The Applications Of Cooperative Communication In Cognitive Radio

The development of wireless mobile communications is facing new challenges, and following the direction of broadband, seamless and intelligent.In order to improve the performance of mobile communication system, diversity should generally be provided at the transmit side. Transmit diversity generally requires more than one antenna at the transmitter, just like MIMO system. However, many wireless devices are limited by size or hardware complexity to one antenna. In order to overcome this disadvantage, a new class of methods called cooperative communication has been proposed that enables single antenna terminals in a multi-user environment to share their antennas and generate a virtual multiple-antenna transmitter that allows them to achieve transmit diversity.Therefore, the performance of mobile communication system can also be improved by this scheme.With the increasing broadband communication demands, the shortage of spectrum is becoming an outstanding problem. The conflict between spectrum supply and demand is increasing. The utilization of licensed spectrums is very low under the present spectrum management mode. Under the above background, the cognitive radio (CR) technology is proposed.Cognitive radio is defined as an intelligent wireless communication system that is aware of its environment and uses the methodology of understanding-by-building to learn from the environment and adapt its internal states by making corresponding changes in certain operating parameters in real time, in order to promote the efficient use of the spectrum and revive the lack of spectral resource. It also makes networks and terminals more flexible. The proposal of cognitive concept would challenge the present spectrum management system, and bring new space for the development of wireless communication.This thesis discusses cooperative communication and cognitive radio in detail,investigates the combination of the two technologies partly.The key work in this thesis is included as following.The cognitive radio has been considered as a promising paradigm to achieve efficient spectrum usage by allowing the coexistence of the primary and secondary networks. One fundamental challenge is to ensure the quality of service (QoS) of the primary link while maximizing the transmission rate of the cognitive radio user. In this paper, we investigate the cooperation based spectrum access at the secondary networks. If the QoS requirement is stringent, low power is allowed at the cognitive radio transmitter and hence the transmission rate is limited. It is beneficial that the cognitive radio user can obtain cooperation from the surrounding cognitive radio users to increase the transmission rate. The best relay which can provide the maximal transmission rate is selected for cooperation. The transmission rate performance is investigated under the peak and average interference constraints. The simulation results show that the transmission rate of cognitive radio link rate can be improved by AF cooperation and DF cooperation, especially when the power of the direct link is limited. Under the average interference constraint, the transmission rate is better than that under the peak interference constraint, due to the power adaptation between the transmitter and relay. And higher gain can be obtained as the number of potential relays increases. Performance of DF cooperation is better than AF cooperation.