Research On CSMA Scheme With Double Back-Off In Cognitive Radio Networks

In recent years with the rapid development of information industry especially the mobile telecommunication industry and the increasing broadband communication demands, the shortage of spectrum is becoming more and more serious. On the hand, the conflict between limited spectrum resource and the increasing requirements for wireless access has become the main obstacle in the development of wireless communication. On the other hand, according to the survey of related organizations the utilization rate of licensed spectrums is very low under the present static spectrum management mode. With the characteristics of flexibility, autonomy and adaptation, Cognitive radio (CR) is considered as a new technique that can solve the heterogeneous networks interconnection problem and greatly improve the networks spectrum efficiency. Consequently, CR technology has become an effective way to solve the problem of scarce spectrum resources.This dissertation focused on MAC protocol in unslotted multi-channel CR network. The performance of double back-off CSMA scheme was discussed. An algorithm for getting the optimal average double back-off time was proposed.Firstly, by analyzing of the unslotted cognitive radio network, CSMA scheme with double back-off in Cognitive Radio networks was proposed. At the beginning, each SU sensed all channels before its packet transmission. If an SU found idle channels, the SU selected one idle channel randomly and transmitted a packet, and then the SU g to back-off state in order to prevent the channel being exclusively used by one SU. If the SU did not find any idle channel, the SU went to double back-off state.Then, the mathematical model of CSMA scheme with double back-off in cognitive radio networks was constructed. By3-demensional continuous time Markov process, the steady state probability of the system was obtained and several performance measures such as throughput and forced termination probability of SUs were gained. Performance of scheme with single back-off time and double back-off time were compared. Comparison results show that double back-off time has important effects on system performance. At last, the algorithm for the optimal average back-off time was designed. In the certain condition of system parameters (the number of channel, the number of SUs, the transfer time and back-off time of SU and PU, etc), the optimal average double back-off time was obtained. According to the current network condition, we can dynamically adjust the SU’s double back-off time by the algorithm of optimal average double back-off time. By the double back-off scheme, we can get the maximum throughput of SU and achieve the purpose of efficient use of spectrum resources.