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Proceedings
Challenges in Establishing Free Space Optical Communications Between Flying Vehicles S. Sheikh Muhammad*, T. Plank**, E. Leitgeb**, A. Friedl**, K. Zettl**, T. Javornik***, N. Schmitt**** *
National University of Computer and Emerging Sciences, Lahore, Pakistan (FAST-NU) of Broadband Communications, Graz University of Technology, Graz, Austria *** Department of Communication Systems, Jozef Stefan Institute, Ljubljana, Slovenia ****EADS Innovation Works, Munich, Germany
** Institute
[email protected] Abstract — Establishing free space optical links between flying vehicles is a technically tough challenge and needs enhancements both in the technology and its features. For unmanned aerial vehicle (UAV) swarms to evolve to effective usage in civil and military applications, a means of high data rate communications need be established between them. The free space optical links due to their huge bandwidth and license free spectrum provide a viable communications solution between the UAV swarm. The swarm UAV scenario due to the continuous motion and changing relative speeds of all its members provide extremely challenging conditions to maintain a line-of-sight free space optical link and this paper reviews the existing related scientific developments and talks about the challenges of utilizing FSO in the swarm UAV environment. Further, this survey paper discusses the utility and challenges of developing free space optical links between HAPs. Keywords — Free Space Optics (FSO), Aerial Vehicles (AV), Tracking, Positioning, UAVs network architectures, High Altitude Platforms (HAPs)
I. INTRODUCTION Free Space Optical (FSO) communication links offer a possibility of enhanced high data rate communication between flying vehicles due to their abundance of bandwidth, confidentiality and absence of regulatory restrictions as compared to microwave links. FSO technology was originally developed as a method to bridge the “last mile” gap between the optical backbone and customer premises that exists in many of today’s communications networks; however, it has now found applications in many diverse scenarios as temporary high bandwidth communication links. As long as there is a clear line of sight between the source and the destination and sufficient transmitter power is available, two-way communication remains possible. Frequency coordination like in RF-based systems is not necessary with the FSO technique. Interference to other RF systems is not a concern, and the point-to-point laser or LED signal is nearly impossible to intercept, therefore the technique is inherently secure. Regarding aerial vehicles (AVs), the FSO communication link has a clear advantages compared to RF communication links due to their large bandwidth, good electromagnetic compatibility, minimum “electromagnetic pollution,” use of the license-free band
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and reliable protection against wiretapping [1, 2]. The carrier media being the (visible) light, it uses modulated laser beams or light-emitting diode beams to transfer information. One great disadvantage is that the reliability is mainly determined by local weather conditions. Link loss is affected by all parameters between receiver and transmitter. It may include (atmospheric) turbulence, fog, smoke, rain and dust. The weather adversely effect the links in the troposphere that are up-to an altitude of about 11 km. Vehicles flying at higher altitudes are not effected by weather, so only turbulences forms the major challenge. For the downlink to the ground station, weather influences are often severe. Turbulence occurs in the atmosphere due to different air pockets heating more than surrounding air pockets. The net result of this temperature differential is a change in index of refraction, which affects the path along which the light propagates through the atmosphere. Atmospheric turbulence causes three different effects on a laser beam. The first is beam wander, in which the beam is deflected as it passes through the turbulent atmosphere. Turbulence further causes variations in the phase of the laser beam, also termed as scintillation. Scintillation produces fluctuations in the laser beam power, which may result in a loss of signal power or receiver saturation. Finally, turbulence causes the beam to spread more rapidly than Gaussian beam divergence calculations predict [3]. A very important aspect in maintaining high performance in the communication channel is the rapid relative movement between aerial vehicles and their high speed over ground. Such high mobility leads to a much shorter coherence time and a larger Doppler spread in the multi-path fading channel. As a direct result, the orthogonality among sub carriers is lost and Inter-CarrierInterference (ICI) degrades the performance [4]. Simulation results although show a tolerating impact of the performance degradation due to huge Doppler spread and ICI, communication scenarios by using FSO inbetween swarm UAVs have not yet been fully investigated. This survey article reviews the developments related to FSO communications among flying vehicles. Section-II of the article discusses the major challenge in acquisition, tracking and positioning of the FSO beams in the flying vehicles scenario where they are in relative motion w.r.t. each other. Section-III talks about FSO communications between the more specific unmanned aerial vehicles
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(UAVs) and the networking possibilities therein. This is followed by a section on simulation tests and a section about FSO links between High Altitude platforms (HAPs). Finally, the possibilities and the need for further work are emphasized in the conclusions. II. TRACKING AND POSITIONING The relative high velocities between AVs and the ground station necessitate tracking and positioning systems, thus ATP (Acquisition, Tracking and Positioning) subsystems are an essential part of any communication links between the flying vehicles. To establish a FSO link to a moving object is a challenging assignment. If it turns out that the moving object is a flying vehicle, the challenge will get even more complicated due to the six degrees of freedom (owing to their pitch, roll and yaw). These six degrees of freedom constraint is further a combination of rotation and translation issues. Firstly, the rotation problems need to be solved in terms of knowing, calculating and predicting the pitch, roll and yaw of the individual flying vehicle in order to keep the FSO terminal stable and secondly, it needs to be translated into appropriate co-ordinates for aligning the FSO units. To keep the communication link active, these parameters need to be constantly monitored and processed. One point, which has to be decided, is whether to use an autonomous stand-alone system that is very independent of other techniques than optics or to use a system operating with RF techniques like GPS to get the accurate position information of the vehicle. By choosing the RF technique, it has to be considered that all the advantages regarding the optical system and especially the issues for non-detectable FSO transmissions will be lost. On the other hand, to develop an autonomous stand-alone system without holding any information of the opposite FSO system, is a major challenge in itself and there is currently no reliable system available with this capability. In the rather harsh scenario of UAV swarms, the acquisition, tracking and pointing of FSO links provide a huge challenge. Since, the UAV’s own stability depends on its size and weight, maintaining a line-of-sight FSO link between flying UAVs that might experience a relative motion among themselves is a tough technical requirement that can limit the practical implementation of the technology. The UAV needs to exchange precise information about their position before take off and during the flight, which can be done with a complementary RF link or a GPS terminal. The ATP subsystem developed by JPL [5] for the UAV-to-ground link is highly sophisticated and comprises of six assemblies responsible for acquiring and tracking the beacon laser from the ground terminal receiver and accurately pointing the downlink transmit laser to the receiver. Scanning mirrors have been proposed to steer laser beams in free space optical links between unmanned aerial vehicles [6]. Oneaxis and two-axis scanning mirrors have been fabricated in a wafer bonding process that would enhance the security and reliability of the FSO links between the swarm UAVs. A mechanical gimbal has been proposed for aligning and tracking a ground-to-UAV FSO communication link [7]. Recently, a new method for minimizing link acquisition time for a wavelength diversified FSO link between a ground station and a mobile aerial platform has been proposed [8]. However, such sophisticated solutions would exponentially raise the
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cost of the systems, and more economical ways like the GPS based tracking and automatic telescope tracking might be more feasible. An appropriate scheme can be to utilize an inertial navigation system (INS) and a global positioning system (GPS) to provide the position, velocity, orientation and angular velocity of the UAV by measuring the linear and angular accelerations applied to the system in an inertial reference frame [9]. In addition, having a larger beam divergence would ease the requirements for tracking. If in the swarm formation, the inter-UAV distance remains within a kilometre, the ATP requirements reduce significantly. Some of the tracking systems, particularly those tracking systems working with cameras, also have a stabilization system for an accurate performance integrated. For tracking AVs with a camera or even by using RF; a fast steering and moving gimbal is necessary to follow the flying AV. Moreover, by applying a camera on an AV a stabilization of at least the camera is necessary. III. UAVS Unmanned aerial vehicles - a subgroup of the topic aerial vehicles - are a possible future application for both civil and military use. Flying in swarm formations and carrying a huge variety of sensors and monitoring tools, they can be used for covering landscapes, exploring nuclear polluted areas, observing traffic conditions on the roads and many other fields of application. For interaction and data exchange between the single UAVs, FSO is the appropriate technique. With current systems, 2.5 Gbits per second are achieved [5] in contrast to the much lower data rates supported by the microwave technology. Recently, data rates of 80 Gbps were experimentally achieved [10]. The unmanned aerial vehicles besides their huge advantage in defense missions are also gaining importance in civil applications like remote environmental research, pollution assessment and monitoring, oceanography, scientific missions, border monitoring and law enforcement and in particular, communication relays for wideband applications. We are just beginning to understand the potential impact of UAV operations on the 21st century aviation. The preferred mode of operation of the UAVs is in swarm formation and applications and services emerging out of UAV swarms need a high data rate communication infrastructure between the individual units inside the swarm. The high data rate becomes essential due to the rapid sharing needs of sensor and map information generated by a variety of sensors for monitoring and surveillance. The main scenarios for the use in military are to monitor and sense the battlefield, to find out about the military goals behind the enemy’s lines, to keep peace in critical regions, to support forces and carrying weapons or other useable things for supply. Additional civil purposes are also introduced. Applications like observing important buildings e.g. airports, pipelines (oil companies in Russia already use UAVs to protect important pipelines), bridges, or dams would be conceivable. Especially in times of high pollution because of cars and factories, air-observing systems are highly recommended. With special airplanes for delivering of meteorological data, analysis and rectifications mechanisms can become better [11].
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Sensing functions can also include electromagnetic spectrum, biological and chemical sensors. Typically the electromagnetic sensors scope contains infrared and/or near-infrared as well as visual spectrum cameras. Additionally radar systems are also in some cases installed. Other electromagnetic wave detectors such as microwave and ultraviolet spectrum sensors may also be used; they are in particular assigned for military use. FSO links from ground to an unmanned aerial vehicle in flight have already been proposed and investigated in [1, 3] and wavelength diversity scheme was implemented in FSO systems for better performance in the presence of atmospheric turbulence and fog. Free space optical communication links have been tested and utilized for a LEO-GEO scenario (SILEX experiment) [12], for High Altitude platforms (stratospheric optical payload experiment) [13] and for airborne terminals [14]. These airborne laser links directed to a GEO satellite were established over a distance of 40,000 km during two flights at altitudes of 6,000 and 10,000 meters. In the rest of this section, we explain the interesting challenges generated out of an attempt to establish FSO communications links in a flying UAV swarm. The background research for this work has been published in [2, 11, 15, 16]. A. Networks When flying at lower altitudes, the UAVs need to fly in a swarm formation (Figure 1) to cover bigger geographical territories, and the individual UAVs need to communicate to each other in order to exchange flight and mission information. For these inter-UAV communications, FSO links are the appropriate solution primarily because of their huge bandwidth. This swarm formation produces typical networking scenarios where different network configurations can be utilized to optimize and/or enhance the overall performance of the links. Possible formations are ring, star and the meshed architecture [15].
Figure 1. UAV swarm exchanging data with mother ship Figure 2. Ring architecture
When flying within a ring architecture (Figure 2), all UAVs have bidirectional links, one to the next and one to the previous vehicle. If there is a broken link between two UAVs, an intermediate link could be set up. So a partial backup for security issues is achieved. An optical repeater has then to
Proceedings
be utilized when there is no line of sight between the transmitter and the receiver. Figure 3.
Star architecture
For the case that a star architecture is chosen (Figure 3), one vehicle acts as the Optical Multipoint Unit (OMU), providing the link to the ground station. The other vehicles are permanently connected to the ship in the middle using their optical transceivers. One advantage is having shorter distances between two units, because the OMU is used as a repeater, but the disadvantage is hereby the creation of a single point of failure, which must also handle a high number of links through a multiplexing technique. To improve the reliability, a redundant multipoint unit has to be established. Figure 4.
Meshed architecture
The best network architecture for high reliability is the meshed one (Figure 4), combining the advantages of star and ring architecture, but for the price of increased costs as every single UAV has multiple links established. Different types of connections are possible, so the information flow from one UAV to the other can be realized in several ways. More security against network failure can be achieved, but also the complexity rises and thereby the technical issues for routing algorithms and congestion handling. B. Simulations Many simulations and trials have been carried out in FSO links to flying vehicles. A. Harris et al. [1, 3] investigated the ground-to-UAV links in simulations. In the proposal are three different wavelength implemented (0.85 m, 1.55 m and 10 m) for two different link distances (4 km and 8 km). Results like the mean intensity profile, the off-axis scintillation index, the intensity variance and the scintillation index versus square root of the Rytov variance were presented. Conclusions were that all three wavelengths are adequate for an optical link and that it can be beneficial to use a combination of different wavelengths [3]. In the specific case of radiation fog between the FSO systems, it was pointed out, that the laser beam with 10 m performs much better than the wavelength 0.85 m and 1.55 m. At the altitude of 800 m, the attenuation for the wavelength 0.85 m and 1.55 m already reaches a level where data exchange is impossible (for a transmission power of 20 mW). Whereas a laser with a 10 m wavelength and the same output power successfully transmitted data over a distance of 8 km [1]. There are several topics which need to be simulated in future. For UAV networks that exhibit satisfactory performance, the hardware requirements to keep an optical link active are stringent (especially the speed and accuracy of the steering motors for the adjustment of the
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speed, height and real world condition, like air holes or different wind conditions. After meeting the hardware
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lower altitudes. The distance between the HAPs will be naturally larger than for the UAVs. For this enormous distance, some new effects have to be taken into consideration. Assuming that both HAPs are in the same altitude denotes the link will fall off below this altitude in the central region of the link path. Figure 6 shows the calculations for a two-HAPs scenario in an altitude of 20 km, depicting the variations in altitude above the earth for the FSO-link over the distance between the HAPs for 300 km and 600 km. The refractive index of the different altitudes is not included in Figure 6. Although the link is a horizontal one, it crosses various layers of the atmosphere. Hence, the fluctuation of the refraction index has to be taken into account [16].
Figure 5. HL-20 aircraft simulation model
requirements, the direction of beam propagation in different heights above ground is of prime significance. In this scenario not only obvious parameter like height and direction, but also the humidity and/or the air pressure laser beam between two UAVs). Therefore, simulations of UAVs flying close to each other in different heights and with real world condition can be useful. This simulation should show the variation of the distance between the two UAVs and the angle of the beam wander, caused only by the changing flight track. Variable parameters for such a simulation would be, a fix flying track for both UAVs, should be an input for the simulation. Another step could be to investigate the propagation of the laser beam near to the surface of the UAV. Also, the high maneuver agility of the swarm formation poses an interesting challenge. Hence, models for UAVs needs be defined precisely to include also the aerodynamic component into the simulations. For all these above mentioned simulation requirements MATLAB provides an optimal programming environment. SIMULINK includes the Aerospace Toolbox, which contains simulation blocks for Special Forces and movements to depict real world scenarios. Figure 5 shows a UAV model. IV. HAPS The idea of installing platforms above the troposphere, in a manner that they are not affected by weather conditions as an effective communication platform was successfully demonstrated in the famous CAPANINA project [17]. Horizontal links between the so-called High Altitude Platforms (HAPs) have been explored in the recent past [13, 18, 19]. HAPs operate as the name denotes in a high altitude or more precisely in the height of 20 – 22 km. There are two different versions of the HAPs, one is an unmanned stationary vehicle with an electrical motor and the other category achieves a permanent service over a given area by a circling aircraft scheme. The later category is implemented as an unmanned and a manned airplane [20]. Inter-platform links are reported to achieve a range of at least 600 km. Owing to the high operation altitude, a HAP will observe a large area on the ground and consequently less HAPs are necessary to observe the same area than with UAVs in
Figure 6. Altitude of a FSO link between HAPs for 300 and 600 km distance
h(α ) =
rearth + 20 sin( β ) − rearth sin(π − α )
Where rearth is the average earth radius, β denotes the fix angle between the HAPs and the HAP and the earth central point. Finally, the variable α spans a triangle from the middle point of the earth towards the two HAPs. The disadvantage of a long distance link between HAPs is illustrated in Figure 6. Because of the curvature of the earth, the link propagates beneath the altitude of the HAP, if we assume a plane earth surface between the two HAPs. The closer the optical link propagates to the ground the stronger the attenuation will be. As shown in Figure 6 the optical link distance of 600 km has to cross atmosphere layers in a height of 13 km, whereas a link with a distance of 300 km will always stay beyond 18 km height above the ground. This fact denotes that the link budget must cater for long path length through the turbulent medium (e.g. 300 km), which implies significant changes in the calculation. Furthermore, sun shields should be mounted on the HAPs to avoid the impact of the sun, which can be significant for these long link distances [18]. HAPs can also be used as the central communication point offering FSO down- and uplinks for multiple purposes. Civil and military use cases are conceivable, combining the communication paths of e.g. satellites in the different orbits, ground stations, aerial vehicles, ships and even access points to LANs or MANs in inhabited areas (Figure 7). Scientific research on extraterrestrial communications means is also progressing. HAPs are combining the best features of satellite and
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fixed wireless access services like a short round trip delay, a small propagation loss and the already mentioned possibility of covering large areas and providing a large capacity. Figure 7 sketches a future scenario with the use of HAPs as an important communication platform. Either the downlink to the Earth can be realized via FSO or e.g. using a RF link if the FSO connection is not available.
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[5]
[6]
[7]
[8]
[9] [10]
[11]
Figure 7. HAP as connection platform
V. CONCLUSIONS FSO is an experimentally tested and proven way of providing high data rate communication links. In a recent free space optical communication demonstration between an aerostat and a ground terminal data rates in the range of 80 Gbps were achieved [10]. The main emphasis of previous works [1, 3, 5] has been on establishing UAV-toground optical communication links, and the development of FSO as an inter-UAV communication technology has not yet been thoroughly explored [15]. The effectiveness of free space optical communication links between UAV swarms require detailed scientific and theoretical studies for the technology to evolve for real world demonstration and experimentation. Furthermore, as the platforms of communication backhauls migrate from the terrestrial means to flying vehicles (like the HAPs), the importance of developing high rate links between these vehicles would keep increasing and thus, developments of FSO communications for such scenarios pose an interesting challenge to both the academic researchers and the technologists.
[12]
[13]
[14] [15]
[16]
[17] [18]
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