Cellular for the skies: exploiting mobile network infrastructure for low altitude air-to-ground communications

TLDR: In this paper, the authors presented an overview of UAVs for civil applications focusing on the communication component and discussed the potential of mobile networks with their fully deployed infrastructures, wide radio coverage, high throughput, reduced latencies, and large availability of radio modems.

Abstract: In this article we presented an overview of UASs for civil applications focusing on the communication component. We identified several available communication technologies for UAVs, their constraints, and also protocols available for implementing the remote operation of the vehicles. As an attractive solution for the A2G communication link for UAVs, we discussed the potential of mobile networks with their fully deployed infrastructures, wide radio coverage, high throughputs, reduced latencies, and large availability of radio modems. We described how a UAS can be implemented in a flexible and modular approach that allows it to rely on one or several wireless (UAVs and GCSs) and wired (GCSs) technologies. Despite the advantages of a system based on cellular and IP networks, there are problems that must be dealt with, namely, possible loss of radio coverage, presence of NAT, delay, jitter, and packet loss. Following the proposed architecture, we implemented an UAS and conducted some flight tests, which showed that the operation of the vehicles in semi-automatic or fully-automatic modes is feasible. It is expected that future enhancements for 4G networks and evolution to 5G will benefit UAV communications even further with lower latencies, higher throughput, and higher reliability.

Figures

Fig. 3. NAT as an interface between a private network and a public network.

Fig. 4. Flexible UAS architecture with support for cellular radio and other access technologies, multiple operators and multiple UAVs.

Fig. 1. Received power measurements in the 1800 MHz band (a) obtained using weather balloons (b).

Table 2. Performance measurements obtained during the flight tests.

Fig. 2. Profile of the average received power (a) as a function of position along an horizontal line and (b) as a function of UAV altitude.

Fig. 5 Mission plan in 2D (a) and in 3D (b), with the UAV prototype used for the flight tests.

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:
2019-04-12
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Peer-reviewed
Citation for published item:
Afonso, L., Souto, N., Sebastião, P., Ribeiro, M., Tavares, T. & Marinheiro, R. (2016). Cellular for the
skies: exploiting mobile network infrastructure for low altitude air-to-ground communications. IEEE
Aerospace and Electronic Systems Magazine. 31 (8), 4-11
Further information on publisher's website:
10.1109/MAES.2016.150170
Publisher's copyright statement:
This is the peer reviewed version of the following article: Afonso, L., Souto, N., Sebastião, P., Ribeiro,
M., Tavares, T. & Marinheiro, R. (2016). Cellular for the skies: exploiting mobile network
infrastructure for low altitude air-to-ground communications. IEEE Aerospace and Electronic Systems
Magazine. 31 (8), 4-11, which has been published in final form at
https://dx.doi.org/10.1109/MAES.2016.150170. This article may be used for non-commercial
purposes in accordance with the Publisher's Terms and Conditions for self-archiving.
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L. Afonso, N. Souto, P. Sebastião, M. Ribeiro, T. Tavares and R. Marinheiro are with the ISCTE-
University Institute of Lisbon, and with the Instituto de Telecomunicações, Portugal (
luis_afonso@iscte-
iul.pt, nuno.souto@lx.it.pt, pedro.sebastiao@iscte.pt, marco.ribeiro@iscte.pt,
tiagofltavares@gmail.com, rui.marinheiro@iscte.pt
).
Cellular for the Skies: Exploiting Mobile
Network Infrastructure for Low Altitude Air-
to-Ground Communications
Luís Afonso, Nuno Souto, Pedro Sebastião, Marco Ribeiro, Tiago Tavares, Rui Marinheiro
ABSTRACT
In the last few years, unmanned aerial vehicles (UAVs) have been standing out as a pervasive tool
in numerous civil and commercial applications. Although different wireless technologies can be
employed to establish communications between a UAV and a ground control station (GCS), most
either limit the operational radius or increase substantially the complexity of the system. Taking
into account insights from our own real-world experiments and studies carried out within the
scope of the SAAS project from Instituto de Telecomunicações, in this article we discuss the use
of mobile networks for low altitude air-to-ground (A2G) communications as a possible solution to
provide extended mobility and range to operators and UAVs. Besides addressing the advantages
and the associated constraints of using these networks, we propose a flexible architecture for
multiple UAVs and GCSs. Although our experimental results have shown that current mobile
networks can fulfil the basic requirements for many envisioned UAV applications, we discuss
how the evolution towards 5G networks is expected to improve the support for reliable real-time
A2G communications and even for air-to-air communications.

I. INTRODUCTION
Although the concept was born within military use, in recent years we have witnessed an
impressive development of unmanned aerial vehicles (UAVs) for civil and academic applications.
Driving this growth is the myriad of possible scenarios where this technology can be deployed,
such as: fire detection, search and rescue operations, surveillance, police operations, building and
engineering inspections, aerial photography and video for post-disaster assessment, agricultural
monitoring, remote detection (radiation, chemical, electromagnetic), weather services, UAV
photogrammetry, airborne relay networks, and more [1]. Undoubtedly, the increased use of
UAVs has been sustained through the research and development of multiple low cost solutions
for the control of aerial vehicles, the evolution in microelectronics with multiple off-the-shelf
components and sensors and also through a growing global developers community with several
UAV related open source projects.
The safe operation of a UAV requires a communication link to deliver telemetry data, control
commands and other information between the vehicle and a ground control station (GCS).
Different solutions have been studied and implemented, as discussed in [2], but most suffer from
either a restricted operational range or a high implementation complexity. A potential alternative
to overcome the range limitation with reduced complexity, which has not been adequately studied
so far is to use existing wide coverage mobile radio infrastructures, such as GPRS/EDGE, UMTS,
HSPA+, LTE and LTE-A. In this article, we discuss the viability of this approach. Based on our
own experimental studies, performed within the scope of the Portuguese Remotely Piloted Semi-
Autonomous Aerial Surveillance System Using Terrestrial Wireless Networks (SAAS) research
project, we will focus on the advantages of using cellular networks as well as on the problems
that can arise and possible solutions to deal with them. We present a flexible architecture for a
multi-UAV, multi-operator system which can make use of third and fourth generation (3G/4G)
wireless networks and describe some results obtained from experimental tests using our

unmanned aerial system (UAS) implementation. Finally we comment on potential improvements
that can be expected from future cellular networks in this context.
II. UNMANNED AERIAL SYSTEMS
In general, a UAS consists of the vehicle, known as UAV, and the mechanisms, logistic and
equipment regarding its proper operation. UAVs can be grouped into different categories
according to their size [3], with the micro aerial vehicle (MAV), which typically weighs less than
2 kg and operates at low altitudes, being the most appellative for civil use. Several companies and
academic groups have developed proprietary and open source hardware/firmware/software for
UASs. The hardware includes the vehicle, flight avionics (which includes the flight controller)
and the wireless communication subsystem. The firmware running on the flight controller is
responsible for the stabilization of the vehicle, geolocation and preprogramed waypoint
navigation. Additional software is often provided for implementation of GCS functions namely,
mission planning, monitoring and control. A ground operator can use the GCS application to
communicate with the flight controller platform using a bidirectional link.
The most popular open source flight controller is the Ardupilot Mega (APM)
1
, although several
other well-known platforms exist, such as Pixhawk
2
, OpenPilot
3
and Paparazzi
4
. A detailed
comparison of different open source projects is provided in [4]. While most of these flight
controllers are intended for multi-rotors, some also support other airframe configurations such as
fixed-wing, helicopters or even ground rovers.
III. UAV COMMUNICATION ASPECTS
In order to control and monitor UAVs, telemetry and command links are mandatory as they
provide crucial information for the ground operators. Additionally, the transmission of video may
1
Ardupilot Mega, Multiplatform Autopilot. [Online]. http://ardupilot.com/
2
Pixhawk Autopilot. [Online]. https://pixhawk.org
3
Openpilot. [Online]. http://www.openpilot.org/
4
Paparazzi Project. [Online]. http://wiki.paparazziuav.org/

also be required either due to the specific application where the vehicle is being used or as an aid
for its proper operation by providing the operator with an exocentric view of the environment. In
this section we address several relevant aspects regarding air-to-ground (A2G) communications
with UAVs, supported over mobile networks.
A. Wireless Communication Technologies
In military applications, a beyond line of sight (BLOS) connection between the GCS and the
vehicle is often provided through satellite links. However, these links are very expensive, have
high latencies and the hardware is too heavy and complex for most civil applications. For this
reason, systems for the control and monitoring of UAVs in a civil context often rely on radio-
frequency (RF) links, with conventional radio remote control (RC) being the most common
technology employed. These radio systems generally work at the RC reserved frequency bands or
at the industrial, scientific and medical (ISM) 2.4 GHz band, with most flight operations being
accomplished in line of sight (LOS). Due to the different flight controller platforms available,
some open source RC projects (hardware and software) have emerged to support remote vehicle
control with multiple customizable features and telemetry transmission, such as OpenTx
5
and
OpenLRS
6
. The video transmission usually requires separate hardware, which consists of a
dedicated transmitter working at a higher frequency, often at 5.8 GHz.
Besides RC radios, ad-hoc connections are becoming common as they can be easily implemented
and, in some cases, directly support video transmission. The most popular radio system
implementations are based on the IEEE 802.11a/b/g/n [5][6], IEEE 802.15.4 [4] and Bluetooth
standards [7]. Despite the implementation simplicity, these radio technologies were not developed
aiming the aerial environment, their operational range is limited by the transmitter power and the
5
Welcome to OpenTX. [Online]. http://www.open-tx.org/
6
Openlrs - Opensource RC System. [Online]. https://code.google.com/p/openlrs/

Frequently asked questions

Q1. What have the authors contributed in "Cellular for the skies: exploiting mobile network infrastructure for low altitude air- to-ground communications" ?

Taking into account insights from their own real-world experiments and studies carried out within the scope of the SAAS project from Instituto de Telecomunicações, in this article the authors discuss the use of mobile networks for low altitude air-to-ground ( A2G ) communications as a possible solution to provide extended mobility and range to operators and UAVs. Besides addressing the advantages and the associated constraints of using these networks, the authors propose a flexible architecture for multiple UAVs and GCSs. Although their experimental results have shown that current mobile networks can fulfil the basic requirements for many envisioned UAV applications, the authors discuss how the evolution towards 5G networks is expected to improve the support for reliable real-time A2G communications and even for air-to-air communications. 

Q2. What are the future works in "Cellular for the skies: exploiting mobile network infrastructure for low altitude air- to-ground communications" ?

It is expected that future enhancements for 4G networks and evolution to 5G will benefit UAV communications even further with lower latencies, higher throughput and higher reliability. 

Q3. What are the main issues that affect the operation of a UAV?

D. Fail-safe MechanismsWhen exploiting mobile radio infrastructure and a packet-switched network such as the Internet, there are performance issues that are relevant for real time communications, namely latency, packet loss and jitter, which can impact the UAVs’ operation and make manual flight potentially unsafe. 

Q4. What is the advantage of using existing mobile radio infrastructure?

The use of existing mobile radio infrastructure can also enable the implementation of a system with multiple operators and multiple UAVs and simplifies the introduction of redundancy into the communication link, through the use of two or more mobile frequencies. 

Q5. What is the bitrate required to receive video with acceptable quality?

The bitrate required to receive video with acceptable quality may typically lay between 200 kbps and 800 kbps, depending on the codec used, frames per second (fps) and video bitrate. 

Q6. What are the main modules of the system?

The system comprises the following three main modules: UAV Register & UAV Relay are applications located in a ground located server. 

Q7. Why do some open source RC projects exist?

Due to the different flight controller platforms available, some open source RC projects (hardware and software) have emerged to support remote vehicle control with multiple customizable features and telemetry transmission, such as OpenTx5 and OpenLRS6. 

Q8. What was the msg_ping message used to measure?

In their implementation the authors adopted the APM as the UAV flight controller and part of the experiment consisted in using the MAVlink msg_ping message to measure the round-trip time (RTT) at the Application Layer. 

Q9. What is the effect of the antenna pattern on the power of the UAV?

Besides the expected decrease of received power with increasing distance, an abrupt drop is visible above the BS which can be explained by the antenna pattern and respective downtilt. 

Q10. What are the requirements for the control and monitoring of UAVs?

In order to control and monitor UAVs, telemetry and command links are mandatory as they provide crucial information for the ground operators. 

Q11. What is the common way to connect a drone to a ground vehicle?

In military applications, a beyond line of sight (BLOS) connection between the GCS and the vehicle is often provided through satellite links. 

Q12. What are the main problems that must be dealt with with a cellular network?

Despite the advantages of a system based on cellular and IP networks, there are problems that must be dealt with, namely, possible loss of radio coverage, presence of NAT, delay, jitter and packet loss. 

Q13. What type of NAT should be used to deal with network delays?

in order to deal with network delays and possible low reliability of the connection, the operation of UAVs under these networks should be based on autonomous and semi-autonomous flight modes. 

Q14. What is the common technology used for the control and monitoring of UAVs in a?

For this reason, systems for the control and monitoring of UAVs in a civil context often rely on radiofrequency (RF) links, with conventional radio remote control (RC) being the most common technology employed. 

Q15. What are the different protocols available for the communication between MAVs and GCSs?

There are various protocols available for the communication between MAVs and GCSs, e.g., UAVTalk, used by the OpenPilot platform, Micro Air Vehicle Communication Protocol (MAVLink10), adopted by the popular APM, and Ivy, implemented in the Paparazzi platform.