Split shift phase sweep transmit diversity

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

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] 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 se and so, wherein signal se comprises even data bits and signal so comprises odd data bits. Signals se and so are processed to produce signals S14-1 and S14-2. Specifically, se is multiplied with Walsh code w1 to produce signal sew1; a conjugate of signal so is multiplied with Walsh code w2 to produce signal so*w2; signal so is multiplied with Walsh code w1 to produce sow1; and a conjugate of signal se is multiplied with Walsh code w2 to produce se*w2. Signal sew1 is added to signal so*w2 to produce signal S14-1 (i.e., S14-1=sew1+so*w2) and signal se*w2 is subtracted from signal sow1 to produce signal S14-2 (i.e., S14-2=sow1−se*w2). Signals S14-1 and S14-2 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 γ1(S14-2)+γ2(S14-2), wherein γ1 and γ2 are distortion factor coefficients associated with the transmission of signals S14-1 and S14-2 from antenna elements 14-1 and 14-2 to mobile-station 16, respectively. Distortion factor coefficients γ1 and γ2 can be estimated using pilot signals, as is well-known in the art. Mobile-station 16 decodes signal R with Walsh codes w1 and w2 to respectively produce outputs:

W 1 =γ1se+γ2so  equation 1

W 2 =γ1so*−γ2se*  equation 1a

[0005] Using the following equations, estimates of signals se and so, i.e., ŝe and ŝo, may be obtained:

ŝ e =γ1*W1−γ2W2*=se(|γ1|2+|γ2|2)+noise  equation 2

ŝ o =γ2*W1+γ1W2*=so(|γ1|2+|γ2|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 s1 and s2 and processed to produce signals S24-1 and S24-2, where s1=s2. Specifically, signal s1 is multiplied by Walsh code wk to produce S24-1=s1wk, where k represents a particular user or mobile-station. Signal s2 is multiplied by Walsh code wk and a phase sweep frequency signal ej2πfst to produce S24-2, i.e., S24-2=s2wkej2πfst=s1wkej2πfst=S24-1ej2πfst, where fs is a phase sweep frequency and t is time. Signals S24-1 and S24-2 are transmitted at substantially equal power levels over antenna elements 24-1 and 24-2, respectively. Note that the phase sweep signal ej2πfst is being represented in complex baseband notation, i.e., ej2πfst=cos(2πfst)+j sin(2πfst). 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 γ1S24-1+γ2S24-2. Simplifying the equation for R results in

R=γ 1 S 24-1 +γ2S24-1ej2πfst  equation 3

R=S 24-1 {γ1+γ2ej2πfst}  equation 3a

R=S 24-1 γeq  equation 3b

[0009] where γeq is an equivalent channel seen by mobile-station 26. Distortion factor coefficient γeq can be estimated using pilot signals and used, along with equation 3b, to obtain estimates of signal s1 and/or s2.

[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, 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

[0011] 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

[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] FIG. 1 depicts a wireless communication system employing space time spreading techniques in accordance with the prior art;

[0014] FIG. 2 depicts a wireless communication system employing phase sweep transmit diversity in accordance with the prior art; and

[0015] 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

[0016] 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.

[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 split shift PSTD, as will be described herein.

[0018] 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 Sk are intended for mobile-stations k located in sector α and, thus, the present invention will be described with reference to signals Sk being processed for transmission over sector α.

[0019] Processor 32 includes software for processing signals Sk in accordance with well-known CDMA techniques to produce an output signal Sk−1. Note that, in another embodiment, processor 32 is operable to process signals Sk in accordance with a multiple access technique other than CDMA, such as time or frequency division multiple access.

[0020] Signal Sk−1 is split by splitter 34 into signals Sk−1(a), Sk−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 Sk−1(a) is identical to signal Sk−1(b) in terms of data. In one embodiment, signal Sk−1 is unevenly power split by splitter 34 such that the power level of signal Sk−1(a) is higher than the power level of signal Sk−1(b). For example, signal Sk−1 is power split such that signal Sk−1(a) gets ⅝ of signal Sk−1's power and signal Sk−1(b) gets ⅜ of signal Sk−1's power, i.e., Sk−1(a)={square root}{square root over (⅝)}(Sk−1) and Sk−1(b)={square root}{square root over (⅜)}(Sk−1). In another example, signal Sk−1 is power split such that signal Sk−1(a) gets ⅔ of signal Sk−1's power and signal Sk−1(b) gets ⅓ of signal Sk−1's power. In one embodiment, signal Sk−1 is unevenly power split by splitter 34 such that the power level of signal Sk−1(b) is higher than the power level of signal Sk−1(a), or signal Sk−1 is evenly power split into signals Sk−1(a), Sk−1(b).

[0021] Signal Sk−1(a) and phase sweep frequency signal ejΘs(t) are provided as inputs into multiplier 36 where signal Sk−1(a) is phase swept with phase sweep frequency signal ejΘs(t) to produce signal S36=Sk−1(a)ejΘs(t), wherein Θs=2πfst, ejΘs(t)=cos(2πfst)+j sin(2πfst), fs represents a phase sweep frequency and t represents time. Signal Sk−1(b) and phase sweep frequency signal e−jΘs(t) are provided as inputs into multiplier 38 where signal Sk−1(b) is frequency phase swept with signal e−jΘs(t) to produce signal S38=Sk−1(b)e−jΘs(t). In another embodiment, phase sweep frequency signal e−jΘs(t) is used to phase sweep signal Sk−1(a), and phase sweep frequency signal ejΘs(t) is used to phase sweep signal Sk−1(b).

[0022] Note that phase sweep frequency signals ejΘs(t), e−jΘs(t) phase sweeps signals Sk−1(a), Sk−1(b) an equal amount but in opposite directions. Advantageously, this choice of phase sweep frequency signals ejΘs(t), e−jΘs(t) results in the energy of the transmitted signals at mobile-stations to be concentrated at or near a carrier frequency fc. In other embodiments, the phase sweep frequency signals used to phase sweep Sk−1(a), Sk−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.

[0023] Signal S36 and carrier signal ej2πfct are provided as inputs into multiplier 40 to produce signal S40, where S40=Sk−1(a)ejΘs(t) ej2πfct, ej2πfct=cos(2πfct)+j sin(2πfct). Similarly, signal S38 and carrier signal ej2πfct are provided as inputs into multiplier 42 to produce signal S42, where S42=Sk−1(b)e−jΘs(t) ej2πfct.

[0024] Signals S40, S42 are amplified by amplifiers 44, 46 to produce signals S44 and S46 for transmission over antennas 48, 50, respectively, where signal S44=A44Sk−1(a)ejΘs(t) ej2πfct, S46=A46Sk−1(b)e−jΘs(t) ej2πfct, A44 represents the amount of gain associated with amplifier 44 and A46 represents the amount of gain associated with amplifier 46.

[0025] In one embodiment, the amounts of gain A44, A46 are equal. In this embodiment, signal Sk−1 may be split by splitter 34 such that the power level of signal Sk−1(a) is higher than the power level of signal Sk−1(b), or vice-versa, so that differences in power level between signals S44 and S46 are not as large compared to an even power split of signal Sk−1. Alternately, signal Sk−1 may be equally split by splitter 34.

[0026] In another embodiment, the amounts of gain A44, A46 are different and related to how splitter 34 power splits signal Sk−1. For example, the amount of gain A44, A46 applied to signals S36, S38 may be an amount that would cause the power levels of signals S44 and S46 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., S36 or S38, associated with a greater power level is amplified more than the other signal.

[0027] 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.

Claims

1. A method of signal transmission comprising the steps of: splitting a signal s1 into signals s1(a) and s1(b), wherein the signal s1 is split unevenly such that the signal s1(a) has an associated power level greater than a power level associated with the signal s1(b); phase sweeping the signal s1(a) using a first phase sweep frequency signal to produce a phase swept signal s1(a); and phase sweeping the signal s1(b) using a second phase sweep frequency signal to produce a phase swept signal s1(b), wherein the phase swept signal s1(a) has a different phase from the phase swept signal s1(b).

2. The method of claim 1, wherein the first phase sweep frequency signal phase sweeps the signal s1(a) in a direction opposite to a direction the second phase sweep frequency signal phase sweeps the signal s1(b).

3. The method of claim 2, wherein a first phase sweep frequency associated with the first phase sweep frequency signal is identical to a second phase sweep frequency associated with the second phase sweep frequency signal.

4. The method of claim 2, wherein a first phase sweep frequency associated with the first phase sweep frequency signal is not identical to a second phase sweep frequency associated with the second phase sweep frequency signal.

5. The method of claim 2, wherein a first phase sweep frequency associated with the first phase sweep frequency signal is a fixed phase shifting rate.

6. The method of claim 2, wherein a first phase sweep frequency associated with the first phase sweep frequency signal is a variable phase shifting rate.

7. The method of claim 2, wherein a second phase sweep frequency associated with the second phase sweep frequency signal is a fixed phase shifting rate.

8. The method of claim 2, wherein a second phase sweep frequency associated with the second phase sweep frequency signal is a variable phase shifting rate.

9. The method of claim 1, wherein the first and second phase sweep frequency signals phase sweep the signals s1(a) and s1(b) in a same direction.

10. The method of claim 9, wherein a first phase sweep frequency associated with the first phase sweep frequency signal is identical to a second phase sweep frequency associated with the second phase sweep frequency signal.

11. The method of claim 9, wherein a first phase sweep frequency associated with the first phase sweep frequency signal is not identical to a second phase sweep frequency associated with the second phase sweep frequency signal.

12. The method of claim 1 comprising the additional step of: amplifying the phase swept signals s1(a) and s1(b).

13. The method of claim 1 comprising the additional step of: transmitting the phase swept signals s1(a) and s1(b) over a pair of diversity antennas.

14. A method of signal transmission comprising the steps of: splitting a signal s1 into signals s1(a) and s1(b), wherein the signal s1 includes a communication signal; phase sweeping the signal s1(a) using a first phase sweep frequency signal to produce a phase swept signal s1(a); and phase sweeping the signal s1(b) using a second phase sweep frequency signal to produce a phase swept signal s1(b), wherein the phase swept signal s1(a) has a different phase from the phase swept signal s1(b).

15. The method of claim 14, wherein the first phase sweep frequency signal phase sweeps the signal s1(a) in a direction opposite to a direction the second phase sweep frequency signal phase sweeps the signal s1(b).

16. The method of claim 15, wherein a first phase sweep frequency associated with the first phase sweep frequency signal is identical to a second phase sweep frequency associated with the second phase sweep frequency signal.

17. The method of claim 15, wherein a first phase sweep frequency associated with the first phase sweep frequency signal is not identical to a second phase sweep frequency associated with the second phase sweep frequency signal.

18. The method of claim 15, wherein a first phase sweep frequency associated with the first phase sweep frequency signal is a fixed or a variable phase shifting rate.

19. The method of claim 15, wherein a second phase sweep frequency associated with the second phase sweep frequency signal is a fixed or variable phase shifting rate.

20. The method of claim 14, wherein the first and second phase sweep frequency signals phase sweep the signals s1(a) and s1(b) in a same direction.

21. The method of claim 20, wherein a first phase sweep frequency associated with the first phase sweep frequency signal is identical to a second phase sweep frequency associated with the second phase sweep frequency signal.

22. The method of claim 20, wherein a first phase sweep frequency associated with the first phase sweep frequency signal is not identical to a second phase sweep frequency associated with the second phase sweep frequency signal.

23. The method of claim 14 comprising the additional step of: amplifying the phase swept signals s1(a) and s1(b).