(Editor's note: This article, first published at the beginning of 2007, is being presented to our readers again with minor updates to provide an additional perspective on the present controversy regarding to choice of Wi-Fi for the high-speed Bluetooth specification presently under development.)
Bluetooth and 802.11 b/g Wi-Fi have both cemented themselves as essential wireless technologies, leading to the natural convergence of both into devices such as laptops, PDAs, personal multimedia players and cell phones.
Certain devices such as wireless VoIP phones and multi-standard cell phones even demand simultaneous operation of both Bluetooth and Wi-Fi, putting heavy demands on chip design. Therefore, the co-existence of these two technologies can no longer be achieved through limited usage models or simply by creating a distance between the radios.
Without care during development, embedding both Bluetooth and Wi-Fi into a device can cause interference issues that affect user experience.
Both Bluetooth and Wi-Fi operate in the unlicensed 2.4 GHz industrial, scientific and medical (ISM) band and send data in packet form. Although Bluetooth and Wi-Fi use the spectrum differently, interference still occurs when a Wi-Fi receiver senses a Bluetooth signal at the same time as a Wi-Fi signal is being received.
The same applies to a Bluetooth receiver. In addition to the challenges presented by coexistence with other wireless standards, Bluetooth communication links may also be disrupted by other household devices such as microwave ovens, which radiate RF energy as a by-product of their operation and can only be limited to a certain level due to cost and engineering constraints.
In spite of this ambient RF interference, Bluetooth and Wi-Fi have gained increasing popularity with consumers, especially over the past six years where Bluetooth products and wireless LAN networks have appeared in more homes. As both technologies are placed in close physical proximity, coexistence is a priority and a number of detailed mechanisms have been introduced to counteract any interference.
In order to limit the amount of power transmitted in any single area of the ISM band, spread-spectrum techniques of data transmission are mandatory for both Bluetooth and Wi-Fi. Bluetooth employs Frequency-Hopping Spread Spectrum (FHSS) to transmit data packets across a comparatively narrow bandwidth of 1 MHz.
The frequency of the narrow band signal is then changed at a rate of 1600 hops per second within the 79 channels available within the range. By hopping frequently around the spectrum, the signal power is spread across the band.
When normal interference occurs, reception of part of a transmitted packet of data may be interrupted due to an overlap in the Bluetooth and 802.11 b/g signals, resulting in packet errors. Closely located antennae can cause front-end overload interference on the second system running. However, this interference requires a stronger interfering signal and is therefore a less common problem than normal interference.
As the Bluetooth specification has developed, new techniques have been added which allow Bluetooth to coexist easily with Wi-Fi and other potential sources of interference. A number of measures which have been implemented to this end are explained below.
Adaptive Frequency Hopping (AFH)
Adaptive Frequency Hopping (AFH) was introduced in the v1.2 Bluetooth specification developed by the Bluetooth Special Interest Group (SIG) and provides an effective way for a Bluetooth radio to counteract normal interference. AFH identifies "bad" channels where there are either other wireless devices interfering with the Bluetooth signal or where the Bluetooth signal is interfering with another device.
The AFH-enabled Bluetooth device will then communicate with other devices within its piconet, to share details of any identified bad channels. The devices then switch to alternative available "good" channels, away from the areas of interference, thus having no impact on the bandwidth used. For AFH to work, the classification of the bad channels must be accurate and 'normal' interference should be the only form of interference. Figure 1 demonstrates AFH working effectively.
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Figure 1: Proper operation of adaptive frequency hopping (AFH).
Default settings for CSR's BlueCore Bluetooth silicon adapt to interference coming from a new source within approximately 4 seconds.
Channel skipping offers some of the benefits of AFH to Bluetooth v1.1-qualified devices although there is some sacrifice of Bluetooth bandwidth necessary to minimize disruption to Wi-Fi signals. Time-critical media applications such as stereo audio streaming and mono audio headsets are typically not effected as far as the user is concerned when the AFH is switched on.