Quantization Degradation of GNSS Signal Quality in the Presence of CW RFI

Abstract

In a digital matched filter (DMF) the degradation of signal quality depends on the pre-sampling filter bandwidth and on the number of analogue to digital) converter (ADC) bits. Digital matched filters are used in the acquisition of Global navigation satellite systems (GNSS) signals. Automatic gain control (AGC) in the GNSS receivers is used to determine the maximum level of quantization by the ADC. In (Chang, 1982), the quantization degradation for different pre sampling bandwidths and adc bits is analysed in the presence of Additive Gaussian white noise (AGWN). Bandwidths of more than once (or twice) the data bit rate contribute only about 0.4 dB (or (0.2 dB) to the overall degradation of the DMF. Also 2 bit uniform step quantization with optimum ADC threshold setting is shown to recover most of the digital implementation degradation. In this paper by focusing on pre-sampling filter bandwidth equal to the data bit rate and 2 bit ADC, the effect of quantization on the received GNSS signal quality in the presence of continuous wave (CW) radio frequency interference (RFI) is analysed. It is shown that a one bit ADC can have much higher degradation than the 3.5 dB in the AGWN case. It is also shown that the effect of AGC on the quantization degradation has to do with both power and frequency of CW RFI. In the presence of CW RFI, the optimal value for ratio of maximum threshold of ADC to the effective noise RMS is shown to be about 1.35 which is higher than the case when RFI does not exist (0.8).