Multipacket Hybrid ARQ: Closing Gap to the Ergodic Capacity

Abstract

In this work we consider incremental redundancy (IR) hybrid automatic repeat request (HARQ), where transmission rounds are carried out over independent block-fading channels. We propose the so-called multi-packet HARQ where the transmitter allows different packets to share the same channel block. In this way the resources (block) are optimally assigned throughout the transmission rounds.This stands in contrast with the conventional HARQ, where each transmission round occupies the entire block. We analyze superposition coding and time-sharing transmission strategies and we optimize the parameters to maximize the throughput. Besides the conventional one-bit feedback (ACK/NACK) we also consider the rich, multi-bit feedback. To solve the optimization problem we formulate it as a Markov decision process (MDP) problem where the decisions are taken using accumulated mutual information (AMI) obtained from the receiver via delayed feedback. When only one-bit feedback is used to inform the transmitter about the decoding success/failure (ACK/NACK), the Partial State Information Markov Decision Process (PSI-MDP) framework is used to obtain the optimal policies. Numerical examples obtained in a Rayleigh-fading channel indicate that, the proposed multi-packet HARQ outperforms the conventional one, by more than 5 dB for high spectral efficiencies.

How to Boost the Throughput of HARQ With Off-the-Shelf Codes

“…As observed before, e.g., in [2], [3], [15], HARQ is particularly useful when the probability of error in the first round f 1 is high, as then the throughput can be notably increased with K.…”

“…The practical problem is thus to increase the throughput for a given and finite rate R. This problem is particularly challenging for the throughput in the vicinity of R, where the conventional HARQ fails to provide any improvement [2], [3].…”

“…However, the throughput increase is obtained with variable-length channel blocks which may be a challenge in those systems which have to keep the block size constant. The second venue harnesses the channel coding to overcome this very difficulty: the works [3], [10]- [14] keep the block size constant but increase the coding rate, i.e., the number of bits encoded in each HARQ round. This may be seen as a joint encoding of various packets into a single channel block.…”

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

“…To show the existence of an (η BRQ (T ), T )-zero outage EMS protocol, we need to demonstrate that BRQ-EMS satisfies (10) and (11). Both of these conditions follow from (23) if the conditions of Lemma 2 can be verified.…”

AMC and HARQ: How to Increase the Throughput

“…2 Indeed, the literature already recognizes that, to improve the throughput, the coding across the HARQ rounds must be modified. The most relevant solutions may be classified as i) a multi-packet coding [2], [3], [20], [21], where many packets with variable contents are jointly encoded into fixed-length codewords which then use the fixed resources (channel blocks) or, as ii) a variable-length coding [22]- [27], where rather the codewords length varies throughout the HARQ rounds and the packet content is fixed. 3 We focus this work on the adaptive multi-packet coding whose advantage over the relatively well-studied variablelength coding will be discussed in Sec.…”