the following applications filed concurrently and assigned to the same assignee hereof: U.S. patent application Ser. No. 09,918,393 entitled, “Biased Phase Sweep Transmit Diversity,” inventors Roger Benning, R. Michael Buehrer and Robert Atmaram Soni; U.S. patent application Ser. No. 09/918,392 entitled, “Symmetric Sweep Phase Sweep Transmit Diversity,” inventors Roger Benning, R. Michael Buebrer, Paul A. Polakos and Mark Kraml; and U.S. patent application Ser. No. 09/918,391 entitled, “Space Time Spreading and Phase Sweep Transmit Diversity,” inventors Roger Benning. R. Michael Buehrer, Paul A. Polakos and Robert Atmaram Soni. 
   BACKGROUND OF THE RELATED ART 
   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). 
     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. 
   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
 
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
 
   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. 
   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 j2πf     s     t  to produce S 24-2 , i.e., S 24-2 =s 2 w k e j2πf     s     t =s 1 w k e j2πf     s     t =S 24-1 e j2πf     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 j2πf     s     t  is being represented in complex baseband notation, i.e., e j2πf     s     t =cos(2πf s t)+j sin(2πf s t). It should be understood that the phase sweep signal may also be applied at an intermediate frequency or a radio frequency. 
   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   j2πf     s     t   equation 3
 
 R=S   24-1 {γ 1 +γ 2   e   j2πf     s     t }  equation 3a
 
 R=S   24-1 γ eq   equation 3b
 
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 .
 
   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, PSTD causes the energy of the transmitted signals to be concentrated at some frequency between the carrier frequency and the phase sweep frequency. If the frequency at which the transmitted signals are concentrated is not within some frequency tolerance of a mobile-station or receiver to which the signals are intended, the mobile-station or receiver may not be able to or may have difficulty receiving or processing the signals which, in turn, may degrade performance. Accordingly, there exists a need for a transmit diversity technique that is backward compatible without degrading performance. 
   SUMMARY OF THE INVENTION 
   The present invention is a method and apparatus of transmit diversity that is backward compatible and does not degrade performance using a transmission architecture that incorporates a form of phase sweep transmit diversity (PSTD) referred to herein as split shift PSTD. Split shift PSTD involves transmitting at least two phase swept versions of a signal over diversity antennas, wherein the two phase swept versions of the signal have a different frequency or phase sweep rate. In one embodiment, a signal is split into a first and a second signal. The first and second signal are phase swept in equal and opposite directions using different phase sweep frequency signals, which would allow energies associated with the transmitted signals to be concentrated near a carrier frequency. In other embodiments, the phase sweep frequency signals may have a fixed or varying phase shifting rate, may have an identical or different phase shifting rate, may be offset from each other and/or may be phase shifting in the same or opposite direction. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 
       FIG. 1  depicts a wireless communication system employing space time spreading techniques in accordance with the prior art; 
       FIG. 2  depicts a wireless communication system employing phase sweep transmit diversity in accordance with the prior art; and 
       FIG. 3  depicts a base station employing split shift phase sweep transmit diversity (PSTD) and code division multiple access (CDMA) in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 3  depicts a base station  30  employing split shift phase sweep transmit diversity (PSTD) and code division multiple access (CDMA) in accordance with the present invention. Split shift PSTD involves transmitting at least two phase swept versions of a signal over diversity antennas, wherein the two phase swept versions of the signal have a different phase. In one embodiment, a signal is split into a first and a second signal. The first and second signal are phase swept in equal and opposite directions using different phase sweep frequency signals, which would allow energies associated with the transmitted signals to be concentrated near a carrier frequency. In other embodiments, the phase sweep frequency signals may have a fixed or varying phase shifting rate, may have an identical or different phase shifting rate, and/or may be phase shifting in the same or opposite direction. Advantageously, split shift PSTD is backwards compatible from the perspective of mobile-stations. CDMA is well-known in the art. 
   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 split shift PSTD, as will be described herein. 
   Base station  30  comprises a processor  32 , a splitter  34 , multipliers  36 ,  38 ,  40 ,  42 , 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 α. 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. 
   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. 
   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 ,  40 ,  42 , and amplifiers  44 ,  46  in accordance with split shift 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−1  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 over (⅝)} (S k−1 ) and S k−1 (b)= √{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 phase sweep frequency signal e jΘ     s     (t)  are provided as inputs into multiplier  36  where signal S k−1 (a) is phase swept with phase sweep frequency signal e jΘ     s     (t)  to produce signal S 36 =S k−1 (a)e jΘ     s     (t) , wherein Θ s =2πf s t, e jΘ     s     (t) =cos(2πf s t)+j sin(2πf s t), f s  represents a phase sweep frequency and t represents time. Signal S k−1 (b) and phase sweep frequency signal e −jΘ     s     (t)  are provided as inputs into multiplier  38  where signal S k−1 (b) is frequency phase swept with signal e −jΘ     s     (t)  to produce signal S 38 =S k−1 (b)e −jΘ     s     (t) . In another embodiment, phase sweep frequency signal e −jΘ     s     (t)  is used to phase sweep signal S k−1 (a), and phase sweep frequency signal e jΘ     s     (t)  is used to phase sweep signal S k−1 (b). 
   Note that phase sweep frequency signals e jΘ     s     (t) , e −jΘ     s     (t)  phase sweeps signals S k−1 (a), S k−1 (b) an equal amount but in opposite directions. Advantageously, this choice of phase sweep frequency signals e jΘ     s     (t) , e −jΘ     s     (t)  results in the energy of the transmitted signals at mobile-stations to be concentrated at or near a carrier frequency f c . In other embodiments, the phase sweep frequency signals used to phase sweep S k−1 (a), S k−1 (b) may have a fixed or varying phase shifting rate, may have an identical or different phase shifting rate, may be offset from each other and/or may be phase shifting in the same or opposite direction. 
   Signal S 36  and carrier signal e j2πf     c     t  are provided as inputs into multiplier  40  to produce signal S 40 , where S 40 =S k−1 (a)e jΘ     s     (t)  e j2πf     c     t , e j2πf     c     t =cos(2πf c t)+j sin(2πf c t). Similarly, signal S 38  and carrier signal e j2πf     c     t  are provided as inputs into multiplier  42  to produce signal S 42 , where S 42 =S k−1 (b)e −jΘ     s     (t)  e j2πf     c     t . 
   Signals S 40 , S 42  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 jΘ     s     (t)  e j2πf     c     t , S 46 =A 46 S k−1 (b)e −jΘ     s     (t)  e j2πf     c     t , A 44  represents the amount of gain associated with amplifier  44  and A 46  represents the amount of gain associated with amplifier  46 . 
   In one embodiment, the amounts of gain A 44 , A 46  are equal. In this embodiment, signal S k−1  may be 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 . Alternately, signal S k−1  may be equally split by splitter  34 . 
   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  may 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. 
   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.