# Low-Cost SDR for GNSS Interference Mitigation Using Spatial Diversity Techniques +
## 1 Introduction  
Despite the emergence of new positioning approaches experienced in recent years, Global Navigation Satellite Systems (GNSSs) are still the standard and reference technology when implementing navigation and positioning solutions. Today, the number of infrastructures that rely on these systems to properly function is substantial and, with new use cases continuously appearing in the civil domain, the requirements in terms of accuracy and reliability of the position, velocity, and time (PVT) solutions are intensified. The aforementioned demands are not new for certain GNSS sectors, where high accuracy positioning was already a need in the past, but offering precision and robustness in low-cost GNSS receivers aimed at the mass-market is still a challenge. The tendency towards miniaturization, along with the cost and complexity constraints characteristic of these devices, severely restrict the performance of GNSS mass-market receivers. This is particularly accentuated in hostile scenarios, where the coexistence of a high number of technologies tends to degrade the performance of the services. Under these circumstances, it is common to find undesired contributions impinging on the receiver along with the GNSS signal, causing interference, which pose a major threatwhen it occurs inside the bandwidth used by the receiver. When this occurs, the interference cannot be filtered out in the frequency domain, and its presence will inevitably affect the performance. In cases where these contributions are too powerful, the front-end of the receiver is saturated, resulting in an erroneous digital conversion of the received signal that does not allow the receiver to operate at all [1]. In less extreme cases, i.e., where the front-end of the receiver can still function properly, the problem arises during the despreading process, where the power of the interference will be spanned across the bandwidth and lead to noisier measurements that result in inaccuracies in the final positioning solution. Measures should therefore be taken to ensure a proper provision of the service even under these circumstances. In this scope, exploiting spatial diversity has proved to be a very powerful approach [2; 3]. Originally, the use of multi-antenna solutions in the GNSS domain was linked to the defense sector, where controlled radiation pattern antennas (CRPA) are typically used to filter out undesired contributions based on their direction of arrival (DoA) [4; 5; 6; 7]. However, the technological advances in recent years have enabled the exploitation of the spatial domain through digital beamforming techniques and applications such as 5G, Internet of Things (IoT), and the most recent IEEE 802.11 standards have greatly benefited from them [8; 9; 10; 11]. A crucial aspect in the deployment of said multi-antenna techniques is to maintain a separation between consecutive elements of half the wavelength to ensure an undistorted spatial response, and so services using higher frequencies bands take advantage of this. An inter-element spacing of half-wavelength in the GNSS domain requires a distance of \(\sim\)96 mm and \(\sim\)128 mm between antennas, for the L1/E1 and L5/E5 bands, respectively. These values are much larger than those observed in the 5G field, where the millimeter wave nature allows a minimum separation of the sensors of \(\sim\)5 mm, and so moving these multi-antenna configurations into mass-market GNSS receivers comes with several challenges. However, the technological advances experienced in this domain, along with the projections for the near future in terms of miniaturization and lower cost elements, position spatial diversity in smaller GNSS receivers as a great solution to tackle the concerns aroused by undesired contributions [12; 13].