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The Use of Antenna Arrays in the Detection of
Code Division Multiple
Access Signals

Tyler Brown, Mostafa Kaveh, and Vafa Ghazi-Moghaddam
University of Minnesota
Department of Electrical Engineering
200 Union St. SE
Minneapolis, MN 55455
email: tylerb@mozart.ee.umn.edu

Abstract Code division multiple access (CDMA) has been proposed as an efficient access method for cellular and personal communications systems. In contrast to conventional time and frequency division access methods, CDMA systems are interference limited. Consequently, antenna arrays have the potential to provide substantial capacity increases. We then describe a multichannel detection scheme which, in contrast to detectors based on beamforming methods, has performance which is independent of the strength of interfering users. Instead of spatially filtering out the interference of other users, this detector, termed the multichannel decorrelating detector, jointly estimates the information bits of all users simultaneously. It is therefore immune to interfering users which, by their proximity to the antenna array, have large received signal power. The complex fading gains across the array of each user are assumed known in this formulation either through a pilot tone or training sequence. The multichannel decorrelator may however be extended to the case when these fading gains are not known. Instead, the joint temporal/spatial correlation functions of each user are assumed known. The fading gains are then estimated based on past received data and previous bit decisions. This decision-directed approach allows the detector to operate over channels with rapid fading. As with the non-fading case, this decision-directed multichannel decorrelator is immune to near-far interference.

This presentation concludes with a discussion of the performance of these detectors. In the non-fading case the effective power gain of the multichannel decorrelator is found to grow linearly with the number of array elements. The performance improvement available with the multichannel decorrelator is shown to be the result of two factors: spatial diversity and improved fading estimation.



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