# Wide-Band Interference Mitigation in GNSS Receivers Using Sub-Band Automatic Gain Control +
## 1 Introduction  
Interference mitigation is gaining importance in the research and development of GNSS receivers since incidents related to jammers are more and more frequent and are also widely documented in the literature [1, 2, 3, 4, 5]. Simpler mitigation methods like a PB or a notch filter are ineffective against complex interference signals. These include pulsed noise, frequency-modulated continuous-wave (FMCW) (also referred to as \"chirp\" or \"swept-frequency\" signals), frequency hopping, matched spectrum, or a combination of any of these [5]. Additionally, new GNSS signals present a higher complexity, typically associated with a larger bandwidth. It makes the development of IM methods even more challenging.  
The HDDM algorithm provides higher adaptability and wide-band IM [6]. The HDDM is similar to the frequency-domain adaptive filtering (FDAF) method [7, 8, 9]. However, the fast Fourier transform (FFT) is calculated for every newly-received digital sample of thesignal instead of a block of samples. As a result, this oversampling increases the time selectivity and limit ringing and distortion effects.  
Initially, the HDDM was proposed as a software implementation [6], using a simple PB per channel to remove interference signals. Later, the first hardware (HW) implementation [10] for a GNSS receiver was implemented and tested against wide-band noise interferences [11]. However, since then, the HDDM has been proven useful for general signal conditioning methods beyond IM, including equalization, spectrum compression, and signal corrections [12].  
A limitation of the HDDM-PB in IM is that it--like many other IM methods--completely removes the signal in the presence of wide-band noise interferences. Thereby completely disrupting the GNSS receiver. Using an AGC instead of a PB overcomes this limitation [11], and was proposed with an initial HW prototype at the International Conference on Localization and GNSS (ICL-GNSS) 2021. This article extends the conference article by providing an in-depth performance analysis of the HDDM-AGC. It includes an evaluation of the PVT solution as impacted by the interference signal and mitigation methods, determining the position errors and availability. Further, the HDDM-AGC is evaluated using high-end and low-cost commercial-off-the-shelf (COTS) commercial receivers to facilitate a fair performance comparison. The HDDM-AGC mitigates the interference signal in the digital domain on a stand-alone platform before the signal is sent to the COTS receivers.  
This article aims to present an end-to-end analysis of the mitigation performance of the HDDM-AGC against an array of interferences. Therefore, establishing a benchmark to the current state-of-the-art IM capabilities and identifying which interference signals remain challenges for future research. Results confirm that wide-band noise interferences no longer disrupt the entire signal during mitigation, proving the versatility of the HDDM-AGC. However, it also alters the performance against other interference signals. A limitation of this study was the digital-to-analog converter (DAC) of the HDDM-AGC mitigation platform, which, due to limited bit-lengths, resulted in a \(C/N_{0}\) loss of 6 dB, making a direct comparison to the state-of-the-art challenging. Nevertheless, the HDDM-AGC could outperform the state-of-the-art for select interferences, like pulsed interferences, despite this handicap.  
The remainder of the article is structured as follows: Section 2 provides a background to the HDDM and related mitigation methods, and Section 3 details the HW design and implementation. Section 4 describes the test setup and the evaluation methodology. The results are shown in Section 5, and discussed in Section 6. Finally, the conclusions are drawn in Section 7.