Study On Transmission Link Adaptive Based On OFDM In Cognitive Radio

With the rapid growth of people's needing of the Wireless communications services, spectrum resources becomes scarce, and how to effectively improve the spectrum efficiency, has become one of the important issues of concern in the mobile communications industry. Recently, cognitive radio (CR) as an effective means of improves spectrum efficiency, eases the strain on the resources, and has become a hot topic of academic research. Cognitive radio is an intelligent wireless communication technology, it can utilize the spectrum sensing of surrounding environment, detecting the existence of the spectrum holes, at the same time change their operating parameters to adapt to the changing of the communications environment, and opportunistic access to the spectrum hole, improve the spectrum efficiency, ease shortage of the spectrum resources status.Transmission Link Adaptation (TLA) technique is an effective method of against the fading effect, and improving communication system performance and spectrum efficiency, it can use the limited power, spectrum and other resources to maximize the data transmission rate. As cognitive radio system may work in very large bandwidth, and the spectrum is generally not continuous, so the data transmission must be adaptive to this feature. As OFDM not only can dynamic use of spectrum resources and adaptive load according to the sub-channel characteristics, but also can quickly analyze the spectrum occupancy of the primary users, OFDM is considered to be a potential techniques of link adaptation in Cognitive Radio. This Papers research Transmission Link Adaptive based on OFDM in Cognitive Radio. The main contents include:1) Based on the summary and comparison of the typical transmission link adaptive algorithm, analysis the transmission link adaptation algorithm based on multi-user OFDMA system and achieve the performance and characteristics of the algorithm.2) Cognitive radio system including primary users and cognitive users, the prerequisite of the application of Transmission Link Adaptation technology is don't affecting the primary users. So when do adaptive resource allocation, then adding the interference temperature constraint. The accuracy estimation of the interference Temperature distribution, and get the state information of the spectrum occupied. Cognitive users access to the spectrum holes opportunistic and share the spectrum resources with the primary users. Combined with Kriging, the interference temperature sample values obtained by wireless sensors distribute in space are utilized to estimate the interference temperature in the total area.3) Using the primary user's interference temperature threshold as the constraint conditions, then adaptive allocate the total resources(including spectrum holes and total power)of the cognitive users. Detail the adaptive algorithm based on the optimal solution and the tree branch algorithm based on greedy algorithm, then improve the tree branch algorithm.