Why Coexistence Protocol Strategies are Critical in IoT Design

Smart devices and home automation are growing in popularity, and new access points and gateways are making their way into environments where multiple devices communicating across multiple wireless protocols are commonplace. Modern wireless devices rely on a combination of technologies, and the growing numbers of signals, increasing bandwidth, and overlapping bands are all factors that could impact performance. 

IoT gateways and hubs benefit from enabling multiple protocols such as Wi-Fi, Zigbee, Thread, and Bluetooth to support a diverse end-node landscape. Because these protocols share the same 2.4-GHz ISM band, adopting Wi-Fi coexistence strategies can help minimize performance degradation. Degradation happens when one or more collocated radios that may be operating with a Wi-Fi radio operate simultaneously.

And in the modern gateway we’re not always just talking about one or two radios—there may be as many as four or more radios within a single gateway to accommodate the Zigbee, Thread, Bluetooth, proprietary, and Wi-Fi communications. These radios, all supporting the 2.4-GHz ISM band, require effective coexistence strategies to ensure the products operate robustly and as advertised.

Radios operating within the same confined space can create interference for the other system radios, affecting performance and reliability. For the consumer, simultaneous and colocated operation of these different 2.4-GHz radio standards can degrade the performance and could result in signal loss or a product simply not working as intended. This ultimately limits the automation the device was purchased to deliver in the first place.

Interference can also lead to slower product-wide responsiveness and increased device power consumption as messages are resent. For example, when a Zigbee network shares a channel with a Wi-Fi network, the Wi-Fi network will usually block Zigbee communication.

Wi-Fi Coexistence

To improve interference immunity, each of the 2.4-GHz radio standards supports some level of collision avoidance and/or message retry capability. The 802.15.4 specification requires retries at the MAC layer and to boost delivery. Bluetooth messages require responses, and if a response isn’t received within programmable time, the application can resend the message up to a programmable limit. At low data-throughput rates, low power levels, and/or sufficient physical separation, these 2.4-GHz ISM standards can coexist with no significant impact on performance.

Managed Wi-Fi coexistence reduces interference when multiple radios operate in a single, small form-factor device alongside a Wi-Fi radio, such as in an IoT gateway or hub. It does this through a signaling system that coordinates access to the 2.4-GHz spectrum when needed by the appropriate radio. 

The IEEE 802.15.2 standard describes collaborative solutions including packet traffic arbitration (PTA), which is an effective managed coexistence solution. A separate PTA block authorizes all transmissions from the different interfaces using the same channel. The PTA block coordinates the sharing of the medium depending on traffic load and priority.

One approach to that is Silicon Labs’ managed Wi-Fi coexistence solution, in which PTA implements a coordination scheme that enables the EFR32 Gecko SoC to signal the Wi-Fi device before receiving or transmitting. Once the Wi-Fi device is aware of the request, Wi-Fi transmit can be delayed, improving the other radio system’s message reliability.

Such an approach supports up to three wire PTA implementations. It helps designers improve IoT wireless gateway and hub performance by improving responsiveness and reducing power consumption.

Conclusion

The number of Wi-Fi-enabled gateways utilizing some combination of Bluetooth, Zigbee, Thread, or other wireless protocols in homes and buildings is creating a sense of urgency in solving the coexistence challenge. The enthusiasm for home and building automation systems translates to more and more controllers adding Wi-Fi to existing radios. Consequently, there will be enormous growth in the number of gateway/controller type devices that include multiple 2.4-GHz radios and protocols.