Patent Document

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, “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, “Biased Phase Sweep Transmit Diversity,” inventors Roger Benning, R. Michael Buehrer and Robert Atmaram Soni; and 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 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.  
           [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   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  
           [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, both transmit diversity techniques, i.e., STS and PSTD, improve 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 does not provide the same amount of overall performance improvement as STS. Additionally, in additive white gaussan noise (AWGN) conditions, PSTD can significantly degrade performance, whereas STS neither improves nor degrades performance. Accordingly, there exists a need for a transmission technique that provides the performance of STS and the backwards compatibility of PSTD without significantly degrading performance in AGWN conditions.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention is a method and apparatus for transmission that provides the performance of space time spreading (STS) or orthogonal transmit diversity (OTD) and the backwards compatibility of phase sweep transmit diversity (PSTD) without significantly degrading performance in additive white guassan noise (AWGN) conditions using a transmission architecture that incorporates STS/OTD and a form of PSTD referred to herein as biased PSTD, which 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 normally seen in AWGN conditions when PSTD is utilized.  
           [0012]    In one embodiment, a signal s 1  comprising a non-STS/OTD signal and a first STS/OTD signal belonging to an STS/OTD pair is split into two signals s 1 (a) and s 1 (b), wherein the power level of signal s 1 (a) is higher than the power level of signal s 1 (b). The signal s 1 (b) is phase swept using a phase sweep frequency signal. Thus, signal s 1  is processed in accordance with biased PSTD. The phase swept signal s 1 (b) is added to a signal s 2  to produce a summed signal, wherein signal s 2  comprises a second STS/OTD signal belonging to the STS/OTD pair. The summed signal and the signal s 1 (a) are amplified and transmitted over a pair of diversity antennas. The amount of gain applied to the summed signal and the signal s 1 (a) may be equal or unequal such that the amplified summed signal and the amplified signal s 1 (a) are at approximately equal power levels. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    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  
         [0014]    [0014]FIG. 1 depicts a wireless communication system employing space time spreading techniques in accordance with the prior art;  
         [0015]    [0015]FIG. 2 depicts a wireless communication system employing phase sweep transmit diversity in accordance with the prior art;  
         [0016]    [0016]FIG. 3 depicts a base station employing code division multiple access (CDMA), a form of phase sweep transmit diversity (PSTD) referred to herein as biased PSTD, and space time spreading (STS) or orthogonal transmit diversity (OTD) in accordance with one embodiment of the present invention;  
         [0017]    [0017]FIG. 4 depicts a base station employing CDMA, biased PSTD, and STS or OTD in accordance with another embodiment of the present invention; and  
         [0018]    [0018]FIG. 5 depicts a base station employing CDMA, biased PSTD, STS or OTD, and split shift PSTD in accordance with another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]    [0019]FIG. 3 depicts a base station  30  employing code division multiple access (CDMA), a form of phase sweep transmit diversity (PSTD) referred to herein as biased PSTD, and space time spreading (STS) or orthogonal transmit diversity (OTD) in accordance with the present invention. CDMA, PSTD, STS and OTD are well-known in the art.  
         [0020]    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 incorporates STS or OTD and biased PSTD, as will be described herein.  
         [0021]    Base station  30  comprises a processor  32 , a splitter  34 , multipliers  36 ,  38 ,  40 , adder  42 , amplifiers  44 ,  46 , and a pair of diversity antennas  48 ,  50 . Note that base station  30  also includes configurations of splitters, multipliers, adders, amplifiers and antennas for sectors β, γ that are identical to those for sector α. For simplicity sake, the configuration 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.  
         [0022]    Processor  32  includes software for processing signals S k  in accordance with well-known CDMA and STS/OTD techniques, where STS/OTD indicates STS and/or OTD. The manner in which a particular signal S k  is processed by processor  32  depends on whether mobile-station k is STS/OTD compatible, i.e., mobile-station capable of decoding signals processed using STS/OTD. Processor  32  may also include software for determining whether mobile-station k is STS/OTD compatible. If mobile-station k is not STS/OTD compatible, then signal S k  is processed in accordance with CDMA techniques to produce signal S k-1 , which is also referred to herein as a non-STS/OTD signal S k-1 .  
         [0023]    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. In this embodiment, when mobile-station k is not STS/OTD compatible, then signal S k  is processed in accordance with such other multiple access technique to produce the non-STS/OTD signal S k-1 .  
         [0024]    If mobile-station k is STS/OTD compatible, then signal S k  is processed in accordance with CDMA and STS/OTD. Specifically, if mobile-station k is STS compatible, then signal S k  is processed using STS. Such process includes alternately dividing signal S k  into signals s e  and s o , wherein signal s e  comprises even data bits and signal s o  comprises odd data bits. Signal s e  is multiplied with Walsh code w 1  to produce signal s e w 1 , and a conjugate of signal s e  is multiplied with Walsh code w 2  to produce s e *w 2 . Signal s o  is multiplied with Walsh code w 1  to produce s o w 1 , and a conjugate of signal s o  is multiplied with Walsh code w 2  to produce signal s o *w 2 . Signal s e w 1  is added to signal s o *w 2  to produce signal S k-2 (a)=s e w 1 +s o *w 2 . Signal s e *w 2  is subtracted from signal s o w 1  to produce signal S k-2 (b)=s o w 1 −s e *w 2 ). Signals S k-2 (a), S k-2 (b) are also referred to herein as STS signals, and together signals S k-2 (a), S k-2 (b) collectively comprise an STS pair.  
         [0025]    If mobile-station k is OTD compatible, then signal S k  is processed using OTD. Orthogonal transmit diversity involves dividing signal S k  into signals s e  and s o , and multiplying signals s e  and s o  using Walsh codes w 1 , w 2  to produce signals S k-3 (a), S k-3 (b), i.e., S k-3 (a)=s e w 1  and S k-3 (b)=s o w 2 , respectively. Signals S k-3 (a), S k-3 (b) are also referred to herein as OTD signals, and together signals S k-3 (a), S k-3 (b) collectively comprise an OTD pair.  
         [0026]    For illustration purposes, the present invention will be described herein with reference to STS and signals S k-2 (a), S k-2 (b). It should be understood that the present invention is also applicable to OTD and signals S k-3 (a), S k-3 (b).  
         [0027]    The output of processor  32  are signals s α-1 , s α-2 , where signal s α-1  comprises of signals S k-1  and S k-2 (a) and signal s α-2  comprises of signals S k-2 (b), i.e., S α-1 =ΣS k-1 +ΣS k-2 (a) and s α-2 =ΣS k-2 (b). That is, signals intended for STS compatible mobile-stations are included in both output signals s α-1 , s α-2  and signals intended for non-STS compatible mobile-stations are included in only signal s α-1 . Alternately, signal s α-1  comprises of signals S k-1  and S k-2 (b) and signal s α-2  comprises of signals S k-2 (a).  
         [0028]    Signal s α-1  is split by splitter  34  into signals s α-1 (a), s α-1 (b) and processed along paths A and B, respectively, by multipliers  36 ,  38 ,  40 , adder  42  and amplifiers  44 ,  46  in accordance with bias PSTD techniques. Basically, biased PSTD involves transmitting a signal and a frequency swept version of the same signal over diversity antennas at different power levels. 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.  
         [0029]    In one embodiment, signal s α-1  is unevenly power split by splitter  34  such that the power level of signal s α-1 (a) is higher than the power level of signal s α-1 (b). For example, signal s α-1  is power split such that signal s α-1 (a) gets ⅝ of signal s α-1 &#39;s power and signal s α-1 (b) gets ⅜ of signal s α-1 &#39;s power, i.e., s α-1 (a)={square root}{square root over (⅝)} (s α-1 ) and s α-1 (b)={square root}{square root over (⅜)} (s α-1 ). In another example, signal s α-1  is power split such that signal s α-1 (a) gets ⅔ of signal s α-1 &#39;s power and signal s α-1 (b) gets ⅓ of signal s α-1 &#39;s power. In another embodiment, signal s α-1  is evenly power split by splitter  34 . Note that signal s α-1 (a) is identical to signal s α-1 (b) in terms of data. Signal s α-1 (a) and carrier signal e −j2πf     c     t  are provided as inputs into multiplier  36  to produce signal S 36 , where S 36 =s α-1 (a)e −j2πf     c     t , e −j2πf     c     t =cos(2πf c t)+j sin(2πf c t), f c  represents a carrier frequency and t represents time.  
         [0030]    Signal s α-1 (b) and phase sweep frequency signal e −jΘ     3     (t)  are provided as inputs into multiplier  38  where signal s α-1 (b) is frequency phase swept with signal e −jΘ     s     (t)  to produce signal S 38 =s α-1 (b)e −jΘ     s     (t) , wherein Θ s =2πf s t, e −jΘ     s     (t) =cos(2πf s t)+j sin(2πf s t) and f s  represents a phase sweep frequency.  
         [0031]    Signal S 38  is added to signal s α-2  by adder  42  to produce signal S 42 =s α-1 (b)e −jΘ     s     (t) +s α-2 . Signal S 42  and carrier signal e −j2πf     c     t  are provided as inputs into multiplier  40  to produce signal S 40 , where S 40 =(s α-1 (b)e −jΘ     s     (t) +s α-2 )e −j2πf     c     t .  
         [0032]    Signals S 36 , S 40  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 α-1 (a)e −j2πf     c     t , S 46 =A 46 (s α-1 (b)e −jΘ     s     (t) +s α-2 )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 .  
         [0033]    In one embodiment, the amounts of gain A 44 , A 46  are equal. In this embodiment, signal s α-1  is split by splitter  34  such that the power level of signal s α-1 (a) is higher than the power level of signal s α-1 (b) 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 α-1 .  
         [0034]    In another embodiment, the amounts of gain A 44 , A 46  are different and related to how splitter  34  power splits signal s α-1 . Specifically, the amount of gain A 44 , A 46  applied to signals S 36 , S 40  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. For example, suppose the power levels of both signals s α-1 , s α-2  are x and splitter  34  splits signal s α-1  such that the power levels of signals s α-1 (a), s α-1 (b) are 7/8x and 1/8x, respectively. After signal signals s α-2 is added to signal S 38  by adder  42 , the power level of the resultant signal S 42  is 9/8x. In this example, the amount of gains A 44 , A 46  might be 8/7 and 8/9, respectively.  
         [0035]    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.  
         [0036]    [0036]FIG. 5 depicts a base station  70  employing CDMA, biased PSTD, STS/OTD and split shift PSTD in accordance an embodiment of the present invention. In this embodiment, , a form of PSTD referred to herein as split shift PSTD in also utilized. Spilt shift PSTD involves shifting both signals split from a single signal using phase sweep frequency signals that sweeps both signals in opposite direction. As shown in FIG. 5, signals s α-1 (a), is phase swept by multiplier  39  using phase sweep frequency signals e jΘ     s     (t) . Although this embodiment depicts phase sweep frequency signal e jΘ     s     (t)  equal and opposite to phase sweep frequency signals e −jΘ     s     (t) , it should be understood that the phase sweep frequency signal used to phase sweep signals s α-1 (a) need not be equal in magnitude. In another embodiment, signal s α-1 (a) is phase swept using a phase sweep frequency signal that results in phase swept signal s α-1 (a) with a desired or other phase difference to phase swept signal s α-1 (b). Note that that the phase sweep frequency signal used to phase sweep signals s α-1 (a), s α-1 (b) may be phase shifting at an identical or different rate from each other, may be phase shifting at fixed and/or varying rates, or may be phase shifting in the same or opposite direction.  
         [0037]    Although the present invention has been described in considerable detail with reference to certain embodiments, other versions are possible. For example, phase sweeping may be performed along path A instead of path B, i.e., signal s α-1 (a) is phase swept with signal e −jΘ     s     (t) . FIG. 4 depicts a base station  60  in which phase sweeping is performed along path A instead of path B. Therefore, the spirit and scope of the present invention should not be limited to the description of the embodiments contained herein.

Technology Category: 5