IEEE - Aerospace and Electronic Systems - April 2022 - 51

Wu et al.
Figure 7.
Illustration of the impact of a nonzero timing offset on the signal sampling at a UE.
between the permutations. Another challenge is to develop
effective receiving schemes for UE to reliably demodulate
the permutation-based information symbol, particularly
when the issues discussed sequentially are present.
TIMING OFFSET
The prior works on FH-MIMO DFRC [10]-[16] assume perfect
timing, which is also the case in our elaboration provided
in the section " Channel Estimation for FH-MIMO DFRC. "
However, there can be a nonzero timing offset in practice,
particularly when a synchronization link between the radar
and the UE is unavailable; see the section " Information
Demodulation " for details. In a packet-based communication,
the UE can attain a coarse timing by performing conventional
methods like energy or correlation-based detection [17]. Providedanonzerotimingoffset,asampledhop
spans overtwo
actual hops, as illustrated in Figure 7. As a consequence, there
will be interhop interference introduced to the received signals.
Moreover, due to the incomplete sampling ofa hop, the
waveform orthogonality of the FH-MIMO radar, as ensured
by the integer multiple of cycles of different sinusoidal signals
in each hop (seeNode A ofFigure 3), is destroyed, hence
introducing interantenna interference [17].
It is noteworthy that the impact of timing offset is frequency-varying,
which can result in great difficulty for
channel estimation and information demodulation. Based
on the baseband radar signal given in (3), we can model
the received signal with a nonzero timing offset, as given
by ej2pðiLhÞkhmD, where Lh
is the number of samples in
the nonzero timing offset denoted by h. Since khm varies
across hop h and antenna m, the phase nuisance incurred
by Lh also varies over hops and antennas.
A seemingly straightforward solution to this issue is to
perform additional timing offset estimation, which helps
recover a complete hop and revive the methods for channel
estimation illustrated in the section " Channel Estimation for
FH-MIMO DFRC. " However, the estimation of the timing
offset is challenging. As mentioned earlier, interantenna and
interhop interference exists for any given nonzero timing offset.
On the other hand, the antenna- and hop-varying phase
resulted from the timing offset is coupled with the phase of
channel response in a pointwise manner. This makes the estimation
ofeither part challenging.
APRIL 2022
To address the above challenges, we may resort to the
DoF available in the FH-MIMO radar waveform. For
example, if khm ¼ 0, then the phase caused by the timing
offset is removed according to ej2pðiLhÞkhmD. Note that, in
any hop h, only one khm ð8m ¼ 0; 1; ...;M 1Þ can
take zero, so as to comply with the waveform orthogonality
constraint of an FH-MIMO radar, as given in (2).
Moreover, the specific values of h and m, need to be
designed in the development of the methods for estimating
the timing offset and communication channel.
MULTIPATH CHANNEL
Thus far, only the LoS-dominated AWGN channel is considered
for FH-MIMO DFRC. As discussed in the section
" Channel Models Suitable for FH-MIMO DFRC, " it can be
practical to consider the LoS channel for some scenarios,
such as the G2A communications between a ground-based
radar and an high-altitude platform; see Figure 1. However,
as also illustrated in the figure, multipath fading can happen
in FH-MIMO DFRC, when the airborne UE, e.g., UAV, flies
at a low altitude. Under multipath fading, the signal can be
severely attenuated, when arriving at the receiver.
Figure 8 illustrates the signal attenuation caused by
multipath fading by plotting jYhðlÞj, namely, the
Figure 8.
Illustration of the severe signal attenuation caused by multipath
fading, where the Rician channel is considered with the Rician
factor of 5 dB and the LoS path takes the unit power. Two
receiver (Rx) antennas with the spacing of half a wavelength are
considered at the UE, each with an independent AWGN of10
dB noise power. The radar parameters are set according to
Table 1.
IEEE A&E SYSTEMS MAGAZINE
51

IEEE - Aerospace and Electronic Systems - April 2022

Table of Contents for the Digital Edition of IEEE - Aerospace and Electronic Systems - April 2022