Optimization of Time Slot and Transmit Power at Secondary Users for Dynamic Spectrum Access

This paper addresses the issues of coexistence for cognitive radio networks including both the coexistence between primary users (PUs) and secondary users (SUs), as well as the coexistence among SUs. The coexistence between SUs and PUs addresses the interference from SUs to PUs, whereas, the coexistence among SUs addresses sharing of identified spectrum opportunity among SUs to achieve efficient spectrum usage. In this study, we first introduce a new parameter named as quality of coexistence (QoC), which is defined as the ratio of quality of SU transmission over the negative interference to PUs. Assuming that a time slot of spectrum opportunity has been found by multiple SUs, we partition this time slot into subslots and allocate time slots and transmit power to multiple SUs to maximize the QoC. This leads to a "wise" spectrum usage among SUs and minimized interference to PUs. In this study we assume that the SUs have multiple antennas and employ fixed transmit power control (fixed-TPC). By using the approximation of the approximation to the distribution of weighted sum of chi-square random variable (RV) we develop the analytical model for the time slot allocation among SUs. Results show that both the time slot and transmit power allocation can be optimized for achieving the maximum QoC. In a particular scenario the QoC can be increased by 30%.