Pulse or digital communications – Transmitters – Plural diversity
Reexamination Certificate
2000-01-27
2004-10-12
Tse, Young T. (Department: 2634)
Pulse or digital communications
Transmitters
Plural diversity
C375S146000, C375S295000
Reexamination Certificate
active
06804307
ABSTRACT:
BACKGROUND
The present invention relates to transmit diversity in wireless transmitters and communication systems. More particularly, the present invention relates to a method and apparatus for increasing transmit diversity efficiency using complex orthogonal space-time block codes.
Transmit diversity using multiple transmit antennas and, particularly, space-time block codes has recently attracted a remarkable attention in the communication literature and in the standardization bodies. Transmit diversity provides greatly improved performance on channels subject to multipath fading through the provision of a number of spatially separated replicas of the transmitted signal as may further be described in V. Tarokh, N. Seshadri and A. R. Calderbank, ‘Space-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction, ’IEEE Trans. on Information Theory, Vol. 44, No. 2, pp. 744-765, March 1998; S. Alamouti and V. Tarokh, ‘Transmitter diversity technique for wireless communications’, U.S. patent application, International Publication Number WO 99/14871, 25 Mar. 1999; and S. M. Alamouti, ‘A Simple Transmit Diversity Technique for Wireless Communications’, IEEE Journal on Selected Areas in Communications, Vol. 16, NO. 8, pp. 1451-1458, October 1998.
Transmit diversity involves replicas of the transmitted signal being communicated to a receiver on different and independent communication channels, each with a separate transmit antenna, thus increasing the probability that the receiver will receive at least some of the transmit signal replicas which have been less attenuated by fading and related multipath anomalies. Simultaneous signals transmitted from different transmit antennae in a transmit diversity environment have the same information content and differ only in the time domain. Such signal conditions are favorable for maximum likelihood decoding and allow maximum spatial diversity gains to be achieved provided some kind of space-time coding is applied to generate the simultaneous signals.
A simple form of space-time block coding is incorporated in a transmit antenna diversity scheme accepted for the third generation cellular standard (UTRA/FDD) as described in the 3
rd
Generation Partnership Program (3GPP)'s, Techn. Spec. TS 25.211, Physical channels and mapping of transport channels onto physical channels (FDD), September 1999. The transmit diversity scheme described therein, using two transmit antennas, is generally equivalent to another block coding scheme proposed, for example, by Alamouti as further described in “Transmitter diversity technique for wireless communications”, supra and “A Simple Transmit Diversity Technique for Wireless Communications”, supra. Compared to space-time trellis codes as described in ‘Space-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction,” supra, space-time block codes allow a much less complex decoding scheme to be used in the receiver. Despite certain performance losses compared to the space-time trellis codes, space-time block codes nevertheless provide the ability to achieve much lower decoding complexity making them a very attractive alternative for the practical applications and have merited further study as described in V. Tarokh, H. Jafarkhani and A. R. Calderbank, “Space-Time Block Codes from Orthogonal Designs”, IEEE Trans. on Information Theory, Vol. 45, No. 5, pp. 1456-1467, July 1999.
Accordingly, lower decoding complexity transmit diversity may be achieved based on space-time block-codes having “m” information symbols which are coded into “n” codewords, each of length “p” code symbols. All codewords may then be transmitted simultaneously from “n” antennas. The minimum receiver complexity can be obtained if the codewords are orthogonal. At the same time, the codewords must be unique (and orthogonal) for any particular content of information symbols. Accordingly, space-time block codewords may preferably be obtained by weighting, conjugation and repetition of information symbols, or by combination of these three operations. The transmission rate R, defined as the ratio between the number of information symbols to the number of transmitted symbols in a codeword, is desirable to be as large as possible, ideally equal to 1. It should be noted that the receiver complexity is proportional to the length of codewords. For given number of antennas n, the minimum length of orthogonal codewords IS P
min
=n.
A larger transmit diversity gain may be obtained using a larger number of transmit antennas. Space-time block codes useful for larger numbers of transmit antennas are studied in “Space-Time Block Codes from Orthogonal Designs”, supra, where a systematic complex space-time block coding method is developed, that for any number of antennas n>2 codes of rate R=0.5, having the codeword's length p greater than or equal to 2n may be produced. In addition to systematic code construction, two sporadic codes of rate R=0.75, for n=3 and n=4 are presented. It should be noted that the code for n=3 may actually be obtained from the code for n=4 by deleting a single codeword. The sporadic space-time code for n=4 has codewords of length p=4. However, information symbols in the codewords are weighted by the multilevel coefficients, so the actual transmitted signals from each of the antennas have non-constant envelopes regardless of whether the information symbols are associated with a constant-envelope modulation format (e.g. BPSK, MSK, etc.).
It should be noted that all the above codes may provide maximum spatial diversity order (gain) for a given number of transmit antennas. The conditions for maximum spatial diversity order is that each codeword contains all of information symbols, and that each information symbol is repeated an equal number of times within the codeword. It should further be noted that all the above codes may be valid for any arbitrary modulation format of information symbols. If the modulation format is restricted to be a constant-envelope modulation scheme, an alternative space-time block code of minimum length may be derived by using any Hadamard matrix as described in J.-C. Guey, M. P. Fitz, M. R. Bell and W.-Y. Kuo, ‘Signal design for transmitter diversity wireless communication systems over Rayleigh fading channels,’ in Proc. 1996 Veh. Technol. Conf., Atlanta, Ga., 1996, pp. 136-140. Namely, if each column of some “n×n” Hadamard matrix is multiplied symbol-by-symbol with the same string of n information symbols with constant envelope, the resulting matrix is the orthogonal space-time block code of minimum length and of rate R=1. It will be appreciated that the space-time block codewords are the columns of the resulting orthogonal matrix. It should further be noted that information symbols transmitted according to a constant-envelope modulation format all have a constant absolute value.
Problems arise however, in that systematic complex space-time codes proposed in “Space-Time Block Codes from Orthogonal Designs”, supra may have twice or more times longer codeword's length than the minimal possible (p
min
=n). Hence they are not optimal from the decoder complexity point of view. Further problems arise in that a sporadic space-time block code for n=4, may have the minimum codeword length (p=4), however, information symbols are weighted by the multilevel coefficients leading to more difficult requirements for associated power amplifiers. Power amplifiers configured to deal with transmission based on sporadic space-time block codes, must deal with the more stringent requirements associated with higher peak power and higher range of linearity.
Still further problems arise in that space-time block codes described, for example, in ‘Signal design for transmitter diversity wireless communication systems over Rayleigh fading channels”, supra are both of a minimum length and of a maximum rate, however, they are valid only for constant-envelope mo
Ahn Sam K.
Telefonaktiebolaget LM Ericsson (publ)
Tse Young T.
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