IEEE - Aerospace and Electronic Systems - September 2023 - 8

Opportunities and Limitations in Radar Sensing Based on 5G Broadband Cellular Networks
Table 2.
Mapping Between SCS and Symbol Length in NR
Numerologym 01 2
3
SCS ¼ 15 2m
[kHz]
OFDM symbol
duration [ms]
Cyclic prefix
duration [ms]
in [7]. The basic cell information is advertised to the UE
through the SSB.
SSB
4
15 30 60 120 240
66.67 33.33 16.67 8.33 4.17
4.69 2.34 1.17 0.59 0.29
The SSB is the first signal that the UE has to receive to be
able to attach to the 5G cell. It is the only signal, which is
always on and is sent periodically with the default interval
of 20 ms. The synchronization block consists of 20 RBs
(NSSB
RB ) in the frequency domain, each with 12 subcarriers
(240 subcarriers altogether), and four OFDM symbols
(NSSB
symb) in the time domain.
The SSB itself is constructed of two synchronization
Maximum carrier bandwidth is 100 MHz for low/mid
(middle)-band and 400 MHz for high band in Release 15
and 16. Release 17 introduces band n263 with the optional
channel bandwidth up to 2 GHz and 960 kHz subcarrier
spacing.
INITIAL DOWNLINK CHANNELS AND SIGNALS
A5G BTS periodically sends cell and system specific information.
Every UE that wants to attach to the network needs
to receive and decode them. This means that it is possible
to decode them without knowing network or terminal specific
parameters. First, the synchronization signal block
(SSB) is sent to ensure time and frequency synchronization
between the BTS and the UE and to provide the master
information block (MIB) with basic cell information (see
Figure 3). Next, The Type#0-physical downlink control
channel (PDCCH) is transmitted, which carries the downlink
control information (DCI) with the cyclic redundancy
code (CRC) scrambled by the system information radio
network temporary identifier (SI-RNTI). Because SI-RNTI
is constant and defined in the standard, each UE can decode
it. Finally, with all this information, a UE can find and read
the system information block1 (SIB1) carried by the physical
downlink shared channel (PDSCH).
TIMEANDFREQUENCYSYNCHRONIZATION
The 5G NR cellular network uses OFDM in its physical
layer to aggregate many narrowband subcarriers into a
wideband signal [27]. The signal structure can be represented
by an RG, divided into resource blocks (RBs). An
RB is a group of 12 consecutive subcarriers in the frequency
domain and one OFDM symbol in the time
domain.
A precise time and frequency synchronization is thus
required to obtain the exact positions of RBs. This allows
one to find reference signals and physical channels which
could be further used for radar purposes, as described
8
Figure 3.
SSB structure.
IEEE A&E SYSTEMS MAGAZINE
SEPTEMBER 2023
signals (SS): primary (PSS) and secondary (SSS). Their
purpose is to provide a reference for time and frequency
synchronization for listening devices. The rest of the SSB
structure is reserved for the physical broadcast channel
(PBCH), which carries basic cell information. In a multibeam
environment multiple SSBs are transmitted, one in
each beam-they have the same frequency position but
different time locations inside the 5G frame.
The reception and location of the SSB are straightforward.
It is a good candidate for passive radar goals. The
signal is not user specific, therefore, it is not encrypted
and a third party user can also receive and decode it. Furthermore,
the exact position of the SSB is defined and
then sent periodically.
TYPE#0-PDCCH
After the acquisition of basic cell information, the UE
can proceed with PDCCH detection and decoding. The
PDCCH carries DCI. Its correct reception is mandatory
for the correct demodulation of the PDSCH, which carries
the user's data. Each DCI is CRC scrambled by
RNTI. It is impossible to decode it without the prior
knowledge of RNTI. Each UE is assigned with a set of
dedicated DCIs. However, as described previously, the
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