A coherent spread-spectrum diversity-receiver with AFC for multipath fading channels

A direct-sequence spread-spectrum (DSSS) coherent RAKE-receiver demodulating M-ary phase-shift keyed (MPSK) signals is presented for microcellular communication systems with frequency-selective fading channels. Since the channel fading is due to multipath propagation, the received signal contains multipath or time diversity which is resolved at the receiver by a RAKE-structure. The maximum-likelihood (ML) rule is invoked to estimate the phases, and the combining factors of the receiving paths, as well as the frequency offset between the carrier of the receiver and the transmitter. The ML algorithm is simplified by using decision-directed feedback. The resulting digital algorithms for synchronization, data demodulation, and frequency error detection (FED) are well suited for digital implementation because of their fairly low numerical complexity. Analytical expressions of the bit error probability versus signal-to-noise ratio (SNR) and versus frequency offset are presented. In addition, experimental results using Monte-Carlo simulations have also been carried out. The results show a negligible influence of the decision-direction above a certain SNR, as well as a “robust” transient behavior of the phase estimation and the automatic frequency control (AFC) scheme