An overview of remote interference management for 5G
What is remote interference management?
As New Radio (NR) in many cases rely on time division duplex (TDD) for dividing radio resources between uplink and downlink, there is a risk that downlink transmissions interfere uplink reception. As the downlink generally has much higher transmit power than the uplink, the effect of such interference can be adverse. To protect the uplink from downlink interference, a guard period is used when switching from downlink to uplink. However, during certain atmospheric conditions, downlink transmissions can travel large distances and interfere uplink reception despite the guard period. This interference is referred to as remote interference (RI). To ensure robust NR deployments, it is necessary to implement some functionality to mitigate the impact of RI, referred to as remote interference management (RIM).
Remote interference due to tropospheric ducting
During certain atmospheric conditions, layering of the air may form a waveguide in the troposphere, a phenomenon called tropospheric ducting. A tropospheric duct can last for anything between a few minutes to several hours. Radio signals that enter the duct can travel hundreds of kilometers with low propagation losses, and hence cause significant interference far away from the transmitter. The effect of tropospheric ducting is illustrated in Figure 1, where downlink transmission from one base station interferes another distant base station. In commercial LTE TDD networks, degradation of coverage and connection success rate due to RI has been observed, indicating a need for some mitigation mechanisms.
RI is a large-scale phenomenon and thousands of base stations may interfere with each other at the same time. The interference at a certain base station has a time domain “sloping” character, i.e. the closer the uplink symbol is to the guard period, the higher the interface level becomes, as illustrated in Figure 2. This is because the interference is caused by accumulated signals from multiple base stations with different propagation delays, which means that the uplink symbols close to the guard period suffer more from interference compared to later symbols.
Static and adaptive remote interference management
Static RIM relies on planning of the network so that it is inherently robust against RI. This can be done, for example, by configuring a large guard period that covers the long propagation delays associated with RI or increasing the base station antenna downtilt to reduce the power radiated into the tropospheric duct. Static RIM schemes come with a cost in the form of reduced capacity or reduced coverage. It is therefore of interest to consider adaptive schemes, which have little or no performance impact when RI is not present.
Adaptive RIM schemes are applied only when RI is present. A straightforward adaptive RIM scheme is for the victim (interfered base station) to increase the guard period by starting uplink transmissions later, as shown in the middle plot in Figure 3. This will however reduce the uplink capacity and may not be feasible if the TDD pattern has a short uplink duration.
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Other potential adaptive schemes for the victim include utilizing the spatial domain for interference suppression by for instance forming receive beams which attenuates the received signal in directions where the RI is strong or dropping the interfered part of the signal. Adaptive RIM schemes can also be applied at the aggressor (interfering base station), for example by stopping downlink transmissions earlier, as shown in the bottom plot of Figure 3 or reducing downlink transmit power.
Remote interference coordination frameworks
To enable adaptive RIM schemes at aggressor side, coordination between the victims and aggressors is needed. This can either be done in centralized or distributed manner.
A centralized framework relies on a central coordinator, for example OAM (operations, administration and maintenance), which decides which base station should apply which RIM scheme. The centralized framework is likely to outperform a distributed framework, given that sufficient information about the interference situation is available and an appropriate decision can be taken in a timely manner, i.e. in an automated fashion rather than relying on manual intervention.
In a distributed framework, each base station will take this decision by itself, based on signaling from other base stations in the network and its own interference measurements. The signaling between base stations may take place over the air and via backhaul signaling. Such a framework may be more adaptive but on the other hand make decisions based on less information than a centralized framework.
The RIM schemes implemented in current TDD-LTE networks, although they are centralized solutions, rely on manual control via OAM interfaces, which means that there is significant room for improvements. Therefore, one of the main motivations for including RIM as a study and work item in 3GPP was to standardize components enabling an automated solution to handle RI, which has potential to significantly improve performance.
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In the RIM mechanism specified by 3GPP, when RI is detected by a base station, it starts transmitting a special RIM reference signal. This signal can be used by aggressors to identify the victim base station and how many symbols it interferes. It may additionally be used to indicate from the victim to the aggressor if enough RI mitigation has been applied or not. The RIM reference signal may also be sent by the aggressor after it has applied RIM, to indicate to the victim that the tropospheric duct still exists.
Experience from LTE TDD networks indicate that some type of RIM is necessary to ensure reliable network performance. The tools specified in 3GPP will facilitate this.
However, RI does not stop at national borders. This means that coordination between countries will be key, especially in Europe. This challenge may be significantly bigger than the technical one.
If you have a few more minutes, I recommend taking a look at our earlier blog post on 3GPP’s Release 16 and how this will shape 5G new radio.
Read more about Ericsson’s role in 3GPP on our standardization page.
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