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Multichannel Adaptive Beamforming for Interference Mitigation and Spatial Diversity in Multiuser CDMA Systems

Keith W. Forsythe, Daniel W. Bliss, and Catherine M. Keller
MIT Lincoln Laboratory
244 Wood Street
Lexington, MA 02420
tel: (781) 981-3300
email: forsythe@ll.mit.edu

Abstract Multiuser communication systems pose several challenges to signal processing. In CDMA systems, multiple users produce cochannel interference that is only partially mitigated by user-dependent spreading codes. The variation in range between users and the base-station, as well as limitations in any closed-loop transmitter power control, can create received power variations that are potentially large. Antenna arrays can provide adaptive beamformers to spatially filter cochannel interference. In addition to the problems of self-interference, significant levels of multipath associated with non-line-of-sight propagation lead to large variations in signal levels with small changes (several wavelengths) in the location of the base-station antenna. Antenna arrays can thus provide the additional benefits of spatial diversity.

Multichannel, data-adaptive receivers that provide interference mitigation as as well as spatial and temporal diversity are discussed in the context of CDMA applications. A maximum likelihood formulation of multiuser detection with antenna arrays is introduced to provide motivation and intuition. The proposed multiuser receiver incorporates space-time beamforming, estimation/subtraction, and subspace tracking.  Performance examples are given along with comparisons with other types of receivers.

Presentation (pdf format)



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