Analytical Evaluation of Saturation Throughput of a Cognitive 802.11-based WLAN Overlaid on a WiMAX-TDD Network

This paper analyzes the saturation throughput of a cognitive single hop WLAN overlaid on a primary IEEE 802.16e TDD WiMAX network. After the contention among the secondary nodes, the winner node transmits its data packet in the empty slots of downlink subframes of WiMAX. Regarding the OFDMA structure as well as time-scheduled resources in WiMAX, the time duration of opportunities for the secondary network does not follow simple exponential on-off pattern. To model the dynamic behavior of opportunities for secondary nodes as well as contentions to exploit the opportunities, we propose an analytical model comprised of a discrete-time Markov chain and two inter-related open multi-class queueing networks. The effects of random number of empty slots at different frames as the result of random amount of download data, random packet transmission time at WLAN due to random opportunities in different frames, the dependency of the number of empty slots at consecutive WiMAX frames, and the details of 802.11 MAC protocol are included in our analytical approach. We compare the effect of two resource allocations, i.e., horizontal and vertical striping on the saturation throughput of the cognitive WLAN. Simulation results confirm the accuracy of our analytical approach in different conditions.