High-Performance RF Filters Will Secure True Promise of 5G

Why the 5G neighbourhood needs effective fences

In an abundance of caution, the US Federal Aviation Administration (FAA) cited safety concerns in late 2021 to force a delay in the much-anticipated deployment of new mid-band 5G spectrum that another government agency, the Federal Communications Commission, had previously auctioned off to wireless operators.

This rather embarrassing intergovernmental clash came about because the new C-band 5G spectrum, at 3.7 GHz to 4 GHz, resides close to the 4.2 GHz to 4.4 GHz spectrum used by the aviation industry for altimetry applications. Despite more than 200 MHz of separation between them, the FAA was not convinced that all chances of radio frequency (RF) interference had been eliminated.

The seeds of the FAA’s mistrust appears to be sown in the performance of RF filters in aviation equipment and 5G network and phones. To date, cellular RF filters are just not effective enough to eliminate any doubt of radio interference in mid- to high-band frequencies, which are relatively new to 5G phone manufacturers.

The kerfuffle serves as an apt starting point for the third blog in my three-part series on 5G RF front-end technology, in which I look at the role of high-performance filters in realizing the true potential of 5G communications.

The biggest difference between 5G and previous wireless standards is the sheer volume of wireless spectrum put to use. For more details, see the first post in our series. Wider radio bandwidths and the use of more radio spectrums equate to faster data speeds and better mobile experiences — plain and simple. To take advantage of all the radio spectrum being opened up for 5G communications, phone-makers had to completely rethink the design of the radio front-end of the 5G device.

Further complicating matters are the vast amounts of disparate frequencies 5G phones have to corral to realize those amazing data speeds and experiences. From low-band frequencies measured in the hundreds of megahertz up to single-digit gigahertz ranges, 5G phones have had to expand the scope, performance and overall capability of their RF front-end designs to accommodate a plethora of spectral assets.

In Praise of Microacoustic RF Filters

The story of cellular phones cannot be fully told without a homage to the humble microacoustic filter. They are one of the reasons we can carry phones that fit comfortably in our pockets. Before the advent of microacoustic filters, mobile RF design relied on large and bulky components, which meant large and bulky phones.

A microacoustic filter transforms an RF signal input into acoustic waves, which can be isolated to specific frequencies as part of RF front-ends’ efforts to home in on specific licensed spectrum. The tunable characteristics and the small physical size of these components have made them the perfect filter solution to be designed into mobile phones for the past three decades.

Microacoustic filters have evolved over the years from 3G to 4G and now into the 5G era. In the days of 3G, when frequencies used were largely at 2 GHz or below, surface acoustic wave (SAW) acoustic resonators were sufficient to serve as filtering components in the RF front-end. Moving into 4G, as addressable spectrum increased and moved up in frequency to over 2.5 GHz, a more advanced version based on bulk acoustic wave (BAW) filters came to the fore.

In the age of 5G, the use of frequencies above 3 GHz and upward of 7 GHz is upon us, and the design envelope of current microacoustic filter technology is being pushed to its limits. To date, phone-makers and RF component suppliers have been using BAW-based filter designs from the LTE era in first- and second-generation 5G RF designs. However, LTE-period microacoustic filters were not designed to handle the performance criteria of higher frequencies, steep filtering performance and the larger bandwidths in 5G. As the FAA’s actions show, 5G filter performance has not proven that it’s precise enough to categorically dispel any fears of potential RF interference.

Enter High-Performance Microacoustic Filters

With network operators keen to take advantage of larger swathes of spectrum in the frequencies from 2.7 GHz to 5 GHz, the RF component industry must deliver a high-performance version of the existing microacoustic filter design. Much like the difference between the athletic performance of a professional athlete from that of an amateur, the demands placed on these high-performance filters are much more stringent than current BAW-based solutions. A handful of RF component suppliers have stepped into the breach, and I expect their designs to start entering 5G smartphones in 2022, helping to address the hesitation brought on by government agencies such as the FAA.

So, what do I mean by “high performance”? One measure of RF filter technology performance is its response to a specific frequency. For example, band n78, a very common global 5G band spanning 3.3 GHz to 3.8GHz, needs a filter with a sharp response at both ends of this range, allowing only signals between these two points and rejecting everything else.

Recently, developments by RF component manufacturers Murata and Qualcomm have yielded new high-performance microacoustic filters that offer the level of 5G RF filtering needed by the industry. These high-performance products are expected to appear in 5G smartphones next year and become the standard RF filtering technology as 5G matures throughout the decade ahead.

Source: Qualcomm

The Importance of Getting Along

Another major reason why we need effective 5G RF filtering is that high-frequency spectrum is getting very crowded. Not only does high-band 5G occupy a big chunk of the airwaves, but other services such as Wi-Fi (5 GHz and 6 GHz), ultrawide band (6 GHz to 8 GHz), V2X vehicular networking (4.7 GHz to 4.9 GHz) as well as aviation altimetry are all located in the same multi-gigahertz range.

For 5G communications to work well, high-performance filters are critically important to the overall handset design. It’s just as crucial that these filters perform well to eliminate any type of interference with Wi-Fi, ultrawideband or V2X services.

In many cases, 5G communications will be used in conjunction with these other wireless services, and it’s important that each service takes full advantage of all of the capabilities available to it. Therefore, it’s imperative that high-performance filtering technology exists not only to allow neighbouring applications to get along, but to bring out the best in all wireless services.

Why Good RF Front-End Is Critical to 5G

This three-part series has highlighted several points about the importance of the RF front-end design in smartphones, and how advances in RF design have enabled a slew of wonderful capabilities and experiences.

The RF front-end is very much the unsung hero in the story of phone hardware design. It has taken on an exponentially complex problem and through clever engineering abstracted it all away from your average user. People think that phones “just work”, but a big part of the magic is the amazing technologies that enable or improve features and experiences. I hope this series has helped to illuminate some interesting parts of the RF design and impart a new appreciation of the technology-dense device in our pockets.