Generalized HARQ Protocols with Delayed Channel State Information and Average Latency Constraints

TL;DR

This paper introduces generalized EMS protocols for HARQ systems with delayed CSI, optimizing throughput under latency constraints, and demonstrates their near-capacity performance with limited feedback.

Contribution

It proposes a new family of EMS protocols that adapt rates based on delayed CSI, extending HARQ capabilities and analyzing their performance with dynamic programming.

Findings

EMS protocols achieve throughput close to ergodic capacity.

Optimal EMS protocol with full delayed CSIT has a simple interpretation.

Limited feedback EMS protocols nearly match BRQ throughput.

Abstract

In many wireless systems, the signal-to-interference-and-noise ratio that is applicable to a certain transmission, referred to as channel state information (CSI), can only be learned after the transmission has taken place and is thereby delayed (outdated). In such systems, hybrid automatic repeat request (HARQ) protocols are often used to achieve high throughput with low latency. This paper put forth the family of expandable message space (EMS) protocols that generalize the HARQ protocol and allow for rate adaptation based on delayed CSI at the transmitter (CSIT). Assuming a block-fading channel, the proposed EMS protocols are analyzed using dynamic programming. When full delayed CSIT is available and there is a constraint on the average decoding time, it is shown that the optimal zero outage EMS protocol has a particularly simple operational interpretation and that the throughput is identical to that of the backtrack retransmission request (BRQ) protocol. We also devise EMS protocols for the case in which CSIT is only available through a finite number of feedback messages. The numerical results demonstrate that the throughput of BRQ approaches the ergodic capacity quickly compared to HARQ, while EMS protocols with only three and four feedback messages achieve throughputs that are only slightly worse than that of BRQ.