Adaptive transmission strategies to maximize packet throughput of cognitive radio under primary user queue stability constraint

In a cognitive radio system, secondary user should use spectrum allocated to primary user in such a way that primary user´s quality of service is maintained at all times. In a system where primary user stores packets in a queue for transmission, the queue´s stability has to be ensured. We consider joint interweave-underlay mode where secondary user transmits when primary user is absent and also when primary user is present, by adopting its transmit power to keep interference to primary below a threshold. We compare the performance of joint interweave-underlay mode with only-interweave mode. We also use a technique of adaptively spreading packets in time to increase received signal to interference plus noise ratio which increases achievable packet throughput. By comparing only-interweave mode with combination of joint interweave-underlay mode and time spreading, we propose adaptive mode selection to select the mode that gives higher average packet throughput for secondary user while keeping primary queue stable. We present simulation results and analyze proposed method for different channel conditions.