  Integrated Sensing and Communication (ISAC) systems are recognised as one of
the key ingredients of the sixth generation (6G) network. A challenging topic
in ISAC is the design of a single waveform combining both communication and
sensing functionalities on the same time-frequency-space resources, allowing to
tune the performance of both with partial or full hardware sharing. This paper
proposes a dual-domain waveform design approach that superposes onto the
frequency-time (FT) domain both the legacy orthogonal frequency division
multiplexing (OFDM) signal and a sensing one, purposely designed in the
delay-Doppler domain. With a proper power downscaling of the sensing signal
w.r.t. OFDM, it is possible to exceed regulatory bandwidth limitations proper
of legacy multicarrier systems to increase the sensing performance while
leaving communication substantially unaffected. Numerical and experimental
results prove the effectiveness of the dual-domain waveform, notwithstanding a
power abatement of at least 30 dB of the signal used for sensing compared to
the one used for communication. The dual-domain ISAC waveform outperforms both
OFDM and orthogonal time-frequency-space (OTFS) in terms of Cram\'{e}r-Rao
bound on delay estimation (up to 20 dB), thanks to its superior resolution,
with a negligible penalty on the achievable rate.
