Research On Quasi-Orthogonal Time Division Multiplexing Techniques

Broadband wireless communication is an important technique for driving the informationization society toward a high level developing stage, so that it arrests people's great attention in the area of electronics and information. It has several advanced techniques for supporting and must be promising. Orthogonal Frequency Division Multiplexing (OFDM) is supposed to be an un-replaceable technique in the next generation broadband mobile wireless communications. It is indeed possible that OFDM find very wide applications, but it will not be an universal tool to be applicable for every where, since it has some remarkable disadvantages, e.g., the PAPR(Peak-to-Average-Power-Ratio) of the OFDM signal is quite high, and it means that the system must have very great linear dynamic range, which forces the power efficiency of the radio power amplifier decreasing and its size and cost increasing. Besides, it is quite sensitive to the frequency offset and phase wobble of the carrier, which will affect its transmission performance in some situation.The China invention of No.1845487 proposed a Quasi-Orthogonal Time Division Multiplexing (QOTDM) technique, which is an allelomorph of OFDM in principle, i.e., it is proposed according to the principle of allelomorph between frequency and time in Fourier Transform. QOTDM is a time-division-multiplexing method based on sample-interleaving. It can not only satisfactorily solve the challenging problem of how to effectively transmit multiple continuous signals via a continuous channel with a time-division-multiplexing mode, but also constitute a broadband communication system in the case of channel with serious distortion. QOTDM has no disadvantages of high PAPR and sensitive to frequency offset like OFDM. It can conveniently overcome channel distortion including multi-path-fading to achieve very good performance by channel estimation and equalization. Therefore, it would have good prospect.This dissertation researches in depth on QOTDM technique, including its principle and some related key issues, as well as how to combine it with Multi-Input-Multi-Output(MIMO), etc. The author's main contributions are as follows:1) Through researching the principle of QOTDM, aiming at the deficiencies of sensitivity to sampling time offset in QOTDM system, the dissertation deduces a formula for the sample error probability based on the separable features of the joint probability density function of independent variables, due to the statistical independence of samples and noise in the QOTDM system. Since the Modified Chernoff bound is significantly tighter than the Chernoff bound, the upper bound of sample error probability is also obtained, which can provide an efficient method to evaluate the sample error performance of QOTDM systems.2) In the research of channel equalization of QOTDM, the dissertation proposes a variable step-size adaptive channel equalization algorithm based on the nonlinear functional relationship between the step-size and the error signal. The step-size of the algorithm increases adaptively at the beginning of the algorithm or when the channel is varying with time, while it is very small during the steady state. The algorithm can avoid the effects of the irrelevant noise.3) Basing on deducing the expressions of the QOTDM signal in both time and frequency domain and analyzing its frequency spectrum, the dissertation analyzes the performance of the QOTDM system in fading channels, moreover, compares its performance with that of OFDM in the same channel condition by simulation. The results show that the anti-multi-path-fading ability of QOTDM system is better than that of OFDM in some conditions, but poorer in other conditions.4) Based on the study of QOTDM and Space Time Block Coding (STBC), a scheme of QOTDM-STBC communication system is proposed. At sender of the system, after QOTDM is finished, the QOTDM sample sequences were encoded with Space Time Block Coding (STBC) method before transmission; at the receiver, after the received signal is decoded with STBC decoder, QOTDM demultiplexing is performed. The dissertation deduces a closed form solution for symbol error probability (SEP) of the QOTDM-STBC system due to the sample errors over spatially uncorrelated fading channels or over spatially correlated fading channels. Theoretic analysis and the results of Monte Carlo simulations show that the proposed QOTDM-STBC system has performance remarkably better than that of the Alamouti (STBC-QPSK) scheme.