Research On Performance Of Multiple Relay Systems Based On Cognitive Environments

The spectrum is needed as a medium for wireless transmission.However,with the growing demand for radio resources,all the frequency spectrum available almost gone.Due to the irrationality of the spectrum allocation,the spectrum has become a scarce resource.The crisis is relieved to a certain extent after the proposing of cognitive radio technology.Generally,it is suggested that the secondary user can access to the unoccupied spectrum licensed to the primary user for its own signal transmission without causes any conflict or harmful interference to the primary user.But once the primary user has the necessity of transmission,the secondary user must stop its transmission process.Thus,the utilization of spectrum is greatly improved.The mutual cooperation of multiple relays can not only improve the quality of signal transmission,but also expand the coverage of transmission.Herein,multi-relay system has been a hot field of wireless communication.Relay selection systems and multi-hop networks are very typical in multi-relay systems and have been gained much attention.In study of relay selection and multi-hop systems,the performance of the communication system is a vital problem for us to consider,which relates to the quality of communication directly.The key benchmarks to judge the performance of a system include the outage performance,the error data rate and the channel capacity.Among them,the outage performance is the most important.Though a lot of existed literatures have studied the related worksfor the past a few years,a lot of these works only consider Rayleigh and Rice fading,thus have ignored the fading parameters on system performance.Moreover,the direct transmission link to the performance of the considered system hasalso been overlooked in some existed works.Based on the above considerations,the performance of multi-relay system has been investigated and the main works are outlined as follows:1.A relay selection communication system with direct transmission is studied based on decode-and-forward protocol.Also,the performance of the system is analyzed under spectrum sharing environment.In this dual-hop cognitive cooperative communication system,all terminals operate in half-duplex mode,source and all the relay nodes are equipped with a single antenna,meanwhile,the selection combining is adopted at the destination to receive signals from direct transmission and relaying link so as to improve the system performance.What's more,both the exact and the high signal-to-noise ratio asymptotic expressions for the outage probability are derived over non-identical Rayleigh fading channels.The correctness of the proposed analysis is corroborated via Monte Carlo simulations.Results show that the expressions derived in this paper can be used to evaluate the impact of the key system parameters on the end-to-end system performance,and the correctness and effectiveness of the method is more obvious at high signal-to-noise ratio.2.The outage performance of a cognitive amplify-and-forward cooperative system over independent non-necessarily identically distributed(i.n.i.d)Nakagami-m fading channels is investigated.With limited interference from transmit power that the primary receiver can tolerate,the upper bondexpression and asymptotic expressionat high signal-to-noise ratio are derived for outage probability.Monte Carlo simulations are presented to verify the correctness of analytical results,which make it available for us to estimate the performance of considered system.Specially,for fixed number of relays,the diversity and coding gain are discussed.3.The outage performance of a multi-hop cognitive relaying system is investigated under independent but nonidentically distributed Nakagami-m fading channels based on DF relaying protocol.The system is under the maximum transmit power constraint as well as the interference constraint from the primary user.The closed-form analytic expression and an asymptotic expression at high signal-to-noise ratio are derived for the outage probability based on a thorough performance analysis.Moreover,the correctness of the results is validated through Monte-Carlo simulations.Results reveal that the system performance degrades as the number of relays increases,but outperforms significantly the system without relays.In addition,the channel quality of the interference links and the transmission links along with the relationship between the interference threshold and the maximum transmit power have a significant effect on the system performance.