5G RFFE modules get more complex

The fifth-generation (5G) mobile network introduces significant changes compared with the previous 4G technology, including the use of multiple sub-1-GHz, 1- to 6-GHz, and above 6-GHz (millimeter-wave frequencies) frequency bands. In addition to increased bandwidth, 5G technology provides low latency (less than 1 ms), high data rates (higher than 10 Gbits/s), and high connection density. To support the high performance required by 5G technology, RF front-end modules (also known as RFFE modules) play a crucial role.

The RFFE modules manage the signal path, both for transmission and reception, between the antenna and the RF modem. The circuit of an RFFE typically includes a low-noise amplifier (LNA) in Rx, a power amplifier (PA) in Tx, and various types of filters, switches, and diplexers, depending on the specific use case. For an LNA, the key factors are high sensitivity and selectivity, and for a PA, the main key factor is nonlinearity.

A simplified RFFE block diagram is shown in Figure 1. The transceiver block is comprised of two mixers, one for up-conversion (in transmission) and one for down-conversion (in reception). In the first case, the intermediate frequency (IF) is summed with the frequency of a local oscillator to obtain the (higher) frequency used for transmission. In the second case, the mixer performs the reverse operation; that is, it subtracts the frequency of the local oscillator from the signal to obtain the intermediate frequency. The advantage of this technique is that the processing of the IF signal is much simpler than the processing of the original signal (which, in the case of 5G, can reach a few tens of gigahertz) and allows the use of high-speed commercial analog-to-digital converters (ADCs) and digital-to-analog converters (DACs).

Figure 1: RFFE simplified block diagram. The RFFE includes a PA, LNA, filters, and a switch to toggle between the Rx and Tx path. The antenna can be a MIMO type. Click for a larger image. (Source: Stefano Lovati)

The complexity of RFFE modules has grown enormously with the introduction of the 5G network, mainly due to the exponential growth in the number of frequency bands and their combinations (which require the use of specific components and filters), in addition to the reduction of available space on the printed-circuit board. With over 10,000 possible combinations of available frequencies, 5G technology requires the use of different electronic technologies, ranging from CMOS (for frequencies up to about 7 GHz) as well as gallium arsenide (GaAs) and gallium nitride (GaN) for higher frequencies.

New challenges for RFFE modules

In addition to the exponential growth of frequency bands, 5G imposes new design challenges like beamforming and 5G NR massive multiple-input multiple-output (MIMO), which require a completely different approach to the design of RFFE solutions.

Beamforming is a technique that allows you to focus a millimeter-wave (mmWave) wireless signal in a specific direction, instead of covering a more or less large area, as occurs with normal radio signal propagation techniques. The high number of antennas available on 5G systems allows the base stations to focus the horizontal and vertical beams toward specific users, increasing both capacity and data rates.

The spectrum above 6 GHz, where mmWave operates, offers wide bandwidth but also increased complexity in signal transmission and propagation. In fact, it requires RFFE designs to minimize propagation losses due to foliage or rain and to lower oxygen and water absorption.

5G NR massive MIMO allows you to increase the number of antennas on the base stations to implement advanced features such as spatial diversity (increased reliability obtained by transmitting the same data over different propagation paths) and spatial multiplexing (simultaneous transmission of multiple messages).

Because these features must also be guaranteed with reciprocal movement between the mobile user and the base station, it follows that RF solutions designed specifically for the 5G network are needed and able to manage both sub-6-GHz and mmWave bands while providing reduced size and power absorption.

Another critical requirement of 5G technology is latency. Ultra-reliable low-latency communication (uRLLC), one of the pillars of 5G NR, requires a latency of less than 1 ms (in 4G systems, the latency requirement is between 50 and 98 ms). This allows 5G to support innovative and critical use cases, such as autonomous driving, robotics, factory automation, and communication between vehicles (V2V and V2X).

The higher number of frequencies inevitably leads to an increase in the number of filters. Unlike the PA, which covers a wide range of frequencies, filters must be designed for each individual frequency band. The current trend is to integrate these filters, made mainly with acoustic-based filter technologies such as surface acoustic wave (SAW) and bulk acoustic wave (BAW), within a single RFFE component, together with an LNA and PA (see Figure 1).

There are three main trends related to the evolution of RFFE modules that support 5G. These are the conversion from analog to digital, adoption of system-in-package (SiP) solutions, and usage of wide-bandgap (WBG) materials.

• Conversion from analog to digital for some RF functionality: Some signal processing (such as filtering and up-down conversion) traditionally implemented in analog form can today be handled entirely by digital circuits, typically digital signal processors. This allows for greater flexibility, lower costs, and reusability of some parts of the project.
• Adoption of SIP solutions: SIP packages are capable of integrating more and more features in the same chip. This allows for reduction in cost, size, and power consumption.
• Usage of WBG materials: WBG materials such as GaN are being adopted for the construction of amplifiers and other power devices, delivering several benefits over silicon, including greater efficiency, smaller footprint, and better thermal management.

Higher integration of RFFE modules

RF components supplied by leading manufacturers have characteristics that are based on the operating frequencies, the carrier channel bandwidth, and the RF application. In the specific case of 5G, OEMs generally offer highly integrated solutions based on different technologies.

Qualcomm Technologies Inc., for instance, has introduced RFFE solutions to support the performance and energy efficiency of its latest Snapdragon X70 Modem-RF System, which builds on the previous X65, X60, X55, and X50 solutions, supporting all commercial 5G bands from 600 MHz to 41 GHz (Figure 2). Qualcomm’s RFFE solutions integrate several RF components between the modem and antenna.

The Snapdragon X70 product portfolio includes:

• The QET7100, claimed as the first multi-mode, multi-output, multi-power amplifier, wideband envelope-tracking solution, which supports global 5G, sub-6-GHz, and LTE bands. This device offers 30% better energy efficiency than comparable competitive solutions and supports 100-MHz bandwidth for the new 5G frequency bands.
• Qualcomm AI-Enhanced Signal Boost, claimed as the first 5G adaptive antenna tuning solution enhanced with artificial-intelligence algorithms. An appropriate AI-trained model detects the positioning of the user’s hand, allowing real-time dynamic adjustment of the mobile device’s 5G antennas and improving accuracy by up to 30% compared with previous solutions. It is part of Qualcomm’s 5G AI Suite that also includes AI-based channel state feedback and optimization, mmWave beam management, and network selection.

Figure 2: Qualcomm’s Snapdragon X70 Modem-RF System. Click for a larger image. (Source: Qualcomm)

Other suppliers are building their product portfolios based on WBG materials. Qorvo Inc., for example, offers several components based on GaN in its 5G RF solution portfolio. GaN is a WBG semiconductor capable of providing higher efficiency and power density than silicon, plus the ability to operate at higher operating voltages.

The QPF4001, for instance, is a monolithic microwave integrated circuit (MMIC) front-end module (FEM) specifically designed for 28-GHz phased-array 5G base stations and terminals. Built on Qorvo’s 0.15-µm GaN-on-silicon-carbide (SiC) process, the QPF4001 (Figure 3) includes a low-noise, high-linearity LNA, a low-insertion–loss high-isolation T/R switch, and a high-gain, high-efficiency multi-stage PA. This device is the ideal solution for phased-array applications in which reduced space and excellent thermal management are mandatory.

Figure 3: Qorvo’s QFP4001 GaN-based RFFE. Click for a larger image. (Source: Qorvo)

Expanding its mmWave RFFE portfolio for 5G wireless infrastructure applications, pSemi Corp., a Murata company, has recently introduced three beamforming ICs and two up-down converters, available as discrete components or as part of the Murata’s 28-GHz antenna-integrated module (Figure 4). The eight-channel beamformer RFICs integrate PAs, LNAs, phase shifters, and switches into a single die that provides optimal signal strength for up to 1,024-element antenna arrays. The dual-channel, up-down converter RFICs integrate frequency multipliers, quadrature mixers, amplifiers, and switches into a single die that can be paired with up to 16 pSemi beamforming RFICs, or 128 total beamformer channels, to support massive MIMO, hybrid beamforming, and other active antenna configurations.

Murata and pSemi have also co-designed a 5G mmWave antenna-integrated module that supports the 28-GHz band. In a 4 × 4 antenna array, each module integrates high-performance antennas and pass-band filters with pSemi beamforming ICs and an up-down converter. Multiple modules can be combined, allowing designers to quickly scale and build antenna arrays of any size.

Figure 4: pSemi’s 5G mmWave RFFE module. Click for a larger image. (Source: pSemi)

Conclusion

With the introduction of 5G technology, the design complexity of RFFE modules has grown significantly. The new solutions will have to be smaller, more efficient, and easier to produce on a large scale. To meet the requirements of IoT applications, they will also need to be low-cost and highly integrated, thus reducing the number of components required.