A Dual-Frequency Interference Suppression Unit for E1/E5a Designed with National Instruments Software Defined Radios

Thomas Kraus, Stefan Sailer, Bernd Eissfeller

Abstract:	Interferences to GNSS receivers has become a much highlighted topic on GNSS conferences since the last years. Looking only to the single antenna methods, many papers presented numerous DSP algorithm techniques like filtering in the time- or frequency-domain, pulse blanking, wavelet based or Karhunen-Loève transformation (KLT). Mostly one pre-requisite for DSP methods are the linearity of the front-end hardware, which is often limited for high power interferences, and finally the real-time capability of the DSP algorithms. This work is presenting an overall solution in form of a dual-frequency interference suppression unit (ISU), which is switched in between the GNSS antenna and the receiver. The ISU operates in the E1 and E5a band for Galileo and GPS and is using the software defined radios (SDRs) of National Instruments. The bandwidths are for the E1-band 25 MHz and E5a-band 21 MHz, so that the specified receiver bandwidths of the Galileo Open Service Signal in Space document is guaranteed for an E1/E5a GNSS receiver. The algorithm is implemented in the on-board FPGA and consist of pulse blanking and frequency-domain filtering also known as FCME (frequency domain forward consecutive mean excision) or FDAF (frequency domain adaptive filtering). The strongest interference power is usually caused by personal privacy devices (PPD), especially when they are close to the receiver. The ISU shall demonstrate their capability to deal with these interferences of high signal power. The expected C/N0 improvements are up to 45dB for continuous wave and up to 20dB for chirp PPD signals. Finally, it is worth to mention, that the ISU could also operate directly as a front-end for a software receiver. The ISU implementation is done on two different SDR platforms from National Instruments. The low-budget version is the NI USPR RIO 2952R and the high-end version is the NI PXI FlexRIO transceiver 5791. Both systems are equipped with the FPGA Kintex-7 K410T from XILINX programmable with the LabVIEW development system. Due to the different allowed maximum input power of -10dBm and +20dBm of the USRP and FlexRIO, the maximum allowed interference to noise ratio SNR(max) is around 50dB and 70dB, respectively. The antenna used in this setup is the Trimble Zephyr 2, which supports a unique stability in presences of interferences already. The GNSS receiver used in the final tests are the geodetic receiver PolarRx4 from Septentrio and the scientific navigation software receiver from IFEN GmbH. The last part provides the GNSS performance of the receivers with and without ISU (mitigation), which includes the acquisition and tracking performance indicated by the signal to noise ratio C/N0 and the code-tracking noise. The GNSS performance will also be compared with different clocks applied to the SDRs, which are the internal low quality OCXO with and without GPS disciplined clock and an external high quality OCXO. Additionally to the maximum allowed interference to noise ratio SNR(max), the interference to noise ratio for the linear region SNR(linear) and the non-linear region SNR(non-linear) will be given. The closing analysis is about the FPGA resources needed for the E1-band with 25 MHz and E5a-band with 21 MHz configuration, which is finalized with an estimation of the maximum achievable bandwidth with a Kintex-7 FPGA, so that in the future the complete E5 band could be covered.
Published in:	Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015) September 14 - 18, 2015 Tampa Convention Center Tampa, Florida
Pages:	1634 - 1655
Cite this article:	Kraus, Thomas, Sailer, Stefan, Eissfeller, Bernd, "A Dual-Frequency Interference Suppression Unit for E1/E5a Designed with National Instruments Software Defined Radios," Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015), Tampa, Florida, September 2015, pp. 1634-1655.
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