  This paper investigates a unified pilot signal design in an orthogonal
frequency division modulation (OFDM)-based integrated sensing and
communications (ISAC) system. The novel designed two-dimensional (2D) pilot
signal is generated on the delay-Doppler (DD) plane for sensing, while its
time-frequency (TF) plane transformation acts as the demodulation reference
signal (DMRS) for the OFDM data. The well-designed pilot signal preserves
orthogonality with the data in terms of resource occupancy in the TF plane and
quasi-orthogonality in terms of codeword in the DD plane. Leveraging these nice
properties, we are allowed to implement sensing detection in the DD plane using
a simple 2D correlation, taking advantage of the favorable auto-correlation
properties of the 2D pilot. In the communication part, the transformed pilot in
the TF plane serves as a known DMRS for channel estimation and equalization.
The 2D pilot design demonstrates good scalability and can adapt to different
delay and Doppler resolution requirements without violating the OFDM data
detection and can overcome the fractional Doppler with limited sensing
resources. 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.
