In this Tech Talk session, Kris Young, Field Applications Engineer for Silicon Labs, talked about the increasing challenges in wireless coexistence, its impacts on IoT application, and how Silicon Labs manage and offer built-in support for solving these challenges. Click here to watch the complete webinar and register now for future Tech Talks. Here are some key points from Kris’ session.

The Wireless Coexistence Challenge

What challenges exist in wireless coexistence? We have an ISM band at 2.4 GHz that’s very heavily used mostly because of different wireless protocols that share or coexist in the same band: Wi-Fi, Bluetooth, and IEEE 802.15.4 (Zigbee and Thread).

Although these wireless protocols have different modulation schemes, channel frequencies, and bandwidths, they all overlap when co-located, making a signal from one protocol sounds like noise interference to the other protocol. This causes problems in receiving messages between protocols because if the desired received signal is weaker than the noise, the radio is then unable to receive messages properly.

In the past, wireless devices seem to work even without specifically addressing coexistence issues. Unfortunately, ignoring the issues does not work anymore because of the following trends:

• Greater device integration and radio colocation
• Increased Wi-Fi transmit power (+30dBm)
• Larger throughput needed for Wi-Fi streaming

Coexistence Impacts

Looking further on the impacts of coexistence in IoT device development, we’ve determined two categories:

• Co-Channel – As far as IoT is concerned, IoT receivers are blocked if the Wi-Fi interference exists in the same channel and is stronger at the receiver than the signal being received from the remote IoT device. Also, Zigbee uses Clear Channel Assessment (CCA) to test the channel before transmit; and transmit is blocked with energy detection > -75 dBm per 802.15.4 specifications.
• Receiver Blocking/Selectivity – issues exist even when protocols are not on the same channel. For instance, even when Wi-Fi and IoT devices are on different channels, the Wi-Fi energy is still in-band for the LNA, AGC, Mixer, etc. because 2.4 GHz IoT RX sensitivity is being degraded by high amplitude 2.4GHz Wi-Fi. Multiprotocol gateways present a different issue – high power 2.4 GHz Wi-Fi TX in the same enclosure as IoT is difficult to isolate.

Coexistence Issues

• For End Devices – this issue can be clearly illustrated with a use case of a Zigbee security system with battery-powered sensors. In this case, Wi-Fi is co-located within radio range, and the traffic is used for streaming videos. The effect on the customer is manifested through delayed or missing reports as well as reduced battery life. This results in a high rate of retries and latency due to CCA failures. We recommend an Unmanaged Coexistence strategy to mitigate this issue.
• For Gateways – in a use case wherein Zigbee, Bluetooth, and Wi-Fi coexist in a small form factor, and the device is used as a Wi-Fi Access Point and as a home IoT gateway simultaneously (e.g., communicating with sensors), the effect to the customer is demonstrated through poor command responsiveness, delayed or missing Zigbee reports, dropped Bluetooth connections, and reduced battery life on Zigbee devices due to retries. We recommend deploying both an Unmanaged Coexistence and Managed Coexistence strategies to solve these issues.

Improving Coexistence

• Unmanaged – a key value of this strategy is frequency separation. This means selecting a Zigbee channel as far from the 2.4 GHz channel as possible and using a Bluetooth Low Energy channel map. Another scheme in this setup is antenna isolation, where you provide as much isolation between IoT and 2.4 GHz Wi-Fi antennas as possible. If IoT and 2.4 GHz are in separate physical units, it is recommended to implement install guidelines for minimum clearance. Another idea is to use a 20 MHz Wi-Fi bandwidth (avoid 40 MHz) and rely on protocol Retry Mechanisms.
• Managed – this setup is recommended in addition to (not in place of) the unmanaged coexistence techniques if you have a device that has all the radios built into it. The strategy calls for the separation of radio activity in time, which requires coordination between the radios. We use Packet Traffic Arbitration (PTA) to accomplish this. PTA is a recommendation provided in 802.15.2: “Coexistence of Wireless Personal Area Networks with Other Wireless Devices Operating in Unlicensed Frequency Bands.” PTA establishes that the IoT device asserts REQUEST and optionally asserts the PRIORITY signal and that if the Wi-Fi device can grant airtime, it asserts the GRANT signal back to the IoT device(s). The Wi-Fi device is expected to stop transmitting before asserting GRANT and is expected not to begin a new transmission while GRANT is asserted. Thus, when the IoT transaction is completed, the IoT device de-asserts REQUEST, and the Wi-Fi device follows by de-asserting GRANT.

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