Drones and networks: mobility support

In November 2018, we published a White Paper, Drones and networks: Ensuring safe and secure operations, discussing cellular radio networks and Drone Traffic Management (UTM), the enablers of the drone industry. There we describe how the radio environment for drones in the sky can be very different from that experienced by terrestrial user equipment (UE). In this blog post we dive deeper into these phenomena.

Challenges

At higher altitudes, the two main effects that lead to a different radio environment are close to free-space propagation and antenna sidelobes.

For a terrestrial UE, the signal to and from the base station is often obstructed or diffracted due to objects blocking the direct (line-of-sight) path. As a consequence, the received signal strength will be considerably weakened at the UE. Base stations are often placed in elevated positions, such as on cell towers or on top of buildings. If the UE moves to a higher altitude, as in the case of a hovering drone, the likelihood of objects obstructing the line-of-sight path becomes much smaller, as illustrated in the figure below.

Radio coverage from base stations to a terrestrial UE and a drone UE.

Since the signal propagation in the sky is close to free-space propagation, the signal strength becomes stronger due to the reduced path loss. The stronger signal strength from the serving base station is desirable. The drone, however may have line-of-sight paths to many non-serving base stations in the area as well. Since the cells share the same radio resources, the increased likelihood of line-of-sight paths to many non-serving cells increases the interference for the drone. The high level of interference might cause a low signal-to-interference-plus-noise ratio (SINR), which might make it difficult for the drone UE to promptly receive and decode mobility management related messages (for example, handover commands).

The other effect making the radio environment in the sky different from that on the ground is due to base station antenna sidelobes. Every directional antenna emits radiation also in unwanted directions, known as sidelobes, as illustrated in the figure below. A terrestrial UE is usually served by the main lobe of the base station antenna. Drones flying in the sky may move in the areas where the sidelobes are pointing to, and the drones might be served by the sidelobes most of the time.

Radio coverage from the base station antenna beam pattern.

The sidelobes give rise to the phenomenon of scattered cell associations particularly noticeable in the sky. The UE cell association is based on strongest received signal power, i.e., each position is associated with the cell from which the strongest signal is received at that position. Below, you can see the simulated cell association patterns for different altitudes.

Figure from the White Paper "Drones and networks: Ensuring safe and secure operations"

Cell association patterns at different altitudes.

The cell association pattern on the ground is ideally a nicely defined and contiguous area where the best cell is most often the one closest to the UE. As we move up in height, the antenna sidelobes start to be visible, and the best cell may no longer be the closest one. The cell association pattern in this particular scenario becomes fragmented especially at the height of 300 m, and above.

We would like to stress that the cell association pattern shown above only represents one specific scenario. The association pattern strongly depends on the deployment parameters such as inter-site distance, antenna patterns, antenna height, and down-tilt angles of the base station antennas. For example, we show a different cell association pattern for another scenario in our article Mobile-Network Connected Drones: Field Trials, Simulations, and Design Insights. The key takeaway is, that in general, the cell association patterns in the sky are quite different from the patterns on the ground.

The fragmented cell association pattern itself is not necessarily a problem. However, a drone UE served by a sidelobe might experience very sharp drops in signal strength when moving in the sky. A simulated example is shown below: the UE's measurements of the signal strengths of the cells in reach. At the beginning of the simulation (marked by the dashed blue vertical line), the UE selects cell 0 as the serving cell. After a few seconds, the signal strength begins to drop rapidly, and before the UE can be handed over to another cell, it declares radio link failure at the time instant marked by the thick dashed red line.

Sudden drop in signal strength – RSRP (Reference Signal Received Power).

Solutions

Since mobility is a key requirement for many drone use cases, the network should offer quality mobility management service for seamless drone connectivity.

As detailed in the previous section, the best cells may change frequently at the flight altitude of a drone. This requires fast and robust handovers between the cells to maintain connection. Mobility challenges due to rapid changes in signal strength and deep antenna nulls between sidelobes can be solved by ensuring that the handover procedures are executed promptly enough.

To provide seamless connectivity for drones in the sky, it is also important to properly manage the interference issue caused by the high likelihood of line-of-sight paths between the drones and non-serving cells.

In June 2018, in Release 15 of its specifications, 3GPP introduced a set of enhancements for LTE connected drones including

1. Subscription-based aerial UE identification and authorization
2. Height and location reporting based on the event that the UE's altitude has crossed a network-configured threshold altitude
3. Interference detection based on a measurement reporting that is triggered when a configured number of cells fulfils the triggering criteria
4. Open loop power control enhancements including UE specific pathloss compensation factor and extended range of nominal target received power
5. Signaling of flight path information from the UE to the network

For example, the above feature 2) enables the network to configure the UE to report when the UE goes above a certain height. The height threshold for triggering the report can be configured by the network that may take into account the interference scenario. In the above feature 3), UE measurement reporting is triggered when the received signal strengths from a configured number of cells are above a configured threshold. This feature may facilitate interference detection.

The rationale of these features is that the network should be informed when the UE might be in flight mode or interference-limited environment so that the network can dimension the radio resources properly to maintain quality of service. For example, the network may use dedicated radio resources that are free of interference from terrestrial UEs to serve the UE that reported its height is above the configured threshold. In addition, the network may use an optimized setting of uplink power control parameters for the UE to reduce uplink interference.

An illustration of Rel-15 features: a) the drone UE is configured to report when it is above 100 m height; b) the drone UE is configured to report when the received signal powers from 3 or more cells are stronger than a threshold.

In summary, we have evaluated the performance of mobile networks for airborne drone communication and found that LTE networks are already capable of supporting the initial deployment of low-altitude drones.

Next generation 5G networks have higher capacity for providing connectivity services to both terrestrial and aerial devices. New advanced technologies have been introduced in 5G networks. Key 5G New Radio (NR) features include ultra-lean transmission schemes, spectrum flexibility, support for low latency communication, and advanced antenna technologies. Mobility enhancements are being introduced in 5G New Radio to further improve handover reliability and robustness.

Overall, the 5G networks will be able to provide more efficient and effective connectivity for wide-scale drone deployments.

Ericsson will continue to work actively in the relevant forums to align mobile network capabilities with drone communication and traffic management requirements.

To learn more about mobility support for cellular connected drones, please read our article Mobility Support for Cellular Connected Unmanned Aerial Vehicles: Performance and Analysis .

An overview of some of the new features in the 5G NR specifications can be found in our article 5G New Radio: Unveiling the Essentials of the Next Generation Wireless Access Technology.