Performance Analysis and Optimization for Cognitive Radio and Device-to-Device Communications

Abstract

Cognitive radio (CR) and device-to-device (D2D) communications are promising technologies to enhance high spectral efficiency and regarded as key technologies for the upcoming fifth generation (5G) wireless communications. In CR, unlicensed users, a.k.a. secondary users, are allowed to opportunistically reuse underutilized spectrum bands which are allocated to licensed users, a.k.a. primary users. In D2D communications, D2D users directly communicate each other without going through a base station, typically by using the cellular spectrum. Since both CR and D2D communications take place in the spectrum band already occupied by legacy users, interference management is necessary. Especially, in CR, reducing mutual interference between primary and secondary network is one of the most important factors to improve network reliability. In D2D communications, a comprehensive interference management scheme is needed which limits not only the mutual interference between them but also the interference to cellular users. The dissertation consists of two main results. First, we investigate an underlay CR network consisting of a single-hop secondary network co-existing with a multi-hop primary network. In the secondary network, the secondary destination receives the same interference signals from primary terminals over different time. To improve reliability of the secondary destination, it cancels the interference by using successive interference cancellation. We analyze the outage probability of the primary network in an integral expression and obtain its closed form for a special case. Also, we approximate the outage probability of the secondary network in a closed form. The validity of our analysis is verified by computer simulations. It is shown that the analytical results for the outage probability of the primary network perfectly match the simulation results. Also, it is shown that approximate outage probability of the secondary network is close to the simulation results. Second, we investigate underlay D2D communications in cellular networks where D2D transmitters transmit data to their receivers using cellular spectrum. We analyze the average sum throughput of D2D receivers in interference-limited channels. To enhance it, we propose a semi-distributed spectrum access scheme which consists of two stages. In the first stage, a cellular base station divides whole D2D transmitters into multiple groups and assigns a different subchannel to each group. In the second stage, D2D transmitters in each group randomly access the subchannel assigned to it with predetermined access probability. We formulate an optimization problem to find the groups and access probabilities which maximize the average sum throughput. To overcome the prohibitive computational complexity to obtain its optimal solution, we decompose it into two subproblems: one to find groups and one to find access probabilities. A heuristic grouping algorithm is adopted to solve the former, and a branch-and-bound based algorithm is proposed to solve the latter. The validity of the branch-and-bound based algorithm is shown by performance comparison with an exhaustive search. It is shown that the heuristic grouping and the branch-and-bound based algorithm achieve higher average sum throughput than conventional methods.