January 30, 2018
This is the second blog post in a 2-part series looking at design challenges for Wi-Fi front-end designs. Part 1 covered thermal considerations.
There are any number of strategies that consumers can try to fix interference problems with Wi-Fi in their homes — moving the router, reconnecting the device to their Wi-Fi network, power cycling the modem … and calling their service provider when nothing works and they don’t know what else to try. But as an RF engineer, how can you design a Wi-Fi access point that addresses the biggest interference issues from the outset?
This blog post examines the following factors that can impact Wi-Fi interference:
When developing Wi-Fi access points, designers must consider many wireless technology standards:
Many of these standards can interfere with each other, leading to connectivity problems for users.
And then there’s unlicensed spectrum to contend with. Licensed and unlicensed networks are becoming more important factors as constrained wireless communications offload data to continually expand capacity. Also, the new Internet of Things (IoT) realm draws heavily on this unlicensed spectrum.
The challenge is to keep all these licensed and unlicensed bands and
multiple protocols working in conjunction with each other without interference
Interference can occur within a device or between devices, including between wireless carrier signals or between wireless standards. The most common interference scenario is Bluetooth and LTE with Wi-Fi because these technologies are so widespread. Let's look at some of these in more detail.
Desensitization is a degradation of the sensitivity of the receiver due to external noise sources, and results in dropped or interrupted wireless connections. It isn't a new problem — early radios encountered receiver sensitivity when other components became active — but now it’s particularly troublesome for today’s wireless technologies, including smartphones, Wi-Fi routers, Bluetooth speakers and other devices.
The primary “desense” scenarios are:
One way federal governments have tried to help consumers is by regulating the emissions and spectrum of many electronic devices and requiring consumer products to undergo compliance testing.
In the United States, the Federal Communications Commission (FCC) requires that most RF devices undergo testing to demonstrate compliance to FCC rules. They enforce strict band edges by requiring steep skirts on the lower and upper Wi-Fi frequencies, to help with coexistence with neighboring spectrum.
There are two ways for Wi-Fi access points to meet this FCC requirement:
Our approach is to use high-performance coexistence and bandedge filters, to allow Wi-Fi transmitters to operate close to the upper and lower FCC band edges.
Customers have had success using high-Q bulk acoustic wave (BAW) bandpass filters, which offer many advantages:
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?
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.
Learn more about BAW versus SAW in our free e-book, RF Filter Technologies For Dummies® (Volume 1).
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.
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:
In a connected world with more and more devices and wireless standards, coexistence and interference issues will not go away. To make use of every bit of spectrum available, Wi-Fi designs with high-Q BAW filters can improve the performance of Wi-Fi access points.
Learn more in this detailed Chalk Talk video (37:56), sponsored by Mouser Electronics and Qorvo, how Qorvo technology can help you achieve a Wi-Fi product that mitigates these coexistence and bandedge challenges.
"Chalk Talk" with Qorvo: Wi-Fi Connectivity (37:56). Watch Now >
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