Abstract
Orthogonal frequency division multiplexing (OFDM) has become one of the most promising techniques to afford high-rate multimedia services due to its superior multipath resistance and flexible modulation properties. A major disadvantage of OFDM is its high sensitivity to frequency synchronization errors or the Doppler effect, which would induce amplitude attenuation as well as intercarrier interference (ICI). Recently, two-path diversity transmission schemes, known as conjugate cancellation (CC) and phase rotated conjugate cancellation (PRCC), were proposed for ICI cancellation in OFDM systems. Although these schemes have advantages such as good performance, backward compatibility with the existing OFDM systems, and low receiver complexity, they might not provide satisfactory performance under channels where the frequency offset changes with time. To overcome this problem, Wang et al. proposed an adaptive receiver for OFDM systems using conjugate transmission, which can outperform CC and PRCC at many aspects. Nevertheless, there would still be some impairments in this adaptive receiver if the frequency offsets seen in both paths were not identical, which often occurs in fast time-varying channels. In this thesis, we not only investigate the existing two-path transmission schemes for ICI cancellation in OFDM systems, including the benefits and drawbacks under different channel environments, but also propose a modified version of Wang et al.’s ii adaptive receiver. The proposed scheme introduces two phase rotations to the two transmission paths, instead of using only one common phase rotation for both paths in Wang et al.’s scheme, where the two phase rotations are adaptively updated using the normalized block least mean-squared algorithm. Computer simulation results show that the proposed scheme outperforms CC, PRCC, and Wang et al.’s work under time-varying channels, especially for those cases with high-order modulation. It is also demonstrated that the proposed scheme is robust against channel estimation errors.