Published in IET Communications Received on 19th March 2010 Revised on 22nd September 2010 doi: 10.1049/iet-com.2010.0223 ISSN 1751-8628 Throughput analysis of network coding enabled wireless backhauls T.A. Le M.R. Nakhai Centre for Telecommunications Research, King’s College London, Strand WC2R 2LS, UK E-mail: tuan.le@kcl.ac.uk Abstract: This study derives upper bounds on the throughput of the wireless backhaul link within a cluster of inter-connected three base stations (BSs) or a cluster of a controlling BS and three fixed relay stations. The authors propose and analyse the Ring protocol for the former and the Star protocol for the latter using network coding concept. These protocols can be used individually or in an overlaid fashion in a coordinated multi-cell system or in a cellular-distributed-antenna system to exchange information in the backhaul. To avoid the weakest link acting as a bottleneck and determining the backhaul throughput, as a result, the authors derive throughput optimising time sharing factors to compensate for the resulting bit imbalances in the backhaul. 1 Introduction Recently, the idea of turning harmful inter-cell interferences into useful signals has motivated multi-cell processing (MCP) allowing inter-cell cooperation [1–6]. In MCP, multiple base stations (BSs) are interconnected via a backhaul and their spatially distributed antennas are coordinated via joint decoding and encoding of messages, both in the uplink and in the downlink, for example, [5–7]. Recent studies, that is, [1–3], have confirmed high achievable gains of MCP in terms of coverage, performance and data rate. Furthermore, MCP in conjunction with a cellular-distributed-antenna system, that is, [6, 8, 9], can achieve a substantial energy saving at BSs through distributed antenna beam-forming towards the intended users, forming nulls towards the unintended ones and avoiding long-range transmissions in the entire cellular network. Distributed beam-forming that plays a key role in coordinated multi-cell and distributed antenna systems relies on the exchange of information in an inter-connecting backhaul [4, 10, 11]. Unlike conventional beam-forming, where array antenna elements are co-located on one BS/ relay station (RS), distributed beam-forming is performed by a system of array antennas in several geographically separated BSs/RSs. In such scenarios, the transmitted data to the users need to be available to all BSs/RSs participating in the distributed beam-forming process [11]. This paper contributes to the wireless backhaul by introducing a Ring protocol to exchange information among three BSs and a Star protocol to exchange information among three fixed RSs and one BS as a controlling unit. Throughout the paper, a node refers to a BS or a RS, for simplicity. In literature, although network coding [12, 13] and, in particular, physical network coding [14] has been used for two-way communications between two source nodes [15–21], to the best of our knowledge, it has not yet been utilised for sharing information among more than two nodes in wireless backhaul. Our work is inspired by [15], where physical network coding was used by a relay to establish a two-way communication between two nodes. We, first, derive time sharing principles to achieve the maximum throughput in our Star and Ring protocols and, then, find the throughput maximising expressions for these protocols. By derivation, we conclude that the imbalance in the number of bits received by the controlling unit from any two source nodes has to be minimised in the MAC phase of both protocols in order to achieve the highest backhaul throughput. We also propose backhaul transmission strategies for these models based on the signal-to-noise ratios (SNRs) of the wireless links between the BSs. The rest of this paper is organised as follows. Targeted scenarios and System model are introduced in Section 2. We propose backhaul transmission protocols for Star and Ring models in Section 3. Then, in Section 4, throughput analysis for the proposed models are described. The paper is concluded in Section 5 and the proofs to the lemma and theorems are given in three Appendices. 2 Scenarios and system model 2.1 Scenarios The proposed wireless backhaul protocols and their respective throughput characterisations are motivated by the coordinated multi-cell scenarios and their associated clusters of cooperative BSs and/or fixed relay nodes, as depicted in Fig. 1. In Fig. 1a, a cluster of three nearest neighbour BSs, that is, BS1, BS2 and BS3 whose adjacent sector areas have common boundaries, jointly support the users in the vicinity of their inter-cell borders by distributed beam-forming. As a result of this coordination, a consistent QoS, in terms of reception quality and data rate, can be guaranteed for the 1318 IET Commun., 2011, Vol. 5, Iss. 10, pp. 1318–1327 & The Institution of Engineering and Technology 2011 doi: 10.1049/iet-com.2010.0223 www.ietdl.org