  This paper studies the processing principles, implementation challenges, and
performance of OFDM-based radars, with particular focus on the
fourth-generation Long-Term Evolution (LTE) and fifth-generation (5G) New Radio
(NR) mobile networks' base stations and their utilization for radar/sensing
purposes. First, we address the problem stemming from the unused subcarriers
within the LTE and NR transmit signal passbands, and their impact on
frequency-domain radar processing. Particularly, we formulate and adopt a
computationally efficient interpolation approach to mitigate the effects of
such empty subcarriers in the radar processing. We evaluate the target
detection and the corresponding range and velocity estimation performance
through computer simulations, and show that high-quality target detection as
well as high-precision range and velocity estimation can be achieved.
Especially 5G NR waveforms, through their impressive channel bandwidths and
configurable subcarrier spacing, are shown to provide very good radar/sensing
performance. Then, a fundamental implementation challenge of
transmitter-receiver (TX-RX) isolation in OFDM radars is addressed, with
specific emphasis on shared-antenna cases, where the TX-RX isolation challenges
are the largest. It is confirmed that from the OFDM radar processing
perspective, limited TX-RX isolation is primarily a concern in detection of
static targets while moving targets are inherently more robust to transmitter
self-interference. Properly tailored analog/RF and digital self-interference
cancellation solutions for OFDM radars are also described and implemented, and
shown through RF measurements to be key technical ingredients for practical
deployments, particularly from static and slowly moving targets' point of view.

Signal Processing