WiFi / Bluetooth Coexistence

Analysts predict that in 2010 out of 1.4 billion mobile phones shipped over 200 million will have WiFi (News - Alert). To distinguish them from non-cellular WiFi phones, these are called “dual-mode” phones. All dual-mode phones will also have Bluetooth.

When you use a Bluetooth headset for a cellular voice call, the handset’s cellular and Bluetooth radios stream simultaneously. Similarly, when you are using a dual-mode phone to make a call over WiFi, the WiFi and Bluetooth connections are simultaneously active. That’s a problem, since they run in the same frequency band. Their technologies are quite different and incompatible.

WiFi shares the medium (the radio spectrum) using a contention method that transmits only after first sensing that the spectrum is idle. If the packets get corrupted, it backs off to a lower speed that is less susceptible to interference, and retransmits. Bluetooth uses a “frequency hopping” technique that transmits very briefly on a narrow slice of spectrum, then instantly hops to a different frequency for its next transmission. If it hops into the middle of a transmitting WiFi packet, it can corrupt it. After a few such occurrences, the WiFi transmitter backs off to a lower speed. The lower speed transmission takes longer to deliver a complete packet, so it is statistically more likely to get zapped by another Bluetooth hop.

This leads to a vicious circle, of WiFi slowing down and consequently being more likely to be interfered with, consequently slowing down more. Chip manufacturers have come up with a variety of “coexistence” techniques that mitigate this problem. The Bluetooth 1.2 specification includes a feature called Adaptive Frequency Hopping (AFH) which tries to avoid channels where it senses WiFi activity. More explicit coexistence techniques have the Bluetooth and WiFi chips in the phone communicating with each other directly to arbitrate transmissions.

Even so, the current generation of dual-mode phones has difficulty with coexistence. It is common for organizations implementing FMC to ban Bluetooth. The standards bodies have been at work on this (IEEE (News - Alert) 802.15.2 and IEEE 802.19), and the next generation of combo WiFi/Bluetooth chips have improved coexistence capabilities, but the definitive solution is to move the WiFi out of the 2.4 MHz spectrum by using 5 GHz 802.11n. 5 GHz has the added benefit of 20 non-interfering WiFi channels compared to just 3 in 2.4 GHz. If you have a compelling reason to stay in the 2.4 GHz band, you can get around the problem by abandoning Bluetooth for headsets and using WiFi instead, an approach promoted by Nanoradio for several years, and now by an interesting startup called Ozmo. IT

Michael Stanford (News - Alert) has been an entrepreneur and strategist in Voice-over-IP for over a decade.

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