Non-Data-Aided Feedforward Carrier Frequency Offset Estimators for QAM Constellations: A Nonlinear Least-Squares Approac
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Non-Data-Aided Feedforward Carrier Frequency Offset Estimators for QAM Constellations: A Nonlinear Least-Squares Approach Y. Wang Department of Electrical Engineering, Texas A&M University, College Station, TX 77843-3128, USA Email: [email protected]
K. Shi Department of Electrical Engineering, Texas A&M University, College Station, TX 77843-3128, USA Email: [email protected]
E. Serpedin Department of Electrical Engineering, Texas A&M University, College Station, TX 77843-3128, USA Email: [email protected] Received 21 February 2003; Revised 17 March 2004; Recommended for Publication by Tomohiko Taniguchi This paper performs a comprehensive performance analysis of a family of non-data-aided feedforward carrier frequency offset estimators for QAM signals transmitted through AWGN channels in the presence of unknown timing error. The proposed carrier frequency offset estimators are asymptotically (large sample) nonlinear least-squares estimators obtained by exploiting the fourthorder conjugate cyclostationary statistics of the received signal and exhibit fast convergence rates (asymptotic variances on the order of O(N −3 ), where N stands for the number of samples). The exact asymptotic performance of these estimators is established and analyzed as a function of the received signal sampling frequency, signal-to-noise ratio, timing delay, and number of symbols. It is shown that in the presence of intersymbol interference effects, the performance of the frequency offset estimators can be improved significantly by oversampling (or fractionally sampling) the received signal. Finally, simulation results are presented to corroborate the theoretical performance analysis, and comparisons with the modified Cram´er-Rao bound illustrate the superior performance of the proposed nonlinear least-squares carrier frequency offset estimators. Keywords and phrases: synchronization, cyclostationary, non-data-aided estimation, harmonic retrieval, carrier frequency offset.
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INTRODUCTION
In mobile wireless communication channels, loss of synchronization may occur due to carrier frequency offset (FO) and/or Doppler effects. Non-data-aided (or blind) feedforward carrier FO estimation schemes present high potential for synchronization of burst-mode transmissions and spectrally efficient modulations because they do not require long acquisition intervals and bandwidth consuming training sequences. For these reasons, non-data-aided carrier compensation schemes have found applications in synchronization of broadcast networks and can be used in many practical receivers where a coarse carrier FO correction is applied in front of the matched filter. Non-data-aided feedforward carrier FO estimators have been proposed and analyzed in various contexts by many
researchers [1, 2, 3, 4, 5, 6, 7, 8]. The common feature of these algorithms relies on the cyclostationary (CS) statistics of the received waveform that have been extensively exploited in communication systems to perform tasks of synchronization, blind channel identification, and equalization (se
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