Pulse or digital communications – Transmitters – Plural diversity
Reexamination Certificate
2002-02-20
2004-11-09
Ghebretinsae, Temesghen (Department: 2631)
Pulse or digital communications
Transmitters
Plural diversity
C375S267000, C455S101000
Reexamination Certificate
active
06816557
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and apparatus for achieving transmit diversity in telecommunication systems and, more particularly, to a method and apparatus for non-zero complex weighting and space-time coding signals for transmission on multiple antennas.
BACKGROUND OF THE INVENTION
As wireless communication systems evolve, wireless system design has become increasingly demanding in relation to equipment and performance requirements. Future wireless systems, which will be third and fourth generation systems compared to the first generation analog and second generation digital systems currently in use, will be required to provide high quality high transmission rate data services in addition to high quality voice services. Concurrent with the system service performance requirements there will be equipment design constraints, which will strongly impact the design of mobile terminals. The third and fourth generation wireless mobile terminals will be required to be smaller, lighter, more power-efficient units that are also capable of providing the sophisticated voice and data services required of these future wireless systems.
Time-varying multi-path fading is an effect in wireless systems whereby a transmitted signal propagates along multiple paths to a receiver causing fading of the received signal due to the constructive and destructive summing of the signals at the receiver. Several methods are known for overcoming the effects of multi-path fading, such as time interleaving with error correction coding, implementing frequency diversity by utilizing spread spectrum techniques, or transmitter power control techniques. Each of these techniques, however, has drawbacks in regard to use for third and fourth generation wireless systems. Time interleaving may introduce unnecessary delay, spread spectrum techniques may require large bandwidth allocation to overcome a large coherence bandwidth, and power control techniques may require higher transmitter power than is desirable for sophisticated receiver-to-transmitter feedback techniques that increase mobile terminal complexity. All of these drawbacks have negative impact on achieving the desired characteristics for third and fourth generation mobile terminals.
Antenna diversity is another technique for overcoming the effects of multi-path fading in wireless systems. In diversity reception, two or more physically separated antennas are used to receive a transmitted signal, which is then processed by combining and switching to generate a received signal. A drawback of diversity reception is that the physical separation required between antennas may make diversity reception impractical for use on the forward link in the new wireless systems where small mobile terminal size is desired. A second technique for implementing antenna diversity is transmit diversity. In transmit diversity a signal is transmitted from two or more antennas and then processed at the receiver by using e.g. maximum likelihood sequence estimator (MLSE), minimum mean square error (MMSE) receivers, Maximum-a Posteriori receivers, or their approximations. Transmit diversity has more practical application to the forward link in wireless systems in that it is easier to implement multiple antennas in the base station than in the mobile terminal.
Transmit diversity for the case of two antennas is well studied. Alamouti has proposed a method of transmit diversity for two antennas that offers second order diversity for complex valued signals. S. Alamouti, “A Simple Transmit Diversity Technique for Wireless Communications,”
IEEE Journal on Selected Areas of Communications
, pp. 1451-1458, October 1998. The Alamouti method involves simultaneously transmitting two signals from two antennas during a symbol period. During one symbol period, the signal transmitted from a first antenna is denoted by S
0
and the signal transmitted from the second antenna is denoted by S
1
. During the next symbol period, the signal −S
1
* is transmitted from the first antenna and the signal S
0
* is transmitted from the second antenna, where * is the complex conjugate operator. A similar diversity transmission system may also be realized in code domain. As an example, two copies of the same symbol can be transmitted in parallel using two orthogonal Walsh codes. Similar techniques can be also used to construct a space-frequency coding method.
Extension of the Alamouti method to more than two antennas is not straightforward. Tarokh et al. have proposed a method using rate=½, and ¾ SpaceTime Block codes for transmitting on three and four antennas using complex signal constellations. V. Tarokh, H. Jafarkhani, and A. Calderbank, “Space-Time Block Codes from Orthogonal Designs,”
IEEE Transactions on Information Theory
, pp. 1456-1467, July 1999. This method has a disadvantage in a loss in transmission rate and the fact that the multi-level nature of the ST coded symbols increases the peak-to-average ratio requirement of the transmitted signal and imposes stringent requirements on the linear power amplifier design. Additional techniques that mitigate these problems are proposed in O. Tirkkonen and A. Hottinen, “Complex space-time block codes for four Tx antennas,” Proc. Globecom 2000, November 2000, San Francisco, USA. Other methods proposed include a rate=1, orthogonal transmit diversity (OTD)+space-time transmit diversity scheme (STTD) four antenna method. L. Jalloul, K. Rohani, K. Kuchi, and J. Chen, “Performance Analysis of CDMA Transmit Diversity Methods,”
Proceedings of IEEE Vehicular Technology Conference
, Fall 1999, and M. Harrison, K. Kuchi, “Open and Closed Loop Transmit Diversity at High Data Rates on 2 and 4 Elements,”
Motorola Contribution
to 3GPP-C30-19990817-017. This method requires an outer code and offers second order diversity due to the STTD block (Alamouti block) and a second order interleaving gain from use of the OTD block. The performance of this method depends on the strength of the outer code. Since this method requires an outer code, it is not applicable to uncoded systems. For the case of rate=⅓ convolutional code, the performance of the OTD +STTD method and the Tarokh rate=¾ method ST block code methods are about the same. Another rate
1
method is proposed in O. Tirkkonen, A. Boariu, and A. Hottinen, “Minimal non-orthogonality rate
1
space-time block code for 3+Tx antennas,” in Proc. ISSSTA 2000, September 2000. The method proposed in this publication attains high performance but requires a complex receiver.
It would be advantageous, therefore, to have a method and apparatus that provided the advantage of transmit diversity on greater than two antennas while at the same time not greatly increasing the complexity of system design.
SUMMARY OF THE INVENTION
The present invention presents a method and apparatus for non-zero complex weighting and space-time coding signals for transmission on multiple antennas. The method and apparatus provides expansion of an N×N′ space-time block code, where N is the number of transmit paths and N′ is the number of output symbols per transmit path, to a M×M′ space-time block code, where M>N, generated by using repetition and non-zero complex weighting of the symbols within the N×N′ space time block code, to allow transmission of the space time block code on a number M of diversity transmit paths. The diversity transmit paths may comprise separate antennas or beams. The temporal length of the larger code M′, may equal the temporal length of the original code, N′. In the method and apparatus, a transform is performed on an input symbol stream, to generate a transform result comprising a space-time block code. The N output streams of the space-time block code, each consisting of N′ output symbols, are then repeated and at least one of the repeated streams non-zero complex weighted over time to generate M streams of N′ output symbols for transmission on M diversity transmi
Hottinen Ari
Kaipainen Yrjo
Kuchi Kiran
Kuusela Markku
Tirkkonen Olav
Fraccaroli Federico
Ghebretinsae Temesghen
Nokia Mobile Phones Ltd.
Rivers Brian T.
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