Cross-link interference in TDD networks and what to do about it

In cellular communication between a base station and user equipment (UE), simultaneous transmission and reception is typically accomplished using different resources for uplink and downlink, i.e. different frequencies, called FDD, or time resources, namely TDD. While FDD networks have separate uplink and downlink frequency bands, TDD networks utilize the same bandwidth, but allocate different time slots for uplink and downlink. It’s here that TDD networks experience so-called cross-link interference, where the base stations interfere with each other as they transmit and receive in the same frequency band.

End users constantly require improved coverage, capacity and throughput. Service providers need to develop the radio access networks accordingly, but in a way that reduces the cost per bit. One of the main advances that 5G New Radio (NR) has brought about is large scale deployments of advanced antenna systems with massive MIMO and state-of-the-art beamforming using time division duplex (TDD).

These deployments are done mainly in the C-Band, or mid-band frequencies (3.4GHz to 4.2GHz) and in mmWave, or high-bands (24.25GHz-29.50GHz) frequencies, depending on the region. The C-Band spectrum is already licensed in Australia, Japan, Korea, Hong Kong, many member nations of the European Conference of Postal and Telecommunications Administrations (CEPT) and in the Middle East. According to the Global mobile suppliers association (GSA), 112 service providers had invested for spectrum in the mmWave band by October 2019, and 67 of them were already licensed, mainly in the USA and associated territories, Japan, Hong Kong, Korea and Italy.

Compared to FDD networks, TDD networks have more flexibility when it comes to uplink and downlink. TDD systems can more easily adapt the ratio between uplink and downlink traffic, and they don’t need paired spectrum for uplink and downlink.

Implementation of TDD systems does introduce a few new challenges. One of them is so-called cross-link interference, which occurs when one base station is transmitting, while another is receiving in the same frequency band. Base stations usually transmit at higher power and have better propagation conditions between them, i.e., lower path loss compared to the link between base station and user equipment. As a result, cross-link interference can be significant when a base station in uplink is interfered by the downlink from another base station.  This interference from another base station that is transmitting is significantly larger than the received uplink from a user to another base station, resulting in a decrease in user throughput.

One way to avoid cross-link interference is to ensure that all base stations are either transmitting simultaneously or receiving simultaneously. This is known as synchronization and refers to sharing a common clock, phase reference and the same frame structure. Let us explain how it works, and which factors need to be considered.

We will start with the outdoor scenario, considering a single network comprising many cells operating in a TDD band. Each cell in the network can adapt its uplink-downlink ratio depending on the traffic. This is known as Dynamic TDD. This results in scenarios where cross-link interference occurs within a network affecting the network throughput. In this case, different base stations of the same network are transmitting in the same frequency band, resulting in large cross-link interference.  Within a network, Dynamic TDD is feasible only under low-load conditions in dense urban environments due to cross-link interference from other base stations. Intra-site interference becomes very large at high loads, which results in significant throughput loss, as explained here. Therefore, the cells in a network must be synchronized in order to avoid co-channel cross-link interference.

Usually, multiple service providers are allocated licenses in the same band and need to co-exist with each other. Typically, the service providers are allocated spectrum in adjacent channels for wide-area deployments. This results in cross-link interference between networks due to the out-of-band emissions from the base station of a service provider in the adjacent bands. In these cases, it’s important to synchronize the outdoor networks to avoid cross-link interference.

When service providers deploy in an area without synchronization, i.e., unsynchronized operation, for example, when the downlink of a service provider interferes with the uplink of another service provider, there is significant interference between the networks, even though they operate in adjacent channels. The uplink of one service provider is affected significantly by the out-of-band emissions from the downlink of the other. If they share the same site, the interference to the uplink can be quite large, especially in the C-band. Unsynchronized operations work only if the networks have large geographical separation. In the mmWave band, cross-link interference is smaller compared to the C-Band, due to deployment and propagation characteristics like, for example, lower transmit power. Nevertheless, unsynchronized operation still results in significant throughput loss in the mmWave band.

The reasoning is illustrated in Figure 1. Let us consider the case where two base stations in the same geographical area from two different operators communicate with their respective subscribers’ terminals.

Figure 1: Illustration of two TDD networks operating in the same band, indicating propagation conditions of the interference paths. When the two networks are not synchronized, meaning the downlink of one network interferes with the uplink of the other, the result is large base station to base station interference.

Typically, the base stations in a macro deployment are above building rooftops, and have larger antenna gain compared to the UE. Therefore, the path loss between the base stations is lower compared to the path loss between the base station and UE, as indicated in Figure 1(a).

Figure 1(b) shows two operators using adjacent channels in the same band. While the operators transmit in their specific bands, the transmit power is not only limited to the operator’s specific band, but there is leakage in the adjacent channel, typically specified as the Adjacent Channel Leakage Ratio (ACLR). The document 3GPP TS 38.104 provides  further details. While operators typically use adjacent frequency bands, the base station emissions in the adjacent band are still large enough to cause interference to the uplink of the operator in the adjacent band.

Figure 1(c) illustrates how the time resources are shared between the networks which use frequency bands adjacent to each other. In time slots T₁ and T₃ both networks are either in uplink or downlink simultaneously, whereas in time slots T₂ and T₄ , one network is in downlink while the other is in uplink, resulting in cross-link interference from downlink of one base station to the uplink of the other.

To conclude, synchronizing the outdoor networks is recommended as it avoids cross-link interference. Typically, the time reference used for synchronization is traceable to the Coordinated Universal Time (UTC). Additional information on synchronization of base stations is available in 3GPP TS 38.401 and ECC Report 216.

For indoor deployments, the situation is different. The indoor base-stations typically transmit at lower power compared to outdoor base stations. Hence, typically, the interference from indoor to outdoor base stations is smaller than the interference from outdoor to indoor base stations. While the interference from outdoor base stations in downlink is attenuated by the outdoor-to-indoor propagation loss, which includes the building penetration loss, co-channel operation in the same area is not feasible due to the large interference. Adjacent channel operation without synchronization is feasible both in the C-band and mmWave bands if the building penetration loss is sufficiently large and the indoor deployment is carefully done.

As a result, unsynchronized operation in an adjacent channel is possible without significant throughput loss with careful deployment of the indoor network which includes measures like ceiling-mounted installation, placement of base station away from windows, additional shielding around buildings, and so on. See ECC Report 296 for further information.

To summarize, synchronization of outdoor networks will avoid cross-link interference, especially from downlink of one operator to the uplink of another operator, thus enabling deployment of TDD systems. For indoor deployments, while co-channel deployment is not feasible,  unsynchronized operation in an adjacent channel is possible with careful deployment of the indoor network.

Learn more

Read more about remote interference management in TDD networks.

Read more about synchronization and impact of unsynchronized operation of TDD networks in ECC Reports 216, 281, 296 and 307.