Tracking And Suppression Of Phase Noise In OFDM System

OFDM(Orthogonal Frequency Division Multiplexing) is a fundamental technology of wireless communications. Because of its high bandwidth efficiency and robustness against the frequency selective channels, OFDM has been chosen as an effective transmission method for several standards. Due to the long symbol duration and the narrow sub-carrier spacing, one drawback of OFDM is its sensitivity to phase noise(PHN). Two PHN effects are distinguished: CPE(Common Phase Error) and ICI(Inter-carrier Interference). The performance of OFDM system suffers a significant degradation because of PHN. As OFDM signals are transmitted in higher frequency, it’s a tough chanllenge to suppress PHN.Based on joint time-frequency method, PHN is tracked and suppressed in this thesis. The ICI is suppressed, and the problem of joint suppression of ISI(Inter-symbol Interference) and PHN is solved. The main contents are listed as following:1. The generation mechanism and models of PHN are introduced. PHN is a randomprocess caused by the fluctuation of both transmitter and receiver oscillators. PHNgenerated by different kinds of oscillators has respective features and is modeled asWiener process or Gaussian-colored noise. 2. The performance of the OFDM system affected by PHN is studied. As PHN ismodeled by Wiener process, the analysis shows that the transmission performance ofthe system is a function of its critical parameters, like sampling frequency, carrierspacing etc. When PHN is modeled by Gaussian-colored noise, the SINR(signal-to-noise-plus-interference ratio) is only relative to the variance of PHN. 3. Joint time-frequency PHN suppression method in CP-OFDM system is proposed.The cyclic prefix(CP) is a copy of a portion of CP-OFDM signal. It is motivatedthat a considerable number of ISI-free samples in CP can be linearly combined withthe corresponding samples in the OFDM symbol to suppress ICI. And the propriatecombined coefftcients are derived. In frequency domain, the frequencyapproximated model of PHN is established. According to the model, consecutivepilots can be inserted to track and suppress PHN. The simulation results show that,joint time-frequency method achieves a better performance. 4. The scheme to suppress PHN when OFDM signals propagate through multi-path channels is introduced. If PN-OFDM signals transmitted in multi-path channels and affected by the PHN, ISI and ICI are overlapped and interfered. In this thesis, PN(pseudo noise) sequence and TPS(transmission parameter signaling) symbols are made use of to distinguish PHN from ISI by iteratively decision-directed method. It can be concluded from simulation results that the new method has a better performance compared with the traditional one.