Solving RF Communications Challenges with GaN SiC Power Amplifiers

This article explores the communications challenges of RF applications such as 5G, satellite communications, aerospace and defense, and how GaN-on-SiC power amplifiers can help.

RF systems require power amplifiers (PAs) to provide linear, efficient, high output power. As systems move to higher-order modulation schemes, such as 64/128/256 quadrature amplitude modulation (QAM), they must provide high linearity and efficiency in denser environments with tight peak-to-average power ratios (PAPR). The new generation of Gallium Nitride (GaN) on Silicon Carbide (SiC) Monolithic Microwave Integrated Circuits (MMIC) PAs offer solutions to these challenges with the highest power density to produce highly linear output power with high efficiency.

Figure 1. Applications of millimeter-wave 5G. 

RF Power Amplifier Opportunities and Challenges
The biggest growth opportunities and challenges for RF power amplifiers come from satellite communications and emerging 5G communications solutions. nasa helps private industry launch thousands of low earth orbit (leo) satellites around the earth to provide broadband internet access, navigation, maritime surveillance, remote sensing and other services. These RF applications are always looking for size, weight, power and cost (SWaP-C) advantages. Large dish antennas for satellite communications are being replaced by phased array antennas, which require smaller sized components for integration and lighter components. High RF power linearly correlates with high P1dB and IP3 to reduce distortion, and high efficiency through high PAE to minimize power consumption, which is critical for these RF applications.

Millimeter Wave 5G Communications
Next-generation millimeter-wave 5G communications solutions dramatically increase the amount of information that can be shared to support real-time decision-making and other military applications thanks to their speed, ultra-wide bandwidth and low-latency broadband communications. While 5G systems operating in lower frequency bands (below 6 GHz) are susceptible to high-powered interfering signals, 5G millimeter-wave (24 GHz and above) systems are bringing 5G networks via millimeter wave into frequency bands where applications on and off the battlefield are less susceptible to high-powered interfering signals. Examples include battlefield sensor networks for command and control data collection and augmented reality displays for enhanced situational awareness for pilots and infantry. 5G will also provide virtual reality solutions for remote vehicle operations in air, land, and maritime missions. Beyond the battlefield, 5G will support a variety of smart warehouse, telemedicine and troop transportation applications.

5G Millimeter Wave Bands
The 5G millimeter wave band varies from country to country. In the U.S., 28 GHz is the first 5G millimeter wave band to be deployed, followed by 39 GHz. china is deploying 5G millimeter wave at 24.25 - 27.5 GHz, but is already lagging behind in the adoption of 5G millimeter wave.

Figure 2. 5G mmWave frequency bands globally. 

5G Network Architecture
5G networks consist of macro base stations and small base stations. Macro base stations connect to the core network using millimeter wave backhaul or fiber links. Macro base stations can communicate directly with user equipment cellular phones or small cells, which communicate with user equipment mobile devices that provide last mile connectivity. Microcell base stations and femtocell base stations provide network connectivity in office buildings where connectivity may be weak or user density is high.

Femtocell base stations are typically installed by the user to improve coverage in smaller areas, such as home offices or blind spots within buildings. femtocells are designed to support only a small number of users and can only handle a few calls at a time - they have a very low output power of up to 0.2 Watts.

Femtocells offer greater capacity and coverage, supporting up to 100 users at ranges of up to 300 meters. Microcell base stations are often deployed indoors to improve poor wireless and cellular coverage in buildings, such as office floors or retail spaces. Microcells can be deployed temporarily to cope with high traffic in a limited area (e.g., sporting events), but they can also be installed in heterogeneous networks as a permanent feature of the mobile cellular network, working in tandem with macrocells to provide uninterrupted coverage to end-users. They have an output power of up to 2 watts.

Macrocells cover an area of >km and have an output power of up to >100 watts.

Figure 3. 5G network architecture comprising small cells and macro base station. 

Radar Communications
Radar systems operate in the 1 GHz to 2 GHz L-band for applications such as "friend or foe", distance measuring devices, and tracking and surveillance.
The S-band (2 GHz to 4 GHz) is used for selective response S-mode applications and weather radar systems. the X-band (8 GHz to 12 GHz) is used for weather and aircraft radar, and the C-band (4 GHz to 8 GHz) is used for 5G and other sub-7 GHz communications applications.
5G millimeter wave offers the highest bandwidth and data rates and operates in the 24 GHz and higher frequency bands. leo satellite communications and geosynchronous communications operate in the K-band, which ranges from 12 GHz to 40 GHz.

Figure 4. Marine Radar communication uses frequencies in the S-band, L-band, C-band, and X-band up to Ku/Ka-band. 

RF Beam Formation
The different types of phased array beam fouling architectures used in these RF applications include:
-Analog
-Digital
-Hybrid

Analog beamforming
For any phased array, the ideal spacing between elements is the wavelength lambda multiplied by two.

Figure 5 shows analog beamforming. There are four phased array elements with a wavelength lambda spacing of 2. For a 30 GHz signal, the spacing between the phased array elements is 5 mm. In analog beamforming, a phase shifter performs beamforming by changing the phase to interfere with the received and transmitted signals by focusing the energy from the beam in a specific direction for the phase length. This is all done at RF frequencies; therefore, it is most sensitive to interconnect losses. The signal from the phase shifter then enters the power synthesizer/distributor and then passes through the down converter and ADC/DAC to the baseband. In this case, only one digital front end exists for the N phased array element. The advantage of this architecture is that it minimizes the number of components and consumes the least amount of power, as shown in Figure 5, where there is only one digital front-end containing the ADCs/DACs of four phased-array elements. However, this beamforming architecture is the most sensitive to interconnect loss and phase shift complexity since the phase shift is accomplished in the RF band.

Figure 5. Block diagram of analog beamforming with four-phased array elements. 

Digital Beamforming
Digital beamforming has conventional up and down conversion to baseband frequencies and accomplishes digital phase shift. Since digital beamforming is done in the baseband, this architecture provides higher accuracy. However, each phased array unit has an ADC/DAC, resulting in a high number of components and high power consumption. In this case, for N phased array cells, there are N digital front ends. Fig. 6 shows four digital front ends containing ADC/DACs for four phased array elements.

Figure 6. Block diagram of digital beamforming with four phased array elements. 

Hybrid beamforming
Hybrid beamforming, which combines digital and analog beamforming, is optimal for large phased arrays, allowing the efficiency of analog beamforming to be obtained with fewer components, power consumption, and the accuracy of digital beamforming. Figure 7 shows two digital front ends containing the ADCs/DACs of four phased array elements. compared to analog beamforming, where there is only a single digital front end ADC/DAC; for digital beamforming, there are four digital front end ADCs/DACs.

Figure 7. Block diagram of hybrid beamforming with four phased array elements. 

RF Signal Chain
Figure 8 shows the block diagram of the RF signal chain. At the receiver, the RF signal enters through the antenna, passes through a limiter diode, then a switch, and passes through a sawtooth filter to select the desired RF frequency. The desired signal is then amplified through a low noise amplifier with a very low noise figure to minimize degradation of the signal-to-noise ratio of the received signal. It is then downconverted using a mixer. The local oscillator (LO) signal is generated using a discrete PLL component that includes a phase frequency detector and a prescaler to provide the LO frequency to the mixer, which down-converts the signal to the intermediate frequency (IF), and then signals are processed from IF to baseband.

Figure 8. RF signal chain block diagram. 

At the transmitter, the baseband signal is upconverted to IF and then to the desired RF frequency. The RF signal is amplified using a power amplifier to transmit the signal.    

RF Quality Factors
This table shows the RF quality factor and the benefits of the components used in the RF block diagram.

Power Amplifier Requirements
Power amplifiers play a key role as transmitters in RF applications.
One of the biggest requirements of a PA is that it can operate in the linear region to minimize RF distortion. Satellite communication systems using higher-order modulation schemes such as 64/128/256 quadrature amplitude modulation (QAM) are extremely sensitive to nonlinear behavior. Another challenge is to achieve a satisfactory peak-to-average power ratio (PAPR), which is the ratio of the highest power generated by the PA to its average power.The PAPR determines how much data can be sent and is proportional to the average power. Also, the size of the PA required for a given format depends on the peak power.The 5G millimeter-wave Effective Omni-directional Radiated Power (EIRP) requirements set by the FCC include 43 dBm EIRP transmit power for mobile handsets and 55 dBm EIRP transmittable power at the base station. GaN-on-SiC power amplifiers for satellite communications, 5G, aerospace and defense applications address these and other conflicting challenges.

GaN-on-SiC Power Amplifiers
GaN-on-SiC (Gallium Nitride on Silicon Carbide) offers the highest power density to efficiently generate highly linear output power. GaN-on-SiC power amplifiers operate at high frequencies in the Ka and Ku bands from 12 GHz to 40 GHz for satellite communications and 5G, and have the wide bandwidth, high gain, and improved thermal performance to meet the requirements of RF applications.
Microchip offers RF solutions utilizing GaN-on-SiC technology to meet the SWaP-C requirements of components. the ICP2840 is a flagship device that operates from 27.5-31 GHz and delivers 9 watts of continuous wave (CW) output power and 10 watts of pulsed output power with a gain of 22 dB and a power-added efficiency of 22%.

Figure 9. ICP2840 linear PAE across frequency and output power levels. 

Figure 10. ICP2840 linear gain across frequency and output power levels. 

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