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

With the rapid development of the wireless communication, more spectrum resources is needed and the current bandwidth seems lackage. Cognitive radio(CR) provides a way of sharing and utilizing the spectrum can effectively solve this problem. Cognitive radio technology can sense unused spectrum intelligently in the communications environment, and use it to improve the spectrum efficiency.Orthogonal frequency division multiplexing (OFDM) has been the key technology of the fourth generation of mobile communication system due to its high spectral efficiency and robustness to frequency selective fading. As the constant demands of humans for communication business and communications quality requirements, OFDM technology is used more and more widely. OFDM systems usually adopts the multi-level modulation technology, and the receiver need the channel state information obtained by channel estimation for the coherent demodulation, so power allocation is one of the key technologies in OFDM systems.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 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.(3) 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.