Cramer-Rao bound on timing recovery for GSM receivers

Abstract

The problem of timing recovery in cellular receiver systems is critical. This is because synchronization functions must follow rapid parameter changes inherent in mobile systems. The ultimate goal is therefore to achieve low bit error rate on the recovered information. Traditional timing recovery methods have over-relied on phase-locked loops for timing information adjustment. However, associated schemes do not exploit code properties. This leads to difficulties in synchronizing digital receivers with transmitters’ end separated by lossy channels. In this paper we present a class of fast converging timing recovery algorithm for GSM receivers in low signal to noise ratio. In the proposed method, the receiver exploits the soft decisions computed at each turbo decoding iteration to provide reliable estimates of soft timing signal which in turn improves the decoding time. The proposed method converges in about 10 iterations and the derived sequential minimization technique approaches a theoretical Cramer-Rao bound with unbiased estimates. Though an 8-PSK GSM baseband communication system is simulated, it was found that this model worked well with most modulation schemes. The proposed scheme is also insensitive to carrier offsets recovery. Simulation showed that the proposed method outperforms conventional timing extraction methods with respect to jitter performance.