New techniques for acquiring the codes used in frequency hopping (FH), hybrid frequency hopping/time hopping (FH/TH), and frequency hopping/direct sequence (FH/DS) spread-spectrum systems are presented. Autoregressive spectral estimation is employed to recover the FH code, quadrature processing is used for the DS code, while the TH gating code is recovered by a simple threshold test used in conjunction with the adaptive filter used for the spectral estimation. A wide margin of spectral estimation errors can be tolerated since an acquisition logic is used to estimate the FH code phase. The proposed techniques require acquisition times only of the order of the shift-register generator lengths, whereas other schemes of serial and parallel search require acquisition times of the order of the code lengths. Detailed simulation results will identify the working carrier-to-noise ratio (CNR) range for the different techniques presented as well as the various thresholds involved.

The multipath communication channel for space communication is modeled as a multiplicative channel. This paper discusses the effects of the multiplicative channel processes on the symbol error rate for quadrature modulation (QM) digital modulation schemes. An expression for the upper bound on the probability of error is derived and numerically evaluated. The results are compared with those obtained for additive channels.