Timing recovery for a pseudo-random noise sequence in a direct- sequence spread-spectrum communications system

摘要

In a digital communication system for voice signals, a system and method for recovering the timing of a pseudo-random noise (PN) sequence used for direct-sequence spreading and despreading of the communicated signals. In one embodiment, a received signal is a time- division duplexing (TDD) or time-division multiple access (TDMA) signal, and a receiver performs a complete "sliding correlator" examination of the received signal in a fixed time by using the timing of the TDMA or TDD frames. This examination allows a rapid initial acquisition of the PN synchronization. In another embodiment of the receiver, the initially acquired PN phase is verified by reading a SYNC field from the received signal and by checking that shifting the receiver's local PN phase results in a degraded correlation between the local PN sequence and the received signal. A fast tracking procedure finely synchronizes the receiver's PN sequence with the received PN sequence by repeatedly testing the effects of dithering the receiver's PN phase. The fast tracking is preferably performed for a fixed duration of time and is followed by a slow tracking that continues the dithering to maintain the fine synchronization. In the slow tracking procedure, the receiver preferably adjusts its PN phase only if repeated tests consistently indicate a lag or lead in the synchronization. The slow tracking procedure preferably also includes a second-order correction in the PN phase to allow for frequency differences between a PN clock in the receiver and a PN clock in a remote transmitter.