Performance of Cognitive Radio Network with Novel Hybrid Spectrum Access Schemes

In this paper two novel spectrum access schemes, a hybrid spectrum access scheme (HSAS) and a modified hybrid spectrum access scheme (MHSAS) combining underlay and overlay, are proposed to enhance the performance of cognitive radio (CR) enabled secondary user (SU) in terms of throughput and outage probability in a CR network. The CR employs an energy detection to detect the presence of primary user (PU). If PU is sensed to be present, the CR employs an underlay scheme of co-existence while the CR works in an overlay mode if the PU is sensed to be absent. In underlay mode, the transmit power of CR is controlled so as to maintain the interference at PU receiver within a tolerable limit. An imperfect CSI of the interfering link between CR transmitter and PU receiver is assumed. In HSAS, CR transmits either in underlay scheme or in overlay scheme only during the transmission phase of the detection cycle whereas in MHSAS, the CR transmits for the entire frame including the sensing period of a detection cycle. Closed form analytical expressions for the throughput and outage of CR have been developed. Impact of several sensing parameters such as sensing time, channel estimation error, tolerable interference threshold, maximum allowable transmit power of CR is investigated. The impact of collision probability on the performance of CR network due to re-arrival of PU under proposed scheme is investigated. Furthermore the performances of the proposed schemes are also compared with the conventional i.e. only underlay or only overlay schemes. Our schemes are found to outperform the conventional schemes, in terms of secondary performance.