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The frequency synchronization procedure is essential to a mobile set in order to calibrate its reference oscillator. In the TDD Low Chip Rate (LCR) UMTS system after the frequency scan procedure, which alms to find the carrier frequency, an error as great as 20 kHz is expected. The requested maximum frequency offset between the local oscillator and the Node B oscillator is 200 Hz (0.1 ppm.). The aim of this paper is to present a frequency recovery algorithm suited for the TDD/sub LCR/ system and to verify if the requested precision of 0.1 ppm is reachable with the proposed frame format. Many data aided frequency estimators have been proposed in the literature. In this paper the best suitable estimator for TDD/sub LCR/ (Luise et al. (1995)), is fitted to the TDD/sub LCR/ system and an original nonlinear closed loop approach, which improves performance, is proposed. The performance results show that a combination of an open loop estimation with a closed loop configuration, exploiting two different training sequences, ensure a final accuracy of 0.1 ppm, even in the worst environment conditions. The modified nonlinear recursive algorithm, introduced in the closed loop configuration, is a promising approach to increase accuracy and to reduce convergence time. |
