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

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Abstract

In many practical wireless systems, the signal-to-interference-and-noise ratio (SINR) 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as channel state information (CSI), can only be learned after the transmission has taken place and is thereby delayed (outdated).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 CSIT is available and there is a constraint on the average decoding time, it is shown that the optimal 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 BRQ approaches the ergodic capacity quickly compared to HARQ, while EMS protocols with only three and four feedback messages achieve throughput that are only slightly worse than the throughput of BRQ.
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In many practical wireless systems, the signal-to-interference-and-noise ratio (SINR) 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as channel state information (CSI), can only be learned after the transmission has taken place and is thereby delayed (outdated).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 CSIT is available and there is a constraint on the average decoding time, it is shown that the optimal 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 BRQ approaches the ergodic capacity quickly compared to HARQ, while EMS protocols with only three and four feedback messages achieve throughput that are only slightly worse than the throughput of BRQ.
Original languageEnglish
JournalI E E E Transactions on Information Theory
VolumePP
Pages (from-to)1
Number of pages34
ISSN0018-9448
DOI
StateE-pub ahead of print - 2018
Publication categoryResearch
Peer-reviewedYes

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