The performance and reliability of wireless communication links can be improved by employing multiple antennas at both ends, thereby creating multiple-input multiple-output (MIMO) channels. However, once multiple co-channel users are added to the system it can be difficult to provide as many receive antennas as transmit antennas, resulting in a so-called overloaded (rank-deficient) system. Under overloaded conditions, maximum likelihood (ML) detection works well, but its exponential complexity prohibits its use and suboptimal linear detectors perform poorly. In this thesis, new signal processing techniques for multi-user overloaded systems using hybrid automatic repeat request (HARQ) protocols are investigated. The HARQ retransmissions are used to form virtual receive antennas, which can efficiently transform an overloaded system into a critically loaded system (i.e. a system with an equal number of transmit and receive antennas). In the first part of the thesis, a multi-user non-cooperative overloaded system is considered. At first, it is demonstrated that the suboptimal linear minimum mean square error (MMSE) detector leads to significant performance degradation compared to an ML detector for such systems. To overcome this drawback, two multi-user transmission schemes are proposed that work well under overloaded conditions. The proposed schemes allow us to apply linear multi-user detection (MUD) algorithms without requiring additional antennas or hardware chains. Monte-Carlo simulations demonstrate that the proposed schemes can result in significant gains in terms of bit-error-rate (BER) and dropped packet performance. In the second part, the performance of multiple HARQ processes for a two-hop multi-source multi-relay decode-and-forward (DF) relaying network with no direct link are analyzed. Dealing with multiple HARQ processes at each relay, a retransmission scheme is proposed that utilizes virtual antennas to achieve increased receive diversity and improved throughput compared to traditional orthogonal (time division) retransmissions. A novel forwarding strategy at the relay(s) to destination link is proposed with the objective of further improving throughput. Finally, the end-to-end outage probability and throughput efficiency of the proposed retransmission and forwarding schemes are found analytically and confirmed with Monte-Carlo simulations.

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