by Gary Echo, Wavestream
Whether land-mobile, maritime or airborne, military or commercial, the need to communicate on moving platforms at increasing data rates is becoming more and more critical. These “disadvantaged” platforms present difficult challenges to be solved. Platforms incorporating Wavestream amplifiers have dramatically improved the solutions being deployed.
The Army’s vision of a rapidly advancing military force capable of remaining in constant communication with commanders and other services became a key driver for satellite terminal development. Wavestream amplifiers have been at the heart of many deployed on-the-move terminals. These systems are operating in some of the most demanding conditions of very high temperatures and the continual shock and vibration of wheeled and tracked vehicles.
Systems deployed on maritime and airborne platforms present similar challenges. Satellite communications on ships and aircraft is not a new concept. But there is an evolving need for much higher data rates and lower profile solutions. Wavestream amplifiers are allowing sub-meter terminals to transmit at T1 (1.544 Mbps) and above while at sea or in flight.
News gathering, disaster recovery, and homeland security are applications that are benefitting from having communications on-the-move (COTM), allowing personnel to be much more effective and safe.
The Challenges
System engineers tasked with developing satcom systems for COTM applications have many challenges to overcome. These are impacted greatly by the amplifier and its performance. The challenges boil down to five key categories: thermal, mass, vibration, power source, and reliability. All of these categories are inter-related; and gains in one area can lead to gains in other areas with a cascade effect.
Thermal
COTM terminals have one thing in common: the antenna system is in a sealed enclosure. As a result, heat exchange to ambient air is not direct. Unless thermal loading is controlled, a runaway condition can occur. The most significant heat source in the system is the power amplifier, and its efficiency directly drives the required thermal solution.
Today’s systems are moving to smaller and smaller antennas to reduce terminal height. This change has doubled or quadrupled the RF output power required to keep data rates constant. But demand for data rates has increased in most cases by a factor of four or more. Combined, these two changes have increased the required RF power by a factor of 8 to 16 times.
Wavestream has responded to this challenge with high-efficiency amplifiers in small packages. Efficiency gains typically remove one-half the heat dissipation of other amplifiers. Additionally, in most terminal designs, Wavestream amplifiers are able to be mounted directly on the feed of the antenna, minimizing (or eliminating) the loss between the amplifier and the feed found in prior systems. Mounting on the feed also eliminates costly (and lossy) RF rotary joints capable of handling high power and replaces them with low-power Intermediate Frequency (IF) designs.
Improving the system equation is the superior linearity of Wavestream amplifiers. Spectral regrowth and intermodulation performance define the operating point of the amplifier. Wavestream amplifiers deliver 1 or 2 dB better linear performance to comparable Solid-State Power Amplifiers (SSPAs) and up to 4 or 5 dB better linear performance over Traveling Wave Tube Amplifiers (TWTAs).
Mass
COTM applications require gimbaled, high-speed tracking antennas. Analysis of instrumented vehicle dynamics shows that tracking rates of up to 200 degrees per second, and tracking accelerations of 400–600 degrees per second are required to maintain communications performance while in motion. These requirements place a premium on reducing moving mass in the antenna’s gimbaled assemblies.
Further, in many designs, mass that is placed on final antenna axis must be offset to maintain balance. Mass savings in the amplifier often leads to mass savings in counterweight.
Overall, mass savings lead to simplified antenna designs with minimized motor requirements; which lowers thermal load, improves cost, and provides better tracking performance.
Vibration
Land-mobile, maritime, and airborne applications impose varied and difficult shock and vibration requirements on the amplifier. Many commercially available amplifiers will not stand up to the harsh environment presented by these platforms. Wavestream’s amplifier architectures are inherently robust — spatial power combining yields compact amplifiers that are less susceptible to shock and vibration. Wavestream designed amplifiers use only high military/industrial-grade components and processes. No matter if the application is military or commercial, Wavestream applies the same design heritage to its amplifiers.
Power Source
Often overlooked is the power supply. It can be the Achilles’ heel of any system and often is the cause of system failures. Wavestream has recognized this challenge and applied design methodologies used by space programs where power supplies must be efficient and completely trouble-free (once you launch you don’t see it again). Wavestream power supplies typically have 90 percent or better efficiency and are designed with the highest grade capacitors and other components to meet high Mean Time Between Failures (MTBF) in the challenging, high-temperature environment of COTM platforms.
Reliability
A significant challenge in COTM applications is repair and replacement. COTM systems, as the name suggests, are not fixed. Systems deployed with the warfighter, or used covering a news story, can be miles (of land or ocean) away from a depot or major city. Furthermore, the radome and mounting position of these systems present additional complications. The bottom line is the system MTBF must be high as the cost to diagnose and repair a failed unit is costly in time and in money.
As mentioned earlier, all the factors in the system design are inter-related. Improvements in each category lead to an overall improvement in reliability. Similarly, deficiencies in one category can undermine the overall reliability of the system.
A driving factor to MTBF is temperature. A decrease of 10 degrees in the operating temperature inside the radome can yield a 10-fold increase in the MTBF of a component, perhaps taking a component from 100,000 hours to 1,000,000 hours MTBF. The key heat source is the amplifier. The mass, efficiency and linearity improvements Wavestream provides work together to decrease the thermal load by up to a factor of four. This is huge — it can be the difference between a design with thermal runaway and a high MTBF design that meets the objective requirements.
The Solution
While several approaches for building high-power transmit amplifiers exist, many do not meet the particularly demanding needs for COTM applications. Conventional amplifier approaches use binary combining of several solid-state MMICs to build low-power modules and then binary combining of these modules into high-power amplifiers. The size and mass of these amplifiers preclude mounting them on the moving portion of the tracking antenna. The inefficiencies in this method of combining result in power dissipations that cannot be handled inside the radome.
Spatial Power Combining
Wavestream uses a different technique for combining the transistor outputs. Spatial power combining is the means for advancing solid-state solutions to the next level and the basis for the Powerstream™ amplifiers. Rather than combining in multiple steps, increasing loss and size with each combining stage, all transistor outputs are combined in a single step. Many amplifying elements synchronously amplify the input signal, and outputs are combined in free space for very high efficiency.
Wavestream’s patented Grid Amplifier™ chip couples the output from a grid of hundreds of transistors in free space with very low combining loss. Figure 1 shows a Grid chip and the individual unit cells. The resulting power-added efficiency (PAE) is a 2-4x improvement over traditional amplifier designs.
Simple Design
Each of the amplifier components is manufactured using standard techniques and these are assembled in a straight-forward manner. The signal passes directly through the chip, never touching a bond wire or other tunable element. No post-assembly tuning is necessary. The amplifiers can be manufactured in very high volume.
Heat from the amplifier chip is removed radially in a nearly optimal configuration significantly reducing junction temperatures and therefore leading to higher MTBFs. Figure 2 on the next page shows the assemblies of a two-stage Grid amplifier.
The Total Package
The result is an amplifier that is compact and much lighter than the traditional amplifier. Figure 3 is a picture of a 12W Ka-band amplifier, which measures a mere 2.25”L x 2.0”W x 1.25”H and weighs approximately 16 ounces. With power supply and integral driver, the power draw is only 125 watts.
The results at Ku-band are similarly impressive. Figure 4 is a picture of a 40W Ku-band amplifier, which measures 3.75”L x 2.5”W x 1.5”H and weighs 1.5 lbs. With power supply and integral driver, the power draw is only 275 watts.
These core amplifier modules are used directly in many COTM terminals with smaller apertures. On larger platforms, packages integrating the SSPA and Block Upconverter (BUC) with thermal management are used. Wavestream offers a wide range of frequency bands and output power levels.
Mission Effectiveness Enhanced
The new generation of COTM terminals enabled by Wavestream amplifiers is greatly enhancing mission effectiveness for land-mobile, maritime and airborne users. The challenges have been met. High data rates and high reliability are being deployed in the most demanding environments.
About the author
Gary Echo, Vice President Business Development at Wavestream, has spent his career in satellite communications. At Tachyon, Inc., a pioneer in IP-based satellite networking, Gary was Director of Product Marketing and General Manager of Mexico operations. He also served as Director of Business Development for Titan Wireless, a leading provider of satellite-based rural telephony. Gary was an early employee at ViaSat, Inc., where he filled various roles building leading-edge, military and commercial satellite communication systems. Gary started his career at Linkabit, where he worked on advanced modulation and coding algorithms. He earned his bachelor’s and master’s degrees in electrical engineering from Rice University.
