CLI handling and RIM for NR
Rel-14 NR study showed that duplexing flexibility with cross-link interference mitigation shows better user throughput compared to static UL/DL operation or dynamic UL/DL operation without interference mitigation in indoor hotspot (4GHz and 30GHz) and urban macro scenarios (4GHz and 2GHz). Furthermore, semi-static and/or dynamic DL/UL resource assignments should also consider coexistence issues particularly among different operators where tight coordination are challenged. For efficient coexistence, not only coexistence requirements need to be understood but also advanced mechanisms to mitigate interference such as TRP-to-TRP measurement and adaptation based on measurements should be considered. The Rel-16 Work Item Cross Link Interference (CLI) handling to support flexible resource adaptation for unpaired NR cells achieves the following objectives:
• CLI measurements and reporting at
a UE (i.e., CLI-RSSI and SRS-RSRP), and network coordination mechanism(s)
(i.e., exchange of intended DL/UL configuration) are developed.
• Perform coexistence study to
identify conditions of coexistence among different operators in adjacent
channels is accomplished.
In NR
deployment on lower TDD frequency, the impact of the troposphere bending will
continue existing if no special mechanisms are introduced. In the RIM SI, the
frameworks for mechanisms for gNBs to start and terminate the
transmission/detection of reference signal(s), the functionalities and
requirements of the corresponding RS(s) as well as the design of the RS(s), and
the backhaul-based coordination mechanisms among gNBs have been studied. It is
recommended to specify RIM RS(s) to support identifying remote interference
related information, it is also recommended to specify the inter-set RIM
backhaul signalling via the core network for backhaul-based solution. The
Rel-16 Work Item Remote Interference Management (RIM) to deal with mitigation
of the remote interference caused by gNBs achieves the following objectives:
• RIM RS resource and
configurations, and the inter-set RIM backhaul signalling via the core network
to convey the messages of “RIM-RS detected” and “RIM-RS disappeared are
developed.
• Corresponding OAM functions to
support RIM operation is identified.
CLI
handling
Once
different TDD UL/DL configurations are applied among neighboring cells, UL
transmission from a UE in a cell causes interference to DL reception of some
other UEs in the rest of the neighboring cells. The interference is referred as
inter-cell UE-to-UE cross link interference (CLI). To mitigate an inter-cell
UE-to-UE CLI, gNBs can exchange and coordinate the intended TDD UL/DL
configuration over Xn and F1 interfaces. Taking into account the exchanged
information, a gNB may decide the transmission and reception pattern in order
to avoid CLI to neighboring cell or from neighboring cell.
For CLI
handling, two types of CLI measurements and reporting (i.e., CLI-Received
Signal Strength Indicator (RSSI) measurement and SRS-Reference Signal Received
Power (RSRP) measurement) are specified. For CLI-RSSI measurement, the victim UE
measures the total received power over configured CLI-RSSI measurement resource.
For SRS-RSRP measurement, the victim UE measures the RSRP over configured SRS
resource(s) which is/are transmitted from one or multiple aggressor UE(s).
Then, Layer 3 filtering can be applied to the measurement result for both
CLI-RSSI measurement and SRS-RSRP measurement. For measurement result reporting
for both CLI-RSSI measurement and SRS-RSRP measurement, event triggered and
periodic reporting are supported. Furthermore, CLI measurement and reporting
can be configured for NR cells in multi-carrier option.
Semi-static and/or dynamic DL/UL resource assignment causes interferers between networks on adjacent channels. It has been tasked to investigate the adjacent channel co-existence effects arising when CLI, or more generically dynamic TDD is operated in an aggressor network. The technical report captures a description of the adjacent channel interference effects that arise with dynamic TDD as well as a simulation investigation of adjacent channel interference in a number of different deployment scenarios.
RIM
Due to
the atmospheric ducting phenomenon, the DL signals of an aggressor cell in
unpaired spectrum can interfere with the UL signals of a victim cell in the same
unpaired spectrum bandwidth that is far away from the aggressor cell. Such interference
is termed as remote interference. A remote interference scenario may involve a
number of victim and aggressor cells, where the gNBs may execute Remote
Interference Management (RIM) coordination on behalf of their respective cells.
To
mitigate remote interference, RIM frameworks for coordination between victim
and aggressor gNBs are specified. The coordination communication in RIM
frameworks can be wireless- or backhaul-based. In both frameworks, all gNBs in
a victim set can simultaneously transmit an identical RIM reference signal
carrying the victim set ID over the air.
In the
wireless framework, RIM-RS (RIM reference signal) type 1 and RIM-RS type 2 are
specified. Upon reception of the RIM reference signal (RIM-RS type 1) from the
victim set, aggressor gNBs undertake RIM measurement, and may send back a RIM
reference signal (RIM-RS type 2) carrying the aggressor set ID if configured,
and may undertake interference mitigation actions. The RIM reference signal
(RIM-RS type 2) sent by the aggressor is able to facilitate estimation whether
the atmospheric ducting phenomenon between victim and aggressor sets exists.
In the
RIM backhaul framework, upon reception of the RIM reference signal (RIM-RS type
1) from the victim set, aggressor gNBs undertake RIM measurement, establish
backhaul coordination towards the victim gNB set. The backhaul messages which
carry the detection or disappearance indication are aggregated at gNB-CU via F1
interface and sent from individual aggressor gNBs to individual victim gNBs via
NG interface, where the signalling is transparent to the core network.