PHY-layer Detection Threshold Optimization And Mac-layer Multi Users Access In Cognitive Radio

In recent years, as the development of 3G and 4G mobile communications, the users'demand is increasing. As a result, the fixed spectrum allocation strategy is starting to appear bottleneck, which makes some new communication technologies have no extra band to use. Instead, from the monitoring of assigned spectrum, we can see that the utilization efficiency is very low. This paradox makes the new spectrum utilization technology necessary. Cognitive Radio (CR) makes use of Opportunistic Spectrum Access (OSA) manner and admits Cognitive User (CU) to use licensed spectrum, which solves the problem effectively.In Cognitive Radio, spectrum sensing in physical layer and spectrum access in Mac layer are important. Once CUs detect the licensed spectrum idle, they would transmit data on it. However, no matter what methods CUs use to sense, there are always missing detection to Primary User (PU) and false alarm to access opportunity. Consequently, how to improve sensing performance and channel utilization efficiency is one of the key research points. Different from other papers, we focus on spectrum sensing in physical layer and spectrum access in Mac layer. To improve channel utilization efficiency of CU, we propose adaptive threshold scheme based on different channel occupied probability. Meanwhile, we propose queuing scheme to solve the low successful access efficiency of multi-user access in Mac layer.In physical-layer spectrum sensing, missing detection probability and false alarm probability are characterized as the criteria of sensing performance. In order to improve sensing performance, the minimum average error probability rule is used. We derive the optimal detection threshold, and analyses the missing detection probability and false alarm probability of different detection threshold. Based on different channel occupied probability of primary network, we propose an adaptive threshold scheme to improve channel utilization efficiency of CU. With the analysis of detection threshold and channel occupied probability, exponential relation is derived. The numerical results verify the confidence of the proposed scheme and perform better, in terms of sensing performance and the measurement of channel utilization efficiency.Additional, in Mac-layer access, there are multi CUs to compete the channel simultaneously, which leads to information collision and then decrease the successful access efficiency of CUs. To solve the problem, we propose a scheme of queuing access. In this scheme, multiple CUs queen from short to long according to their packet length, and then the CU with the shortest packet length accesses the channel with the priority. Meanwhile, truncate the long packet on the basis of no affection for CUs. From the simulation, this scheme dramatically increases the successful access efficiency on the limit of a certain collision probability of the system.