Efficient combining techniques for multi-input multi-output multi-user systems employing hybrid automatic repeat request

The authors consider chase-combining hybrid automatic repeat request (HARQ) schemes over multiple antenna multi-user systems. The focus is on a multiple-access channel, where the users as well as the base-station are equipped with multiple antennas. In such chase-combining HARQ systems, the transmitters (users) re-transmit their codewords when requested by the receiver (base-station). The receiver may choose to enforce blanking, that is, it may choose to request only a subset of the transmitters to re-transmit and the remaining ones to be silent. Moreover, subject to the complexity and latency constraints, the receiver may be able to perform codeword cancellation wherein it can re-encode, re-modulate a subset of decoded codewords and subtract them from the received observations. A candidate combining technique is the conventional chase-combining, which in contrast to the optimal combining requires significantly less memory and processing capability at the receiver but can result in substantial performance degradation. The authors propose efficient combining techniques that cater to all the various scenarios that arise in such multi-codeword systems. The proposed techniques impose similar memory and complexity demands as the conventional combining but yield a significant performance improvement. The issue of limited feedback in multi-codeword multi-antenna chase-combining HARQ systems is also addressed and it can be used to further improve the system throughput.