A novel differentially coherent (DC) detection scheme for rapid code acquisition is proposed for a direct-sequence (DS) spread-spectrum system being used to communicate over a fast Rayleigh fading mobile radio channel. The scheme processes the received signal at baseband using a complex differential detector with one-chip time delay and then performs a coherent partial correlation of the detector output with the product of a local DS code and its one-chip delayed phase to form a test sample. While stil-1 preserving the pseudo-noise attribute at its output, the differential detector is able to effectively suppress slowly-fluctuating phase components due to fading and carrier frequency offset, thereby enhancing the in-syne correlation at the following correlator. The performance of this detection scheme is analyzed based on a central-limit-theorem argument, and verified by simulation. Both serial and parallel acquisition systems employing the proposed DC detector are compared with the conventional parallel I-Q system by mean acquisition time performance. It is shown that the serial DC acquisition system significantly outperforms the parallel I-Q system in very fast Rayleigh fading environments.
