Optimization of spectrum management in cognitive radio networks.

The rapid growth of wireless services has resulted in the increasing demand on spectrum. However, a recent study has shown that most of the allocated frequency bands are significantly under-utilized. Cognitive radio has been proposed as a potential technology to mitigate the spectrum scarcity by allowing the secondary users (SUs) to opportunistically utilize the licensed spectrum without causing interference to the primary users (PUs). Spectrum sensing is required to be performed first before accessing the channels. If more than one channels are detected, the SUs need to decide which channels are suitable for data transmission. As soon as the return of the PU is detected, the SU is required to vacate the channel, and find another opportunity to resume its unfinished transmission. These are the so called spectrum management problem in cognitive radio networks (CRNs). In this dissertation, the author widely exploits the spectrum sensing, spectrum sharing and spectrum handoff in the network design to improve the system performance. It is shown that the spectrum, if well managed, is able to enhance the network performance, increase the achievable throughput, reduce the cooperation overhead as well as provide sufficient protection to PUs. According to the specific challenges of spectrum management in the cognitive radio networks, different models and solutions are provided. In this work, the author studies the optimization of spectrum management in CRNs. The first problem addressed is spectrum sharing among the SUs and PUs. The author proposes a cluster-based adaptive multi-spectrum sensing and access strategy, in which the SUs seeking to access the channel can select a set of channels to sense and access with adaptive sensing time. Specifically, the spectrum sensing and access problem is formulated into an optimization problem, which maximizes the utility of the SUs and ensures sufficient protection of the PUs and the transmitting SUs (The SUs who have already gained access to the channel and are transmitting.) from unacceptable interference. Moreover the author explicitly calculates the expected number of channels that are detected to be idle, or being occupied by the PUs, or being occupied by the transmitting SUs. Spectrum sharing with the primary and transmitting SUs is accomplished by adapting the transmission power to keep the interference to an acceptable level. In addition, simulation is conducted to demonstrate the effectiveness of the proposed sensing and access strategy as well as its advantage over conventional sensing and access methods in terms of improving the achievable throughput and keeping the sensing overhead low. MAC sensing-transmission protocols have been widely investigated for the SUs to efficiently utilize and share the spectrum licensed by the PU. One important issue associated with MAC protocols design is how the SUs determine when and which channel they should sense and access without causing harmful interference to the PU. The author's second contribution focuses on jointly considering the MAC-layer spectrum sensing and channel access. Normal Spectrum Sensing (NSS) is required to be carried out at the beginning of each frame to determine whether the channel is idle. On detecting the available transmission opportunity, the SUs employ CSMA for channel contention. The novelty is that, Fast Spectrum Sensing (FSS) is inserted after channel contention to promptly detect the return of the PUs. This is unlike most other MAC protocols which do not incorporate FSS. Having FSS, the PU can benefit from more protection. A concrete protocol design is provided, and the throughput-collision tradeoff, utility-collision tradeoff problems are formulated to evaluate its performance. Finally, the author studies sequential sensing based spectrum handoff in multiple users scenario. Spectrum handoff occurs when the PUs appear in the licensed spectrum temporarily occupied by the SUs. Efficient spectrum handoff aims to help the SUs to vacate the spectrum rapidly and to resume unfinished transmission on newly selected available channels. However, a spectrum handoff policy that comprehensively considers spectrum sensing, target channel selection as well as spectrum estimation has yet to be developed. Thus, in this work, the author presents a sequential sensing based spectrum handoff policy for multiple users in CRNs. The author first selects the appropriate candidate channels for each SU, then their associated optimal sensing order together with the best target handoff channel is determined through sequential sensing using Dynamic Programming (DP). Note that many spectrum handoffs will occur during one SU transmission and the objective is to minimize the total number of spectrum handoff. The sequential sensing based spectrum handoff policy is evaluated through a comprehensive simulation study. The results reveal significant improvements in the system performance by reducing the number of spectrum handoff over conventional approaches. Moreover, the proposed DP method can significantly lower the computational complexity compared to exhaustive search and common DP.DOCTOR OF PHILOSOPHY (SCE