  This paper investigates a novel underlaid sensing pilot signal design for
integrated sensing and communications (ISAC) in an OFDM-based communication
system. The proposed two-dimensional (2D) pilot signal is first generated on
the delay-Doppler (DD) plane and then converted to the time-frequency (TF)
plane for multiplexing with the OFDM data symbols. The sensing signal underlays
the OFDM data, allowing for the sharing of time-frequency resources. In this
framework, sensing detection is implemented based on a simple 2D correlation,
taking advantage of the favorable auto-correlation properties of the sensing
pilot. In the communication part, the sensing pilot, served as a known signal,
can be utilized for channel estimation and equalization to ensure optimal
symbol detection performance. The underlaid sensing pilot demonstrates good
scalability and can adapt to different delay and Doppler resolution
requirements without violating the OFDM frame structure. Experimental results
show the effective sensing performance of the proposed pilot, with only a small
fraction of power shared from the OFDM data, while maintaining satisfactory
symbol detection performance in communication.
