Adaptive Analog-to-Digital Conversion and pre-correlation Interference Mitigation Techniques in a GNSS receiver

Kurzfassung

The objective of this diploma thesis was the development of pre-correlation interference mitigation techniques for a GNSS receiver. Since these developed algorithms shall be implemented in the real world DLR Galileo receiver, some pre-defined parameters were given that respects specifics of the hardware on which these algorithms shall work. So, a ideal 20 dB AGC and a ideal 14 bit ADC were available for the adaptive A/D-conversion, gain steering should be performed on the ADC output data in 1 ms intervals. Further, the digitized signal should own a 6 bit resolution at the circuit output, provided that no RFI is present. Other specifics such as the circuit’s complexity did not exist, hence it was possible to take use of the full advantages of the software defined radio. Based on these given specifics, AGC steering strategies and interference detection and mitigation techniques were developed. The AGC steering was performed with look-up-table because with this it is able to reduce the gain completely with 1 ms of ADC output data. Interference detection was performed in the frequency and time domain as well as on the PDF of the ADC bins distribution. In this context also the Chi-Square Test was investigated, but due to its lack that it cannot determine the specifics of the interference, which there are frequency or point in time when it occurs, more complicated techniques had to be developed. Therefore two different interference detection and mitigation strategies were developed taking place in the frequency as well as in the time domain. Of major concern was thereby the development of frequency excision to mitigate the impact of narrowband and pulsed RFI in the frequency domain. Thereby it was shown that the best performance of these techniques is achieved when overlapped Hanning data windows are used to reduce spectral leakage in the discrete spectrum computation. A simulative performance evaluation showed that these techniques are able to mitigate the impact of CW-like interference up to an ISR of � 83dB, when only a 14 bit ADC is implemented, and up to an ISR of � 107 dB, when also an AGC is available. For both of these ISRs the ADC was oversteered, but simulations showed that the frequency excision techniques and the GPS C/A-code can handle saturation effects in the ADC up to a certain level. Furthermore is was shown that these techniques are also able to mitigate the impact of pulsed RFI on the correlator, but the achieved C/N0 suffered a degradation of � 3 dB. Hence different techniques were developed to mitigate pulsed RFI in the time domain. In this context temporal blanking was introduced, which was performed on sample-by-sample basis as well as monitoring power fluctuations in the received signal. Both of these techniques showed an excellent performance, so that the achieved C/N0 was only slightly degraded compared to the RFI free case. But for a implementation in the DLR Galileo receiver one should use that method that monitors power fluctuations in the received signal, because it can identify the start and end of a pulse, and so perform a better temporal blanking (the achieved C/N0 is about 0.4 dB as for the sample-by-sample technique) and further one can better distinguish between pulsed interference and other interference types in the time domain.