Optimal Cooperative Strategy in Energy Harvesting Cognitive Radio Networks

We consider a cognitive radio system, in which a secondary transmitter harvests energy from a primary transmitter's wireless signals. If the achievable rate of the primary transmitter's direct link to the primary receiver is smaller than a target rate, the secondary transmitter can provide decode-and-forward relaying service for the primary system and transmit its own data as well, in which the secondary transmitter uses a time-switching protocol to harvest energy and decode primary transmitter's information. Our target is to achieve maximal secondary throughput by selecting the time portion used for energy harvesting and the percentage of the secondary transmitter's power used for relaying. The formulated problem is nonconvex. To solve the problem, we show that, after some math manipulations, the initial problem can be converted to a problem with the objective function being theoretically proved to be quasiconcave. Then we propose a two-level bisection search algorithm to find the maximal objective function. The efficiency of our proposed method is demonstrated by computer simulation.