IEEE - Aerospace and Electronic Systems - September 2023 - 16

Opportunities and Limitations in Radar Sensing Based on 5G Broadband Cellular Networks
It should be noted, however, that with the relatively
short integration times of 5G NR signals, one can
obtain, " for free, " the ability to refresh measurements
very frequently. Assuming an integration
period of50 ms, one gets a maximum of20 measurements
per second. In the case of real-time radar
operation, it may turn out that the processing time
for such an amount of data (the operating bandwidth
is of the order of tens or even hundreds of MHz), is
much longer than the integration time. What can be
proposed is to skip part ofthe data in order to realize
real-time operation. In selected applications (e.g.,
car detection or UAVs), a refresh rate of a few Hz is
sufficient.
Working with a TDD mode may also bring some
benefits. A fixed carrier frequency for uplink and
downlink allows hardware simplification for
applications different from radar sensing. For
example, terminal localization can be performed
using the same receiver. For this purpose, one
can remove the downlink signal, leaving only the
signal from the terminal. Thanks to this, such
techniques as the direction of arrival can be used
for user localization both in outdoor and indoor
conditions [32].
In the case of selected frequency bands of 5G NR network
operation (sub-6 GHz), one deals with the frequency
division duplex mode. The operation of passive radar in
such conditions seems to be much simpler, since there is
no need to provide fine time and frequency synchronization
to remove signal fragments coming from the mobile
terminal (uplink).
PULSESIGNAL-BASEDPROCESSING
Previously described challenges mainly referred to passive
radars operating in PCL mode, focused on processing continuous
signals. As mentioned in the " Principles of Active
and Passive Radars in a 5G Network-Based Sensing
Perspective " section, passive radars can also operate in
pulse mode (options 3 and 4 in Figure 2), which brings
new difficulties for designers working with 5G NR, such
as the following.
Selection of signals, which can be applied in pulse
operation mode. This is a vast issue and relates to a
number of concerns. The " 5G Signal Morphology "
section describes a number of signals that are potentially
suitable for use in pulse-based passive radar
operation. Only signals that can be decoded are considered
for re-synthesis without knowing any user
IDs or codes for decryption. The only one that is
always present in a 5G frame, and meets the presumed
requirements, is SSB. A detailed analysis of
its possible use is presented in the " Radar
16
Characteristics of 5G Signals " section. However,
the most serious problem with this signal is its limited
operating bandwidth, limited duration, and too
long repetition period (for some applications). Also
in the " Radar Characteristics of5G Signals " section,
analyses are presented on the use of Type#0PDCCH
and SIB1 signals. These are other potential
signals on which radar processing can be based.
However, the presence of these signals is not
guaranteed in every case.
A chance to increase the pool of usable signals is by
using those that require knowledge of user IDs (not
completely accessible to an outside observer). In the
case ofPDSCH (SIB1), the modulation of the signal
is known (QAM), through which it is possible to
reconstruct the constellation and its resynthesis. The
disadvantage of such a solution is that it is impossible
to verify that the resynthesis process took place
without error (no CRC check), but with a sufficiently
high SNR of the signal, a negligible share of
errors can be assumed.
IEEE - Aerospace and Electronic Systems - September 2023

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