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

Titolo della tesi: ADVANCED SIGNAL PROCESSING TECHNIQUES FOR OFDM RADAR SYSTEMS

The popularity of Orthogonal Frequency Division Multiplexing (OFDM) radar has increased rapidly in recent years being driven by two main aspects: the advancement in hardware capabilities and the increased demand for radio frequency resources. In this regard, two different perspectives have been considered. On one hand, the possibility to parasitically exploit OFDM transmitters as illuminators of opportunity for passive radar (PR). On the other hand, the co-existence of radar and communication systems using the same waveform for both purposes. Examples of such systems are joint radar and communication (JRC) and integrated sensing and communication (ISAC). However, since OFDM waveforms are primarily designed for communication purposes, their use for radar systems requires the development of appropriate signal processing techniques that allow performing the typical radar functions while exploiting the peculiar characteristics of such waveforms. The latter constitutes the main goal of this thesis. In particular, a significant focus is dedicated to addressing the main issues of two essential OFDM radar signal processing stages: the disturbance cancellation and the range-Doppler map evaluation. In the first part of this thesis, we explore the use of an alternative, cost-effective processing chain for OFDM radar systems. This architecture proposes the application of a reciprocal filter (RF)-based range compression before the disturbance cancellation. We show that this change in the typical order of the processing stages allows a significant simplification of the cancellation step since it exploits the time-invariant response of the RF to static disturbances. Moreover, we introduce two alternative clutter removal algorithms that take advantage of the novel architecture to achieve improved performance with limited computational cost. Finally, we evaluate and compare multiple schemes using the proposed and conventional architectures against simulated data. The second part of this work is dedicated to addressing the main issues of the existing range-Doppler map implementations. In particular, we focus on the suboptimal batches algorithm used with the OFDM fragmentation batching strategy. First, we evaluate the performance of the matched filter (MF) and RF when applied to the OFDM fragmentation. This analysis reveals the main limitations of the OFDM fragmentation and motivates the use of an unconstrained batching strategy that we referred to as non-OFDM fragmentations. We show that when the RF is applied with these fragmentations it maintains its advantage of reducing the random sidelobes floor level (SFL) compared to the MF. However, we demonstrate that the RF suffers a significant signal-to-noise ratio (SNR) degradation. To overcome this issue, we propose a set of strategies to supervise the RF implementation which are able to preserve its advantages while reducing the SNR degradation. Moreover, we provide a theoretical characterization of the supervised RF techniques performance and an evaluation of their effectiveness by applying them against simulated and experimental data collected by a PR exploiting a DVB-T signal as the illuminator of opportunity.