Partial decorrelation for a coherent multicode code division...

Details Partial decorrelation for a coherent multicode code division... Partial decorrelation for a coherent multicode code division...

1997-05-01

2001-03-13

Patel, Ajit (Department: 2732)

Multiplex communications

Communication over free space

Combining or distributing information via code word channels...

C375S346000

Reexamination Certificate

active

06201799

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to Coherent Multicode (MC) Code Division Multiple Access (CDMA) receivers and, more particularly, to a Coherent MC-CDMA receiver which uses a partial decorrelator.
BACKGROUND OF THE INVENTION
Code-Division Multiple Access (CDMA) has become one of the most prominent schemes for wireless communication systems. In a given radio frequency channel, individual CDMA data channels are distinguished from one another by different code sequences. A multiple code (MC) CDMA receiver allows for communication at J>1 times the base data rate by simultaneously demodulating J data channels. Due to the wide band nature of the CDMA signal, the receivers can be made robust against fading by exploiting the multipath diversity using a RAKE receiver. In a coherent CDMA system, a pilot signal is often transmitted so that the RAKE receiver can obtain amplitude and phase estimates of the channel needed for coherent detection. This pilot signal is usually designed to be orthogonal to the data channel spreading codes so that in the rare case of no multipath dispersion, the pilot signal will not cause interference at the matched filter output of the desired data channel. If there is multipath dispersion, there will be unwanted interference at the RAKE receiver's matched filter output due to a variety of multipath components. Specifically, for a given multipath component of a desired data channel at the receiver, its RAKE receiver output will have unwanted contributions due to other multipath components of itself, the other channels, and the pilot signal. As a result, the performances of the conventional RAKE and MC-CDMA receiver suffer as a consequence of ignoring the interchannel multipath interference .
In our recently allowed patent application entitled PILOT INTERFERENCE CANCELLATION FOR A COHERENT WIRELESS CODE DIVISION MULTIPLE ACCESS RECEIVER, Ser. No. 08/700,266, filed on Aug. 20, 1997, we introduced a pilot interference canceller which improved the performance of a coherent CDMA receiver by reconstructing and canceling out pilot multipath interference prior to demodulation by the RAKE receiver. Notwithstanding the interference reduction provided by the pilot interference canceller, there is a continuing need to eliminate the interchannel multipath components of interference in MC-CDMA receivers.
SUMMARY OF THE INVENTION
In accordance with the present invention, a partial decorrelation (PD) interference cancellation method and apparatus is described which eliminates interference for a multiple code (MC) CDMA receiver that simultaneously demodulates J>1 data channels. Our decorrelator is partial in that the detector decorrelates with respect to only the known J users as opposed to all K users. Our invention expands on prior multiuser decorrelation techniques for synchronous CDMA channels (as described by R. Lupas and S. Verdú, “Linear Multiuser Detectors for Synchronous Code-Division Multiple-Access Channels,” IEEE Transactions on Information Theory, Vol. 35, No. 1, pp. 123-136, January 1989) and its application for fading multipath CDMA channels (see the article by H. C. Huang and S. C. Schwartz, “A Comparative Analysis of Linear Multiuser Detectors for Fading Multipath Channels,” Proceedings of the 1994 IEEE Globecom Conference, pp. 11-15, November 1994). Our invention is also an enhancement of the technique described in the article by C. -L. I, S. ten Brink, S. Nanda, C. A. Webb III, H. C. Huang, and R. D. Gitlin, “IS-
95
Enhancements for Multimedia Services,” Bell Labs Technical Journal, Winter 1997, which is incorporated by reference herein .
More specifically, in demodulating a particular MC data channel, our PD eliminates interference due to the other J-1 MC data channels. According to our invention, a MC CDMA receiver receives L time-delayed, complex-amplitude-modulated replicas of the transmitted CDMA signal which is the sum of K≧J data channels signals and a pilot signal. A decision statistic for the desired J MC data channels is obtained using a bank of J RAKE receivers. Thereafter, a partial decorrelator (PD) processes the decision statistic to suppress, for each of one of the J RAKE receiver outputs, interference caused by the other J-1 data. The resulting decorrelator output vector is a vector in which for each of the J components, the interference caused by the other J-1 multicode channels has been suppressed. This vector is used to determine the data transmitted on the J MC data channels.
According to a feature of the invention, when a pilot signal is transmitted to aid in obtaining coherent channel estimates, a pilot interference canceller (PIC) cancels a received pilot signal from the J RAKE receivers. In this manner the PD can be used in conjunction with the PIC so that the combined PIC/PD detector encounters interference due only to the non-MC data channels and enjoys the compound benefits of both techniques.


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