Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1998-04-21
2001-02-13
Chin, Stephen (Department: 2734)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S130000, C375S346000, C375S267000, C455S303000, C455S304000, C702S191000
Reexamination Certificate
active
06188736
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to wireless systems and, more particularly, to systems having more than one antenna at the receiver and at the transmitter.
Physical constraints as well as narrow bandwidth, co-channel interference, adjacent channel interference, propagation loss and multi-path fading limit the capacity of cellular systems. These are severe impairments, which liken the wireless channel to a narrow pipe that impedes the flow of data. Nevertheless, interest in providing high speed wireless data services is rapidly increasing. Current cellular standards such as IS-136 can only provide data rates up to 9.6 kbps, using 30 kHz narrowband channels. In order to provide wideband services, such as multimedia, video conferencing, simultaneous voice and data, etc., it is desirable to have data rates in the range of 64-144 kbps.
Transmission schemes for multiple antenna systems may be part of a solution to the problem of the currently available low data rates. Such schemes were first proposed in papers by Wittneben, and by Seshadri and Winters, where the problem was addressed in the context of signal processing.
One prior art arrangement having a single transmitter antenna and multiple receiver antennas is shown in FIG.
1
. Each of the receiver antennas receives the transmitted signal via a slightly different channel, where each channel i is characterized by transfer function &agr;
i
. Using an approach known as “Maximum Ratio Combining”, the prior art approach to detection contemplates multiplying each received signal that had been influenced by &agr;
i
j
by the complex conjugate signal, a
i
*, summed, and then processed.
In a co-pending application titled “Method and Apparatus for Data Transmission Using Space-Time Codes and Multiple Transmit Antennas”, filed on May 6, 1997, bearing the Ser. No. 08/847,635, and assigned to the assignee of this invention, a coding perspective was adopted to propose space-time coding using multiple transmit and receive antennas. Space-time coding integrates channel coding, modulation, and multiple transmit antennas to achieve higher data rates, while simultaneously providing diversity that combats fading. It may be demonstrated that adding channel coding provides significant gains over the schemes of Wittneben and Seshadri and Winters. In said co-pending application, space-time codes were designed for transmission using 2-4 transmit antennas. These codes perform extremely well in slowly varying fading environments (such as indoor transmission media). The codes have user bandwidth efficiencies of up to 4 bits/sec/Hz which are about 3-4 times the efficiency of current systems. Indeed, it can be shown that the designed codes are optimal in terms of the trade-off between diversity advantage, transmission rate, decoding complexity and constellation size.
It can also be shown that as the number of antennas is increased, the gain increases in a manner that is not unlike a multi-element antenna that is tuned to, say, a particular direction. Unfortunately, however, when maximum likelihood detection is employed at the receiver, the decoding complexity increases when the number of transmit and receive antennas is increased. It would be obviously advantageous to allow a slightly sub-optimal detection approach that substantially reduces the receiver's computation burden.
SUMMARY
Such an approach is achieved with a receiver arrangement where signals received at a plurality of antennas are each multiplied by a respective constant and then summed prior to being applied to a maximum likelihood detector. The respective constants, &lgr;
j
, where j is an index designating a particular receiver antenna, are derived from a processor that determines the largest eigenvector of the matrix A, where &Lgr; is a vector containing the values &lgr;
j
, and A is a matrix containing elements &agr;
ij
, which is the transfer function between the i
th
transmitter antenna to the j
th
receiver antenna. The &agr;
ij
terms are determined in the receiver in conventional ways.
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“Space-time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction” Tarokh, V.; Seshadri, N.; Calderbank, A.R. Info. Theory, IEEE Trans. on, vol. 44 No. 2, Mar. 1998, pp.: 744-765.
“Diversity for the direct-sequence spread spectrum system using multiple transmit antennas” Weerackody, V., Communications 1993, IEEE Intntl. Conf. on, ICC '93 Geneva, Tech. Program Conf. Record, vol. 3, 1993, pp.:1775-1779 vol. 3.
“The diversity gain of transmit diversity in wireless systems with Rayleigh fading” Winters, J.H., Communications, IEEE Int. Conf. on, 1994. ICC '94, SUPERCOMM/ICC '94, Conference Record, ‘Serving Humanity Through Communications.’ pp.: 1121-1125 vol. 2.
“A new bandwidth effect transmit antenna modulation diversity scheme for linear digital modulation” Wittneben, A. Communications, IEEE International Conf. on, 1993. ICC '93 Geneva. Technical Program, Conference Record, vol. 3, pp. 1630-1634 vol. 3.
“The Impact of Antenna Diversity on the Capacity of Wireless Communication Systems” Winters, J.H.; Salz, J.; Gitlin, R.D. Communications, IEEE Transactions on, vol.: 42 No. 2, Feb./Mar./Apr. 1994, pp. 1740-1751.
Lo Titus
Tarokh Vahid
AT&T Wireless Svcs. Inc.
Chin Stephen
Rupert Paul N.
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