Dottorato in TECNOLOGIE DELL'INFORMAZIONE E DELLE COMUNICAZIONI (ICT)

Titolo della tesi: Signal processing techniques and design strategies for passive radar systems based on non-uniform linear arrays

In response to the rising demand for low-cost civil surveillance, this doctoral thesis explores innovative signal processing techniques and design strategies for multichannel radar systems with reduced complexity. When dealing with multichannel radar systems based on array antenna receivers, the need for low-budget, lightweight and compact systems often imposes severe limitation on the maximum number of receiving elements. Therefore, in this work we explore the use of Non-Uniform Linear Array (NULA) configurations, as a way to strategically positioning a very limited number of elements to trade-off performance and cost-effectiveness. The first part of the thesis addresses Direction of Arrival (DoA) estimation under poor Signal to Noise Ratio (SNR) conditions, for which outlier estimates sensibly degrade the DoA estimation accuracy. The flexible three-element NULA design strategy devised in Chapter 2 allows to mitigate the problem of outliers, showing enhanced DoA accuracy compared to conventional three-element design solutions. Moreover, the Outlier Rejection Algorithm (ORA) introduced in Chapter 3 addresses this challenge downstream the estimation process, leveraging NULA spatial diversity to discard outlier-affected estimates, further improving the DoA estimation accuracy in real-world applications. The second part of the thesis investigates the use of NULA configurations for clutter suppression in multichannel mobile passive radar systems based on Orthogonal Frequency-Division Multiplexing (OFDM) waveforms. Both non-adaptive techniques like Displaced Phase Centre Antennas (DPCA) and adaptive ones like Space-Time Adaptive Processing (STAP) have been explored, showcasing the effectiveness of NULA configurations in managing the trade-off between the clutter notch width and the distance between blind velocities in both cases. Despite non-adaptive DPCA techniques being best suited for low-cost radar systems, they require the array inter-element distances and the batch duration to be synchronized with the platform velocity to effectively achieve clutter suppression. This imposes severe quantization constraints on the NULA element positioning, not allowing to exploit their full potential. This issue is properly addressed in the third part of the thesis, where arbitrary fragmentations for OFDM radar signal processing are introduced, decoupling the array design from the OFDM symbol structure, and unlocking the full potential of NULA configurations in multi-channel OFDM radar systems deployed on moving platforms.