Low SNR Timing and Frequency Synchronization for PIP-OFDM System

We proposed a precoded in-band pilots design for OFDM signal (PIP-OFDM) in [1]. With a special design, user identification in cognitive network is easily achieved by demodulating unknown information on identification pilot subcarriers embedded in the transmitted PIP-OFDM signal. In this paper, we discuss synchronization issues of PIP-OFDM system. Different from conventional OFDM system, synchronization in PIP-OFDM system is realized at low SNR. Benefit from the redundant information on pilots, performance of synchronization in PIP-OFDM system is greatly improved by processing multiple OFDM symbols. Coarse timing and frequency synchronization are briefly introduced using cyclic prefix correlation and pilots´ energy correlation in frequency domain. Fine timing and frequency synchronization are realized with modified maximum likelihood estimator based on phase shift estimation in frequency and new method using the similar principle, respectively. System performance is evaluated by theoretical formulation and computer simulations in terms of mean square error. Results show that synchronization algorithms in this paper work effectively at low SNR from -5dB to 5dB.