Power allocation and subcarrier pairing for AF-OFDM based cognitive radio systems

An OFDM based cognitive radio(CR) system with amplify-and-forward relay was established.Optimal subcarrier pairing and power allocation algorithms for the system were investigated,while the total power of CR user was under the constraint and the interference introduced to the primary user remained within a tolerable range.The Lagrange dual method and subgradient method were employed to design the optimal power allocation algorithm,and the Hopcroft-Karp algorithm for subcarrier pairing was studied with power allocation algorithm.Besides,a suboptimal power allocation algorithm with regard to total system power and interference constraint was proposed,which has low computational complexity.Then the optimal power allocation algorithm was compared with the suboptimal power allocation algorithm,and the Hopcroft-Karp subcarrier pairing algorithm was compared with the fixed subcarrier pairing algorithm,which indicates that the signal transmitted by the source on one subcarrier is forwarded on the same subcarrier by a relay to the destination.Finally,simulation results verify the optimization of combining power allocation with subcarrier pairing can effectively improve the system spectrum efficiency,and is feasible for practical system.