Research On Decode-and-Forward Cognitive Cooperative Communication Technology

Emerging wireless applications and consumer expectations fueled an endless desire for higher data rates. The enabling technologies for these high data rate services have consumed almost all the available and accessible spectrum and the inefficient utilization of huge chunks of licensed spectrum have made it a very scarce resource. The technology of cognitive radio can use the untapped spectrum resource to increase the cognitive radio technology efficiency dramatically. Generally, it is suggested that if the licensed or primary user is not using the dedicated spectrum or part of it, the secondary or cognitive user can exploit this unused spectrum. The primary and secondary users can also share the spectrum simultaneously in underlay approach as well. In this approach, the secondary user has to satisfy strict interference constraint, and its transmission power should be below a certain threshold all the time.In order to obey the interference threshold, secondary users in underlay mode are required to transmit at low power, which limits their area of coverage. Therefore, it is very likely that the secondary users in underlay networks would incorporate relaying techniques to increase their area of coverage. There exist various techniques and protocols for relaying a signal in cooperative networks among which amplify and forward(AF) is the most popular one because of its simplicity. As the name suggests, in this technique, the received signal at the relay is just amplified and forwarded to the destination. Another better performing but computationally expensive approach is decode and forward(DF) in which the relay decodes and reproduces the received message before forwarding it to the destination or the next node. Nevertheless it is found that the existing papers have focused on AF systems, and the DF cognitive cooperative communication networks have seldom been considered. Based on the above considerations, to enhance the performance, some new schemes are proposed in this dissertation, and the main works are outlined as follows:1. A new dual-hop communication scheme is proposed and analyzed for a cognitive cooperative system where multiple primary-user receivers coexist. In this proposed scheme, both source and relay are equipped with a single antenna, but multiple receive antennas are equipped at the destination; maximum ratio combining is adopted to improve the system performance. Moreover, the outage probability performance is investigated over Rayleigh fading channels and the exact closed-form expression is derived, then an asymptotic expression for the outage probability is obtained too at high signal-to-noise ratio. Finally, simulation results are presented to verify the theoretical analysis.2. With multiple primary users coexisting, a new dual-hop cognitive cooperative communication system is proposed and analyzed. In this proposed system, multiusers are existed at the destination and the optimal channel coefficient scheme is adopted to improve the system performance. Moreover, the outage probability performance is investigated over Rayleigh fading channels and the exact closed-form expression is derived, then an asymptotic expression at high signal-to-noise for the outage probability is obtained too. Finally, simulation results are presented to verify that the system performance is improved with multiuser condition.3. In a new two-hop DF cognitive cooperative communication system with a direct channel, a relay selection scheme is proposed and investigated over fading channels. Based on the closed-form distribution function of the end-to-end signal-to-noise ratio obtained herein, the outage probability of the system under study is analytically investigated. Then we compare the performance in terms of outage probability of the use of fixed transmit power(FTP) and adaptive transmit power(ATP). Finally, all analytical results are exhibited and corroborated by simulation results.