Multi-User Scheduling for Network Coded Two-Way Relay Channel in Cellular Systems

There has been a growing interest in analog network coding, which can improve system throughput and spectrum efficiency significantly in wireless systems. Two-way relay scenario is considered in this paper, where the base station (BS) and the relay are both equipped with multiple antennas, and each of K user equipments (UEs) in the cell has a single antenna. An advanced multi-user scheduling scheme is proposed for the analog network coded two-way relay channel, where UEs and BS exchange information via the relay node during two time slots. Because each UE with single antenna has poor transmission and reception capability, the scheduling scheme is carefully designed to decrease multi-user interference and optimize beamforming gain from BS to relay. Analytical results of sum rate and outage probability have been developed to demonstrate that the multi-user diversity gain can be achieved by the proposed scheme. Meanwhile, the user fairness of the proposed scheme is on the same order as that of round robin (RR), and the computation complexity only increases linearly with the number of users in the serving cell. Monte-Carlo simulation is conducted to exhibit the performance gains from the proposed scheduling scheme. Results show that the proposed channel correlation based user selection scheme outperforms existing traditional scheduling schemes significantly, even when the cellular user payload is low.

I. Introduction

Network coding [1] is a promising technique for wireless networks to improve the spectrum efficiency. By allowing the intermediate node to relay the mixed signals received from multiple source nodes, instead of transmitting them separately, the sum throughput of the whole networks can be enhanced significantly. Since conventional network coding schemes need to decode each data stream and generate the mixtures by bit-wise exclusive-or (XOR) operation, the computational complexity of these scheme is high and three time slots are needed [2]. In order to overcome this problem, the physical network coding (PNC) [3], which is also named as analog network coding (ANC)[4], is proposed. The difference between ANC and PNC lies in their corresponding relay operations, which are amplify-and-farword (AF) and estimate-and-forward (EF), respectively. For two-way relay channel scenario, two source nodes transmit their messages simultaneously in the first time slot. Then, the combined messages from two sources are relayed in the second time slot. Rather than attempting to decode both of the packets, ANC just amplifies the observations of the mixed messages, while PNC decodes the mixtures and re-codes them. The duration for one round information exchange is reduced from three to two. Hence, the spectrum efficiency can be further improved by the ANC and PNC. In fact, the broadcast property of wireless propagation is suitable to deploy ANC and PNC in radio transmissions.