IEEE - Aerospace and Electronic Systems - September 2023 - 15

Ksi ˛e_zyk et al.
Removing or skipping part of the signal transmitted
by the mobile terminal (uplink transmission). From
the point of view of passive radiolocation, for a 5G
NR network, the ideal situation would be for BTSs
to be the only source of the transmitted signal. Signals
transmitted by mobile terminals, due to their
relatively low power and possible target detection
range, seem to be, in the case under analysis, even
interference, which should be eliminated as efficiently
as possible. Due to the transmission method
of the 5G network (TDD), part of the time slots are
provided for uplink transmission, and this is a loss
that must be reconciled with in the case of passive
radar operation. Because signals transmitted by
mobile terminals can be received by both the reference
and observation antenna, additional artifacts
from these signals may be visible on the CAF. For
this reason, this phenomenon can be reduced by
zeroing or filling the uplink slots with white noise.
Figure 12.
Example of spectrograms for different amount of content in 5G
signal. (a) No data content. (b) Lot of data content.
realization of the goal, such as detecting targets
of a given kind (e.g., drones) with a preset probability
of false alarm.
Some help in evaluating the amount of occupied
link during transmission can be provided by the
image of the signal spectrogram. Figure 12(a)
shows the spectrogram of the 5G signal for
low link occupancy and Figure 12(b) shows the
result for high link occupancy. However, some
difficulty here may be the computational complexity
of the algorithm for determining the spectrogram,
calculated based on the short-time
Fourier transform.
A decrease in signal filling also results in the reduction
of average transmitted power. As a result, the
maximum range of the radar system is also reduced.
This effect is presented in more detail in the
" Coverage prediction " section.
Another difficulty is also working in the TDD mode,
which requires the following.
Ensuring synchronization to the received signal
(detection of synchronization blocks); it is necessary
here to know the numerology, including the
TDD pattern, intervals, frequencies, etc. Without
this basic knowledge, it will not be possible to properly
synchronize the receiver and potentially modify
the received signal. For this reason, the performance
of the passive radar can be significantly reduced.
SEPTEMBER 2023
Removing/omitting the uplink is associated with
a decrease in effective integration time, resulting
in a decrease in resolution of the bistatic rate
measurement and lower processing gain. The
bistatic velocity resolution DVb is defined as follows
[1]:
DVb ¼
:
Tint
(6)
The selection of the proper integration time is also
related to the range and velocity cell migration. For
this reason, the coherent processing time cannot be
too long [1], [4]. Assuming bistatic velocity resolution
of about 1 m/s, the integration time should be
equal to 87 ms for a 5G station operating in the Sband
(carrier frequency of 3.44 GHz) [7]. When the
signal bandwidth is equal to 38.16 MHz, the processing
gain [defined in (5)] is as high as 65 dB.
However, when uplink slots are not used in signal
processing, only part of the whole signal is covered.
In the previously discussed case [7], only 74% of
the whole signal is occupied by downlink, which
yields effective Tint ¼ 65
ms, effective
Vb ¼ 1:35 m/s, and effective processing gain
Gint ¼ 64 dB. However, these calculations assume
that the content of the transmission is fully loaded,
which is not always true. Less content is an additional
aggravating factor for the bistatic range resolution.
Integration time is strongly related with the
number of samples that must be processed. Regardless
of the effective duration of the integration time,
the number of samples remains the same. This
should be kept in mind when designing a real-time
radar system and assuming a sufficiently high resolution
of bistatic velocity measurements.
IEEE A&E SYSTEMS MAGAZINE
15
IEEE - Aerospace and Electronic Systems - September 2023

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