Resource Allocation for Two-Way OFDM Relay Networks with Fairness Constraints

In this paper, a new resource allocation scheme is proposed for a two-way OFDM relay network. In the proposed scheme, two terminals exchange information with the help of multiple relays, and subcarriers and their power are successively allocated to the relays to maximize sum capacity. To achieve fairness for the relays, power constraint on each relay is imposed. Simulation results show that the proposed scheme improves sum capacity and fairness significantly. I. INTRODUCTION Orthogonal frequency division multiplexing (OFDM) pro- vides reliable and high data rate communications in wireless networks (1). In an OFDM system, bandwidth for data trans- mission is divided into a number of orthogonal subcarriers and each subcarrier experiences independent frequency-flat fading. In the multiuser OFDM system, frequency diversity and multiuser diversity are obtained by adaptive resource allocation (2), (3). Cooperative relaying obtains spatial diversity without hav- ing a multiple input multiple output (MIMO) antenna array but additional channel resources are required for relay trans- mission (4). By combining cooperative relaying with either superposition coding or network coding, two-way relaying provides improved spectral efficiency compared with one-way cooperative relaying (5). In two-way relaying, two terminals exchange information with each other via a single or multiple relays. In a single relay network, various relaying protocols are proposed to reduce the required channel resources (5). In (6), two-way relaying is combined with OFDM so that sum capacity is increased by power allocation and tone permutation. In a multiple relay network, it is known that diversity gain is increased as the number of relays increases, and the distributed space time coding scheme and the opportunistic relaying scheme are proposed for two-way relaying (7), (8). However, combining two-way relaying with OFDM in the multiple relay network has not been investigated yet. In this paper, we propose a new resource allocation scheme for a two-way OFDM relay network. In the proposed scheme, a number of relays in good channel condition are selected, and the subcarriers and their power are successively allocated to the selected relays based on the instantaneous channel condition. Compared with the opportunistic relaying scheme in which a single best relay is selected, multiuser diversity may be exploited as the number of selected relays increases. To achieve fairness for the relays, maximum power constraint is imposed on each relay. This paper is organized as follows. Section II describes a system model and problem formulation. A new resource allocation scheme is proposed in Section III. Section IV shows simulation results and Section V concludes this paper. II. SYSTEM MODEL AND PROBLEM FORMULATION