Network-Coding-Aware Dynamic Subcarrier Assignment in OFDMA-Based Wireless Networks
IEEE Transactions on Vehicular Technology
Orthogonal Frequency Division Multiple Access (OFDMA) has been integrated into emerging broadband wireless systems such as the 802.16 wirelessMAN. A critical problem in OFDMA is to assign multiple frequency bands (called subcarriers) to different users. Taking advantage of the frequency diversity and multiuser diversity in OFDMA systems, dynamic subcarrier assignment mechanisms have shown to be able to achieve much higher downlink capacity than static assignment. A rich literature exists that
... ature exists that proposes MAC and physical layer schemes aiming at exploiting the diversity gain with low implementation complexity. In this paper, we propose a cross layer approach that explores the joint advantage of network coding and dynamic subcarrier assignment. With network coding, it becomes possible to assign the same subcarriers to different downlinks without causing any interference. Consequently, our codingaware assignment scheme improves the bandwidth efficiency and increases the downlink throughput by a substantial margin. In designing the scheme, we identify a tradeoff between diversity gain and the network coding advantage, which is critical to the network performance in terms of throughput and fairness. To explore the tradeoff, we formulate the coding-aware assignment scheme as a mixed integer program, and design a polynomial time approximation algorithm that can be used in practical systems. We prove the asymptotic performance bound of the algorithm, and demonstrate that it closely approximates the optimum under realistic experimental settings. Index Terms-Network coding, OFDMA networks, WiMax/802.16 1 We use the term "frame" and "packet" synonymously, to denote a group of information bits transmitted through the wireless link.