TABLE OF CONTENTS A spread-spectrum receiver must generate a spreading sequence or frequency-hopping pattern that is synchronized with the received sequence or pattern; that is, the corresponding chips or dwell intervals must precisely or nearly coincide. Any misalignment causes the signal amplitude at the demodulator output to fall in accordance with the autocorrelation or partial autocorrelation function. Although the use of precision clocks in both the transmitter and the receiver limit the timing uncertainty in the receiver, clock drifts, range uncertainty, and the Doppler shift may cause synchronization problems. Code synchronization, which is either sequence or pattern synchronization, might be obtained from separately transmitted pilot or timing signals. It may be aided or enabled by feedback signals from the receiver to the transmitter. However, to reduce the cost in power and overhead, most spread-spectrum receivers can acquire code synchronization from the received signal.
In the first part of this chapter, we consider direct-sequence systems. To derive the maximum-likelihood estimate of the code phase or timing offset of the spreading sequence, several assumptions are made. Since the presence of the data modulation impedes code synchronization, the transmitter is assumed to facilitate the synchronization by transmitting the spreading sequence without any data modulation. In nearly all applications, noncoherent code synchronization must precede carrier synchronization because the signal energy is spread over a wide spectral band. Prior to despreading, which requires code synchronization, the signal-to-noise ratio (SNR) is unlikely to be sufficiently high for successful carrier tracking by...
