Reduced-rank signature-receiver adaptation

Interference in code-division multiple access (CDMA) and multi-antenna systems can be avoided by choosing a signature (in space and/or time), which lies in the direction of least interference plus noise. If the interference statistics are unknown a priori, then the signature can be adaptively estimated with a training sequence. We present an iterative scheme for joint signature-receiver adaptation with an adaptive reduced-rank Multi-Stage Wiener Filter (MSWF) at the receiver. We establish convergence of the iterative signature-receiver optimization scheme for a single user with fixed interference, and show that the limiting performance for any filter rank D ≥ 2 is the same as that obtained with a full-rank receiver. To reduce feedback requirements, we also consider joint signature-receiver adaptation with a reduced-rank signature, which is confined to a randomly chosen subspace. Numerical results are presented for both single- and multi-user (group) adaptation, which show that reduced-rank signature-receiver estimation can achieve near-optimal performance with relatively little training and low complexity.