Spread spectrum communications using CMOS digital correlators

CMOS digital correlators with extended on-chip signal processing capabilities strongly increase the applicability of digital matched filters (DMFs) in noncoherent direct sequence spread spectrum receivers. However, the digital concept gives rise to problems caused by time- and amplitude-quantization at analog-to-digital (A/D) conversion prior to correlation. In the present work, a noncoherent receiver structure, using a sampling window technique to prevent timing errors due to oscillator uncertainties, is reviewed. In order to improve the system performance in the presence of CW (continuous wave) interference, a 2-bit A/D-converter (ADC) with adaptive thresholds is proposed. The performance of a DMF receiver using 2-bit A/D conversion as well as hard limiting is compared with that of an analog matched filter receiver. By the use of the 2-bit ADC instead of the hard limiter, a performance improvement is obtained, and, when interferences have a strong CW content, the analog counterpart is even outperformed