  The convergence of sensing and communication in the millimeter-wave (mmWave)
and low terahertz (THz) bands has been envisioned as a promising technology,
since it incorporates high-rate data transmission of hundreds of Gbps and
mm-level radar sensing in a spectrum- and cost-efficient manner, by sharing
both the frequency and hardware resources. However, the joint radar sensing and
communication (JRC) system faces considerable challenges in the mmWave and
low-THz scale, due to the peculiarities of the propagation channel and
radio-frequency (RF) front ends. To this end, the waveform design for the JRC
systems in mmWave and low-THz bands with ultra-broad bandwidth is investigated
in this paper. Firstly, by considering the JRC design based on the co-existence
concept, where both functions operate in a time-domain duplex (TDD) manner, a
novel multi-subband quasi-perfect (MS-QP) sequence, composed of multiple
perfect subsequences on different subbands, is proposed for target sensing,
which achieves accurate target ranging and velocity estimation, whilst only
requiring cost-efficient low-rate analog-to-digital converters (A/Ds) for
sequence detection. Furthermore, the root index of each perfect subsequence is
designed to eliminate the influence of strong Doppler shift on radar sensing.
Finally, a data-embedded MS-QP (DE-MS-QP) waveform is constructed through
time-domain extension of the MS-QP sequence, generating null frequency points
on each subband for data transmission. Unlike the co-existence-based JRC system
in TDD manner, the proposed DE-MS-QP waveform enables simultaneous
interference-free sensing and communication, whilst inheriting all the merits
from MS-QP sequences. Numerical results validate the superiority of the
proposed waveforms regarding the communication and sensing performances,
hardware cost as well as flexibility of the resource allocation between the
dual functions.

Signal Processing