Read part 1
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:
- The need to support multiple wireless standards
- Different types of interference
- Why band edges matter
- The importance of high-performance bandedge and coexistence BAW filters
One access point, many standards
When developing Wi-Fi access points, designers must consider many wireless technology standards:
Standards that operate at short and mid coverage ranges, such as Bluetooth, Zigbee and Z-Wave- Standards that operate at higher power levels and short and long ranges, including Wi-Fi, 3G/4G LTE and 5G
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 difficulties.
Different types of interference: From in-device to LTE and Bluetooth
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.
- In-device coexistence: For in-device coexistence, the system’s multiple antenna architectures can interfere with each other. As a result, the coupling between the affected antennas (antenna isolation) is compromised. The foreign transmit (Tx) signal increases the noise power at the affected receiver, which has a negative impact on the signal-to-noise ratio. The receive (Rx) sensitivity decreases, which causes what engineers call “desensitization.”
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:
- Two radio systems occupy bordering frequencies, and carrier leakage occurs.
- The harmonics of one transmitter fall on the carrier frequencies used by another system.
- Two radio systems share the same frequencies.
- LTE and Wi-Fi: As shown in the below figure, several LTE bands — Bands 40, 7 and 41 — are very close to the Wi-Fi band channels. Leakage into the adjacent Wi-Fi radio band is very probable at both the high and low end of the 2.4 GHz band. Without proper system design, the cellular and Wi-Fi channels 1 and 11 can interfere with each other’s transmissions and receive capability.
Bluetooth and Wi-Fi: Bluetooth and Wi-Fi transmit in different ways using differing protocols, but they operate in the same frequency ranges, as shown in the following figure. As a result, when Wi-Fi operates in the 2.4 GHz band, Wi-Fi and Bluetooth transmissions can interfere with each other. Because Bluetooth and Wi-Fi radios often operate in the same physical area (such as inside an access point), interference between these two standards can impact the performance and reliability of both wireless interfaces.
ISM, Wi-Fi & Bluetooth Channel Frequencies
Why band edges matter for Wi-Fi coexistence
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:
- Back off the power level on Wi-Fi channel 1 and 11, because they’re at the edge of the Wi-Fi spectrum.
- Use filters with very steep band edges.
Design tips to overcome interference challenges: Use high-Q BAW filters
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.
