A green cognitive radio that is solely powered by an energy harvester which extracts energy from the RF signal of a primary user transmitter is considered. The cognitive radio operates in a harvesting-transmitting fashion with the period of one time slot. In each period, it first harvests energy for a fixed duration and then transmits data using the harvested energy for the rest of the period. Considering the intrinsic harvestingthroughput tradeoff, we optimise the harvesting ratio of the cognitive radio to maximise its achievable throughput, subject to the interference constraint by the primary user receiver. It is shown that the formulated optimisation problem is convex and we derive a closed-form expression for the optimal harvesting ratio. Simulations validate the correctness and analyse the achievable throughput and optimal harvesting ratio under different parameters. Introduction: As spectrum efficiency and energy efficiency are increasingly critical for wireless networks, RF-powered cognitive radio networks (RF-CRNs) have attracted a great deal of attention in recent years. Unlike overlay schemes for RF-CRNs [1] [2] [3] in which the secondary users (SUs) operate in a fashion of 'saving-sensing-transmitting', this Letter studies an underlay RF-CRN that works in a 'harvestingtransmitting' fashion, i.e. SUs first harvest energy from the signals of primary users (PUs) and then transmit data using the harvested energy. A similar 'harvest-then-transmit' protocol was proposed in [4] where all users first harvest the wireless energy broadcast by the hybrid-access point in the downlink and then send their independent information to the hybrid-access point in the uplink. Different from [4] , this Letter considers the presence of PUs acting as both the energy supplier and interference to the studied RF-CRNs, which makes the 'harvesting-transmitting' in RF-CRNs more complex and challenging. Motivated by the seminar work on the sensing-throughput tradeoff for overlay CRNs [5], this Letter for the first time formulates a novel harvesting-throughput tradeoff optimisation problem for an underlay RF-CRN. Similar to [5] , the formulated optimisation problem is also convex and has a unique optimal solution. However, its optimal solution in this Letter is given in a closed form, while the optimal solution in [5] has to be numerically evaluated.
In a similar model, the reference REF aimed to maximize the secondary achievable throughput and revealed an inherent energy harvesting-throughput trade-off.
Harvesting-throughput tradeoff for RF-powered underlay cognitive radio networks
Electronics Letters
Mathematics