The Key Techniques Of High Efficient OFDM And Its Space Time Processing

Recent years, as an effective high-speed digital transmission mode, orthogonal frequency division multiplexing (OFDM) has attracted many attentions. Compared with traditional serial transmission mode, OFDM systems obtain many advantages through dividing the entire channel into many narrow sub-channel and transmitting digital stream in parallel. At the same time, as a new technique, space-time coding has also become popular in wireless communication. Not only space-time coding is adopted in 3GPP, but also in other wireless communication systems. In this paper, the attention is focused on some key techniques of OFDM and also the transmission diversity of OFDM is discussed. The main research works and results are listed as follows: 1. Study on channel estimation and Turbo detection for high efficient OFDM system.Several equalization schemes are analyzed and compared. In the slow time variable channel, based on cyclic reconstruction and tail cancel, a new method for OFDM Turbo detection that does not need cyclic prefix is given. When there is no cyclic prefix in OFDM symbol, this new scheme can compensate the received symbol in time domain and a new high efficient transmission structure can be constructed. Also based on LMS algorithm, an adaptive estimation method applied to this Turbo detection is presented. The channel estimation methods for fast fading channel are also discussed in this paper, and several interpolation functions are introduced when estimating channel. Then a Turbo detection method in fast fading channel is presented. Finally two channel estimation methods, which do not need cyclic prefix and can be adapted to the Turbo detection scheme, are proposed. Finally, the Turbo detection is applied in fast fading channel 2. Study on synchronization of time and frequency for OFDM and its transmitter diversity techniques.Methods of synchronization of OFDM system are investigated. Firstly, the effect of symbol offset is analyzed, and the common methods of symbol synchronization are discussed. Then a modified symbol synchronization method, which can get better performance under multi-path environment in time domain, is studied. Also, in frequency domain, a symbol synchronization methods based on frame selection is present in this paper. The performance of OFDM systems with frequency offset is also discussed. Based on the analysis of several frequency synchronization algorithms, an unbiased adaptive frequency synchronization algorithm with no training sequence is presented. The effect of frequency offset for space-time (frequency) OFDM systems is also studied in this paper. And with the help of two training sequences, a method for the joint estimation of fine timing and coarse frequency-offset is finally obtained.3. Study on space-time processing for OFDM.The application of space-time coding in OFDM systems is studied and emphasis is on combination of space-time block coding with OFDM. The performance of space-time and space-frequency OFDM systems is analysis and then a band-efficient space-time (frequency) system, which based on Turbo detection, is discussed. Simulation result indicates that without cyclic-prefix this Space-time (frequency) Turbo detection scheme can get higher efficient and sacrifice less signal noise ration. Finally, by using the property of amicable orthogonal matrix, a simple differential modulation diversity scheme in the presence of unknown multi-path is presented.