| OFDM is anther key technology after CDMA due to its ability to overcome the problem of increasing ISI as the data rate increases and its advantages of high bandwidth efficiency and simple hardware implementation. It has been used in many wireless communication systems, and is supposed to be adopted in the fourth mobile communication system. However, OFDM systems are very sensitive to phase noise, carrier frequency offset and Doppler frequency of time-varying channel. They destroy the orthogonality between sub-carriers, and thus introduce inter-carrier interference, which causes an error floor and significantly degrades the system performance. This thesis mainly concentrates on the analysis of phase noise influence on OFDM systems and its corresponding mitigation methods, and simply investigates the performance of OFDM systems under carrier frequency offset and time-varying channel as well.Firstly, the detailed analysis of the phase noise influence on OFDM systems is presented. Properties of phase-noise-caused common phase error and inter-carrier interference are discussed respectively, and the effect of system parameters to system performance under phase noise is discussed as well, to provide theoretical reference to phase noise mitigation research and optimum system parameter setting. Next, two iterative phase noise mitigation algorithms based on Turbo Coded OFDM system and IEEE 802.11 PHY standard are separately proposed. Algorithm proposed based on the Turbo Coded OFDM makes full use of the soft output of Turbo decoder to reconstruct the OFDM symbols, which are then used to estimates the phase noise by LMMSE method. Simulation result shows, the proposed algorithm can eliminate the error floor caused by ICI through few times of iteration. However, since the LMMSE method is used in the algorithm, it has very high complexity and is only applicable to Wiener phase noise. Algorithm based on the IEEE 802.11a PHY is similar to the algorithm proposed for the Turbo Coded OFDM system, but its implementation complexity is greatly reduced and it is applicable both to the Wiener phase noise and PLL phase noise. It makes full use of the four pilot subcarriers to estimate the common phase noise, and hence no additional pilot information is required. Certain mathematical method is used to simplify the matrix computation in the algorithm. Simulation result shows, the proposed algorithm can also eliminate the error floor effectively through few times of iteration.Two ICI self-cancellation algorithms, i.e. data-conversion ICI self-cancellation algorithm and data-conjugate ICI self-cancellation algorithm, are studied, and their effectiveness under phase noise, carrier frequency offset and time-varying channel is investigated respectively. Finally, a new phase noise mitigation algorithm based on these two algorithms is proposed. Simulation result shows, the proposed algorithm is able to eliminate both common phase error and inter-carrier interference, and thus achieves better performance than ICI self-cancellation algorithms. |
