The Story of Filters in the World of 5G

The Story of Filters in the World of 5G

As more and more equipment are working under standards based on ultra-high frequencies (UHF), which according to definitions of ITU, imply a band that extends from 300MHz to 3GHz, the radio spectrum has been incrementally congested. In order to alleviate the problem of slow connection, gurus are eager to develop new specifications for the next generation of wireless communication systems, incubating the idea of 5G.

In reference to the requirements set by NGMN (The Next Generation Mobile Networks), 5G devices ought to be able to attain extremely high data rates, low latency and improved coverage. Through leveraging wide range of bands, including those with frequencies beyond 10 GHz, it is estimated that data can be rapidly transferred. An analogue of transition from 4G to 5G is a highway with more lanes. As the number of channels for interchange of data surges, assuming amount of data is kept constant, we should witness improvements in data rate.

However, the biggest problem encountered is the short wavelengths of extremely high frequency (EHF) waves. According to the simple physics equation v=fλ, with the velocity of electromagnetic wave fixed at 3 x 10^(8) m s^(−1), the higher the frequency, the lower the wavelength. As frequencies continue to increase, diffraction of waves becomes insignificant, meaning that a single base station’s network coverage will be massively dwindled as frequency rises. The implication behind is that much more base stations are required, which directly drags the demand for advanced filters that are capable of delivering excellent performance.

Under the above mentioned arduous circumstances, it is of no doubt that only a pinch of RF solution providers with state-of-the-art technologies can embrace the opportunities arose from emergence of 5G concepts. In my opinion, the key content growth driver for companies will be leading BAW filtering techniques.

To readers who have been closely tracking the development process of filters, you might have heard of SAW (Surface Acoustic Wave) and BAW (Bulk Acoustic Wave) filters. Both are passive filters that utilize piezoelectric substrate and interdigital transducers (IDTs), a special kind of transducers that transform acoustic waves into digital signals or vice versa, so as to remove unwanted waves while keeping the waves that lie in passband. For SAW filters, like how its name suggested, acoustic waves travel through the surface of the substrate, unlike BAW filters, which store acoustic waves inside the substrate.

Due to the variance in working principle, it comes to no one’s surprise that the two filters serve for different purposes. In fact, SAW filters are better for frequencies below 1.5 GHz. But as the frequency rises, for instance, at the level beyond 2.5 GHz, SAW filters start drifting much more significantly than BAW filters, meaning that performance is largely affected by temperature change. As we begin using EHF band, BAW filters plainly outperform SAW filters. Technically, BAW filters are known to have higher quality factor, a parameter that exhibits the relationship between center frequency and bandwidth, meaning that there is lower energy loss throughout the process of filtering. Above that, the sharper filter skirts, as reflected by the value of Q factor, causes the rejection curve of the filter to be steeper. This feature enables BAW filters to amplify desired waves more efficiently. Last but not least, the low insertion loss also implies that BAW filters perform excellently even under conditions with poor signals.

Back into the topic that we initiated at first, by refining the design of BAW filters, companies with such kind of technologies will have their golden age, as demand for the BAW components will skyrocket. At the same time, speaking of competition, unlike smartphone industry with numerous corporations, BAW filter industry is predominantly occupied by Avago Technologies and TriQuint, a subsidiary of Qorvo. The outrageously high complexity in developing related technologies has made new comers difficult to win over the existing companies, unless it is heavily funded and is willing to make substantial investments. Otherwise, the oligopoly is quite likely to sustain.