Power Allocation And Performance Analysis Of Cognitive Relay System

Wireless communication depends on the wireless spectrum. However, at present the available wireless spectrum is becoming very scarce due to the increase of user demand and the access of a large number of wireless devices. Cognitive radio(CR) which does not rely on fixed spectrum, allows the unlicensed users(secondary users, SUs) to share the spectrum with the licensed users(primary users, PUs) without impacting the quality of service(Qo S) of PUs. It is recognized as one of the best technologies to solve the spectrum scarcity. At the same time, relay cooperation expands the wireless coverage and combats the wireless propagation fading through relays amplifying and forwarding the received signals. Therefore, the cognitive relay systems(CRS), which is the combination of CR and RC technologies, have been considered as a promising method for improving the system performance. On one hand, the coverage and reliability of wireless propagation is improved by RC. On the other hand, the spectral efficiency is enhanced by CR. However, all of these excellent profits are basis of the efficient power allocation. The thesis focuses on the investigation of the power allocation(PA) of cognitive relay systems. The main contributions are as follows.Two PA schemes are first proposed for the decoded-and-forward cognitive relay systems, which are independent power allocation(IPA) and joint power allocation(JPA), respectively. In IPA, the PUs for the source and relay of SUs are mutually independent, which is based on their respective interference power constraints at PUs; while in JPA the ones for the source and relay are considered jointly. Then, by comparing the spectrum and energy efficiencies of the two power allocation schemes, we propose a hybrid interweave-underlay spectrum access method. In this hybrid spectrum access scheme, the SUs send its signal with the maximum power under the case where the Qo S of PUs is not guaranteed; The SUs no longer transmit signal when the outage probability of SUs is very high due to the severe interference from PUs; When the Qo S of both SUs and PUs is guaranteed simultaneously, the PUs and SUs work in Underlay mode. At the same time, for the proposed schemes, the outage probabilities and ergodic capacities are investigated in detail.Secondly, a novel relay gain allocation(RGA) scheme for amplify-and-forward CRS is proposed based on the ergodic capacity constraint of primary system and the average symbol error rate(ASER) minimization of secondary system. By using Lagrange multiplier method, the exact optimization solution of the relay gain is achieved as well as the corresponding upper and lower bounds. With upper and lower bounds, we can obtain the insight about the effect of systems on the relay gain allocation. The derivations show that the RGA is determined by not only the transmission power of SUs and the secondary source-relay channel coefficients, but also the transmission power of PUs and the channel coefficients of primary systems. Especially, in our scheme the channel coefficient of secondary relay-sink is also a crucial factor for the relay gain. Based on the achieved relay gain allocation scheme, the ASER of AF CRS is investigated as well as the effect of the primary and secondary systems parameters on ASER and relay gain.