臺灣博碩士論文加值系統

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.

1 Introduction . . . 1

2 IQ Imbalance and Its Impact on OFDM Systems . . . 5
2.1 Why IQ Imbalance ? . . . 6
2.1.1 Effect of IQ Imbalance on the Received
Baseband Signal . . . 8
2.2 Introduction to OFDM Systems . . . 10
2.3 Introduction to SC-CP Systems . . . 15
2.4 Impacts of IQ Imbalance on OFDM and SCCP
Systems . . . 17
2.5 Compensation of IQ Imbalances Effect . . . 21
2.5.1 Compensation with K1, K2 Known [18] . . . 21
2.5.2 Compensation with K2 K¤1 Known . . . 24
2.5.3 Adaptive Compensation [19] . . . 25
2.5.4 Least-Squares Compensation [19] . . . 26

3 Joint IQ Imbalance and Channel Estimation for OFDM
Systems . . . 27
3.1 Some Existing Methods . . . 28
3.1.1 Post-FFT Estimation [19] . . . 28
3.1.2 Estimation for Channels with Smooth Frequency
Responses [18] . . . 29
3.2 Proposed Methods for Joint IQ Imbalance and Channel
Estimation . . . 41
3.2.1 Method 1 . . . 41
3.2.2 Method 2 . . . 44
3.2.3 Time-Domain Estimation Method . . . 56
3.3 Simulation Results and Comparison . . . 68
3.3.1 OFDM Systems . . . 69
3.3.2 SC-CP Systems . . . 74

4 Conclusion . . . 83

Bibliography . . . 85

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