An efficient network coded ARQ for multisource multidestination relay networks over mixed flat fading channels

Abstract

This paper proposes a new reliable automatic repeat request (ARQ) transmission protocol for wireless multisource multidestination relay networks over mixed fading channels. Conventional application of ARQ protocols to retransmit lost or erroneous packets in relay networks can cause considerable delay latency with a significant increase in the number of retransmissions when networks consist of multiple sources and multiple destinations. To address this issue, a new ARQ protocol based on network coding (NC) is proposed where the relay detects packets from different transmission sources, then uses NC to combine and forward lost packets to their destinations. An efficient means for the retransmission of all lost packets is proposed through two packet-combination algorithms for retransmissions at the relay and sources. The paper derives mathematical formulation of transmission bandwidth for this new NC-based ARQ protocol and compares analytical and simulation results with some other ARQ protocols over both mixed Rayleigh and Rician flat fading channel. The mixed fading model permits investigation of two typical fading scenarios where the relay is located in the neighbourhood of either the sources or the destinations. The transmission bandwidth results show that the proposed NC-based ARQ protocol demonstrates superior performance over other existing ARQ schemes.

Introduction

Relay techniques are normally deployed to increase coverage between remote transmission and reception nodes as well as improve service quality and link capacity for local users [1], [2]. Recently spatial diversity gain methods have been considered in an attempt to extend relay transmission coverage and further improve transmission integrity [3], [4].

Basically, relays transmit packets through a store-and-forward mechanism, and thus do not increase the network throughput. In an attempt to improve throughput, network coding (NC) techniques have been investigated at the relays [5], [6], [7]. The basic concept of NC is that the relays perform algebraic linear/logic operations on received packets from multiple transmission sources in order to create a new combined packet, which is then forwarded to the destination nodes in the subsequent transmission. Various NC-based protocols have recently been proposed for some particular relay channel topologies such as relay-assisted bidirectional channels [8], broadcast channels [9], multicast channels [10] and unicast channels [11].

Automatic repeat request (ARQ) techniques permit information to be reliably delivered over multicast or broadcast networks. However, lost packets require to be retransmitted with ARQ protocols which may introduce significant packet latency since each packet is retransmitted individually. In addition, for ARQ, retransmissions are repeated until all packets are received correctly at each reception node. For multisource multidestination relay networks (MMRNs), in [12], the beamforming matrix was designed to minimize the sum transmit power at the relays subject to signal-to-interference constraints at the destinations to reliably support multiple parallel data streams. Also, in [13], stop-and-wait ARQ, go-back-N ARQ and the selective-repeat ARQ were investigated and compared to evaluate the maximum achievable throughput and the steady-state throughput of butterfly networks, a specific model of the MMRNs. However, the design of reliable transmissions over MMRNs that can achieve high network throughput efficiency and reduced retransmission packet latency has received little attention in the previous literature.

As an improved solution to these issues, we propose a new ARQ protocol based on NC for MMRNs. In this new protocol, the relay detects packets, combines information through NC, and transmits the lost packets from different sources to the destinations. Additionally, to achieve an optimal performance, multi-user detection (MUD) techniques [14] are implemented at both the relay and destinations. Thus along with MUD, lost packets can be combined and retransmitted to achieve an improved ARQ mechanism. The representation of lost packets in MMRNs may be categorized into two classification types: Type-I – packets that are successfully received at the relay but lost at the destinations, and, Type-II – packets that are lost at both the relay and destinations. Retransmission of Type-II packets is undertaken by the source, but the issue of how the relay retransmits Type-I packets with the lowest number of retransmissions requires to be addressed. To solve this retransmission problem, we propose a relay algorithm and also a source algorithm to enable retransmission of Type-I and Type-II packets, respectively. As an example of the protocol implementation, a two source, relay, two destination configuration is considered. Specifically, for this scenario, the proposed algorithm employed for retransmission at the relay is based on a combination of NC and packet detection from the two different sources.

A further contribution of this paper involves a performance comparison between our proposed NC-based ARQ protocol and other typical ARQ protocols for MMRNs. The other typical ARQ protocols considered are the direct transmission (DT)¹ and the relaying transmission (RT)² protocol. The performance comparison is achieved through deriving principally the complex analytical expressions of the transmission bandwidth for the new NC-based ARQ protocol and comparing it with the general analytical formulations for the other two protocols. The paper also extends the analytical performance analysis to include channel fading for the situations when the sources and destinations are located near to, or distant, from the relay. In these scenarios, it is approximated that the links from the sources to the relay, or the links from the relay to the destinations are line-of-sight (LOS) transmissions (close by, Rician fading), or non-line-of-sight (NLOS) transmissions (distant, Rayleigh fading).³ Accordingly, the considered fading channels are modelled as a mix of both Rayleigh and Rician fading, or are both Rayleigh or Rician fading.

It is shown through appropriate analytical and simulation examples, that our proposed ARQ protocol when applied to two-source two-destination single-relay network, significantly reduces the number of retransmissions for all fading situations, when compared with the DT and RT protocols.

The paper is organized as follows: Section 2 describes the system model and the different retransmission protocols of MMRNs; Section 3 derives the transmission bandwidths; Section 4 presents the numerical evaluation results and Section 5 concludes the paper.