Subcarrier allocation algorithms based on graph-coloring in Cognitive Radio NC-OFDM system

Cognitive Radio (CR) system can be combined with Non-Contiguous Orthogonal Frequency Division Multiplexing (NC-OFDM) technology to improve the spectrum utilization efficiency. In this paper, we introduce graph-coloring theory and max-min algorithm in multiuser OFDM system to CR NC-OFDM system for subcarrier allocation to gain the better system performance. We build a subcarrier allocation model based on graph-coloring theory. And then, considering the interference between cognitive users (CUs) and time and space difference between the available spectrums of CUs as two constraints, we propose the rand algorithm, greedy algorithm and Max-Min algorithm for subcarrier allocation in CR NC-OFDM system. The rand algorithm is simple, but can't optimize system throughput. The greedy algorithm can gain the best throughput performance but has poor fairness performance. The Max-Min algorithm can not only achieve better throughput performance but also good user fairness.