Studies On Equalization For OFDM Systems In Wide-sense Time-Varying Fading Channels

Orthogonal Frequency Division Multiplexing (OFDM) is a high efficiency multi-carrier transmission technology. It is wildly studied and used in wireless communication because it can effectively combat the inter-symbol interferences and it has high spectral efficiency. OFDM is considered to be one of the most expecting solutions in the 4th generation of communication.However, the future communication is expected to be used with high frequency, high speed and high quantity, there is a higher carrier frequency offset and the Doppler frequency shift caused by the relative movement between the transmitter and the receiver. Therefore, the orthogonality of the subcarriers are destroyed, resulting in a serious inter-carrier interference (ICI), and lead to irreducible performance degradation. To mitigate the ICI, the use of equalizer is considered to be one of the most effective ways. This paper makes an in-depth research on the channel equalization of OFDM systems in wide-sense time-varying fading channels.Firstly, the characteristics and model of wireless channel and basic principle, structure and characteristics of OFDM systems are introduced.Then the generation of mechanism of ICI of OFDM in wide-sense time-varying channels are analyzing in detail. Based on the analysis, ICI self-cancellation scheme is studied. In order to improve the spectral efficiency, the structure of pre-codeing matrix is changed, and combined with the serial equalizing method, the system performance of the ICI self-cancellation is also enhanced.This paper focus on the MMSE equalization of OFDM system in wide-sense time-varying channels. MMSE equalization has weakness when Multi-mode modulation is used, and with the increase of the modulated modes, the effect of the weakness becomes larger. To solve this problem, a so-called Modified MMSE equalization is proposed. For the first MMSE round, the estimation of the input signals are used to estimate the energy of separate input signals, for the second round, the separate energies are used to form the equalizer. This method great increases the symbol error rate (SER). Iterative and diagonal methods are used to reduce the computation complexity of the Modified MMSE equalization.At last, the principle of Turbo equalization of OFDM system is studied. Combined with the iterative and diagonal method, the low complexity solution is proposed. Theoretic analysis and simulation results demonstrate that all the equalization algorithms proposed in this paper are capable of mitigating the ICI and improving the SER performance, also, some of the algorithms reduce the computation complexity.