The case against alphabet awareness for detection of direct-sequence spread-spectrum signals

Abstract

In this paper we investigate the detection of a direct-sequence spread-spectrum signal with QPSK coded chips, whose spreading sequence is unknown to the detector, and whose spreading code period is longer than the detector observation window. We propose a likelihood-ratio-test detector that takes advantage of knowledge of the signal alphabet, whose complexity grows linearly with the observation window length, and that makes use of multiple antennas. The proposed alphabet-aware detector outperforms multi-antenna alphabet-unaware detectors, especially when the SNR is high and the observation window is small. However, the performance advantage is not large, it diminishes further when the SNR is low, and it comes at the cost of higher computational complexity. In most cases, simpler alphabet-unaware detectors such as the energy detector provide comparable detection performance with less complexity. Due to the i.i.d. signal model this paper utilizes, the results in this work are also applicable to single-carrier unspread QPSK signals, with direct-sequence spread-spectrum signals as a specific case in which the SNR is low.