Understanding and Mitigating Ducting Interference in 5G

When 5G was initially introduced, users, CSPs, and vendors alike were excited about the potential new use cases and advantages of this revolutionary technology. Discussions quickly turned to gigabit speeds, smart cities, massive machine-type communications, ultra-reliable low latency, and network slicing—innovations expected to transform mobile network capabilities.

Major telecom companies began preparing for the 5G journey as RAN vendors introduced new, revolutionary hardware and software features. While 4G deployments were mainly based on Frequency Division Duplexing (FDD) spectrum bands, 5G specifications required the allocation of more Time Division Duplexed (TDD) spectrum bands. Many of these new TDD spectrum bands, such as the 3.5 GHz band, had previously been used for purposes other than mobile networks in many countries.

While the technology’s fundamentals, advantages, and use cases were widely discussed, the challenges associated with the new spectrum intended for 5G received far less attention.

Mobile spectrum is the most valuable, limited, and costly resource in the telecom industry, and operators are constantly seeking optimal spectrum allocation to increase network capacity and coverage. When we discuss frequency bands, it’s important to understand the differences between TDD and FDD. The primary difference is that with FDD bands, uplink and downlink transmissions occur on separate channels, whereas with TDD, uplink and downlink share the same channel, separated only by time. There are benefits to TDD’s uplink downlink multiplexing. TDD allows the use of various antenna technologies such as massive MIMO and beam forming due to channel reciprocity. It also allows operators to use spectrum more efficiently by providing them with the flexibility to allocate more capacity for downlink (DL) or uplink (UL) based on changing demands.

One major challenge that the incorporation of TDD bands created is its vulnerability to certain types of RF interference, notably interference caused by a phenomenon known as tropospheric ducting.

Tropospheric ducting occurs when a layer of warm air sits above a layer of cooler air. This weather phenomenon creates variations in air temperature and moisture content, resulting in a layered atmospheric structure that behaves like a waveguide. As a result, electromagnetic waves can be refracted, allowing them to travel back toward the Earth and propagate over long distances, particularly affecting all the radio signals with frequencies above 50 MHz.

5G and LTE TDD systems are especially susceptible to ducting interference when the propagation time between distant cells exceeds the guard period separating downlink and uplink transmissions. Downlink signals from distant cells, including those from neighboring markets, can travel more than 200 km and interfere with uplink signals, causing TDD self-interference due to ducting.

Ducting interference is a global issue affecting telecommunications networks today. In regions like North America, Asia-Pacific, and the Middle East, it accounts for more than half of the interference impacting operators’ networks, often affecting thousands of cells simultaneously. Ducting interference can lead to a substantial decrease in data throughput, ranging from ~50% to ~90%, and can also degrade audio quality, resulting in customer dissatisfaction and complaints.

Since tropospheric ducting is heavily influenced by weather conditions, it tends to vary seasonally and can be very dynamic. This adds complexity for operators since ducting interference can appear or dissipate quickly, even within the same day. A near real-time interference mitigation platform has become more critical than ever for operators to identify when ducting interference occurs or subsides. More importantly, it helps quantify the impact on network quality and service assurance, while providing actionable solutions to efficiently resolve the issue.

Today, operators often resort to manual interventions or temporary changes in network configuration to mitigate TDD interference, particularly when dealing with tropospheric ducting. These reactive measures are far from ideal, as they are resource-intensive and often fail to keep pace with the dynamic nature of ducting interference. Manual adjustments can also be inconsistent, leading to delays in detecting and resolving problems. Furthermore, operators often depend on RAN vendor software functionalities, which are limited in capability and may not support multi-vendor environments. This results in reduced capacity and a lack of visibility into interference issues.

What operators need is an automated, proactive approach that continuously monitors the network environment, evaluates interference levels, and quickly optimizes performance. This is where Spectrum-NET comes into play. It utilizes advanced AIOps models to continuously enhance the network through a ‘Detect, Analyze, Mitigate’ framework, offering real-time visibility and monitoring across multi-vendor networks.

Mitigation of ducting interference is achieved through parameter reconfiguration. Commonly used techniques include:

• PDSCH blanking at dominant aggressor cells.
• Adjusting the special slot format parameter.
• Reducing victim cells’ footprint and increasing UE transmit power.
• Increasing antenna tilt for both victim and aggressor cells.

When we talk about ducting interference, there are two types of cells involved: aggressor and victim cells. Aggressor cells are those causing the most ducting interference due to factors such as transmitted power, location, and traffic volume. Victim cells are the ones that experience heightened levels of interference and KPI degradation. It’s important to note that any aggressor cell can also be a victim cell.

Mitigating ducting interference is not a simple task, and it often requires a trade-off between reducing interference and maintaining network capacity and coverage. For example, blanking the required PDSCH slots at aggressor cells to eliminate interference at victim cells generally reduces downlink aggressor cell capacity by about 6% to 19%, depending on the number of blanked PDSCH slots. Applying this mitigation approach uniformly across potentially thousands of affected cells can lead to significant reductions in overall network capacity, resulting in issues like decreased network speed and possible congestion.

Therefore, mitigation efforts should target the dominant aggressor cells, as they can provide the most substantial interference reduction for victim cells, ultimately maximizing overall network capacity. One of the key features of Spectrum-NET is its ability to identify these dominant aggressor cells, based on average interference levels and Downlink Physical Resource Block (DL PRB) utilization observed when ducting was detected. By focusing mitigation actions on these dominant aggressor cells, we achieve the greatest interference reduction for victim cells while maximizing network capacity. For example, dominant aggressor cells may account for just over 20% of all aggressor cells, yet increasing the guard time in these cells alone can achieve an 80% overall reduction in interference for all victim cells within the affected market.

Ducting interference can occur not only within an operator’s network but also between different operators. This interference may arise from varying frequency allocations between counties, states, or between operators in neighboring countries—even when their cells are aligned in frequency and synchronized. The latter example, known as cross-border ducting interference, can impact both TDD and FDD cells. To effectively mitigate cross-border ducting interference, cooperation between operators and regulator entities in the involved countries is essential. At Spectrum Effect, we have developed the “QoE Focused Auto-Mitigation feature,” which uses multiple patented techniques applied to the victim cells (considering the aggressor cells are outside of the network) to reduce the impact of persistent cross-border interference on key performance indicators (KPIs) and enhance subscribers’ quality of experience (QoE) through parameter adjustments.

While TDD offers significant advantages for 5G, it also introduces new challenges, particularly ducting interference that can degrade network performance. To address this, a centralized, automated, multi-vendor solution like Spectrum-NET is essential for effective mitigation. It transitions operators from reactive, manual interventions to a proactive approach that continuously monitors and optimizes network performance through AIOps models. By focusing on dominant aggressor cells for self-interference and adjusting victim cells for external ducting interference, Spectrum-NET ensures timely and effective measures are taken, maximizing spectrum usage and enhancing service quality—and doing so in multi-vendor environments. By leveraging Spectrum-NET’s capabilities, operators can increase network capacity, reduce churn, and improve overall QoE, making it an indispensable tool for overcoming ducting interference in modern 5G and LTE TDD networks.