Research Of Power Allocation Algorithm In The OFDM-Based Cognitive Radio System

At present, because of the relative "shortage" and "waste" of the spectrum, the limited spectrum resources becomes a serious shortage as a constraint to the development of wireless communications and next generation network bottleneck. It is the serious problem to maximize the utilization of the wireless spectrum. Cognitive radio technology is put forward to solve this problem, able to sense the external radio environment, learn from the environment and dynamically adjust its internal working performance according to environmental changes. Cognitive radio technology has changed the traditional government-licensed radio spectrum access methods. As the core technology of the next generation wireless communications to the dynamic use of spectrum resources, cognitive radio technology has been more and more the concern of communication scholars, and was predicted for the next most popular wireless technology. Cognitive radio technology can adopt a more flexible way to manage the limited spectrum resources, greatly improve the spectrum efficiency, but cognitive radio technology will require breakthroughs made in key technologies in order to really be applied to practical communication system. OFDM technology is a high-speed wireless environment multi-carrier transmission technology, and has strong ability in anti-frequency selective fading and narrowband interference. It can make full use of spectrum resources through a cutting or combination of the spectrum. OFDM technology is suitable for multipath propagation of wireless high-speed data transmission in mobile communication, the combination of OFDM technology and cognitive radio technology makes the two technologies complement each other, both constitute the transmission system of frequency agility. Therefore, the study of cognitive radio systems based on OFDM has a very important theoretical meaning and realization of value.In the OFDM-based cognitive radio system, there are two power allocation models, one is the sensing- based spectrum sharing model, the other is the opportunitic spectrum access model. This article researches the problem of optimal power allocation under the both models. The main contents are as follows: (1) The article analyses the principle of water-filling power allocation in the traditional OFDM system, and make simulation analysis of water-filling power allocation.(2) Under the overlay-based opportunistic spectrum access model, we make simulation analysis of a improved power allocation strategy. It applies iterative partitioned water-filling algorithm under the conditions of the reasonable nulling mechanism between the subchannels. Simulations show that the proposed scheme maximizes the channel capacity while reducing the interference of subcarriers' sidelobe. In this way, the system optimizes the power allocation and improves the spectrum utilization efficiency.(3) Under the underlay-based spectrum sharing model, we make simulation analysis of the optimum power allocation problem under the PUs' rate loss constraint and hybrid protection constraint. It ensures the PUs'quality of service, and maximizes the channel capacity of the SU link.