NR-U Control plane (Candidates)
Note: NR-Unlicensed is still in the study phase with barely any work done on the TS. It is expected that by 2022 (so Release 18) we may see some specification for the 6GHz band in Europe. As of now, only potential candidates for NR-U implementation can be guessed.
RLM/RLF and mobility
For non-standalone
NR-U deployments, connected mode mobility is supported on licensed spectrum
using the baseline mobility procedure specified for the concerned licensed
radio access technology (LTE or NR).
For standalone NR-U deployments, the
following mobility scenarios shall be supported:
- Inter-cell
handover between NR-U and NR-U;
- Inter-cell
handover between NR-U and NR.
In addition, the following mobility
scenarios shall be supported based on the mobility between NR-U and NR and and
the mobility between NR and (e)LTE, however further optimizations to this
scenarios will be considered possibly with lower priority:
- Inter-RAT
handover between NR-U and LTE connected to EPC;
- Inter-RAT
handover between NR-U and LTE connected to 5GC.
For connected mode mobility, the main issue
identified for NR operation in unlicensed band is the reduced transmission
opportunities for different signalings due to LBT failure.
The following modifications to
mobility-related procedures have been identified as beneficial to study:
- Modifications
to mobility-related measurements considering limitations to the transmission of
reference signals due to LBT. NR-U needs to consider techniques to handle
reduced RS (e.g. SS/PBCH block and CSI-RS) transmission opportunities due to
LBT failure.
- Modifications
to mobility-related measurements and/or triggers considering limitations
related to high channel occupancy. NR-U needs to consider techniques to handle
increased interference levels in the unlicensed channel for mobility-related
decisions.
- Modifications
to mobility-related procedures and/or triggers considering limitations related
to the transmission of control plane signalling due to LBT. NR-U needs to
consider whether NR-U specific techniques to handle additional signaling delays
due to LBT failure are required, if not resolved by general mobility
enhancement solutions [RP-181433].
Potential
modifications to the measurement reporting quantities, to the measurement
reporting triggers and to the condition used by the UE when delaying the time
at which it applies a reconfiguration for mobility that are based at least on
channel occupancy and RSSI should be studied.
For
RRM, RLM, and mobility procedures, NR licensed specification in Rel-15 are
considered as a baseline for NR-U. However, changes to these due to new
physical layer design and LBT for the unlicensed operation can be introduced.
These will support both synchronous and, except for LAA case, asynchronous
deployments.
The RRM
and RLM framework for NR-U will also support multiple beam operation. The measurement of multiple beams in NR-U will use the framework in
TS 38.300 Section 9.2.4 as a baseline and the measurement model captured in
Figure 9.2.4-1 is also applicable for NR-U.
For UE
measurements, it is assumed that recurring transmissions of SSB/PBCH and RMSI
will be available with possibly reduced opportunities due to LBT. The NR
licensed measurement framework (cell and beam quality derivation for RSRP,
RSRQ, and SINR, filtering and combining multiple beams) is used as a baseline.
The handling of missing measurements due to LBT are expected to be captured at
physical layer specifications.
In
addition to the existing measurement quantities, channel occupancy and RSSI,
similar to adopted for LTE LAA, are considered useful.
In
unlicensed spectrum, multiple PLMNs from different operators can share the same
channel and coordination between different operators may not happen. This may
cause PCI collisions or confusion. As one possible solution, the gNBs can scan
different frequencies to identify the PCIs of neighbour cells and use this
information in setting the PCIs of their own cells in order to avoid PCI
collisions. In addition, ANR can be used, as in NR licensed, to detect and
solve PCI collision and confusion.
Other
Since System Information (SI) transmissions
will be subject to LBT, it is beneficial to add more transmission opportunities
in time domain for SI transmission, e.g. by configuring a longer SI window.
If there is need to have multiple SI
messages then with existing NR design, different SI messages require separate LBT
procedures. It may be beneficial not to require multiple LBTs for different SI
messages to increase the success probability of the transmission.
In response to a RAN2 LS requesting study
of system level aspects of NR-U, SA2 has discussed this topic and concluded as
follows:
- Based on
SA2 analysis, only system impact identified specifically for NR-U is the need
for introducing RAT type for NR-U, if desired, for "subscription based
access restriction", policy and charging purpose.
- If a
non-public network operator wants to leverage NR-U, Network Identification
& Network selection aspects for operators with no globally unique PLMN ID
are already being addressed within FS_Vertical_LAN study ongoing in SA2. Thus
NR-U is not resulting in additional system impacts work.
- The same
impact identified for 5GS applies also for EPS. SA2 understanding is that for
NR-U in EPS it is only for NR-U as secondary RAT (ENDC case) following similar
approach in terms of subscription based access restriction, policy and charging
as LAA/LWA. As such, similar solution can be adopted as the one that already
exists in EPS.
Based on the SA2 analysis and response,
there is no impact to RAN for the possible changes to 5GS and EPS for NR-U. The
support for "subscription based access restriction", policy and
charging is contained to CN signalling and the support for non-public operator
network identification will not result in additional work specific to NR-U.
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