IEEE - Aerospace and Electronic Systems - September 2023 - 10

Opportunities and Limitations in Radar Sensing Based on 5G Broadband Cellular Networks
DM-RS
To ensure high-quality signal transmission, the new generations
of mobile systems, for example, LTE and NR, use
DM-RS to provide an accurate reference for channel estimation.
In 5G NR, the DM-RS is content specific and is
transmitted only when data are present. The reference signal
appears in every physical channel, regardless of the
link direction (PBCH, PUCCH, PDSCH, etc.). The exact
position in these channels can be very different.
In the PBCH, the position is dependant on the physical
cell ID, which can be derived from synchronization signals
(PSS and SSS) and supports only eight possible
sequences. Therefore, one can calculate the position of the
PBCH DM-RS performing a simple correlation of the
received signal with known eight templates [27]. The SSB
depicted in Figure 3 contains a PBCH (yellow) with its
DM-RS (light stripes across the whole channel resources).
The PDCCH has to be discussed together with the
CORESET, despite the fact that it is only a placeholder
and does not carry any information itself. Nevertheless,
the CORESET parameters also configure the position and
values of the DM-RS symbols. More precisely, the parameter
pdcch-DMRS-ScramblingID designates the sent bit
sequence. In general, the CORESET parameters are configured
by a radio resource control (RRC) message, which
is sent directly to a dedicated UE. The only exception is
the CORESET#0, which can be derived from the MIB. An
example of the PDCCH resources is shown in Figure 6
(left-hand side) together with its associated DM-RS (light
stripes across the whole channel resources).
The PDSCH is the data plane channel, and it occupies
the largest amount of radio resources in the downlink. The
associated DM-RS could be very valuable in the context
of a reference signal for passive radar. Unfortunately, UEs
are informed by the BTS about the PDSCH. The channel
position within the RG and the coding configuration is
provided by the BTS by a higher layer parameter. Since
the PDSCH is a large channel, it has a bigger number of
associated DM-RS symbols. This gives one the opportunity
to use them for passive radar processing. To be able
to find out, which DM-RS sequence is used (one of 65,535
possible) one has to receive the DMRS-DownlinkConfig
higher layer parameter. An example ofthe PDSCH resources
is shown in Figure 6 (right-hand side) together with
the associated DM-RS (light stripes across some symbols).
CSI-RS
CSI-RS are needed in the 5G NR system for many functionalities.
These include link adaptation, DL precoding
(both codebook and noncodebook based), frequency and
time tracking, and beam management.
CSI-RS was first introduced in LTE release 10 to support
transmission mode 9 (TM9) with up to eight layers. The
10
Figure 6.
PDCCH, PDCCH DM-RS, PDSCH, and PDSCH DM-RS.
reason for introducing CSI-RS (in TM9) was to decrease the
signaling overhead caused by the cell-specific reference signal
(CRS) when using a high multiple input multiple output
(MIMO) order (beyond 4). In the CRS scenario, each
antenna in the MIMO scenario must be assigned with dedicated,
broadcast reference symbols. Increasing the number
of antennas also causes an increasing number of RS and
therefore increasing the signalling overhead for all users in
the cell, not only the 8 8 MIMO ones. On the other hand,
CSI-RS are UE-specific reference symbols. This means that
they are assigned only to users, which need them and are not
constantly broadcast.
This approach was further adapted and extended in 5G
NR where there are no CRS at all. From the telecommunication
perspective, it is advantageous because of the lower
energy consumption and signaling overhead. However,
this approach generates a lot of challenges for 5G-based
active and passive radars.
In order to perform CSI-related measurement, each
UE must be explicitly informed (via RRC signaling),
which resources it should use to measure the channel and
which to measure interference.
As mentioned previously, CSI-RS are user specific.
Each UE, when needed, is assigned with a dedicated set of
CSI-RS via RRC signaling. This indicates that, in general,
it may be hard or even impossible to eavesdrop on selected
UE RRC signaling and get information on assigned CSIRS.
The task becomes even more difficult if passive radar
is obligated to extract a CSI-RS combination from all connected
UEs within its range.
The NR CSI-RS signal aðp;mÞ
k;l , defined in Section
7.4.1.5.3 of [27], is based on the pseudorandom sequence
rðmÞ defined in Section 7.4.1.5.2 and comprises one or
more resource elements (REs) mapped to the RG according
to RRC-configurable attributes: numerology m, number
of antenna ports X, port number p, density r, resource
location in the time l and frequency k domains, cdmIEEE
A&E SYSTEMS MAGAZINE
SEPTEMBER 2023
IEEE - Aerospace and Electronic Systems - September 2023

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