A study on blind and semiblind detection for MIMO systems and ARQ retransmissions

We investigate blind and semi-blind channel estimation and detection techniques for multiple-input multiple-output (MIMO) communication systems. MIMO is an emerging technology that provides significant capacity and throughput gain over single-input singleoutput (SISO) systems without increase in bandwidth and power. To achieve these gains, typically, it requires pilot assisted channel estimation at the cost of bandwidth and power efficiency. Blind and semi-blind channel estimation or direct data detection techniques provide viable alternatives.;We begin with blind zero-forcing (ZF) detection of Space-Time Block (STB) coded signal for MIMO systems over flat-fading channels for single user systems. We propose linear programming-based blind ZF equalization algorithm. Our method is globally convergent. We exploit redundancy and the implicit structure of space-time block codes to cast the problem into a low complexity linear programming. Unlike several known methods, the proposed technique is applicable to full-rate space-time block codes such as the popular Alamouti orthogonal code and the quasi-orthogonal space time code. For orthogonal spacetime codes, our LP algorithm achieves near maximum likelihood detection of orthogonal codes without channel knowledge and does not impose any conditions on the number of receive antennas.;We extend this work to blind detection of STB coded data for co-channel synchronous multiuser MIMO system. We propose a globally convergent algorithm for the blind detection of co-channel multiuser signals under synchronous orthogonal space-time block coded modulation. We exploit the algebraic structure of the orthogonal space-time block codes and the Quadrature Amplitude Modulation (QAM) signal constellation to derive a quadratic programming (QP) algorithm that achieves desired convergence without the aid of training pilots. Focusing on the high rate space-time block codes in a synchronous co-channel multiuser system, we propose a successive processing strategy based on the proposed blind zero forcing (ZF) detector which has low computational complexity and fast convergence. Furthermore, our QP receiver for multiuser detection imposes less stringent conditions on the number of receive antennas of the multi-input-multi-output (MIMO) system.;After investigating blind detection of STB coded signals for single and multiuser MIMO system, we focus on bandwidth and power efficient Hybrid Automatic Repeat reQuest (HARQ) at the physical layer level. The concept of HARQ is adapted in Long Term Evolution (LTE) and WiMAX standard and both standards consider full retransmission of the original copy of the original information. The physical layer performs joint detection called diversity combining. We propose retransmission of partial copy of original message and omit pilot symbols during retransmission. Our receiver jointly estimates channel of the both transmissions in semi-blind fashion. The joint detection of two transmissions improves overall performance of the receiver. We demonstrate bandwidth savings without significant performance loss and mild increase in complexity.