| Summary: | 碩士 === 國立臺灣大學 === 電信工程學研究所 === 95 === Recently, direct-conversion RF receivers become more appealing, because of significant advantages in cost, package size, and power consumption. However, OFDM system with a direct-conversion RF receiver is very sensitive to non-idealities at receiver front-end, such as I/Q imbalance and carrier frequency offset (CFO). These non-idealities at the receiver generate inter-carrier interference (ICI), and as a result the performance of OFDM system will degrade severely if these non-idealities are not properly compensated. Perfect I/Q and carrier frequency matching is impossible in the analog domain, especially when low cost fabrication technologies are used. Therefore, a precise estimation and compensation method is required in the digital domain.
In this thesis, the effect of I/Q imbalance and CFO on OFDM receivers are studied, and the estimation of these parameters are considered. The results of this thesis consist of two parts. In Part I, we solve the problem using preamble. We propose a time domain method for joint estimation of I/Q imbalance, CFO, and channel response as well as the timing synchronization. The proposed method only requires a preamble of only one OFDM block to accomplish both the timing synchronization and joint estimation. Simulation results show that our method is robust against channels with non-smooth frequency response, and the performance of our method does not depend on the severeness of I/Q imbalance and CFO. Moreover, the time domain method can be successfully applied to other broadband communication systems, such as the OFDM system with a zero padding rather than cyclic prefix, the single carrier system with cyclic prefix, etc. In Part II, we solve the problem of timing synchronization and joint estimation using superimposed training sequence. The superimposed training sequences includes pilot symbols as special case. We show that the proposed time domain method can be extended to this case.
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