Spread-Spectrum Diversity-Receiver wit FC for Multipath Fading Channels
U. Fawer · 1994
A direct-sequence spread-spectrum (DSSS) coher- ent RAKE-receiver demodulating M-ary phase-shift keyed (MPSK) signals is presented for microcellular communica- tion systems with frequency-selective fading channels. Since the channel fading is due to multipath propagation, the re- ceived 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 simpli- fied by using decision-directed feedback. The resulting dig- ital algorithms for synchronization, data demodulation, and frequency error detection (FED) are well suited for digital implementation because of their fairly low numerical com- plexity. Analytical expressions of the bit error probability versus signal-to-noise ratio (SNR) and versus frequency off- set 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 robustn transient be- havior of the phase estimation and the automatic frequency control (AFC) scheme.