The atmospheric ducting phenomenon, caused by lower densities at higher altitudes in the Earth's atmosphere, causes a reduced refractive index, causing the signals to bend back towards the Earth. A signal trapped in the atmospheric duct can reach distances far greater than normal. In TDD networks with the same UL/DL slot configuration, and in the absence of atmospheric ducting, a guard period is used to avoid the interference between UL and DL transmissions in different cells. However, when the atmospheric ducting phenomenon happens, radio signals can travel a relatively long distance, and the propagation delay exceeds the guard period. Consequently, the DL signals of an aggressor cell can interfere with the UL signals of a victim cell that is far away from the aggressor. Such interference is termed as remote interference. The further the aggressor is to the victim, the more UL symbols of the victim will be impacted.
A remote interference scenario may involve a number of victim and aggressor cells, where the gNBs execute Remote Interference Management (RIM) coordination on behalf of their respective cells. Aggressor and victim gNBs can be grouped into semi-static sets, where each cell is assigned a set ID, and is configured with a RIM Reference Signal (RIM-RS) and the radio resources associated with the set ID. Each aggressor gNB can be configured with multiple set IDs and each victim gNB can be configured with multiple set IDs, whereas each cell can have at most one victim set ID and one aggressor set ID. Consequently, each gNB can be an aggressor and a victim at the same time.
To mitigate remote interference, the network enables RIM frameworks for coordination between victim and aggressor gNBs. The coordination communication in RIM frameworks can be wireless- or backhaul-based. The backhaul-based RIM framework uses a combination of wireless and backhaul communication, while in the wireless framework, the communication is purely wireless.
In both frameworks, all gNBs in a victim set simultaneously transmit an identical RIM reference signal carrying the victim set ID over the air.
In the wireless framework, upon reception of the RIM reference signal from the victim set, aggressor gNBs undertake RIM measures, and send back a RIM reference signal carrying the aggressor set ID. The RIM reference signal sent by the aggressor is able to provide information whether the atmospheric ducting phenomenon exists. The victim gNBs realize the atmospheric ducting phenomenon have ceased upon not receiving any reference signal sent from aggressors.
In the RIM backhaul framework, upon reception of the RIM reference signal from the victim set, aggressor gNBs undertake RIM measures, and establish backhaul coordination towards the victim gNB set. The backhaul messages are sent from individual aggressor gNBs to individual victim gNB, where the signalling is transparent to the core network. The RIM backhaul messages from aggressor to victim gNBs carry the indication about the detection or disappearance of RIM reference signal. Based on the indication from the backhaul message, the victim gNBs realize whether the atmospheric ducting and the consequent remote interference have ceased.
In both frameworks, upon realizing that the atmospheric ducting has disappeared, the victim gNBs stop transmitting the RIM reference signal.
When different TDD DL/UL patterns are used between neighbouring cells, UL transmission in one cell may interfere with DL reception in another cell: this is referred to as Cross Link Interference (CLI).
To mitigate CLI, gNBs can exchange and coordinate their intended TDD DL-UL configurations over Xn and F1 interfaces; and the victim UEs can be configured to perform CLI measurements. There are two types of CLI measurements:
SRS-RSRP measurement in which the UE measures SRS-RSRP over SRS resources of aggressor UE(s);
CLI-RSSI measurement in which the UE measures the total received power observed over RSSI resources.
Layer 3 filtering applies to CLI measurement results and both event triggered and periodic reporting are supported.