Research Of Distributed Beamforming Optimization Algoritm In Cognitive Network

With the rapid development of wireless communications, people increasingly focus on the growing demands for the quality of wireless communications. The limited spectrum resources and inefficient spectrum allocation have led to the scarcity of radio spectrum. To solve this problem, Dr. Joseph Mitola proposed the concept of cognitive radio in1999. Cognitive radio is an intelligent spectrum sharing technology which can increase the spectral efficiency by discovering idle spectrums and using them rationally. In cognitive radio network, secondary users can use the same spectrum with primary users simultaneously as long as they do not interfere with the communications of primary users. However, in order to satisfy this constraint, the transmitted power of secondary users are usually very low. To solve this problem, we use cooperative relaying techniques in cognitive network. Among cooperative relaying techniques, distributed relay beamforming has attracted wide attention for improve the system performance. In this paper, distributed relay beamforming in cognitive network is our research object, the problem researched are as follows:Firstly, we propose a filter-and-forward (FF) beamforming strategy in the secondary relays for the one-way cognitive relay network with frequency selective fading channels. Our aim is to optimize the beamforming coefficients and the secondary transmitted power to maximize the SINR at the secondary destination, while satisfying the interference constraint on the primary network and the total relays’ transmitted power restriction. To solve this problem, we propose a two-step iterative algorithm. Simulation results show that comparing the amplifyand-forward (AF) strategy, the filter-and-forward strategy (FF) can get higher SINR at the secondary destination by using the algorithm that we proposed Secondly, we concentrate on the physical-layer secrecy in filter-and-forward (FF) relay cognitive network with frequency selective channels, where a secondary user (SU) communicates with its receiver on the same band with a primary user (PU), meanwhile a eavesdropper is trying to get the information transmitted by the SU. In this paper we study the problem of secrecy capacity, which is defined as the maximum rate of reliable information sent from the source to the intended destination while the eavesdroppers obtain zero information. We solve the secrecy capacity maximization problem under the constraints of interference in the primary network and the total relays’ transmitted power by performing a one-dimensional search which involves solving a sequence of semidefinite programs (SDPs). Simulation results demonstrate that FF relay strategy can get higher secrecy capacity performance than amplify-and-forward (AF) strategy in this model.