Research On Key Technologies Of NC-OFDM Systems

Cognitive radio, which has been proposed to solve the problem of low spectrum efficiency caused by static spectrum allocation, provides a powerful solution. NC-OFDM(Non-Contigous Orthogonal Frqeucny Divison Mutilplex) based on OFDM as the underlying transmission technology is an effective solution to cognitive radio communication system environment.The focus of this work is to study key technologies of NC-OFDM systems, mainly divided into channel estimation algorithms, adaptive modulation and coding algorithms and SNR estimation algorithms. This paper is divided into three parts.The first part is the first two chapters of this thesis, which introduced the research background and state-of-the-art researches of NC-OFDM system. Then the basic principle and system model of NC-OFDM system are introduced based on OFDM system and the key technologies of NC-OFDM systems to be studied are summarized.The second part is the third, fourth and fifth chapters,which are the main content of this thesis. In chapter III channel estimation algorithms in NC-OFDM systems are studied, including pilot patterns design, estimation algorithms and interpolation algorithms. Analysis of the theoretical performance of OFDM systems based on the traditional pilot patterns are given, then novel pilots patterns are adapted for NC-OFDM systems. LS estimation algorithms, LS with noise reduction algorithms MMSE algorithms with reduced complexity are studied respectively and their performances in different channels are simulated and analyzed. In interpolation algorithm, the performances of several commonly used interpolation methods and interpolation method based on IDFT are given, which indicates the IDFT-based method has a large advantage over other methods.SNR estimation algorithms are studied in Chapter IV. Firstly, analysis of SNR estimation algorithms of OFDM systems in AWGN channels are given and then SNR estimation algorithms in Rayleigh fading channels are studied. After a simulation analysis of the NC-OFDM system in different channel conditions, the performance of various SNR estimation algorithms are analyzed, giving a method which has good performance in frequency-selective channels.The fifth chapter is the study of adaptive modulation and coding algorithms of NC-OFDM systems. We first derive the bit error performances of different fixed modulation and coding scheme in different channels, hence obtaining thresholds of fixed-threshold algorithms,which indicates that the throughput of NC-OFDM systems improves with SNR improving while meeting the BER performance requirment and more subbands can give a more accurate MCS. The performance of adaptive threshod algorithms are better than fixed-threshold algorithms since the threshold decision is more accurate according to the variant channels. In this thesis, a conbined dual-parameter adapitve algorithm is proposed, which does not need channel estimation and SNR estimation resulting in a less complexity. Simulation shows that its performance is similar to adapitive-threshold algorithms.The third part of this article is a summary of the full thesis, in which the research work done in this paper are summarized and we point out the direction of future researches.