Modern expeditionary military missions generate and exchange massive amounts of data that are used to produce situational awareness and guide decision-making. Much of the data must travel long distances along backbone communications networks composed of high-capacity links that connect command centers.
Fiber optic cables provide the core backbone for military and civilian networks, enabling Internet, phone, video and other data to move at super-high speeds with virtually no degradation over long distances. In deployed environments, where a fiber optic backbone doesn’t exist, other communications modes are used resulting in reduced data-rate capacity for the warfighter. Satellite Communications (SATCOM) services can provide some capacity to remote areas but cannot provide the capacity needed to support the amount of data generated by emerging ISR systems.
Free-space optical (FSO) links using laser communications is another option to project data at such speeds. FSO links have been shown to have fiber-optic-equivalent capacity at long ranges and are expected to play a significant role in the military’s airborne-based data backbone. Still, today’s FSO link technology cannot propagate through clouds, which are present 40 percent of the time in some regions and lead to unacceptable network availability, DARPA researchers point out. Instead, high-speed radio frequency (RF) links may be the best solution.
To provide high bandwidth connections to soldiers, DARPA is developing millimeter wave communication. The Programs sought to capitalize on the technology base already initiated by the allocation of the E-Band ( 71 – 86 GHz) spectrum. The total spectral bandwidth available consisting of 5GHz spectrum at each of two bands 71-76 GHz and 81-86GHz exceeds that of all allocated bands in the microwave spectrum. With such wide bandwidth available, millimeter wave wireless links can achieve capacities as high as 10 Gbps full duplex, which is unlikely to be matched by any lower frequency RF wireless technologies.
Missions in remote, forward operating locations often suffer from a lack of connectivity to tactical operation centers and access to valuable intelligence, surveillance, and reconnaissance (ISR) data. The assets needed for long-range, high-bandwidth communications capabilities are often unavailable to lower echelons due to theater-wide mission priorities.
DARPA’s Mobile Hotspots program aims to help overcome this challenge by developing a reliable, on-demand capability for establishing long-range, high-capacity reachback that is organic to tactical units. The program is building and demonstrating a scalable, mobile millimeter-wave communications backhaul network mounted on small unmanned aerial vehicles (UAVs) and providing a 1 Gb/s capacity. The aim is to to connect dismounted warfighters with forward operating bases, tactical operations centers, ISR assets and fixed communications infrastructure.
DARPA performers recently completed the first of three phases in which they developed and tested key technologies to be integrated into a complete system and flight tested in subsequent phases. “We’re pleased with the technical achievements we’ve seen so far in steerable millimeter-wave antennas and millimeter-wave amplifier technology,” said Dick Ridgway, DARPA program manager. “These successes—and the novel networking approaches needed to maintain these high-capacity links—are key to providing forward deployed units with the same high-capacity connectivity we all enjoy over our 4G cell-phone networks.”
MMW communication for High-altitude, long-endurance (HALE) aircraft and Solar powered stratospheric drones
Military are using High-altitude, long-endurance (HALE) aircraft capable of flying as high as 60,000 feet and can endure missions as long as 32 hours. But they are not weaponized. The two primary HALE aircrafts the military currently operate are the Air Force’s land-based RQ-4 Global Hawk and the Navy’s MQ-4 Triton, the maritime variant of the Global Hawk.
DARPA is developing One of the largest solar powered drones, known as ‘SolarEagle’ designed to cruise at altitudes of 60,000 to 90,000 feet with a thousand-pound payload at a speed of 70 to 80 knots while performing communications, intelligence, surveillance and reconnaissance missions. It is under development at Boeing Phantom Works, under DARPA’s $89 million Vulture II program. The solar-electric-powered drone, with 400-foot wings span carrying solar panel arrays, will be able to sustain the drone aloft in the stratosphere for at least five years.
Unlike microwave links, which cast very wide footprints reducing the achievable amount of reuse of the same spectrum within a specific geographical area, millimeter wave links cast very narrow beams. A key benefit of the highly narrow beam millimeter wave links is the scalability of their deployments. For, example, millimeter wave is well suited for network topologies such as point-to-point mesh, a dense hub-and-spoke or even a ring. Other wireless technologies often reach their scalability limit due to cross interference before the full potential of such network topologies can be realized.
Properties of millimeter wave propagation, either through the atmosphere or through material objects, have been well researched and documented. Weather phenomena that affect millimeter wave propagation, such as rain rates, have also been well characterized and understood regionally throughout the world. With many decades of military and government-funded research behind it, millimeter wave technology has reached a level of maturity comparable to older forms of radio technologies.
100G Fiber Optic class Airborne Network
DARPA’s 100G program is developing the technologies and system concepts to project fiber-optic-class 100 Gb/s capacity via airborne data links anywhere within the area of responsibility (AOR). The goal is to create a 100 Gb/s data link that achieves a range greater than 200 kilometers between airborne assets and a range greater than 100 kilometers between an high-altitude long-endurance aerial platforms (at 60,000 feet) and the ground.
Computationally efficient signal processing algorithms are also being developed to meet size, weight and power limitations of host platforms. Recent progress included a demonstration of key components performing to levels suitable to meet overall 100G system goals, during which DARPA performers set several millimeter-wave modulation and transmission records. The technologies are currently being integrated into a full 100 Gb/s system, to be followed by flight testing.
The 100G program goal is to meet the weight and power metrics of the Common Data Link (CDL) deployed by Forces today for high-capacity data streaming from platforms. Additionally, the system will provide an all-weather (cloud, rain, and fog) capability while maintaining tactically-relevant throughput and link ranges.
US Military currently uses Common Data Link (CDL) with data rate upto 274 Mbit/sec and Tactical Common Data Link (TCDL) upto 10.7 Mbit/sec. Common Data Link (CDL) is a secure U.S. military communications protocol that networks together a deployment, for shuttling around imagery, intelligence, orders, and so on. CDL operates within the Ku band at data rates up to 274 Mbit/s. CDL allows for full duplex data exchange.
The Tactical Common Data Link (TCDL) is a secure data link being developed by the U.S. military to send secure data and streaming video links from airborne platforms to ground stations. It uses a Ku narrowband uplink that is used for both payload and vehicle control, and a wideband downlink for data transfer. The TCDL uses both directional and omnidirectional antennas to transmit and receive the Ku band signal. The TCDL was designed for UAVs, specifically the MQ-8B Fire Scout, as well as manned non-fighter environments. The TCDL transmits radar, imagery, video, and other sensor information at rates from 1.544 Mbit/s to 10.7 Mbit/s over ranges of 200 km. DARPA now wants to push these speeds up to 100 Gbps, while using equipment that retains the same weight/power requirements of CDL — i.e. these 100G systems must be deployable in the field.
The 100G program is exploring high-order modulation and spatial multiplexing techniques to achieve the 100 Gb/s capacity at ranges of 200 km air-to-air and 100 km air-to-ground from a high-altitude (e.g., 60,000 ft./18 km) aerial platform. The program is leveraging the characteristics of millimeter wave (mmW) frequencies to produce spectral efficiencies at or above 20 bits-per-second per Hz. Computationally efficient signal processing algorithms are also being developed to meet size, weight and power (SWaP) limitations of host platforms, which will primarily be high-altitude, long-endurance aerial platforms.
The 100G prototype will operate in the 71-76 GHz and 81-86 GHz bands, using high-order modulation and spatial multiplexing. Use of the millimeter-wave frequencies enables high signal power gain from compact apertures to overcome modest weather impairments on the signal. It also enables the system to take advantage of spatial multiplexing due to the relationship between wavelength and Rayleigh range. High-order modulation further increases spectral efficiency, potentially enabling a 5 b/s/Hz signal in the 5 GHz channel bandwidths to attain 25 Gb/s between each pair of transmit and receive apertures. Thus a 100G prototype with two (2) transmit and two (2) receive apertures, with each aperture transmitting or receiving two orthogonally polarized signals, would create four (4) independent streams and attain a spectral efficiency of 20 b/s/Hz.
Mobile Hotspots to create Gb/s communications backbone that can be carried on UAVs to connect dismounted warfighters
DARPA is also developing 100 Gb/s RF Backbone (100G) program whose goal is to design, build, and test an airborne based Millimeter based RF communications link with fiber-optic equivalent capacity and long reach capable of propagating through clouds and providing high availability.The system will provide 100 Gb/s capacity at ranges of 200 km for air-to-air links and 100 km for air-to-ground links when installed in a high-altitude (e.g. 60,000 ft) aerial platform.
Providing high-bandwidth communications for troops in remote forward operating locations is not only critical but also challenging because a reliable infrastructure optimized for remote geographic areas does not exist. “And while satellite communications services can provide some capacity to remote areas, they cannot provide the high-bandwidth communications needed to support the amount of data generated by emerging ISR systems,” said Dr Arati Prabhakar.
The program goal of Mobile Hotspots program is to demonstrate a scalable, mobile, self-forming and managing network of millimeter-wave air-to-air links of 50 km @ 1 Gb/s range and air-to-ground data links of 40 km @ 1 Gb/s range. The backbone should also provide reliable end-to-end data delivery between hotspots, as well as from ISR sources and command centers.
Darpa’s Mobile Hotspots program retrofits retired Shadow drones with pods that will be able to transfer one gigabyte per second of data — the equivalent of 4G smartphone connectivity — so that soldiers in remote areas will have the same access to tactical operation centers and mission data that others in more central theaters have. “This past year the program made significant progress by building low-power millimeter-wave radios small enough to be carried on UAVs, and demonstrating that these can communicate at tactically relevant ranges at Gb/s rates. We expect to demonstrate a full network built upon these radios later this year in a major Marine Corps exercise,” said Prabhakar.
To achieve this capability, the program seeks to develop advanced millimeter-wave pointing, acquisition and tracking (PAT) technologies that are needed to provide high connectivity to the forward-located mobile hotspots. Advanced PAT technology is key for connectivity to small UAVs, for example, enabling them to serve as flying nodes on the mobile high-speed backbone. Additionally, the program seeks novel technologies to increase the transmission power of millimeter-wave amplifiers to provide adequate ranges within the small size, weight, and power (SWAP) constraints required for company-level unmanned aerial vehicles (UAVs).
The key functional subsystems of the UAV mounted equipment are: the GPS & IMU, network router, discovery radio, LTE networking equipment and the millimeter-wave (mmWave) radios. These subsystems work together to enable the robust PAT function required by the highly directional mmWave radio links.A total of four mmWave radios will provide the gigabit directional links with as wide an operating field-of-view (FOV) as possible. Additionally, highly efficient power supplies and creative cooling techniques are also needed to realize the mission.
In addition to the airborne equipment, there will be stationary ground-based nodes both mobile and compact deployable, as well as portable commercial LTE devices. The ground based nodes will be fully functional gigabit nodes similar in content to the pods but also providing access in and out of the network.
The ability to then tie in and extend the network with LTE will enable multiple “Hotspots” over 1,000 square miles of area to be connected within hours. For the user, these features create a virtual network with high availability that provides a broadband Internet-like experience, leveraging the millimeter-wave gigabit links as the core transport mechanism.
Phase 1 accomplishments include:
Smaller, steerable millimeter-wave antennas: During field testing, the program successfully demonstrated steerable, compact millimeter-wave antennas that rapidly acquire, track, and establish a communications link between moving platforms. Steerable millimeter-wave antennas will enable the formation of a high-capacity backhaul network between aerial and ground platforms.
Low-noise amplifiers: Performers also demonstrated an advanced low-noise amplifier (LNA), which boosts the desired communications signal while minimizing unwanted noise. The prototype achieved the record for the world’s lowest noise millimeter-wave LNA at about half the noise figure of a typical LNA.
More efficient and capable power amplifiers: Efficient millimeter-wave amplification is required to achieve the long ranges (> 50 km) desired in the Mobile Hotspots program. During Phase 1, performers demonstrated output power exceeding 1 watt and 20% power added efficiency (PAE) from a single gallium nitride (GaN) chip operating at E-Band frequencies (71 GHz to 86 GHz). Output powers exceeding 20 watts and approaching 20% PAE were also achieved using power-combining techniques.
New approaches for robust airborne networking: Mobile ad-hoc networking approaches were developed to maintain the high-capacity backhaul network among mobile air and ground platforms. Phase 1 performers developed unique solutions to overcome connectivity and network topology challenges associated with mobility and signal blockages due to terrain and platform shadowing.
Low-Size, Weight, and Power (SWAP) pod design to carry it all: Performers created engineering designs for small, lightweight pods to be mounted on an RQ-7 Shadow UAV. The pods, with all of the Mobile Hotspots components inside, are designed to meet the challenging program goals of widths no more than 8 inches, weight less than 20 pounds, and power consumption less than 150 watts.
Phase 2 of the program began March 2014. Two performers, L-3 Communications and FIRST RF, were chosen to lead teams comprising several Phase 1 performers. Phase 2 goals include the integration of the selected Phase 1 technologies into Shadow-compatible aerial pods and ground vehicles. Phase 2 will conclude with a ground demonstration of at least four Shadow-compatible pods, two ground vehicles and a fixed ground node. A planned third phase will encompass field testing of the Mobile Hotspot systems on networks of multiple SRQ-7 Shadow UAVs and mobile ground vehicles.
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