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On the Achievable Rates of Virtual Full-Duplex Relay Channel

Songnam Hong,D. Hui,I. Marić,G. Caire

Published 2019 in IEEE Transactions on Information Theory

ABSTRACT

We study a multihop “virtual” full-duplex relay channel as a special case of a general multiple multicast relay network. For such a channel, quantize-map-and-forward (QMF) [and its generalization of noisy network coding (NNC) and short message NNC] achieves the cut-set upper bound within a constant additive gap, where the gap grows <italic>linearly</italic> with the number of relay stages <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula>. This gap, however, may not be acceptable for practical communication systems with multihop transmissions (e.g., a wireless backhaul operating at high frequencies). Recently, we improved the capacity scaling by using a forward sliding-window (SW) decoding and by optimizing the quantization level at each relay, obtaining the gap that grows <italic>logarithmically</italic> as <inline-formula> <tex-math notation="LaTeX">$\log {K}$ </tex-math></inline-formula>. Furthermore, the improved scheme has lower decoding complexity and delay than the general QMF and NNC approaches. In this paper, we further improve the performance by presenting a <italic>mixed</italic> scheme in which each relay can perform either decode-and-forward (DF) or the improved QMF (with SW decoding) and can choose to perform rate-splitting to enable partial interference cancellation. In general, the optimization of the relay DF/QMF configuration is combinatorial. Nevertheless, we provide that a simple greedy algorithm finds an optimal configuration under some practically reasonable assumptions. We derive an achievable rate that is easily computable and show that the proposed mixed scheme outperforms the QMF-only schemes. We demonstrate that the performance improvement increases with <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula>, which indicates that the mixed scheme is indeed beneficial for multihop transmission.

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