Research On Cooperative Communication In Cognitive Radio Networks

With an increasing demand of wireless applications, various new types of different wireless networks, different operations and different access technologies in wireless communication are emerging nowadays, leading to the scarcity of spectrum resources. The problem of how to improve spectrum utilization, the system capacity and quality of services in wireless communication needs urgent solution. Cognitive radio(CR), has emerged as a promising technology to improve the spectrum utilization by allowing the cognitive users to coexist with primary users(PU) in the licensed channel.In this dissertation, we focus on the exploration of cooperative transmission schemes in cognitive radio networks. Through the cooperation between cognitive users, we can greatly improve the performance of communication. The main contributions of this dissertation are described as follows:(1) Construction of a system model for cooperative transmission scheme in CRNs, based on which we propose an amplify-and-forward based cooperative transmission scheme.With the increase of primary user’s transmission performance, the cognitive user(CU)’s performance will be decreased rapidly in CRNs. This article against a backdrop of spectrum sensing technology, proposed a new opportunistic cooperative transmission scheme based on spectrum sensing in CRNs. Besides, closed-form expressions of outage probability and average bit error ratio (BER) of CU for the proposed opportunistic cooperative transmission scheme are presented over Rayleigh fading channels. Then, numerical simulations are conducted to compare our scheme with the traditional non-cooperative transmission, showing the merits of the proposed opportunistic cooperative transmission in terms of outage probability and BER, and transmission performance of CU in CRNs can be improved without interfering the regular transmissions of PU.(2) Construction of a system model for multiuser cooperative transmission, base on which a best relay selection based cooperative transmission scheme is proposed.We propose a new AF (Amplify and Forward) cooperative transmission scheme based on optimal relay selection method while PU and CUs coexist in the same CN(Cognitive Networks) and each PU can be either active(ON) or inactive (OFF) in any given slot. Analytical expression of the non-interference transmission durations was given. Non-cooperative and cooperative schemes were derived show the effect of cooperative transmission on the non-interference transmission durations performance. Besides, numerical experiments were conducted to verify the effectiveness of the proposed cooperative schemes, and selection cooperation scheme exhibits higher time gains than that of the single relaying scheme. The increase of non-interference transmission durations by cooperative transmission confirms the advantages of our new scheme and moreover, time gains obtained by the cooperative schemes increase with a rise in the number of candidate relays.(3) Construction of a system modle for cooperation transmission, which joint considered the sensing time and power constrain to examine the impact of the spectrum sensing and cognitive cooperative transmission.We propose a new cooperative transmission scheme for the CRNs in which a cognitive transmitter could opportunistic communication with a cognitive receiver via a direct or a relay channel, depending on the state of a primary transmitter and study the end-to-end performance of cognitive network over Rayleigh fading channels with power allocation which jointly considering the spectrum sensing phase and data transmission phase in cognitive radio networks. Specifically, in the first phase, cognitive resource senses the licensed channel and determines the state of primary user (the licensed channel is occupied or not). In the second phase, cognitive resource transmits its data to destination via cognitive relay according to different spectrum sensing results with interference power constraint (protect the primary user from harmful interference). In cognitive networks, the improvement of outage performance can be obtained by increasing the power consumption and data transmission duration for cognitive users, while the primary user’s interference will be increased using this method. Therefore, the cognitive performance, spectrum sensing duration and power allocation should be jointly considered in our proposed scheme. Closed-form expressions for the outage probability and the average bit error rate are determined. Numerical results show that, there is a tradeoff in determining the spectrum sensing overhead in order to maximize the cognitive performance with the protection of primary user.