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An Overview of 3GPP Release-15 Study on Enhanced LTE Support for Connected Drones
Siva D. Muruganathan, Xingqin Lin, Helka-Liina Maattanen, Jonas Sedin, Zhenhua Zou, Wuri A. Hapsari, Shinpei Yasukawa
TL;DR
Low-altitude UAV connectivity creates interference and mobility challenges for existing LTE networks. This article reviews the 3GPP Release-15 study and work-item features, concluding that LTE connectivity is feasible despite these challenges.
Problem
UAVs can have line-of-sight links to multiple eNodeBs, causing interference that may degrade uplink performance for ground users and requiring detection, mitigation, or admission control.
Method
The article overviews 3GPP Release-15 findings, including interference detection and mitigation solutions, mobility performance, and standardized work-item features.
Results
Existing LTE networks are feasible for connecting low-altitude drones, with Release-15 features enhancing interference detection, uplink interference mitigation, mobility performance, and aerial UE identification.
Takeaways & Limitations
Efficient UAV connectivity while minimizing effects on terrestrial devices requires rethinking cellular-system assumptions, models, and techniques.
Abstract
from arXiv · showhide
Cellular connectivity to low altitude unmanned aerial vehicles (UAVs) has received significant interest recently which has led to a 3GPP study on enhanced LTE support for connected UAVs in Release 15. The objective of the study is to investigate the capability of long-term evolution (LTE) networks for providing connectivity to UAVs. In this article, we provide an overview of the key findings of the 3GPP Release-15 study-item phase. We first introduce UAV connectivity requirements and performance evaluation scenarios defined in the study. We then discuss radio channel models and the key identified challenges of using LTE networks to provide connectivity to UAVs. We summarize potential solutions to address the challenges including interference detection and mitigation techniques, mobility enhancements, and UAV identification. Finally, we also shed light on the key features standardized during the Release-15 work-item phase.
I. INTRODUCTION
The study addresses growing demand for cellular connectivity to low-altitude UAVs and examines LTE requirements, scenarios, and challenges. It covers UAV use cases, evaluation environments, and the Release-15 study's scope.
- Cellular UAV connectivity supports use cases including search-and-rescue, surveillance, wildlife conservation, package delivery, and critical-infrastructure monitoring.
- Aerial vehicles can expose multiple neighboring eNodeBs to uplink interference, potentially harming terrestrial users and requiring mitigation or admission control.Identifying aerial vehicles is a prerequisite for either response.
- The Release-15 study assessed LTE networks with up to Release-14 functionality and documented analysis, evaluation, and field-measurement results.
- The article overviews the study findings and features standardized during the Release-15 work-item phase.
- Command-and-control traffic was assigned an uplink/downlink data-rate requirement of 60-100 Kbps to support aerial-vehicle operation.
B. Channel Modelling
Release-15 defined aerial-UE channel models across urban and rural scenarios using height-dependent LOS, pathloss, shadow-fading, and fast-fading formulations. The models combine reused specifications with ray-tracing and measurement-based additions.
- The study modeled LOS probability, pathloss, shadow-fading, and fast-fading for aerial UE-to-eNodeB channels.
- LOS probability models depend on aerial UE height, with lower thresholds of 22.5 m for UMa-AV and UMi-AV and 10 m for RMa-AV.
- Above upper height thresholds, UMa-AV and RMa-AV assume 100% LOS at 100 m and 40 m, respectively, while UMi-AV has no applicable upper threshold.
- Between height thresholds, LOS models were derived using ray-tracing simulations for UMa-AV and RMa-AV, and above the lower threshold for UMi-AV.
- Above the lower height threshold, pathloss and shadow-fading models for LOS and NLOS conditions incorporated field measurements and ray-tracing results from multiple sources.
- Three alternative fast-fading models differ in angular spreads, delay spreads, K-factor ranges, and modeling methodology.
III. PROBLEMS IDENTIFIED DURING THE STUDY PHASE
The study identified interference problems in both uplink and downlink when aerial UEs operate in LTE scenarios. These problems motivated analysis of their distinct network effects.
- Interference problems were identified in both uplink and downlink for scenarios involving aerial UEs.
A. Uplink Interference
Aerial UEs create broader uplink interference and experience broader downlink interference because airborne LOS propagation exposes more cells. Increasing aerial-UE presence therefore degrades throughput and network performance.
- A. Uplink Interference: Aerial UEs cause interference to more cells than typical terrestrial UEs because airborne LOS propagation reaches more cells.
- A. Uplink Interference: Increasing the aerial UE ratio raises terrestrial-UE uplink interference over thermal-noise ratios and degrades terrestrial uplink throughput.
- A. Uplink Interference: Degraded terrestrial uplink throughput increases resource utilization, which further raises interference and degrades uplink performance for both aerial and terrestrial UEs.
- B. Downlink Interference: Aerial UEs receive downlink interference from more cells, producing poorer five-percentile downlink geometry than terrestrial UEs.Figure 1 compares the five-percentile geometry of aerial and terrestrial UEs.
- B. Downlink Interference: Increased downlink interference degrades aerial-UE throughput, while higher resource utilization subsequently degrades downlink performance for both aerial and terrestrial UEs.
- 3GPP studied interference detection and mitigation, mobility enhancements, and UAV identification, standardizing some solutions during Release-15.
IV. SOLUTIONS FOR INTERFERENCE DETECTION
The study considered UE- and eNodeB-based measurements for detecting uplink and downlink interference. Proposed enhancements included multi-cell triggering events, richer reports, and additional UE information.
- Interference detection can use UE-reported RSRP, RSRQ, and RS-SINR measurements for neighbouring cells.
- Enhanced triggering conditions can depend on measurements from multiple cells rather than a single-cell RSRP.
- TTT is the time during which event criteria must be met before a measurement report is triggered.
- Event A3 occurs when a neighbour cell becomes better than the serving cell by a configured offset.
- Uplink interference can be detected from eNodeB measurements or measurements reported by the UE.
- Potential enhancements included new triggering events, richer measurement reports, mobility history, speed, timing advance, and location information.
B. Network-based Solutions Considered During Study-Item Phase
Network-based solutions used information exchange among eNodeBs to detect aerial-UE interference, including UE measurements, reference-signal configurations, and transmission powers. Release-15 features also enabled multi-cell and height-triggered measurement reporting, though broad information exchange depends on backhaul feasibility.
- Network-based Solutions Considered During Study-Item Phase: eNodeBs could exchange UE measurements and uplink reference-signal configurations to detect interference from aerial UEs.A neighbouring eNodeB could measure interference by measuring the aerial UE’s uplink reference signal.
- Network-based Solutions Considered During Study-Item Phase: Exchanged neighbour-eNodeB transmission power and UE measurements could support estimating uplink pathloss and interference.The estimate uses the UE’s transmission power and uplink pathloss, assuming reciprocity.
- Network-based Solutions Considered During Study-Item Phase: Information exchange across many eNodeBs depends on the backhaul type and the feasibility of exchanging the required data.
- Network-based Solutions Considered During Study-Item Phase: A multi-cell report is triggered when a configured number X of cells satisfies an A3, A4, or A5 entry condition during TTT1.The UE is configured with X, the event, and associated thresholds.
- Network-based Solutions Considered During Study-Item Phase: With X=2, cells A and B entering the configured event condition trigger a report used for interference detection.
- Network-based Solutions Considered During Study-Item Phase: For four-cell A4 reporting, four neighbouring-cell RSRP values exceeded -76 dBm near 4.4 s for a UE at 150 km/h and 200 m altitude.
- Network-based Solutions Considered During Study-Item Phase: Height-based reporting sends aerial-UE height, location, and horizontal/vertical speeds when a configured height threshold is crossed.The report can support early interference detection because uplink and downlink interference increase above a certain height.
- Network-based Solutions Considered During Study-Item Phase: An upward- or downward-moving UE reports when crossing the threshold, whereas a horizontally moving UE does not report without crossing it.
V. INTERFERENCE MITIGATION
This section introduces the interference-mitigation techniques and Release-15 features addressed in the work-item phase.
- The section covers interference-mitigation techniques and features standardized during the Release-15 work-item phase.
A. Uplink Interference Mitigation Solutions Considered During Study-Item Phase
Uplink mitigation considered UE-specific power-control parameters, closed-loop control, FD-MIMO, and directional antennas. These techniques differ in whether they require LTE specification enhancements and how they respond to aerial-UE propagation conditions.
- A. Uplink Interference Mitigation Solutions Considered During Study-Item Phase: UE-specific fractional pathloss compensation lets aerial UEs use factors different from terrestrial UEs, potentially varying by aerial-UE height.
- A. Uplink Interference Mitigation Solutions Considered During Study-Item Phase: UE-specific P0 configures a different target received power for aerial UEs, but LTE’s supported P0 range may need extension.
- A. Uplink Interference Mitigation Solutions Considered During Study-Item Phase: Closed-loop power control adjusts aerial-UE target received power using serving- and neighbour-cell reports.Larger transmit-power-command steps may be needed to handle fast sky-signal changes, potentially requiring specification enhancements.
- A. Uplink Interference Mitigation Solutions Considered During Study-Item Phase: FD-MIMO can use multiple eNodeB receive antennas to mitigate uplink interference without LTE specification enhancements.
- A. Uplink Interference Mitigation Solutions Considered During Study-Item Phase: Directional aerial-UE antennas can reduce uplink signal power across broad angles and therefore reduce interference.Considered tracking modes included alignment with direction of travel and ideal or non-ideal tracking of the serving-cell line of sight.
- A. Uplink Interference Mitigation Solutions Considered During Study-Item Phase: Directional antennas are an implementation technique and therefore do not require specification enhancements.
B. Uplink Interference Mitigation Features Specified during Work-Item Phase
Release-15 specified uplink interference mitigation features that improve control over aerial UEs while protecting terrestrial-user performance. The work included UE-specific power-control enhancements and relied on existing or implementation-based downlink techniques.
- Uplink power control: Combination 2 improved terrestrial UEs’ 5-percential uplink throughput by roughly 45% and mean uplink throughput by 20%.Combinations 3 and 4 instead caused significant performance degradation, emphasizing the importance of configuration.
- Uplink power control: A UE-specific fractional pathloss compensation factor was introduced so aerial and terrestrial UEs could use different configurations.
- Uplink power control: The UE-specific P0 component range was extended from -8 dB to +7 dB to -16 dB to +15 dB, providing greater open-loop power-control flexibility.
- Downlink interference mitigation: FD-MIMO, directional antennas, receive beamforming, intra-site JT CoMP, and coverage extension were studied as downlink interference mitigation techniques.The techniques use antenna directionality, multiple transmit or receive antennas, joint transmission, or improved synchronization and initial access.
- Downlink interference mitigation: Because the studied downlink solutions were already supported in LTE or implementation-based, no further downlink interference features were introduced.
VI. MOBILITY AND AERIAL UE IDENTIFICATION
Release-15 study results showed poorer aerial-UE mobility than terrestrial-UE mobility, particularly at higher aerial-UE densities, and identified procedural and measurement-reporting enhancements as possible remedies.
- Mobility performance: Aerial-UE mobility was worse than terrestrial-UE mobility, especially when the number of aerial UEs was large.Higher downlink interference reduced aerial-UE SINR and contributed to more handover failures, radio link failures, and longer handover interruption time.
- Mobility performance: Aerial-UE mobility performance was better in the RMa-AV scenario than in the UMa-AV scenario.The simulations did not include the interference mitigation techniques discussed elsewhere, which were expected to improve mobility performance.
- Mobility solutions: Potential mobility enhancements included conditional handover and handover parameters using location, airborne status, and flight-path information.
- Mobility solutions: Measurement reporting could be enhanced through new events and modified triggering conditions.
B. Flight path Signaling
Release-15 specified flight-path reporting so networks could request available route information from aerial UEs. The broader work also addressed regulatory identification and concluded that LTE connectivity for low-altitude drones is feasible despite challenges.
- Flight path signaling: The network can request flight-path information, and the aerial UE reports it when available.The UE may first indicate whether flight-path information is available to limit unnecessary signaling.
- Flight path signaling: Reported flight plans contain waypoints and corresponding timestamps.The network can use this information to estimate how many drones will be served in an area and plan resources.
- Aerial UE identification: Aerial UEs may require network identification because of country-specific regulations and possible drone-specific services or charging.
- Aerial UE identification: Identification can combine subscription-based authorization with device radio-capability signaling for aerial functions.The MME can signal authorization information to the eNodeB, while the device can signal support for aerial-related functions.
- Conclusion: 3GPP concluded that existing LTE networks can provide connectivity to low-altitude drones despite identified challenges.Release-15 features targeted interference detection, uplink interference mitigation, mobility performance, and aerial-UE identification.