Coherent DS-CDMA Acquisition in Time-Variant Channels at Low Signal to Noise Ratios

Abstract

This paper presents a novel acquisition method for time-variant channels, which yield a different channel phasor for each transmission symbol due to the non-zero Doppler frequency. The alternating channel phasors prevent a coherent symbol averaging and require an incoherent or a differentially-coherent symbol averaging instead. Compared to the incoherent symbol averaging, the differentially-coherent symbol averaging yields a higher acquisition probability, since it avoids the squaring loss (SL). However, high data rates with rapidly alternating modulation symbols reduce the acquisition probability of the differentially-coherent symbol averaging remarkably. Thus, this paper presents a novel rate independent delay acquisition method, that even yields a higher acquisition probability compared to the differentially-coherent symbol averaging and compared to the incoherent symbol averaging anyway. The increased acquisition probability in our simulation results corresponds to a signal to noise ratio (SNR) gain of at least 0.4 dB compared to the differentially-coherent symbol averaging and of at least 1 dB compared to the incoherent symbol averaging, and we reduce the receiver complexity at the same time.