Sliding correlator

Abstract

A sliding correlator for initial synchronization and tracking of a SS (Spread Spectrum)-modulated signal containing a transmitted pseudo noise (PN) code, digitizes the received signal, and attempts to correlate the PN code in the received signal with a locally generated reference PN code. Initial synchronization employs correlation with three reference PN codes, an early, center and late channel. When correlation is found between the transmitted PN code and one of the reference PN codes, the three reference PN codes are shifted to align the center channel with the correlated channel and to displace the early and late channel reference PN codes to one side and the other of the center channel. In one embodiment, the displacement is one-third chip, and in another embodiment, the displacement is one-half chip. In one embodiment for initial tracking, the three channels are displaced one chip from each other and, after failure to attain correlation in any of the channels, the three channels are displaced three chips to again seek synchronization. Initial Tracking, following initial synchronization, detects changes in correlation in the three channels, and adjust the frequency and/or the phase of the three reference PN codes to return the reference PN codes into a desired relationship with the transmitted PN code. In one embodiment of tracking, clockwise and counterclockwise cyclic permutation of the three reference PN codes is used to maintain alignment between the three reference PN codes and the transmitted PN code. In other embodiments, the condition of correlation, or lack thereof, in each channel is stored in a register and shifting of the reference PN codes is performed in response to the status of the register.