On the Benefits of Partial Channel State Information for Repetition Protocols in Block Fading Channels

TL;DR

This paper introduces new HARQ protocols that leverage partial channel state information via feedback bits to improve throughput in block fading channels, especially at low and moderate SNR levels.

Contribution

It presents explicit protocol constructions that utilize feedback for both retransmission requests and channel quality information, combining HARQ and power control benefits.

Findings

Significant throughput gains at low and moderate SNR.

Repetition not needed at low SNR with a single feedback bit.

At high SNR, combined power control and repetition improve performance.

Abstract

This paper studies the throughput performance of HARQ (hybrid automatic repeat request) protocols over block fading Gaussian channels. It proposes new protocols that use the available feedback bit(s) not only to request a retransmission, but also to inform the transmitter about the instantaneous channel quality. An explicit protocol construction is given for any number of retransmissions and any number of feedback bits. The novel protocol is shown to simultaneously realize the gains of HARQ and of power control with partial CSI (channel state information). Remarkable throughput improvements are shown, especially at low and moderate SNR (signal to noise ratio), with respect to protocols that use the feedback bits for retransmission request only. In particular, for the case of a single retransmission and a single feedback bit, it is shown that the repetition is not needed at low $\snr$ where the throughput improvement is due to power control only. On the other hand, at high SNR, the repetition is useful and the performance gain comes form a combination of power control and ability of make up for deep fades.