Customers have had success using high-Q bulk acoustic wave (BAW) bandpass filters, which offer many advantages:
- Extremely steep skirts that simultaneously exhibit low loss in the Wi-Fi band and high rejection in the band edge and adjacent LTE/TD-LTE bands
- Significant size reductions, which aid designers in creating smaller, more attractive end-user devices for homes and office environments
- Resolves coexistence of Wi-Fi and LTE signals within the same device or near one another
- Unique power handling capabilities, allowing for implementation into high-performance, high-power access points and small cell base stations
These filters address the stringent thermal challenges of multi-user multiple-input/multiple-output (MU-MIMO) systems, without compromising harmonic compliance and emissions performance. This is critical to achieving reliable coverage across the full allocated spectrum.
But why do high-Q BAW filters make such a difference for FCC band edges?
#1: BAW devices have lower insertion loss, steeper band edges and better temperature stability than SAW technology at Wi‑Fi frequencies
As you move into higher bandwidths like Wi-Fi, surface acoustic wave (SAW) devices can suffer from higher insertion losses than BAW due to radiation of acoustic energy into the bulk of the substrate. As shown in the following figure, as you move up (to the right) in frequency, you can see high-Q BAW is a good option for filter designs due to this bulk radiation loss effect. Also, BAW maintains the steep skirts required for FCC band edges; SAW can’t meet the performance requirements at these higher frequencies.
BAW also has better temperature stability than other technologies, which gives it an advantage during FCC certification tests. Most Wi-Fi designs are created at room temperature (20-25°C) on a bench, but the system in its application environment can actually operate around 60-80°C. Insertion loss increases as temperature increases, and failing to estimate for this can cause issues during product certification. Using BAW reduces the shifts in insertion loss and makes certification test results more predictable.
#2: BAW filtering can help engineers provide seamless transitioning between interfering bands
As shown in the following figure, the bandedge response is better using a filter than without it, and it allows designers to push the limit on RF front-end output power while meeting the FCC requirement for power spectral density. This means bandedge BAW filtering allows operators and manufacturers to deliver high-speed data and greater bandwidth by using spectrum that might be lost with no filtering.
#3: High-Q BAW bandedge filters can extend the range in channel 1 and 11 by 2-3 times
Wi-Fi designers normally must set the entire unit power to whatever the lowest bandedge-compliant power is for all channels. So, if the compliant channel at channel 1 is 15 dBm but channel 6 can achieve 23 dBm, the designer settles the entire power control scheme to 15 dBm. Using bandedge filtering allows designers to set the power scheme to much higher powers, thus making it possible to use fewer RF chains to achieve their goals.
BAW bandedge filters can also exhibit power handling capabilities for transmitting up to 28 dBm. This can improve system performance by greater than 15 percent and enable 5G multi-MIMO with less co-channel interference.
CPE developers who don’t use bandedge filtering have difficulty meeting FCC requirements on Wi-Fi band channels 1 and 11. In contrast, when high-Q BAW bandedge filters are used, it allows the CPE designer to keep the power level the same throughout all the channels (1 – 11).
To paint the picture, here’s the difference in user experience with and without bandedge filters:
- Without bandedge filters: Let’s assume you’re in a house with several individuals using Wi-Fi and mobile phones. You’re on Wi-Fi using channel 5, streaming a football game and experiencing no buffering or interruption. But then new mobile users arrive in the house and begin to take over your channel 5 Wi-Fi space. The CPE unit adjusts and bounces you to channel 1 to free up more space on channel 5. If the Wi-Fi unit doesn’t have bandedge filters (as in the block diagram on the left), your Wi-Fi strength and streaming degrade to the point where buffering occurs. Why? Because to meet the FCC requirement, the CPE unit must back off its power in channel 1 so it doesn’t interfere with adjacent cellular bands.
- With bandedge filters: However, if the CPE unit had been designed with bandedge filters (as shown in the block diagram on the right), channel 1 and 11 would not be compromised and the power level would not require backoff. You can watch your streamed football game without any buffering.