by Robert Hammond, Resonant CTO, and Mike Eddy, Resonant VP Product Marketing & Business Development
5G networks operate in new and higher frequency bands, requiring new underlying technology and performance standards for RF filters. The industry must develop new resonating structures in order to filter at these high frequencies, and large bandwidths, with enough power to maximize signal range. Here are five key FAQs regarding 5G filters.
What’s a filter, and how does it work?
Radio frequencies occupy the range of electromagnetic spectrum designated for broadcasting over distance. Using the piezoelectric effect, RF signals are converted into electrical energy that causes minute fluctuations in a substrate. These fluctuations determine which frequencies are processed. Filters can select designated signals from specific frequency bands while rejecting unwanted signals that interfere with the reception of the intended frequency. In a modern smartphone, there are currently more than 50 RF filters allowing operation anywhere in the world. As the number of filters increases then so does the design challenge for the filter and the phone PC board. Good filter performance is important because bad filter design increases power consumption and allows interference.
What are the frequency ranges for 5G filters?
The 3GPP is the standards body for cellular networks. In 3GPP Release 15, it designated two ranges of radio frequency bands for 5G applications. Frequency range one (FR1) is between 450MHz – 6000MHz (6 GHz) and FR2 is from 24250 MHz (24 GHz) to 52600 MHz (52.6 GHz).
Within FR1, there are 34 frequency bands available for 5G, but from an RF filter perspective, the interesting bands are n77 (3.7 GHz), n78 (3.5 GHz) and n79 (4.7 GHz) because these bands offer channel bandwidths of more than 500 MHz, which is essential for the data throughput promised by 5G. However, this represents operation at the very high end of the performance range of acoustic wave resonator structures that are popular in 4G filters.
Within FR2 there are four frequency bands specified: n257 (26 GHz), n258 (24 GHz), n260 (39 GHz) and n261 (28 GHz). These are all designated for 5G millimeter wave (mmWave) services with channel bandwidths over 800 MHz.
Why can’t current resonator structures be used in 5G filters?
Resonator structures support resonance oscillation at defined frequencies. The most popular structures are acoustic wave because of performance and low cost. Surface acoustic wave (SAW) and bulk acoustic wave (BAW) have been practical up to about 2.7 GHz, which is lower frequency than the high bandwidth 5G bands of the FR1 bands. As operating frequencies increase, the physical dimensions decrease, making it more demanding on the processing technologies. Higher frequency signals have higher signal attenuation, which means filters need to support high-power RF signals to boost signal strength. So with the challenges presented by 5G for existing technologies, filter manufacturers are looking for new solutions to meet the demands of the handset manufacturers, which includes new and innovative structures, like the new acoustic wave structure called XBAR, which meets the demands for high frequency, high bandwidth and high power at the band edge as well as other options with more limited performance such as electromagnetic (EM), dielectric, cavity waveguide, on-chip filtering and microstrip (or planar thin film) filters.
What are the RF signal power concerns for 5G filters?
It’s generally true that higher frequency radio signals will need more power to maintain the same coverage as 4G due to higher signal attenuation. This has been born out in early trials where 5G signals operating at 28GHz demonstrate a signal range of up to 2,000 feet from the nearest base station with no obstructions. Because of this performance, it is anticipated that indoor coverage – where 80% of usage occurs – will be very poor. Filters need to have high signal power capability – as much as 1 Watt (30dBm) at the band edge – to achieve maximum transmit distance.
What are the interference and coexistence challenges for 5G filters?
In many use cases, particularly as the full potential of 5G for high data-rate, video applications is realized, the coexistence of 5G and Wi-Fi in the 3-6GHz frequency range will be essential. Bands n77, n79 and 5GHz Wi-Fi are adjacent in frequency with little guard band to separate these bands (see figure below). New Wi-Fi6 standards operate adjacent to the n79 frequency band, which, in turn, neighbors the n77. A high-performance filter, such as an XBAR filter, is essential to mitigate interference between the 5G and Wi-Fi bands, enabling maximum bandwidth operation, preventing Wi-Fi signals from bleeding into the n79 data path and vice versa.
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