Repetition protocols and channel state information in block-fading channels

Abstract

In today's networks, error correction is achieved by a combination of FEC (forward error correction) and ARQ (automatic repetition request). In classical repetition protocols, a receiver requests a retransmission in case FEC-decoding fails. In this work we revise and summarize our recent results that show the benefits of using the retransmission request bit(s) not only to request a retransmission, but also to inform coarsely the transmitter about the actual channel condition. Our goal is to simultaneously enable the performance gain due to retransmission and to power control at the transmitter. We give an explicit protocol construction for any number of retransmissions and any number of feedback bits, and show remarkable throughput improvements, especially at low and moderate SNR (signal to noise ratio). For the case of a single retransmission and a single feedback bit, we show that the repetition is not needed at low SNR and that the throughput improvement is due to power control only, which allows to turn off transmission when the channel is in deep fade. On the other hand, at high SNR, the repetition turns out to be useful, and the performance gain comes form a combination of power control and ability to send more redundancy bits through repetition.