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The influence of signal quantization on the performance of digital receivers for CDMA radio networks

In an all-digital implementation of a data modem for Direct-Sequence Spread Spectrum (DS/SS) Code-Division, Multiple Access (CDMA) systems, the in-phase and quadrature components of the received signal are customarily digitized just after IF matched filtering and baseband conversion. To reduce the processing effort in the data demodulator, it is of interest to represent the signals in the digital domain with as little a number of bits as possible. On the other hand, coarse signal quantization (down to 2-3 bits/sample) causes a non-negligible performance degradation as far as the receiver BER is concerned. The aim of this paper is thus the investigation of the impact of signal quantization on the BER performance of a DS/SS CDMA receiver in the presence of Gaussian noise and co-channel interference. Based on a general theoretical analysis, a few multiple access strategies are taken into consideration, and the theoretical findings are also checker by computer simulation. For all those systems, a four-bit ADC, yielding less than 0.5 dB signal-to-noise ratio degradation at a BER = 10 -3 appears to provide a good trade-off between performance loss and demodulator complexity.