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MIMO WIRELESS TRANSCEIVER DESIGN INCORPORATING HYBRID ARQ
23.1 INTRODUCTION
Hybrid automatic repeat request (HARQ) combines ARQ and forward error correction (FEC) in order to benefit from the advantages offered by both schemes [1]. ARQ aims at improving reliability by retransmitting packets that arrive at the receiver with errors. FEC schemes are based on a different approach: The objective is to avoid retransmission by including sufficient parity data into the packets so that not only errors be detected (as in ARQ), but also packets be corrected at the receiver. ARQ is preferable in good channels because few retransmissions occur and there is no need to include additional error-correcting parity in each transmitted packet. On the other hand, FEC works better over channels where errors are frequent and many retransmissions would result in low throughput and long delays. Moreover, in some systems, the use of feedback, which is required by ARQ, may not be desirable, especially for channels with long delays. For HARQ, both error correction and retransmissions are employed. When errors at the receiver cannot be corrected by the FEC part, a retransmission is requested. Moreover, contrary to ARQ, previous packets are kept at the receiver and are used together with the retransmitted ones to decode the data. By appropriately choosing the FEC scheme, HARQ can achieve better throughput performance compared ...
