  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.
