Solving Military Satellite, Radar and 5G Communications Challenges with GaN-on-SiC MMIC Power Amplifiers
Developers of aerospace and defense systems need RF power amplifiers (PAs) to perform much better across both existing and emerging applications such as military 5G and satellite communication. Systems need to meet higher gain targets but not if it comes with any increases in cost and complexity, or size and weight. As systems move to higher-order modulation schemes, they also must deliver adequate linearity and efficiency in an environment that is even more susceptible to distortion than was the case with earlier schemes. Reducing board space is another critical issue that has required challenging peak-to-average power ratio (PAPR) tradeoffs
A new generation of Gallium Nitride (GaN) Monolithic Microwave Integrated Circuits (MMIC) PAs offers a solution to these challenges, which are especially difficult for bringing 5G networking to both on-battlefield and off-battlefield applications in the unused millimeter wave (mmWave) band that is not as vulnerable to high-power jamming signals.
New Applications, Tough Challenges
In the aerospace and defense sector, some of the biggest challenges and opportunities for RF PA technology are in satellite communications and radar systems, as well as emerging 5G communications solutions for both on-battlefield and off-battlefield applications.
For instance, NASA has enabled private-sector companies to launch thousands of low-Earth-orbit (LEO) satellites that are now circling the Earth and delivering broadband Internet access, navigation, maritime surveillance, remote sensing and other services. New types of radar systems are also in demand (Figure 1).
One example of these radar systems is used to alert pilots to any hostile or foreign radar activity and whether they are being “painted” by the radar of a friend or foe. This can be accomplished with both primary and secondary radar systems. A primary radar system transmits pulsed RF power and receives backscatter data that is used for tracking, surveillance and weather. In contrast, a secondary radar system transmits RF signals at one frequency, which is received by an antenna and decoded, and then responds on a different frequency. In addition to performing friend-or-foe identification using 1030 Megahertz (MHz) and 1090 MHz frequencies, secondary radar systems can be used for distance-measuring equipment using the 960 MHz to 1090 MHz frequencies, and general communications using transponders.
RF PAs are also needed for a new generation of mmWave 5G communications solutions that, by virtue of their speed, ultra-wide bandwidth, and low latency for broadband communication, will substantially increase how much information can be shared in support of real-time decision-making and other military applications. 5G systems operating in wide bandwidths have been vulnerable to high-power jamming signals, but jammers will now have to move into the mmWave range for these close-range 5G-based systems. Examples include battlefield sensor networks for command-and-control data gathering, and augmented reality displays that enhance situational awareness for pilots and infantry soldiers (Figure 2).
5G will also enable virtual reality solutions for remote vehicle operation in air, land, and sea missions. Off the battlefield, 5G will enable a variety of smart warehouse, telemedicine, and troop transportation applications. Each of these applications requires high-performance power technology to meet the high-speed data rates required for video and broadband data. RF PA suppliers have had to balance a mixture of conflicting requirements in order to increase performance from one end of the transmission path to the other.
RF PA Requirements Vary by Application
Defense and aerospace applications operate in different frequency bands ( Figure 2). Satellite communications for LEO and geosynchronous communication operate in the K band, which spans from 12 GHz to 40 GHz. Radar systems operate in the 1 GHz to 2 GHz L band for applications including “identify friend or foe,” distance-measuring equipment, and tracking and surveillance. S band (2 GHz to 4 GHz) is used for selective response Mode S applications and for weather radar systems. X band (8 GHz to 12 GHz) is used for weather and aircraft radar, while C band (4 GHz to 8 GHz) is used for 5G and other sub-7 GHz communications applications. 5G mmWave provides the highest bandwidths and data rates of these applications, operating in the 24 GHz and higher frequency bands.
Each application also has different needs. For instance, one of the critical figures of merit for 5G applications is the PA’s linear output power. Power density must be as high as possible across a broad frequency range. The table in Figure 3 shows the key points of merit used in components for today’s existing and emerging aerospace and defense applications.
One of the biggest PA requirements is that it can operate in its linear region where distortion products are minimal. This increases complexity, cost, size and weight though, since more gain stages are then required to offset the reduction in RF output power that can be delivered in this region. Even then, gain distortion can come into play. Also known as AM/AM and AM/PM distortion, it describes the output phase variation against input power and is often caused by a PA’s nonlinear capacitors. This occurs when the PA is operated near or even beyond its saturation point to maximize conversion efficiency and generate as much power as possible, which leads to device nonlinearities and a compression or peaking of the PA with input power. Compensation is required using techniques like digital predistortion.
Developers face another flavor of distortion with satellite communications systems that use higher-order modulation schemes. This includes 64/128/256 Quadrature Amplitude Modulation (QAM), which is extremely sensitive to non-linear behavior. Another challenge is achieving satisfactory peak-to-average power ratio (PAPR) – that is, the ratio of the highest power the PA will produce to its average power. PAPR determines how much data can be sent, and is proportional to the average power. At the same time though, the size of the PA needed for a given format depends on the peak power.
These and other conflicting challenges can only be met with GaN MMIC PAs, especially for satellite and 5G applications.
GaN MMIC PA Benefits for Ka-Band Satcom Applications
Most of today’s LEO satellites operate in the 27.5 to 31 GHz Ka-band spectrum where they are playing a major role in supporting the tsunami of traffic being generated by video and other data-intensive applications. Unlike the traveling-wave tubes (TWTs) traditionally used as power sources at these frequencies, GaN offers higher efficiency and much lower operating voltages. GaN is also ideal for satellites in geosynchronous orbit that need their inherent radiation tolerance.
GaN-based PAs are also much smaller than TWTs and better aligned with the needs of active phased array antennas. They remove any requirement for complex and burdensome power combiners. They also deliver more RF power in a smaller footprint than gallium arsenide (GaAs) options, while operating at higher voltages.
GaN MMIC PA Benefits for Military 5G Networks
The mmWave (24 GHz to 100 GHz) frequency spectrum is much less congested than lower frequency bands that are struggling to keep up with the signal traffic from TV, radio, and current 4G LTE networks operating between 800 and 3,000 MHz. The higher the frequency bands, the more data can be carried (albeit over much smaller areas). This is what the military needs in its coming generation of close-range 5G-based systems. The mmWave band can be used to increase data bandwidth over smaller, densely populated on-battlefield and off-battlefield networks.
GaN can extend 5G New Radio (NR) femto- and pico-cell base stations deployments into the mmWave band for these military applications, where they will deliver the necessary bandwidth and data rates. Laterally-Diffused Metal-Oxide Semiconductor (LDMOS) MOSFETs are insufficient at >3.5 GHz. GaAs is incapable of delivering high enough power in the mmWave band without moving to an extremely large die. GaN offers the right balance of higher frequencies and power, wide bandwidth, and the required thermal properties, gain, low latency, and high switching speeds.
To fully realize their promise though, GaN MMIC PAs also need silicon carbide (SiC) substrates that improve power density by enabling MMICs to offer better thermal conductivity than is possible with silicon-based wafers.
Effects of Adding SiC Substrate
The use of SiC-based substrates improves GaN MMIC PA power density through better thermal conductivity than is possible with silicon-based wafers. Additional benefits include higher wafer yields because of SiC’s better lattice match with GaN, and a 20 percent smaller package size as compared to LDMOS technology, plus greater efficiencies. Air and spacebased systems get the optimum combination of high-power density and yield in the smallest footprint, along with lower weight, the highest possible power support, superior efficiency, and support for high-voltage operation with a longevity of at least 1 million hours at a junction temperature of 255 °C.
Component suppliers are bringing GaN MMIC PAs to market across more frequency options and more choices of bare die and packaged MMIC PA products. These offerings, along with complementary discrete high electron mobility transistor (HEMT) devices and other components, will give military system developers new ways to meet the unique needs of next-generation radar systems, satellite communications solutions, and 5G mmWave networks. GaN MMIC PAs will help solve the difficult linearity and efficiency challenges of higher-order modulation schemes and enable system designers to bypass the shortcomings of GaAs, LDMOS and TWT-based PAs so they can reach the necessary gain improvements for aerospace and defense applications without compromising on cost, size, weight, complexity or PAPR requirements.
This article was written by Michael Ziehl, Senior Manager of Product Marketing; and Baljit Chandhoke, Product Manager, Microchip Technology Inc. (Chandler, AZ). For more information, visit here .
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