COFDM GaN PAs Provide a Platform for Next-Generation UAV Data Links

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Breakthroughs in wireless communications and complex controls systems have allowed for the application of remotely and autonomously controlled vehicles. In a recent MarketsandMarkets report, the unmanned aerial vehicle (UAV) market was estimated to be $13.22 Billion in 2016 and is projected to reach $28.27 Billion by 2022. The increase in the civilian application of drones is partly why the market is expected to double in a matter of six years. The UAV technology that was originally exclusively leveraged in the military has opened up doors in the scientific, recreational, agricultural, traffic monitoring, mining, disaster management, and oil & gas industries. From small to large, tactical to lethal, stratospheric to exo-stratospheric, and rotary to fixed-wing, these aircraft come in all shapes in sizes that are most suitable for their respective applications. Images from drones that are capable of flying a few meters above the ground will fill a gap between expensive, weather-dependent and low resolution images provided by satellites and car-based images limited to human-level perspectives and the availability of accessible roads [1].

There are a vast array of specialized applications UAVs can serve, but there are still hindrances that keep these aircraft from more complex motor control and communications in the ever-increasing airspace congestion as well as terrain difficulty. Typically, civil and military aircraft have room for kilometers of wiring and hundreds of avionics, but all of this architecture must be scaled down dramatically in a UAV layout. The smaller the size of the aircraft, the more integrated the system must be, which includes the power generation and distribution. Scaling issues create problems at the most basic level of autonomy: they limit the ability to simply sustain flight for an adequate amount of time to perform higher-level mission functions [1].

Figure 1: UAVs come in a variety of sizes, including: nano (< 1 ft), micro (<3 ft), small UAS (<10 ft), ultralight aircraft (<30 ft), light sport aircraft (<45 ft), small aircraft (<60 ft), and medium aircraft (>60 ft). In the military, these UAV all have a prespecified mission endurance where the mission can vary with Intelligence, Surveillance, and Reconnaissance (ISR) payloads.
Source: http://www.ga-asi.com/gray-eagle

Power Distribution in UAVs

The power source of a UAV depends heavily upon its size; larger high altitude long endurance (HALE) or medium altitude long endurance (MALE) UAVs will generally rely on conventional airplane engines while smaller UAVs leverage batteries with battery elimination circuitry (BEC) to eliminate the need for a large battery bank to save space. Naturally, most of the power budget is allocated to propulsion for aircraft, which proves a challenge for small UAVs where there is not much room for high powered equipment. Depending on the mission, the payload will require electrical power from the UAV. This power demand may be a few tens or hundreds of watts for sensors or communications, or it may be tens of kilowatts or more for radar, jamming devices, or weapons [4].

Figure 2: Unlike many other UAV subsystems, the power system interfaces with both the platform and the payloads.
Source: https://www.ece.nus.edu.sg/stfpage/elezhang/Publications/UAV%20Communications.pdf

With the propulsion already burdening the power budget heavily, the power distribution circuitry for the control land non-payload communication (CNPC) as well as payload systems must then be stored in smaller spaces. These high altitude applications for wireless communications can bring in a new set of challenges over the already established telecommunications infrastructure on the ground. High power amplifiers (HPAs) are critical components in ISR payloads that can provide robust data links to deliver relevant information with high definition real-time image captures for surface-mapping.

Solid-state power amplifier (SSPA) technology is already a major advancement, particularly for high altitude operations, from the previously used bulky tube-based amplifiers in terms of size, weight, power, and cost (SWAP-C). Gallium Nitride based (GaN) SSPAs have gained momentum recently for their uniquely high linearity and PAE (power-added efficiency), two key factors in the integrity of data transmissions.

UAVs can be controlled a number of ways, including by a remote human pilot, or by a remote human operator where the operator simply inputs flight parameters for the built-in controls of the UAV. The UAVs can also be semi-autonomous in nature where there is human-controlled initiation and termination, but the mission is autonomous, or fully-autonomous control with only human initiation [3]. Along with the constraints of wireless communications, including fading due to multipath, a UAV must include flight regulation and the ability to send and receive telemetry information to an operator in nearly real-time. Communications and power are also tied together as there is often a tradeoff between communication bandwidth and the power handling capability of the UAV. The UAS’s ability to track a UAV with an antenna is often difficult as the control link relies heavily on line-of-sight (LoS) communications. The curvature of the earth inhibits the direct path for proper transmission and reception of signals at range, so beyond line of sight (BLoS) capabilities are necessary—satellites or cell phone networks are often used for seamless connections at high altitudes.

Robust COFDM Data Links

The many permutations between the type of UAS and its respective mission call for a type of communication architecture that allows for interoperability in order to scale with the market growth of UAVs. Air-to-ground (A2G) communications that leverage Internet protocol (IP) have become promising, as a UAV can then interface with an already established network at available frequencies with high coverage. Coded Orthogonal Frequency-Division Multiplexing (COFDM) data links offer an alternative from traditionally used VHF/UHF radio transmissions with comparatively low data rates and satellite communications with low availability and high cost. The COFDM-based systems in UAVs have the added benefit of more effectively eliminating the issue of multipath propagation. COFDM data links used for surveillance applications for wireless video and telemetry purposes can be IP-based and are then able to connect to IP-enabled systems.

Ethernet protocols have already proliferated in the commercial industry and are beginning to permeate the military for voice and data networking on the battlefield as well as in aircraft with the military standard MIL-DTL-32546. Civilian aircraft have also begun to implement Ethernet protocols with the avionics full duplex switched Ethernet (AFDX) data networking standard (ARINC 664P7); even NASA’s Orion is equipped with Ethernet data networking protocols. The modularity and scalability of this platform is partly why UAV communications often leverage IP. The COFDM IP-based technology has the combined benefit of interoperability with different applications and platforms with an elegant solution to wireless communications in cluttered environments. With the bulk of the budget in many civil aircraft put towards software development in order to be able to be hosted on different platforms, next-generation avionics systems will need to rapidly integrate subsystems from a wide array of vendors.

Benefits of COFDM in Difficult Terrain

The COFDM modulation scheme spreads a transmission over a large number of carriers (over 1,000) where the carriers have a precise frequency spacing. This choice of carrier spacing ensures the orthogonality of the carriers, which in turn helps the receiver’s ability to demodulate signals even with the presence of echoes without experiencing inter-carrier and intersymbol interference. Frequency-selective time-varying fading is a random process that occurs when a signal is attenuated at different frequencies across a channel and is often a product of multipath propagation. Multipath propagation occurs when a signal transmission is intercepted (attenuated, refracted, or reflected) by obstacles in a terrain (mountains, rain, trees, buildings, etc.). This type of fading allows the receiver to show multiple signals taking different paths, oftentimes shifted in phase with constructive or destructive interference. Since fading is a random process, it is often modeled with simulators based on a particular environment (e.g., dense urban, high rainfall, etc.). The mapping of these random processes also allows for forward error correction of a signal before transmission in COFDM-based modulation techniques in order to compensate for errors due to lost carriers from frequency-selective fading, channel noise, and other propagation effects [5].

Figure 3: Images from NASA’s high-altitude ER-2 UAV equipped with a W-band HPA. The HPA helped researchers collect data on precipitation distribution in clouds to improve understanding of precipitation over mountainous terrain.
Source: https://spinoff.nasa.gov/Spinoff2017/it_7.html

Benefits of GaN

The UAV industry faces issues both of power distribution and communication links in mission-critical scenarios. While this technology has provided solutions for many applications, both obstacles hinder its proliferation in more commercial applications. Scaling the UAV down in size has the added benefit of maneuverability, but necessitates more power density and thermal management. Gallium Nitride (GaN), a Group III/V semiconductor, is known for its exceptional power density while upholding a high enough electron mobility to function at L-band and C-band (bands that are allocated for UAV communications). Rapidly replacing silicon-based transistors and other III/V semiconductors in power electronics, this substrate can function at much higher temperatures and voltages than many commonly utilized semiconductors. In 2013, the Department of Energy (DOE) dedicated approximately half of a $140 million research institute for power electronics to GaN research, citing its potential to reduce worldwide energy consumption [6]. This is no exception for the UAV industry, where power and space savings are tremendously valuable.

Figure 4: Fury, a long-endurance UAS from Lockheed Martin, employs open, IP-based architectures
Source: http://www.lockheedmartin.com/us/products/fury.html

Figure 5: Multipath fading is a great concern in wireless communications as receivers often cannot properly demodulate signal transmissions due to the obstacles a signal faces in a variety of environments
Source: http://www.datarespons.com/drones-wireless-video/

GaN-Based COFDM Amplifiers

Leveraging the GaN substrate for power amplification allows for a greater bandwidth usage and power-added efficiency (PAE), while the high transmit efficiency of GaN systems reduces the cooling requirements. The U.S. Air Force has already heavily invested in GaN modules with an advanced communications affordability (ACA) program that significantly improves the manufacturing readiness level and affordability of advanced Gallium Nitride (GaN) Monolithic Microwave Integrated Circuits (MMICs) [8]. Where the Department of Defense’s (DoD) integrated roadmap for unmanned systems includes plans to effectively replace GaAs SSPAs with GaN SSPAs for transmitters [9]. As the cost of producing GaN MMICs goes down, it is very possible that GaN-based SSPAs will penetrate the commercial drone industry as well. With the HPA being a cornerstone component in any transmitter, shrinking the size of this component or even increasing the power output is a drastic design improvement. GaN-based SSPAs with COFDM compatibility can overcome both issues of wireless communications in prohibitive environments and range extension on communications while saving on all aspects of SWAP-C. With Ethernet protocols allowing for flexibility across platforms and COFDM modulation schemes with IP compatibility, the use of GaN-based COFDM power amplifiers can be of use in next-generation UAV platforms.

References:

1. Floreano, Dario; Wood, Robert J. (27 May 2015). “Science, technology and the future of small autonomous drones”. Nature. 521 (7553): 460–466. doi:10.1038/nature14542. PMID 26017445. Retrieved 11 February 2016.

2. https://www.enterprisecontrol.co.uk/UserFiles/Next%20Generation%20COFDM%20Microwave%20Links%20for%20Military%20Surveillance(1).pdf

3. https://www.ece.nus.edu.sg/stfpage/elezhang/Publications/UAV%20Communications.pdf

4. https://www.nap.edu/read/9878/chapter/9#72

5. http://sna.csie.ndhu.edu.tw/~cnyang/MCCDMA/tsld004.htm

6. http://news.mit.edu/2015/gallium-nitride-electronics-silicon-cut-energy-0729

7. http://www.militaryaerospace.com/articles/print/volume-24/issue-7/special-report/uav-command-control-communications.html

8. http://www.wpafb.af.mil/News/Article-Display/Article/819117/afrl-program-improves-manufacturing-process-for-air-force-communication-system/

9. https://www.defense.gov/Portals/1/Documents/pubs/DOD-USRM-2013.pdf

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