Estimation of fine-time synchronization error in FH FDMA satcom systems using the early-late filter technique

The problem of fine-time, or epoch, synchronization on the uplink of a frequency hopped FDMA satellite system is examined. All user signals are dehopped by the same device in the satellite receiver. For each transmitted signal the hop transition time must be adjusted so that when the signal arrives at the receiver, it is closely aligned with the transition time of the dehopper. To accomplish this fine-time synchronization, the system is probed by sending a known signal for a number of hop periods, L. For receivers which use a Fourier transform processor (FTP), such as may be realized by SAW devices, the early-late filter method provides a means for processing the probe response so that an estimate of the time error can be made. The resulting time error estimate is then sent via the downlink to the transmitter where it is used to adjust the user hop transition time accordingly. A unique method is introduced which uses estimates of three second order moments to give a low-biased, consistent estimate of the time error. The mean and standard deviation of the estimator are determined by an approximate analysis using the central limit theorem. Results from this analysis were found to agree closely with those from simulation for practical values of L. Performance results are given for several values of signal-to-noise ratio and a range of L. The performance of the estimator is also investigated when the probe is subject to degradations such as amplitude variations and fading. Results show that the method is extremely robust under these conditions