OPTICAL WIRELESS COMMUNICATIONS FOR AIRPORT SURFACE OPERATIONS: OPPORTUNITIES AND CHALLENGES Abdelbaset S. Hamza, University of Nebraska - Lincoln, Lincoln, NE
Abstract Application of optical wireless communication (OWC) for vehicular communications has witnessed tremendous progress and development during the last decade. In OWC vehicular links, lights from vehicles and road infrastructure (e.g., lighting and signages) are used to establish vehicleto-vehicle (V2V) and vehicle-to-infrastructure (V2I) links to exchange information for efficient and safer road operation. In this paper, we motivate the research of using OWC to facilitate the Data Communications (Data Comm) capability of the NextGen framework by investigating the potential of using OWC during ground operations. We discuss the opportunities and advantages of using OWC in airports as well the challenges. We also present and discuss different possible solutions for such challenges.
Introduction Data Communications (Data Comm), an essential part of the NextGen framework, will enable controllers and pilots to exchange digital information that can be visually displayed and interpreted. In addition to requiring less bandwidth, digital data communication can improve the visual, auditory, and cognitive workload for controllers and pilots. This, in turn, leads to a more efficient and safer operation as the digital messages can also interact with the computers of aircrafts enforcing rules and safety measures [1-2]. Data Comm aims to help sharing real-time and forecast operational information among stakeholders (e.g., ATC controllers and pilots) to have a better understanding of the current status of the airport operation. Realizing Data Comm functionality, however, requires a data communication networking infrastructure to carry the data traffic from/to controllers to/from pilots. Almost all controller-pilot interactions are currently performed by voice communication, except for the pre-departure clearance (PDC) [3], and thus the
required networking infrastructure for Data Comm must be different from already existing networks. When the term wireless communication is mentioned, conventionally, radio frequency (RF) technology is the first to come in mind since it is a well-developed and a mature technology. However, recent advances in other technologies, such as optical wireless communication (OWC), have narrowed the gap as OWC presents certain advantages when compared to RF systems. OWC, also known as free space optical (FSO) communication, is a technology in which light is modulated and transmitted in a non-confined medium to propagate wirelessly from one point to another. OWC combines the flexibility of wireless communication and the high speed and bandwidth of optical communication. As its enabling technologies rapidly advance, OWC is finding its place in many indoor, outdoor, space and underwater applications, such as indoor local area networks and data centers [4-5], mobile networks backhaul [6], space communication [7], and underwater sensing [8]. Vehicular communications [9] is one of the emerging OWC applications that has been witnessing tremendous progress and development during the last decade [10]. In OWC vehicular links, lights from vehicles and road infrastructure (e.g., lighting and signages) can be used to establish vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) OWC links to exchange information for improved and safer road operation. Similar to the application of OWC in vehicular communications, we envision that aircraft-to-aircraft (A2A) and aircraft-to-infrastructure (A2I) can be achieved using OWC serving as an infrastructure for the Data Comm module of the NextGen framework. We anticipate that using OWC for airport ground operations can help us: • Utilize airport lights and signages along the taxiways and runways for Data Comm instead of RF links.
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• Improve aircrafts localization on the airport ground and facilitate sharing this information with pilots to enhance their situational awareness (SA). • Improve the airport throughput. The remainder of the paper is organized as follows. In the next section, we review the basic components of an OWC link. Then, we discuss the application of OWC for vehicular communication. Finally, we present the vision of using OWC for airport communication and discuss the advantages and challenges followed by the conclusions in the last section.
OWC Basics In this section, we briefly discuss the preliminaries and basic components (light sources, photodetectors, and modulation schemes) of a generic OWC link. Light Emitting Diodes (LEDs) and Laser Diodes (LDs) are the most commonly used light sources in OWC links. LDs are highly directional sources that have high optical power outputs and broader modulation bandwidths, and therefore can support high data rate transmission. Using LDs, however, poses a potential eye and skin safety hazard. Therefore, used LDs must comply with certain standards and power restrictions. On the other hand, LEDs are large-area emitters and are considered as extended sources that can be operated safely even at relatively high powers. LEDs are cheaper and more reliable as compared to LDs, and thus, are preferred in some indoor applications. In general, LEDs support lower data rates as compared to that of LDs, however, recent research demonstrations show relatively high achievable data rate (up to 1 Gbps) using LEDs and rate-adaptive discrete multitone modulation [11]. Conventionally, there are four types of photodetectors, namely, positive-intrinsic-negative (PIN) photodetectors, avalanche photodetectors (APDs), photoconductors and metal-semiconductor metal photodiodes. However, PINs and APDs are the most commonly used types of photodetectors. Recently, an extensive research effort in being exerted in the field of quantum dot, Nano-particle
and graphene-based photodetectors to develop ultrafast photodetectors that work over a broad range of wavelengths [12]. These detectors could be of great use in ultrahigh bandwidth optical communication systems. Although On-Off keying (OOK) is the most commonly used modulation scheme due to its simplicity, wide range of digital modulation schemes can be used in FSO systems. Different modulation schemes have different energy efficiencies (EE), spectral efficiencies (SE), and transmission reliability. Depending on the application, a suitable modulation scheme can be used. It is worth pointing that for high data rate applications (e.g., deep space communication), Pulse Position Modulation (PPM) or one of its variations, such as, Variable-PPM (VPM), is usually used. Both OOK and PPM are considered as single-carrier pulsed modulation. Multiple-subcarrier modulation such as Orthogonal frequency-division multiplexing (OFDM) can also be used in severe channel conditions since it does not require complex time-domain equalization as compared to PPM.
OWC for Vehicular Communications The research in visible light communication (VLC) in which OWC is designed using LEDs has witnessed significant advancement as a response to the recent development in LEDs design in addition to its attractive attributes, such as low power consumption, long life, low maintenance cost, better visibility and low temperature generation. One of the VLC applications is the Intelligent Transportation Systems (ITS), in which traffic lights, head, tail and brake lights are used as transmitters, and receivers are mounted to the traffic lights and vehicles to establish OWC links between the vehicles and the infrastructure (V2I) and vehicles on the road (V2V). Using these links, traffic safety related information can be continuously broadcasted and integrated with built-in computers in vehicles leading to enhanced traffic flow, and reduced accidents and fatalities. Most proposed VLC systems for ITS utilize high speed cameras for as receivers [10] since cameras facilitate tracking. Information is carried on the pixels of the LED array in the transmitter. As the vehicles move, the camera of the receiving vehicle captures the images of the car in front and/or behind. Using image processing techniques, the receiver detects the LEDs areas and analyze the intensity variation and extract the transmitted signal. Since the camera operates as a
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2. Communication between maintenance platform and aircraft under maintenance. 3. Aircraft-to-infrastructure (A2I) and aircraftto-aircraft (A2A) communication during taxiing to runway to receive updates of the airport status and locations of surrounding aircrafts. Also communication with surround aircrafts to exchange location and speed information for safer operation. 4. A2I communication before takeoff for permissions and radio tuning instructions. 5. A2I communication immediately after landing and during taxiing to receive the gate number, taxiing route, information about the airport status, and other aircrafts locations. The landed aircraft can also send information and reports while taxiing to the controller and the company dispatch.
receiver, the system bandwidth is limited by the frame rate of the camera. Therefore, the data rate is limited by the quality and cost of the cameras installed. Moreover, VLC in ITS faces some other challenges, such as, long link distances and background light which can make the link design more complicated.
OWC for Airport Ground Operations Figure 1 depicts a top view of an airport in which OWC links are used. In addition to utilizing the airport lights and signages for establishing OWC links, additional OWC beacons can be distributed to guarantee connectivity. We believe that OWC can replace or at least complement the conventional RF technology to establish communication links for the following communication scenarios (see Figure 1): 1. Communication between terminal and aircraft before (after) landing to exchange pre (post) flight information.
Figure 1. Example of OWC for Aiport Surface Operation
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By utilizing the airport lights and signages distributed along the taxiways and runways, RF communication can be reduced and/or limited to the air operation. This can reduce the overall operation power consumption of the airport as well the total electromagnetic field density and RF congestion currently experienced at many major airport. Aircrafts localization using conventional airport surface detection systems (e.g., ASDE-X) is performed by gathering data from surface surveillance radar, multilateration sensors, airport surveillance radars, and automatic dependent surveillance broadcast sensors and fuse it with flight data plan data acquired from terminal automation system. The information is then presented to the controllers on a colored display that shows the airport’s map of runways/taxiways with aircrafts and vehicles. The map is continuously updated to show the airport movement area that controllers can use to enhance their situational awareness. Although the ASDE-X system is equipped with visual and aural alarms to alert controllers of possible runway incursions or incidents, such systems makes the controller a single point of failure and overloads him as he must share the data to provide pilots the needed situational awareness using voice communication. Even when Data Comm is enables, sharing the information generated by the ASDE-X with all pilots can be data-intensive and overwhelming for the pilots as they may receive unnecessary information beyond their route or surroundings. We envision that using OWC can help mitigate these problems and help in sharing information needed by pilots based on their location. As the large distributed network of OWC beacons is used, each beacon or signage can be assigned a unique ID. When an aircraft enters the coverage of an OWC beacon, the aircraft can get beacons’ information such as the ID and the address. Using the acquired information, the aircraft can send identification and operational information such as type and speed. Using this information, localization on the airport ground can be improved. Based on the transmitted signals and the position of the aircraft, information needed to improve the pilot’s SA within his surrounding and route of interest can be sent specifically to his aircraft by using the nearest OWC beacon.
In addition to communicating basic and essential data for airport ground operation, the distributed OWC infrastructure can also be used to carry additional data such as performance and troubleshooting reports from aircraft to maintenance centers. This can be done immediately after landing, during taxiing, and passenger disembarking. This in turn can help maintenance and service personnel to perform faster maintenance plan decisions and execution, and thus improve airport throughput.
Challenges of OWC in Airports OWC links for airports can be classified as terrestrial OWC links [13], and thus links are exposed to atmospheric variations. A light beam propagating through turbulent channels can be impaired affecting the system performance significantly. Moreover, OWC links for airports may experience challenges that are unique to the airport scenario and not experienced by other terrestrial OWC links. In this Section, we briefly discuss challenges and impairments that can be faced by OWC links in airports.
Noise and Atmospheric Impairments A terrestrial OWC link can be affected by noise sources such as background radiation, or atmospheric impairments such as, attenuation, and/or scintillation. Although most of the terrestrial OWC links are prone the same impairments, the severity of each of these impairments depends on the length of the link, the day during the year, and the time during the day. Therefore, measurements must be performed in actual airport environment to identify the severity of the following impairments. Background radiation noise is due to the detection of photons generated by localized point sources (i.e., the Sun) and extended sources (i.e., the sky). This type of noise will be significant during the day, however, its impact is the highest when the Sun is co-linear with the OWC link (i.e., during the sunrise and dawn). Therefore, the quality of the link and the received Signal-to-noise-ratio (SNR) will depend on the orientation of the airport and the orientation of the aircraft in the airport during the taxing. In order to overcome the background radiation power, higher transmitted power and highly directional links must be used.
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Another impairment for OWC links in airports is the link power attenuation that can be caused by absorption and scattering. Absorption is mainly due to the interaction between the photons of the radiation and the atoms or molecules of the medium, whereas, scattering is the deflection of incident light from its initial direction, causing spatial, angular, and temporal spread [6], [14]. Any, or a combination, of the fog, rain, snow and dust can lead to absorption, refraction and scattering of a transmitted FSO beam. This in turn leads to a signal attenuation affecting the performance of the link. The impact of absorption and scattering can be mitigated by increasing the transmitted power and/or by temporal and spatial diversity. Spatial diversity can be realized by
establishing OWC links from both sides of the runway and taxiways. Scintillation or intensity fluctuation refer to the temporal variation of intensity observed at an infinitely small point and the spatial variation of intensity within the receiver aperture [15]. This change in light intensity in time and space at the receiver can be due to changes in the index of refraction of the air. One of the main factors that can cause scintillation in airports is the relatively hot and humid exhaust from jet engines. Similar to power attenuation, scintillation can be alleviated using temporal and spatial diversity. Table 1 summarizes the noise and impairments that can be experienced by an OWC link in an airport, and their potential solutions.
Table 1. Summary of OWC Link Impairments in Airports Impairment
Sunlight
Reduced SNR
• •
Solutions Increase transmitted power Highly directional links.
Attenuation
Fog, rain, snow, or dust.
Absorption, refraction, and scattering
• • •
Increase transmitted power Temporal and spatial diversity. Efficient modulation
Atmospheric Turbulence
Refractive index variations due to heat dependency (e.g., engine exhaust)
Beam wandering
• •
Adaptive optics. Temporal and spatial diversity.
Ambient Light
Causes
Effects
Link Distance and Geometry As discussed earlier, OWC links require LOS. Moreover, unlike RF communication in which the received SNR is proportional to the signal amplitude, the received SNR in OWC is proportional to the transmitted power. Depending on the aircraft type, the distance between the fuselage housing the OWC equipment and the OWC beacon can greatly vary, the larger the aircraft type, the shorter the OWC link distance. Therefore, adaptive optics may be required to adjust the transmission power according to the aircraft type. Another issue that must be considered is the orientation of the OWC beacons with respect to the
aircraft direction of motion. Figure 2 depicts two possible OWC communication beacon positioning. In Figure 2(a) the beacon is perpendicular to the aircraft direction of motion. In this case, the time window for transmission is short. However, the OWC beacon transceiver will be facing the OWC transceiver that is mounted in the side of the aircraft. On the other hand, Figure 2(b) depicts a slant positioning of the OWC beacon. This positioning is similar to the way signage are placed in airport ground allowing the longest period of exposure to the pilots, and thus better readability. Similarly, this will give the OWC modules longer exposure, and thus communication window. However, the OWC transceiver mounted in the aircraft must be tilted to establish the link.
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References [1] Willems, Ben, and Sehchang Hah, 2008, “Future En Route Workstation Study (FEWS): Part I,” Automation Integration Research, Atlantic City International Airport, NJ: Fedral Aviation Administration William J. Hughes Technical Center. [2] Hah, Sehchang, Ben Willems, 2010, “The Evaluation of Data Communication for the Future Air Traffic Control System (NextGen),” Proceedings of the Human Factors and Ergonomics Society Annual Meeting, vol. 54, no. 1, pp. 99-103.
(a)
[3] Wargo, C. A. and J. F. D'Arcy, 2011, “Performance of Data Link Communications in Surface Management Operations,” Aerospace Conference, IEEE, Big Sky, MT, pp. 1-10. [4] Hamza, A. S., J. S. Deogun and D. R. Alexander, 2015, “Evolution of Data Centers: a Critical Analysis of Standards and Challenges for FSO Links,” Standards for Communications and Networking (CSCN), IEEE Conference on, Tokyo, pp. 100-105. [5] Hamza, A. S., J. S. Deogun, and D. R. Alexander, 2016, “Wireless Communication in Data Centers: A Survey,” in IEEE Communications Surveys & Tutorials, vol.PP, no.99, pp.1-1.
(b) Figure 2. Different OWC Module Positioning With Respect to Aircraft Direction of Motion
Conclusions In this paper, we motivate the research of using OWC technology to facilitate the Data Communications (Data Comm) capability of the NextGen framework during the airport ground operations. OWC. Using the lights and signages along the taxiways and runways, OWC links can be used to establish aircraft-to-infrastructure (A2I) and aircraft-to-aircraft (A2A) communication links. Such links can help improve the localization of the aircrafts in airports, and thus information sharing to increase the SA of pilots. OWC links in airports can experience impairments such as background radiation and atmospheric impairments. To identify the severity of such impairments, measurements must be performed in actual airport environment.
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[10] Takai, I., T. Harada, M. Andoh, K. Yasutomi, K. Kagawa, and S. Kawahito, 2014, "Optical Vehicle-toVehicle Communication System Using LED Transmitter and Camera Receiver," in IEEE Photonics Journal, vol. 6, no. 5, pp. 1-14. [11] Khalid A., G. Cossu, R. Corsini, P. Choudhury and E. Ciaramella, 2012, “1-Gb/s Transmission Over a Phosphorescent White Led by Using Rate-Adaptive Discrete Multitone Modulation,” Photonics Journal, IEEE, vol. 4, no. 5, pp. 1465–1473. [12] Pospischil, A., M. Humer, M. M. Furchi, D. Bachmann, R. Guider, T. Fromherz, and T. Mueller, 2013, “CMOS-Compatible Graphene Photodetector Covering All Optical Communication Bands,” Nature Photonics, vol. 7, pp. 892–896.
[14] Bouchet, Olivier, Hervé Sizun, Christian Boisrobert, Frédérique de Fornel, Pierre-Noël Favennec, 2010, “Free-Space Optics: Propagation and Communication”, series ISTE. Wiley. [15] Henniger, O.W., 2010, “An Introduction to FreeSpace Optical Communications,” Radio engineering, vol. 19, no. 2.
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[13] Hamza, A. S., J. S. Deogun and D. R. Alexander, 2015, “CSOWC: A Unified Classification Framework for Standardizing Optical Wireless Communications,” Standards for Communications and Networking (CSCN), IEEE Conference on, Tokyo, pp. 112-117.
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