| Orthorgonal Frequency Division Multiplexing (OFDM) becomes one common technology in the high-speed communication system such as digital multimedia broadcasting owing to the high spectrum efficiency, robustness to multipath fading. One of the main drawbacks is sensitive to the time-variant interference, especially in using high level modulation. In OFDM systems, time-variant interference results in the interchannel interference (ICI). The ICI will gives in many system proplem such as spectrum efficience, complexity and error floor. Another wireless system design challenge is how to improve the system spectral efficiency and link quality, using multiple antennas in sending and receiving terminal to transmit signals is an effective solution, multiple-input multiple-output system. MIMO system can be easily obtained wireless channels to achieve spatial diversity gain and capacity of wireless channel gain. OFDM and MIMO technology has become the two cornerstones of advanced wireless transmission technology.Now, the problem of how to use high level modulation in time-viarant mutipath channel and low complexity becomes a hot topic in the OFDM technology, and many schemes have been proposed. These schemes are based on coherent modulation. But, the major proplem of these schemes is low comprehensive performance, that is, the excessive pursuit of high performance, regardless of the side effect. While traditional multi-antenna transmission has obvious advantages, and has been a new generation of wireless communication system adopted by the mainstream protocol, but there are still problems. Specifically, the existing multi-antenna base stations are set up in the end, while the mobile terminal multi-antenna placement is difficult.In this paper, the above problems for differential system are analyzed completely, at the same tiem, base on the classical quaternion theory, we extended study space-time block codes. The main works are described as follow:First of all, the effects of the inter-channel interference (ICI) resulting from Doppler frequency and carrier offset on OFDM systems with various demodulation schemes are analyzed. To evaluate these effects, the signal-to-ICI ratio (SIR) has been derived for different demodulation systems. The results show time-variant characteristic of channel has different effects on different demodulation.Sendly, the bit error rate (BER) of MDPSK-OFDM in frequency domain over HF fading channels is evaluated, the optimum value of subcarrier number is given and the upper and lower bound on BER is obtained. Improved frequency domain differential demodulation (FDDD) scheme for fast fading HF channel is proposed. Extensive computer simulation results show that the proposed scheme is superior to the conventional differential demodulation technique at fast fading HF channel. In addition, the convention differential demodulation scheme is sensitive to the value of subcarrier number in time-variant mutipah channel.Thirdly, According to the analysis of channel interference, new block frequeny differential modulation and demodulation is suggested. New scheme selects the lenth of block according to the knowledge of delay spread. The performance of new scheme has obvious gain in time-variant multipath channel, without any addition of complexity. This scheme is very adapted to the environment of fast fading and strong delay spread such as bad HF channel enviroment.Finally, based on characteristics of the antenna polarization diversity, using Quaternion orthogonal design theory, in the space-time block code into the antenna polarization-dimensional resources, design of a new three-pole polarized orthogonal space-time block code, the code words satisfy the orthogonal relations maximum likelihood detection can be used to decode. Under the same conditions, the classical space-time block code scheme comparison, the program to more effectively reduce the error rate and improve system performance.
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