HomeThe Bluewater BlogBluetooth and Wireless co-existence

Bluetooth and Wifi (802.11b/g in particular) have two things in common: they're both increasingly ubiquitous in embedded devices, and they both use the 2.4 GHz frequency band. Although resident in the same part of the frequency spectrum, Bluetooth and Wifi use markedly different approaches to transmission management and collision avoidance. As a result, devices (or event localised areas) with both Bluetooth and Wifi active are prone to conflict and service degradation - in extreme cases, leading to wireless networking being completely unusable. The basic issue is that Wifi breaks down the frequency range into 14 channels, and transmits on a fixed channel. Bluetooth, on the other hand, hops between 79 channels at a rate of up to 1600 hops per second. When a Bluetooth transmission happens to coincide (in time and frequency) with a Wifi packet transmission, both transmissions may be lost. Due to the longer packet sizes, this has a far greater impact on Wifi transmissions, at worst leading to a cycle of packet loss, Wifi transmission rate dropping (thus increasing the risk of collision), and further packet loss - terminating in complete loss of Wifi connectivity. To address this problem there are a few basic approaches: 1. Limit inter-device interference via distance or shielding. By providing 25dB or greater isolation between antennas, the issue may be avoided entirely, but this is often not practical for small devices. 2. Activity signaling and time sharing. Many Wifi and Bluetooth modules provide signals indicating that transmission is currently in progress, or to defer non-critical transmissions. This avoids interference, since only one device is transmitting at any given point in time, but can adversely impact performance. 3. Channel signaling. Interference only occurs when both Bluetooth and Wifi attempt to transmit on the same frequency range, so notifying the Bluetooth module of what frequency slots to avoid during its hopping sequence can avoid interference with no loss of Wifi bandwidth, and minimal impact on Bluetooth. 4. Adaptive Frequency Hopping. The Bluetooth v1.2 standard outlines a mechanism for Bluetooth devices to automatically mark and avoid frequency slots that exhibit interference from other devices, this preventing interference for both Bluetooth and Wifi. Unlike the other solutions listed, this does not require special board-level support, but is only available where all devices in a network support it. These schemes typically rely on module extensions and require board-level support, and vary in terms of the types of interference they can address effectively. In particular, these solutions may be limited where there is broad-base interference or where communication between interference sources is impossible. A simple general solution is therefore not achievable, so care must be taken when doing board design and module selection to take these issues into consideration, and to address the specific coexistence needs of the product.

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