Research Of Key Techniques For NC-OFDM Based On Cognitive Radio

Cognitive radio is an intelligent wireless communication system that can improve spectrum utilization significantly and solve the scarcity of the spectrum by utilizing a spectrum hole unoccupied by the primary user at the right location and the time .Orthogonal frequency division multiplexing (OFDM)-based transmission is a promising candidate for spectrum controlling and utilizing as well as the primary multiplexing and modulation of cognitive radio. When OFDM is applied to cognitive radio, the whole band will be divided into several subcarriers. In this case, information can be transmitted by those unoccupied subcarriers, thus, a non-contiguous OFDM, namely NC-OFDM, is formed. This paper deals with some fundamental framework of NC-OFDM and challenges it is confronted. More emphasis is laid on some key techniques of NC-OFDM, namely:Firstly, peak-to-average power ratio of NC-OFDM is analyzed. In the process of analyzing the PAPR of the NC-OFDM symbol, the correlation function of the NC-OFDM symbol is derived from the correlation property of the symbol. In the meanwhile, correlation coefficient is applied to measure their relativity and a conclusion is drawn that the signal is approximately considered to be noncorrelation and the CCDF of PAPR can be calculated by formula when the correlation coefficient of the signal is below the threshold value of 0.08. The paper also explores the PAPR reduction technique, and put forward a novel adaptive algorithm combined with partial transmit sequences and interleaving. Simulation results show that the adaptive algorithm can reduce the PAPR substantially and cost less computation.Secondly, synchronization of NC-OFDM is analyzed. The paper gives a systematic analysis of the synchronization of OFDM, and investigates the maximum-likelihood (ML) algorithm based on cyclic prefix. This algorithm reduces the complexity of calculation and is easy to realize, meanwhile, it can realize timing and frequency offset estimation simultaneously. When this is applied to NC-OFDM system, it meets some limitation, for example, the number of active subcarriers must not be too small and a relatively small range of frequency offset estimation. Afterwards, the paper studies synchronization algorithm based on PN repeated sequences. A modification is made to Tufvesson's time synchronization object function so that the target function puts out only one peak value and the false alarm probability is significantly reduced. The paper analyzes the two-step frequency offset estimation algorithm. Sequences with closer interval are used as correlation to attain coarse frequency estimation, and a second estimation is performed by cyclic prefix after the frequency is compensated, thus, a large estimation range and a high estimation precise are achieved. This approach can be perfectly applied to NC-OFDM system. Finally, highly efficient FFT algorithm is analyzed. There are a great number of zeros input and output in the computation of IFFT/FFT of NC-OFDM. In view of this, the paper applies an effective transform decomposition algorithm to NC-OFDM, and makes a comparison between it and the traditional pruning algorithm. This simple and feasible algorithm significantly reduces calculating volume with the absence of any conditional statements.