Optimal Spectrum Access and Energy Supply for Cognitive Radio Systems With Opportunistic RF Energy Harvesting

We consider an energy-harvesting cognitive radio (CR) system coexisting with an unslotted primary user (PU) system. The secondary user (SU) either harvests energy from the PU transmissions or transmits when the PU is sensed to be idle. In this paper, we optimize the sensing intervals to balance between energy harvesting and spectrum access, considering the PU traffic statistics that affects both the energy arrival and consumption in addition to the opportunistic spectrum access. We propose to use different sensing intervals if the SU decides to transmit or harvest energy. The harvesting and transmission durations can be used to optimize the tradeoff between maximizing the utilization of PU spectrum opportunities and harvesting sufficient energy to transmit. We determine a closed-form expression for the probability of SU being active as a function of the PU traffic statistics and SU's access and harvest durations. Optimal transmission and harvesting durations have been found to maximize the CR throughput while protecting the PU. Furthermore, we enhance the system capability by considering that the SU can choose to purchase energy from the PU during the discovered PU idle periods in addition to the energy harvested opportunistically. It can further exploit the difference in the PU busy and idle durations as per its energy needs. The average energy cost spent in a time slot is minimized to achieve a given SU transmission rate while adhering to the PU collision constraint.