Time and Frequency Synchronization for Real-Time Wireless Digital Communication Systems

In this paper, we present a fully field-programmable gate array (FPGA)-based real-time implementation of a wireless time and frequency synchronization scheme for communication and sensor networks. For this purpose, a communication link using frequency division multiplexing is established between three distributed stations. This allows the simultaneous and continuous transmission of quadrature phase-shift keying-modulated data between the stations with no interference. The time difference between the stations is estimated using the precision time protocol. Due to the deterministic processing time of the FPGA implementation, precise time synchronization is achieved, with a standard deviation of 10 ns per communication cycle. Moreover, in the proposed method, successive estimates of the time offset are used to estimate the frequency difference between the stations, which allows the tuning of the digitally controlled crystal oscillators of the distributed stations to the frequency of the central reference station, with a clock rate deviation of approximately two parts per billion. The structure is robust with respect to multipath interference and is extendable to additional frequency bands and stations. The results demonstrate the excellent performance and robustness of the proposed concept.