Designer’s guide: RF front-end modules

Starting from the antenna and finishing at the modem, the front-end module is one of the most important parts of an RF system. Based on the application, RF front-end (RFFE) modules are available in various configurations that include power amplifiers (PAs), low-noise amplifiers (LNAs), switches, couplers, filters, power detectors, baluns and other components.

As the deployment and services provided by 5G technology are becoming increasingly widespread, the design of RFFE modules offers more challenges and opportunities for chipset makers and mobile or base-station device manufacturers.

Design complexity increases with 5G

About 10 years ago, the introduction of 4G LTE technology forced RF engineers to rethink the wireless signaling and transmission architecture inside the RFFE module. This was mainly due to higher data-transfer rates, lower latency and more efficient utilization of the available radio spectrum. With the addition of new advanced capabilities—such as orthogonal frequency-division multiplexing (OFDM) for data transmission, multiple-input multiple-output (MIMO) technology for improved signal reception and throughput, and carrier aggregation—the complexity of the RFFE design increased.

The advent of the 5G era has further raised the bar in design complexity, with RFFE modules capable of supporting more than 20 frequency bands and multiple antennas in increasingly confined spaces. In addition, 5G technology has brought greater bandwidth, extending the spectrum of frequencies used above the 24-GHz threshold, commonly known as the millimeter-wave (mmWave) spectrum.

The frequency bands supported by 5G wireless technology include sub-1 GHz, 1 to 6 GHz and above 6 GHz. All of these frequency bands should be supported by the 5G RFFE. Compared with LTE, which uses 20 MHz per subcarrier, 5G employs a higher bandwidth of 100 MHz per subcarrier.

Radio-signal reception and propagation on millimeter waves require innovative techniques different from traditional ones, with the need to implement advanced solutions like beamforming and beam steering. Making a viable RFFE that supports all three generations of cellular radio technology is more difficult, as 5G increases the number of radios and RF components in phones. Compared with 4G LTE technology, 5G technology offers lower latency (less than 1 ms), a larger data rate (over 10 Gbits/s) and a higher connection density.

Low latency is essential for implementing the ultra-reliable low-latency communication (uRLLC) feature required by mission-critical applications like autonomous vehicles, robotic control, industrial automation and vehicle-to-everything (V2X) communications.

Although the advent of antenna tuners has made it possible to employ already-built antennas for various frequencies, lowering the number of actual antennas, most 5G smartphones typically have multiple antennas to support the different wireless technologies and frequency bands used in 5G. Typically, a 5G smartphone may include:

• Two to four antennas dedicated to the sub-6-GHz frequency band, which offer broader coverage and improved reception
• Multiple antenna elements (up to eight) arranged in an array to deliver beamforming capabilities for improved signal reception and transmission on mmWave
• Other antennas dedicated to 4G LTE and other wireless technologies, such as 3G and Wi-Fi, for backward compatibility

Figure 1: Telecommunication tower supporting a 5G network, including radio modules and smart antennas (Source: Shutterstock)

In 5G, multiple antennas are required to implement antenna diversity, a technique used in wireless communication to mitigate the effects of fading and interference in the wireless channel. Due to the deployment of higher-frequency bands in 5G networks, which are more vulnerable to signal loss brought on by obstructions and reflections, antenna diversity is particularly crucial. The system can increase the overall quality of the received signal by using numerous antennas to take advantage of the diversity in the received signals, which may include varying intensities and phases.

The most commonly used antenna diversity types in 5G systems are the following:

• Space diversity: It involves using multiple antennas that are physically separated from each other. Placing antennas at different locations reduces the chance of all antennas experiencing deep fades or interference simultaneously.
• Polarization diversity: By transmitting and receiving signals with different polarizations, the system can take advantage of the fact that the wireless channel may affect different polarizations differently, mitigating the impact of polarization fading.
• Beamforming diversity: Beamforming implies that the transmit and receive antennas focus on specific directions to improve signal strength and capacity. It involves using multiple beamforming antennas that can dynamically adapt the radiation pattern to track the user’s location and optimize the signal quality in real time.

To achieve the high efficiency required especially by 5G mobile devices, RFFEs also adopt advanced technologies like envelope tracking (ET) and wide-bandgap (WBG) semiconductors. ET dynamically adjusts the PA’s supply voltage based on the input signal’s envelope (amplitude), allowing the amplifier to operate at a lower voltage when the signal amplitude is low, thereby increasing overall efficiency. WBG semiconductors, such as gallium nitride (GaN) and silicon carbide (SiC), offer higher efficiency, higher power density and better thermal management compared with traditional technologies like silicon.

Here are a few examples of RFFE chipsets and modules, targeting 5G applications.

Commercial devices

Movandi mmWave chipsets

Movandi Corp. specializes in delivering 5G mmWave RF chipsets and phased-array antenna modules with performance-optimizing algorithms, reference system designs and cloud software driven by AI that powers the 5G mmWave ecosystem. The RFFEs include phase-locked loops, up/down converters and beamforming optimized for 5G mmWave applications.

One example is the MV2850 front-end beamformer for the 28-GHz frequency band. The device is a fully integrated transmit/receive beamformer IC, supporting four dual-polarization antenna channels (or eight single-polarization antenna channels) for 28-GHz phased-array applications. The MV2850 supports two simultaneous and independent beams in the 3GPP n257 band (26.5 to 29.5 GHz) for 5G cellular systems. With the MV2853, a 28-GHz wideband IF up/down converter IC, and the MV3554 5G synthesizer IC or the MV3504 5G synthesizer IC, a complete mmWave phased-array RFFE chipset can efficiently support a wide range of 5G applications.

NXP BTS7203U

NXP Semiconductors’ BTS7203U is a receiver analog front-end (RX AFE) module provided with two channels operating in the frequency range from 3.3 GHz to 4.2 GHz, supporting the 5G massive MIMO (mMIMO) infrastructure. The RX AFE includes an LNA with varying gain control on each of its two receive channels. It also includes a switch for high-power TX data on each channel. The device features single-ended input/output RF ports matched to 50 Ω and built-in harmonic and out-of-band filtering.

Available in a 32-pin leadframe HVQFN package (5.0 × 5.0 × 0.85 mm), the BTS7203U offers fast switching time between RX and TX operation modes and ESD protection on all pins.

 Due to its low power consumption, the BTS7203U is highly energy-efficient, reducing the system cost of base stations. Traditional base stations have four to eight channels for sending and receiving data, but 5G mMIMO infrastructure plans usually call for 32 or 64 channels for sending and receiving data. The RX front-end module meets the need for more channels by providing a dual-channel solution optimized for low current consumption. This helps both carriers and OEMs reduce their power needs and operating costs.

The RapidRF reference board, shown in Figure 2, helps to accelerate the development of 5G radios that need an average broadcast power from 2.5 to 8 W (34 to 39 dBm) at the antenna. It includes a highly efficient RF PA, a linear pre-driver, an RX LNA with a T/R switch and a circulator, all on a single PCB that can be used for various frequency bands.

Figure 2: NXP RapidRF reference board (Source: NXP Semiconductors)

Qorvo QPF4005

GaN is a WBG semiconductor offering significant advantages in RF applications, in which high power, wide bandwidth, high operating voltage and high thermal capabilities are required. This is due to GaN’s unique material properties, such as high charge density, high electron mobility and high temperature tolerance. GaN power devices allow designers to reduce bill of materials and costs while increasing overall system efficiency.

Qorvo offers several RFFE modules supporting 5G technology. Built on Qorvo’s 0.15-µm process, the QPF4005 is a dual-channel, multi-function GaN MMIC front-end module, addressing 39-GHz phased-array 5G base stations and terminals. Each channel of the MMIC includes an LNA, a high-isolation transmit/receive switch with low insertion loss and a high-gain multi-stage PA. Operating in the frequency range from 37 GHz to 40.5 GHz, the QPF4005 is available in a compact 4.5 × 6.0-mm air-cavity laminate SMD package with an embedded copper heat slug. This enables the QPF4005 to operate in phased-array applications requiring extreme case temperatures.

Skyworks SKY58095-11

Skyworks Solutions Inc.’s Sky5 product family, targeting 5G/5G NR and 4G LTE applications, is comprised of highly integrated RFFEs, combining multiple RF capabilities in a single, small chipset. The SKY58095-11 Sky5 LiTE is a mid- and high-band front-end module for 3G, 4G and 5G applications.

The RFFE includes separate 3G/4G/5G PA modules operating in mid and high bands, a silicon controller containing the MIPI RFFE interface, RF band switches, MB and HB antenna switches, bidirectional couplers and integrated filters for bands 1, 2, 3, 7, 34, 39, 40 and 41. Internal matching of RF I/O terminals to 50 Ω reduces the need for external components.

The SKY58095-11 RFFE is optimized for LTE Advanced and 5G applications, supports ET operation and comprises all required components between the antenna and RFIC transceiver, providing high RX sensitivity.