Advanced Hybrid-ARQ Receivers for Broadband MIMO Communications

Multiple input multiple output (MIMO) and hybrid–automatic repeat request (ARQ) mechanisms play a key role in the evolution of current wireless communications systems towards high data rate wireless packet access. In MIMO techniques, the spatial dimension of the MIMO channel is exploited through the use of multiple antennas at both the transmitter and receiver sides. This translates into an improvement in the spectrum efficiency and/or the link quality Wolniansky et al. (1998). Hybrid–ARQ protocols provide an important source of time diversity through the combination of channel coding and ARQ. This is performedwith the aid of packet combining techniques where erroneous data packets are kept in the receiver to help detect/decode retransmitted frames. In broadband MIMO communications, the MIMO wireless link suffers from intersymbol interference (ISI) caused by multipath propagation. This effect can be mitigated using channel equalization and/or Hybrid–ARQ. In this chapter, we focus on the joint design of the packet combiner and the channel equalizer forMIMOARQ transmission over the broadbandwireless channel. We start the chapter by reviewing the various approaches for joint packet combining and equalization. We introduce the considered broadband MIMO ARQ transmission scheme. Then, we derive the structure of the optimal maximum a posteriori (MAP) turbo packet combiner and study its outage performance. Finally, we introduce a new class of low-complexity minimummean square error (MMSE)-based turbo packet combiners and analyze their implementation requirements and block error rate (BLER) performance.