寬頻通訊系統中實虛部非協調與通道響應之同估測

Abstract

Nowadays, a lot of effort is spent on developing inexpensive OFDM receivers. Especially, zero-IF receivers are very appealing, because they avoid costly IF filters. However, zero-IF front-ends also introduce significant additional front-end distortion, such as IQ imbalance. Unfortunately, OFDM is very sensitive to nonidealities (IQ imbalances) in the receiver front-end. An important effect of IQ imbalances is that achievable BER saturates as the SNR increases, suggesting that the system’s performance at high SNR will be dominated by the IQ imbalances rather than the operating SNR. In this thesis, the effect of IQ imbalances on OFDM receivers is studied, and a framework for deriving OFDM receivers with IQ imbalance correction in the digital domain was presented. We propose three new schemes for joint IQ imbalance compensation and channel estimation. Previous method is sensitive to channels with nonflat frequency response. Besides leading to performance enhancements, our approaches are robust against channels with highly nonflat frequency response with low complexity. Additionally, one or two OFDM training block provides sufficient information to accurately estimate the IQ imbalances and the channel response, allowing to shorten the training time. Therefore, our methods greatly relaxes the mismatch specifications and thus enable zero-IF receivers employing cheap analog components.