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5G RF front-ends: Design challenges and their solutions

Analog front-ends (AFEs), also known as radio frequency front ends (RFFEs), are components that use smart partitioning architectures by integrating high-speed amplifiers, receive analog-to-digital converters (ADCs), and transmit path digital-to-analog converters (DACs) along with ever-shrinking high-frequency filter designs. The AFE solutions are used both in 5G base stations and handsets.

More specifically, in 5G designs, AFEs are needed to support multiple transmit and receive paths in multiple-input multiple-output (MIMO) configurations. Not surprisingly, therefore, the rising complexity of 5G has been limiting the number of manufacturers that have the expertise to develop such complex RF sub-systems. At the same time, however, more suppliers are stepping up to the AFE challenge as 5G designs mature.

That’s because RF designs represent a huge opportunity in 5G networks that will deploy a vast number of base stations in mini-, micro-, pico-, and femto-cell devices to facilitate appropriate subscriber coverage. The 5G networks will also require higher integration and miniaturization than 2G, 3G, and 4G to reduce power dissipation and cost.

A key aspect of 5G technology is its ability to support Internet of Things (IoT) as well as industrial IoT (IIoT) applications. And AFEs are at the heart of this bandwidth innovation. The increased AFE bandwidth in 5G networks no longer serves only cellphones like prior cellular networks; instead, 5G can be used as an Internet service even for laptops and desktop computers.

Figure 1 The 5G Release 16 has ushered in new capabilities while raising the 5G AFE design bar to a whole new level. Source: 3GPP

AFE design challenges in 5G

5G aims to support the substantially higher data rates and the large traffic volume flow; add to this a massive growth in connected devices for very diverse use cases and requirements. To meet this massive boost in wireless data traffic capacity, spectral efficiency and reuse, higher speeds, and lower latency are major considerations for AFE designs.

Spectral efficiency and reuse

While the spectrum under 6 GHz is crowded with several wireless applications, the spectrum above 6GHz, especially the millimeter-wave (mmWave) band, has drawn a great deal of attention for its wide available bandwidth.

In order to support the transmission in the mmWave band, which is most challenging in outdoor areas, the task of an AFE is to improve high path loss, high oxygen and H2O absorption, the losses through the foliage, and fading due to rain. To overcome all these unfavorable channel characteristics in the mmWave frequency, extend transmission distance and improve service coverage, beam-forming and beam-tracking are the two most crucial technologies used in AFE designs.

Speed

Speed is another crucial factor in AFE designs. The 5G AFE architectures operate at higher speeds than prior 2G, 3G, or 4G systems. The present 5G systems are ten times faster than 4G LTE. The 5G has a maximum speed of 10 Gbps—with a strong potential of reaching 20 Gbps—while 4G LTE ran at 1 Gbps, 3G at 42 Mbps, and 2G at 0.3 Mbps.

Latency

The RF front-end architectures in 5G have new elements that will provide faster access and low latency. Latency in 5G AFEs is so much more important than in previous 3G and 4G versions. 5G has a minimum latency of 1 ms or less. On the other hand, latency in 4G systems was at 50 ms to 98 ms, in 3G systems at 212 ms, and in 2G at a whopping 629 ms.

That’s why the new 5G services now use ultra-Reliable Low-Latency Communication (uRLLC) feature, which is especially needed in mission-critical applications in autonomous vehicles, robotic control, factory automation, and vehicle-to-everything (V2X) communications.

Meeting AFE design challenges

The RF chipmakers have risen to this 5G design challenge with new AFE/RFFEs. Data converters in these AFEs/RFFEs support the channel bandwidths available in mmWave bands, and it will open the doors for the generalization of RF architectures that potentially will reduce the complexity of RF circuitry by moving the digital/analog divide closer to the antenna.

Figure 2 The functional block diagram of AFE7988/89 highlights a new level of RF integration in 5G designs. Source: Texas Instruments

Then there are AD9081 and ADR554x RF front-ends for massive MIMO (M-MIMO) radios launched by Analog Devices Inc. (ADI), as shown in Figure 3. These AFEs vastly increase the number of simultaneous transceiver channels operating in multiple bands while squeezing all the necessary hardware into a smaller form factor.

Figure 3 The integrated dual-channel architecture allows RF designers to quickly scale their MIMO capacity to meet 5G bandwidth demands. Source: Analog Devices Inc.

Clearly, with the number of antennas and bands that need to be supported, along with the large number of devices that are necessary to achieve adequate coverage, 5G transceivers must take full advantage of integration to reduce power dissipation and costs (Figure 4).

Figure 4 The 5G AFE has four TX/RX paths with an observation channel. Source: Synopsys

The 5G AFE/RFFEs, with RF sampling getting closer to the antenna, will simplify and shrink radio form factors and enable higher levels of integration. That, in turn, makes the direct RF sampling for 5G a design reality in both base station and smartphone designs.

So, advanced packaging and integrated AFE/RFFE modularization are ever closer to reality as 5G designs continue to shrink. Next, with 6G at tera-hertz frequencies, we will see even smaller AFE form factors in the near future.

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