Energy-Efficient Multicasting in Hybrid Cognitive Small Cell Networks: A Cross-Layer Approach

We study the performance of a cognitive small cell network, catering multicast services to multiple groups of secondary users, using a pre-assigned set of orthogonal channels of primary users present in the corresponding macrocell. We consider the hybrid mode of cognitive radio operation for efficient spectrum access, where cooperative spectrum sensing is performed by the collocated secondary users of each group to ensure better protection to primary users' communication. Considering delay-sensitive application for each multicast group and availability of a rate adaptive application layer and a power adaptive link layer, we aim to maximize the energy efficiency of the small cell base station using a cross-layer approach. More precisely, a joint optimization problem involving sensing time, rate and power allocation is formulated to maximize the energy efficiency of the small cell network under the probabilistic interference constraint of each primary user and heterogeneous quality of service constraints of each SU multicast group. The problem is found to be generally non-convex and an iterative algorithm is proposed to find either an optimal or a near-optimal solution. Simulation results are presented to analyze the performance of our proposed scheme in terms of energy efficiency concerning some key system parameters. The results confirm that our solution is much more energy-efficient than the conventional approach with only transmit power adaptation.