The purpose of this paper is to address the extent in which 4G LTE can be used for air traffic management of small Unmanned Air Vehicles (sUAVs) and the limitations and enhancements that may be necessary. We provide a brief overview of the communications aspects of the Unmanned Aerial System (UAS) Traffic Management Project followed by the evolving trends in air traffic management including beyond visual line of sight (BVLOS) operations concepts and current BVLOS operational systems. Issues and Concerns are addressed including the rapidly evolving global regulations and the resulting communications requirements as well LTE downlink and uplink interference at altitude and how that interference affects command and control reliability as well as application data capabilities and mobility performance

Downey, Alan N.

Ivancic, William D.

Kerczewski, Robert J.

Matheou, Konstantin

Murawski, Robert W.

NASA Technical Reports Server

National Aeronautics and Space Administration
William D. Ivancic, Syzygy Engineering LLC
Robert J Kerczewski
Robert W. Murawski, MTI Systems Incorporated
Konstantin Matheou, Zin Technologies Incorporated
Alan N Downey, NASA Glenn Research Center
https://ntrs.nasa.gov/search.jsp?R=20190025254 2019-08-31T13:40:56+00:00Z
2• Unmanned Aerial System (UAS) Traffic Management (UTM)
– UAVs with a total weight including cargo of under 55 pounds 
– Flying under 400 feet 
– Speeds of up to 100 mph
– Flying in uncontrolled airspace.  
UTM
UAM
UAM up to 18,000 feet
Amazon's Airspace Design for Small Drone Operations with Floating Warehouses
Floating 
Warehouses
3• Can 4G LTE can be used for air traffic management of small 
Unmanned Air Vehicles (sUAVs) as well as support user 
applications.
– Applications include: mapping, surveying, newsgathering, 
surveillance, agricultural and marketing (videos and imagery) 
and package delivery
• If so:
– What are the limitations?
– What enhancements may be necessary? 
– What are the major issues and concerns?
4• Most international government agencies responsible for Airspace 
Safety want to know if current and future cellular 
telecommunications systems and in particular 4th Generation, 
Long Term Evolution (4G LTE ) can be used to provide 
communications necessary to ensure that sUAVs can fly safely 
beyond visual line of sight (BVLOS)
• Can LTE be used to provide the uplink applications 
communication needs and the uplink/downlink command and 
control  communications necessary to ensure sUAVs can fly 
safely beyond visual line of sight (BVLOS)?
o Command and control implies reliability
o Uplink applications stress the LTE network
– Examples: High-definition video or high-quality imagery
5• User Equipment 
– Cell phone, smart phone, UAV with 
2G, 3G, LTE, 5G radio and associated 
protocols
o Radio Access, Network Registration, 
Handovers, Service Level 
Agreements (e.g. data rates and 
data quantity), Billing
• Downlink: the unidirectional radio 
link for the transmission of signals 
from a UTRAN (base station) access 
point to a UE (User Equipment - e.g. 
cell phone). In general the direction 
from Network to UE.
• Uplink:  the unidirectional radio link 
for the transmission of signals from 
User Equipment (UE) to a base 
station
Uplink
Downlink
UE
eNodeB
or
eNB
Uplink
Downlink
6• Pre 1956 – managed aircraft into and out of airports, else see and avoid
• Current ATM – Active Control system with air traffic controllers 
communicating directly with pilots
• Corridors (highways in the sky) 
– Pro: one of the ways to help ensure separation of aircraft. 
– Con:  can increase flight distance (fuel) and time and reduces scheduling 
flexibility, scaling issues
• Free Flight – Pilots could fly paths that are more direct to their 
destinations rather than flying in corridors.
– Yet to be instituted. 
o Security concerns
o Flexible, but difficult scheduling problems – particularly coexisting with current 
practices
• Control-by-Exception
– pilots and aircraft with the use of new situational awareness technologies 
can generally maintain self-separation
– Air traffic controller only needs to be involved when situations become 
abnormal
7• Operating principles for small UAS
– Only authenticated UAS and operators are allowed to operate in the 
airspace 
– UAS stay clear of each other (clear is a vague term)
– UAS and manned aviation stay clear of each other 
– UAS operators or support systems have awareness of all constraints in 
the airspace and of people, animals and structures on the ground and 
UAS will stay clear of them 
– Public safety UAS (e.g. police, first responders, government agencies, 
and military) should be given priority over other UAS and manned 
aviation. 
• Solution must address Scalability Issues.  
– Currently about 5,000 commercial flights in the air over the US at any 
one time
– Forecast 250,000 to 1.6 million commercial drones in the US by 2021
• Free flight for most operations with Geo-fencing
• Corridors expected to be used for high density areas
8
Source: Dr. Marcus Johnson
Full Autonomy
(Control-by-Exception)BVLOSVLOS
2019
Supported by the current
UTM Architecture
9
10
• Current architecture is 
sufficient for operator in the 
loop
• Architecture needs to evolve 
to accommodate autonomous 
operations such as package 
delivery
• Package delivery may be the 
largest application for sUAVs
• Amazon®, FedEx®, UPS, 
Wal-Mart, Domino’s Pizza and 
others are expected to use 
drones for package delivery Amazon Patent Multi-Level Fulfillment 
Center for Unmanned 
Aerial Vehicles
11
• Terrestrial motor vehicle transportation system 
– Corridors (roads … sometimes one-way)
– Coarse regulators (e.g. traffic lights, traffic cops)
– Speed limits  
– The vehicle operating system. 
o Breaks, steering, accelerator
o Sense and avoid system (currently human)
– Blind spot sensors
– Lane changing
– Dynamic Cruse Control
– Driver assist / autonomous parking
– Automatic breaking
• Self-driving cars
o Vehicle-to-vehicle communications 
– Secure and fast (enabled with 5G cellular technology)
o Coordination between groups of vehicles (i.e. eliminate traffic lights or 
make them dynamically controlled)
o Situational awareness only has to be local (Scalability)
– Applicable for sUAVs in UTM airspace, at least for localized traffic below 
200 feet and traveling at relatively low speeds.
12
• Command and Control Requirements are currently undefined … 
at least by FAA or ICAO.  Why?
– Different operation scenarios and traffic management systems 
requires different types and amounts of communication.
– The traditional centralized control system requires communication 
between the aircraft and the controller.
– Autonomous systems may only require communication between 
localized vehicles
• Commanding and control (C2) is expected to put minimum strain 
on the system
– Note: Latency requirements may be quite different depending on 
whether one is commanding to maneuver a UAV vs. commanding 
to update waypoints.
• Applications can place a heavy load on the LTE uplink portion of 
the network – particularly high-quality video.
13
• Current regulations for most major countries can be found on the ICAO UAS Toolkit 
website 
• A 2017 report by the RAND Corporation entitled “International Commercial Drone 
Regulation and Drone Delivery Services” provides the latest BVLOS regulations of 
most major countries
– Most of the information in this report is from 2016 to 2017.  
– Laws are constantly being reevaluated;
– Generally, drone laws are moving toward a more-permissive approach to regulation.
• Most countries have yet to approve BVLOS operations except via waivers
• As of 2017, the only countries that have enacted relatively unrestricted legislation on 
commercial drone are: Costa Rica, Iceland, Italy, Norway, Sweden and the United 
Arab Emirates
• As of October 2018, many, if not most, current operational systems are in counties 
that have rural access needs. 
– 4G LTE infrastructure is not often available if at all.  Here, the use of satellite 
communications is likely necessary. 
• Global UAV has been operating mainly in Europe for over 2 years BVLOS exclusively 
using LTE.
– Mostly of the users are Government and law enforcement as government agencies are 
cleared for such operations
– Uplink rates are advertised at 8Mbps, but with video compression, only 1.5 to 2.0 Mbps is 
required.
14
• 3GPP – 3rd Generation Partnership Project 
• Objectives:
– verify the level of performance (need requirements), 
– identify supportable heights, speed, and densities of aerial 
vehicles, 
– investigate and develop air-to-ground channel models, and 
– study performance enhancing solutions for interference 
mitigation, interference detection, identification, handover, 
and positioning.   
• Requirements (developed by 3GPP by necessity)
• Field trials and simulations by at least 7 different entities 
(Qualcomm, Nokia, Ericsson, ZTE, Huawei, NTT, Docomo)
15
Requirement developed by iterative consensus process among team members
Items Value
Data type
1. C&C: This includes telemetry, waypoint update for autonomous UAV operation, real 
time piloting, identity, flight authorization, navigation database update, etc.
2. Application Data: This includes video (streaming), images, other sensors data, etc.
Latency (NOTE) 1. C&C: 50ms (one way from eNB to UAV) 2. Application data: similar to LTE UE (terrestrial user)
DL/UL data rate 1. C&C: 60-100 kbps for UL/DL2. Application data: up to 50 Mbps for UL  (High Definition Video)
C&C Reliability Up to 10-3 Packet Error Loss Rate (99.9% reliability)
Performance metrics for HO and RLF simulations
KPI Unit Description
Handover rate HO/UE/sec Number of HO attempts over time (including HOF)
HOF rate % Number of HO failures/Total number of HO attempts 
(including HOF)
Radio Link Failure (RLF) rate RLF/UE/sec Number of RLFs over time
Time in handoff % Fraction of time a UE is in HO procedure including time for 
successful HO (HO execution delay) and HOF 
(reestablishment delay)
Time in Qout % Fraction of time a UE is in Qout state
Ping pong rate
(NOTE)
% Number of ping-pongs/Total number of successful 
handovers (excluding handover failures)
16
• At elevation, the RF signals experience free-space propagation 
• Inter-cell interference increases significantly at higher altitudes leading 
to a decreased Signal to Interference Noise Ratio (SINR) at the airborne 
receiver.  
• Reference Signal Receive Powers (RSRPs) for Aerial User Equipments 
(AUEs) are higher than the RSRPs for Terrestrial User Equipments 
(TUEs) 
• Reference Signal Received Qualities (RSRQs), at the higher altitudes are 
lower than the RSRQs at ground level. 
• SINR, RSRP and RSRQ are used to determine handovers and thus affect 
handover operation.
• UE at altitude produces more uplink interference in the network than 
ground UEs (3X in 700 MHz band)
– Results in poor resource utilization.  
– The presence of UAVs has a negative impact on the UL performance of 
the TUEs
17
Urban Macro (Uma)
Source: Naha Kumar
300 M Cell 
Association 
Pattern
Source:
Ericsson
0 M Cell 
Association 
Pattern
Source:
Ericsson
18
Source: IEEE Twenty First National Conference on Communications (NCC), 2015 (DOI: 10.1109/NCC.2015.7084910)
Which Neighbor?
19
Scenario Requires:
Local Radio Control while on the ground
4G LTE  BVLOS in the air
Pilot has LTE connectivity 
20
• Improved interference detection
• Uplink interference mitigation
– Improves utilization of system resources
– Enables increased uplink data rates
• Downlink interference mitigation
– Expected to improve mobility management
• Mobility performance improvement and
– Possible new or different algorithms for AUEs and eNodeBs
• Aerial UE identification
– Enables AUEs to be place in restricted channels thereby mitigating 
interference with TUEs
– Can trigger different mobility algorithms and power control 
algorithms
– Improves security (rouge AUEs can have service terminated)
21
• Optimized Open Loop Power Control (OLPC) is proposed by Qualcomm  
(Note, Qualcomm is a EU manufacturer)
• Extremely attractive solution for interference mitigation, as it does not 
require any changes to eNodeB
• Can be employed by all UEs (terrestrial UEs and aerial UEs – AUEs) 
without differentiation of airborne and ground UEs.
• Issues related to field trials:
– Can one change the power control in current commercial UEs (this may 
require cooperation by UE manufacturers; 
– Does one have to develop a software defined radio (SDR) that operates 
in real-time in order to manipulate the power control algorithms in the 
UE?
– Scalability is hard. It is significantly more challenging to make systems 
work on a global scale than it is for small-scale deployment. 
– How can this be flight tested, as a single AUE or a few AUEs may not 
provide sufficient loading to demonstrate much of anything?  
22
• Nokia  simulations indicated that  pattern diversity (a.k.a. angular 
diversity using 4 or 6 fixed sectorized beams, for both rural and urban 
areas, the achieved reliability is higher than the target 99.9%.
• The amount of interference received in the downlink is limited to the 
beam width of the beam, leading to a reduced overall outage.
• The receiver simply picks the beam direction with the best signal 
quality (RSRP or RSRQ) without adjusting the orientation of the drone
• In the uplink, the antenna provides a gain for the drone and limits the 
interference impact on terrestrial users
Multi-sector fixed beam 
Antennas concept
Environment Terrestrial UE Drone at 
120m
Rural +20% +35%
Urban +51% +56%
Average uplink throughput gains with a grid 
of fixed 6 beams in medium - high load 
traffic conditions
23
• Developing a 6 sector antenna system and integrating that with a UE requires 
coordination of the UE with the antenna control system 
• Determining which sector to activate is a complex problem
– One possible algorithm is for the system to turn all sectors on for scanning to 
determine handover criteria
o Some analytical method must be used to determine from which sector the best 
neighbor is being received.
– Another possibility is to quickly activate each sector to ‘scan’ for towers and 
then turn the selected beam on for UL transmissions
• The gains provided by Pattern Diversity can be field tested by just using one 
directional antenna and performing network measurements to validate the 
simulation results.
– A field test was under development by NASA GRC, but not completed due to 
reprogramming of funding in February 2019.  The antenna (single sector) is 
tested and could be made available to interested parties.
– The critical measurements are the Interference-over-Thermal (IoT) at all 
eNodeBs in view.
o These measurements are needed to determine the overall interference 
generated by the UL transmission and the interference mitigation effects 
provided by the single sector directional antennas.
24
• A limited amount of field testing has been done regarding drones at elevation
– Results have been mixed and reasons unclear
o Qualcomm field trials in an Urban Macro area showed “Handover performance (success rate 
of handovers, and lower frequency of handover events) is superior for airborne UEs than for 
ground Users
o Test by KDDI in Japan over a 100m square route in an Urban Macro area showed that above 
altitude 50m, some handover failures occurred. This is likely because of the interference from 
many neighbor cells.  There were no failures at ground level.
• Six different 3GPP companies performed a number of simulations to evaluate 
handover issues with mixed results
– The results for Source 1, source 2 and source 3 for mobility rates of 3km/h and 
30km/h, the handover rate of TUEs was higher than that of AUEs.
– Source 4, source 5, source 6 and source 3 at mobility rates of 60km/h and 160km/h, 
the handover rate of TUEs was lower than that of AUEs
– The majority of the companies observed higher HOF and RLF rates for aerial UE than 
that for terrestrial UE in most cases. 
o The higher the speed or the height of aerial UE, the higher HOF and RLF rate was 
observed.
– The Majority of the companies observed higher RLF rate for AUE in most cases.
o The higher the speed of AUE, the higher the RLF rate.
– In general, a better mobility performance was observed in rural area networks 
compared to urban area networks.  This is likely due to a limited number of eNodeBs 
and therefore less interference.
25
• 4G LTE can be used to command and control drones and for drone 
applications, but it depends on where the drone is operating and 
what the application is.
• 4G LTE is not ubiquitous. One cannot use LTE where it is not 
available.  Other services such a satellite may be required.
• It is highly unlikely that a drone operating in a UMa area would be 
able to obtain significantly more coverage area by using two 
service suppliers
– In high service areas, such as Urban Macro (UMa) areas, different 
service providers tend to cover the same areas and also tend to NOT 
cover the same areas.
• In highly rural areas with low population densities, there may be 
only one service provider
• Better solutions are needed to address the mobility issues of the 
AUEs and that handover algorithm can be further optimized to 
better support AUE mobility performance.
26
• Studies indicate that for today’s 4G LTE networks in Urban Macro 
setting where LTE is ubiquitous, 3GPP  performance criteria can be 
achieved when only a few AUEs are operating.  However, as the 
number of AUEs increases, without some modifications and 
optimizations of the LTE network or drone antenna technology (for 
interference mitigation), these goals may not be achievable.
• Servicing AUEs using the existing 4G LTE network and associate drone 
antenna technology has significant impact on the terrestrial users.
– Providing efficient and effective connectivity to the aerial UEs while 
minimizing the impact on terrestrial devices requires a rethinking of 
many of the assumptions, models, and techniques used to date for 
cellular systems.
• Applications and operations have to be designed to work within the 
limitations of the network.
• One certainty in drone operations is that, for some drone 
somewhere, the radio link will fail. Sufficient mechanisms must be in 
place to ensure safe operations when radio link failure occurs.


Flying Drones Beyond Visual Line of Sight Using 4G LTE: Issues and Concerns

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