Energy Efficient Adaptive Modulation in Wireless Cognitive Radio Ad Hoc Networks

In this paper, an energy efficient adaptive modulation scheme is proposed for a wireless cognitive radio ad hoc network, where each node is equipped with cognitive radio and the network is an OFDMA system operating on time slots. In each slot, the users with new traffic demand will sense the spectrum and locate the available subcarrier set. Then they choose subcarriers with favorable channel condition individually while avoid introducing harmful interference to the existing users. Given the delay requirement, an adaptive modulation strategy is proposed for each individual user to minimize the energy consumption per bit over the available subcarrier set by selecting the optimal constellation size. The optimal solution of energy efficient adaptive modulation is derived by using Dinkelback- type algorithm, and a sub-gradient search algorithm is proposed to locate the optimal constellation size. Furthermore, a sub- optimal constant bit allocation strategy is presented to address the delay constraints and a distributed power control is performed to manage the co-channel interference among new users when needed. Simulation results demonstrate the effectiveness of our approach.