Codeword and signature design for multi-user multi-antenna communication systems
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Channel sharing by several users in a code-division multiplexing environment introduces multiple-access-interference (MAI). Binary signature sets with minimum total squared correlation (TSC) across all user signatures were recently found for (almost) all signature lengths and set sizes. The designs, however, are scalable only in the underloaded regime. The scalability problem in the context of overloaded systems is addressed by relaxing the strict TSC-optimality requirement and developing new conditionally TSC-optimal overloaded signature sets. In addition to the issue of scalability, in time-varying multipath fading wireless channels static signature set design limits performance inherently. As an alternative, signatures can be designed adaptively to incorporate/withstand on-going channel conditions. A new algorithm for the adaptive design of binary/quaternary signatures is proposed that produces in short polynomial time highly desirable solutions that provide superior SINR performance. MMSE filtering considerations in a multiantenna setup, led to the theory of auxiliary-vector (AV) filter estimators (University at Buffalo developed). In this context, a novel signal subspace basis is proposed using the orthogonal AV algorithm. The AV basis is then applied for signal direction-of-arrival (DOA) estimation with exceptional performance improvement in resolution ability when compared with other DOA estimation algorithms (MUSIC, ESPRIT, grid-ML) of similar and greater complexity. Multiantenna systems also allow for new code design to exploit the added spatial dimension. Orthogonal space-time block codes (STBC) have received considerable attention as means to achieve full diversity with fast (symbol-disjoint) maximum likelihood (ML) decoding. In quasi-orthogonal STBCs, code orthogonality is relaxed in order to obtain full rate; full diversity can still be achieved if the transmitted symbols come from carefully chosen constellations. A new 8x8 quasi-orthogonal STBC is proposed that can be applied simultaneously across 8-transmit-antenna systems and 4-transmit-antenna systems. Also proposed are new criteria for rotation angle maximization. Improvement in error-rate performance for both, 8-transmit-antenna systems and 4-transmit-antenna systems with different rotation angle optimization is demonstrated.