Research On Key Technologies Of Wireless Communication In High-Speed Environment And Broadband Satellite Communication Based On OFDM

Future communication systems aim to use a variety of possible network technology to achieve any person (Whoever) at any time (Whenever), any place (Wherever), with the addition of any one person (Whomever) of any type (Whatever) communications. Therefore, the future communication will be applied in a wider scene and cover all aspects of people’s lives. While maglev and high-speed railway are being used commonly, obtaining high quality communication in a speedy moving environment has become a difficulty in research.With the development of the information and communication technology, the various services like multimedia services, interactive services and high-speed data services continue to increase, the broadband satellite communication system has become hot reseach field of satellite communications. As the one of key technologies being used in B3G and4G, OFDM (Orthogonal Frequency Division Multiplexing) is considered to be applied into the broadband satellite communication system. Not only because of its capability in high speed data transmission, can anti-frequency selective fading and efficient spectrum utilization, but also it can be connected with the land communication network effectively.In this paper, the physical layer algorithms on OFDM wireless communication in high speed moving environment are studied firstly, namely channel estimation algorithm and Doppler frequency shift estimation algorithm. Meanwhile, the air interface technology for broadband satellite communication system based on OFDM/TDM (OFDM combined with Time Division Multiplexing) is researched. The main innovative points and research works of this paper are described as follows:1. Based on the study and analysis of channel estimation algorithm, two algorithms which are suitable for the high speed mobile environment are researched, one is the algorithm of combined with DFT (Discrete Fourier Transform) removing noise and interpolation in time-domain, the other is the algorithm of MMSE (Minimum Mean Square Error) in frequency-domain and interpolation in time-domain. The research results show, these two algorithms have the tracking ability against fast fading channels, can be easily applied into the system with the low complexity and be suitable for the high-speed mobile environment; when the pilots information is enough, the algorithm of MMSE in frequency-domain and interpolation in time-domain has the better performance than the algorithm of combined with DFT removing noise and interpolation in time-domain, and at the same normalized mean square error rate, there will be3-5dB in the increase of performance; but when the pilots information is not enough, both algorithms have the equal performance, so the choice of the algorithm depends on the actual systems and operating environment;2. Based on the study and analysis of Doppler frequency shift estimation algorithm, the level crossing rate estimation algorithm and the autocorrelation function estimation algorithm are discussed, then a novel adaptive Doppler frequency shift estimation algorithm is proposed which adaptively changes estimation length according to the corresponding velocity of the mobile station. The research results show the proposed method has better estimation performance and has a large estimation range even to Doppler frequency shift caused by the mobile station at the speed of500KM/H, at the same time, the normalized mean square error of the estimation results approach to or below to10-3and some estimated points get to10-6. So the proposed method is suitable for the high speed mobile scenario.3. Based on the study of the broadband satellite communication system and OFDM/TDM technology, the air-interface design for the broadband satellite communication system based on OFDM/TDM is presented. The design utilizes OFDM/TDM’s restraining effect on PAPR (Peak-to-Average Power Ratio) and preserves the advantages of OFDM. In the design, the Ka broadband satellite channel is modeled; the PAPR of OFDM/TDM, the scheme on the adaptive coding and modulating (ACM) and the scheme on the channel coding are analysed in performance field. The research results show OFDM/TDM decreases the PAPR, when OFDM with2048subcarriers is divided into32time segments, the probility is1‰as the value of PAPR is about5.6dB, but for the conventional OFDM, the probility is100%when the value of PAPR is about8dB; by adopting ACM, the system bandwidth can be utilized efficiently through real-time adjusting coding and modulating according channel condition and the communication performance can be improved; through channel coding, the system effectively correct burst errors in transmission, ensure the system transmission performance.4. Based on the study of the pilot structure and channel estimation algorithm, an algorithm of filtering in time-domain and combined with DCT (Discrete Cosine Transform) interpolation and removing noise with frequency-domain multiplexing pilot is proposed. Meanwhile, the algorithm of removing noise in transform domain and frequncy-domain interpolation with time-domain multiplexing pilot is researched and noise estimation method is improved in this algorithm. The research results show that the proposed algorithm has the ability to track the fast fading channel, the satisfied performance, and the low complexity; when the Doppler frequency shift changes from30Hz to100Hz, the estimation performance degrades with2dB. So the proposed algorithm is the fit channnel estimation algorithn for the broadband satellite communication system based on OFDM/TDM.