Blind Parametric Estimation in OFDM Systems

• Main Authors: Tzu-Chiao Lin, 林子喬
• Other Authors: See-May Phoong
• Format: Others
• Language: en_US
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/30788042655959909208

博士 === 國立臺灣大學 === 電信工程學研究所 === 104 === In this dissertation, we study blind algorithms for parametric estimation in orthogonal frequency division multiplexing (OFDM) systems. Our works include two parts. In the first part, we discuss the problem of carrier frequency offset (CFO) and in-phase and quadrature-phase (I/Q) imbalance in OFDM systems. In the second part, we study the identification of channels for OFDM-based amplify-and-forward two-way relay network (AF-TWRN) systems. CFO and I/Q imbalance are two common front-end imperfections in low-cost communication devices. It is known that CFO is a crucial problem in OFDM systems. It can destroy the orthogonality between subcarriers and cause significant degradation in system performance. Moreover, the existence of the I/Q imbalance usually reduces the accuracy of CFO estimation. In the first part of the thesis, we study blind estimations for CFO and I/Q imbalance. Firstly, we propose algorithms based on cyclic prefix (CP) for blind estimations of CFO and I/Q imbalance in OFDM transmission over multipath channels. Like other CP-based methods, the proposed methods enjoy low complexity, and closed form formulas are derived. For CFO estimator, the proposed method is very robust to the symbol timing synchronization error. In addition, we also carry out the theoretical mean square error (MSE) analysis and derive the Cramer-Rao bound (CRB). For I/Q parameter estimator, the proposed method does not need to know the CFO value. Hence, by combining these two estimators, we can solve the joint estimation problem of CFO and I/Q imbalance in two stages. First the I/Q parameter is blindly estimated in the presence of CFO and then the CFO can be blindly estimated after compensating the I/Q effect. Another popular technique for blind estimation of CFO and I/Q imbalance is by exploiting constant modulus (CM) constellations. An effective low cost method, called frequency analysis (FA) method was proposed. The FA methods are simple and have closed form solutions. However, in many existing standards for communications, virtual carriers (VC) are introduced for the purpose of system design. When there are VCs, the FA methods degrade significantly. We propose the algorithms based on FA for blind estimation of CFO in the presence of VCs. The cost functions can be well approximated by sinusoidal and parabolic functions and thus the closed form formulas can still be exploited. In the last decade, research on wireless relay networks became popular. In particular, the TWRN has drawn a lot of attention because its overall communication rate is approximately twice of that achieved in the one-way relay network (OWRN). In the second part of the thesis, we propose a blind channel estimation algorithm for AF-TWRN. The OFDM modulation is adopted for frequency selective channel. The proposed method can be also applied to zero-padded OFDM systems. The two cascaded channels are estimated in two steps. First the cascaded channel causing the self-interference is estimated using a proposed power reduction method. Then the other cascaded channel from source to destination is estimated by subspace method. Closed form formulas for channel estimates are derived. In addition, we also carry out the theoretical MSE analysis and derive the approximated CRBs.