# Low-Cost SDR for GNSS Interference Mitigation Using Spatial Diversity Techniques +
## 4 Experimental Analysis
### Results  
With the aim of acquiring a reference on the performance of the receiver in regular conditions, this is, without any external impairment that may hinder its operation, the first experiment conducted consisted in processing the Spinert-simulated GPS signals when no interference was present. The results obtained in this case are presented in Figure 1, in terms of C/N0 in graph (a) and in terms of deviation from the true position in each of the East (E), North (N), and Up (U) in (b). It can be appreciated from Figure 1a that under favorable conditions, nine GPS satellites are visible--satellites 2, 4, 5, 10, 12, 24, 29, 30 and 31--all achieving C/N0 levels around 45 dB-Hz for the entire simulation. This leads to the promising accuracy in the positioning solution observed in Figure 1b, where the error in all three ENU coordinates lies below 2 m.  
However, the positive performance observed in this setting will inevitably deteriorate when an undesired contribution appears in the scenario. The consequence of a CW interference with the characteristics presented at the beginning of this section is exhibited in Figure 2, where the power spectral density (PSD) of the received signal in the presence of the CW interference is shown together with the PSD when only GPS contributions imping the array, in plots (a) and (b), respectively. The impact that the CW has on the receiver can be observed though the PSD of Figure 2a, where the contribution of the interference is clearly seen at the central frequency. Comparing this result with the one in Figure 2b, which is essentially the PSD of the noise since the GPS signals are still buried, one can perceive a large difference in terms of  
Figure 1: Results when only GPS L1 signals are present: (**a**) in terms of C/N0; (**b**) in terms of deviation from the true position. power. The 70 dB increase observed in the presence of the CW interference completely saturates the receiver, making it impossible to acquire the GPS signals and thus impeding the computation of the positioning solution. It is in this case where beamforming plays a key role. The deployment of spatial diversity techniques such as the ones introduced in the previous section can significantly improve the performance of the receiver, effectively mitigating the interference and allowing the processing of the GPS signals. When the PI and PREW beamforming techniques are implemented and applied to the received samples, the resulting power level of the CW contribution is greatly decreased, since the algorithms are capable of spatially filtering out the undesired component. This can be observed in Figure 3, where the PSD at the output of the PI and PREW beamformers is presented, in Figure 3a for the PI, and in Figure 3b for the PREW. In both cases, the spectrum of the received signal affected by the interference before applying the techniques is plotted along, with the purpose of facilitating the comparison. Figure 3: PSD of the received signal before and after applying (**a**) PI beamformer; (**b**) PREW beamformer. Figure 2: PSD of the received signal (**a**) in the presence of a CW interference; (**b**) when only GPS L1 signals arrive. The PSD obtained in the two techniques presented clearly shows that the power of the interference has been largely diminished, almost 50 dB for the PI beamformer in Figure 3a and around 35 dB for the PREW beamformer in Figure 3b. This meaningful attenuation in the power of the interference achieved exploiting spatial diversity allows the GNSS receiver to properly process the GPS signals, thus making it able to compute the positioning solution. In Figure 4, the deviation from the true position of the user is depicted, in Figure 4a for the case of the PI beamformer and in Figure 4b for the PREW. The fact that applying these two low-complexity beamforming techniques, where no additional information is needed to compute the coefficients of the spatial filter, allows the receiver to acquire and track the GPS signals is, in itself, a great achievement.