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Tips for a peaceful Bluetooth/Wi-Fi coexistence

Posted: 16 Jul 2007 ?? ?Print Version ?Bookmark and Share

Keywords:avoid RF interference? prevent wireless interference? Bluetooth Wi-Fi design tips?

Both Bluetooth and Wi-Fi operate in the unlicensed 2.4GHz industrial, scientific and medical (ISM) band, and send data in packet form. Although they 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. Besides 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.

Despite this ambient RF interference, Bluetooth and Wi-Fi have gained increasing popularity with consumers, especially over the past six years. As both technologies are placed in close physical proximity, coexistence is a priority, and many detailed mechanisms have been introduced to counteract any interference.

As the Bluetooth specification has developed, new techniques have been added, allowing Bluetooth to coexist easily with Wi-Fi and other potential sources of interference. This article discusses the measures that have been implemented.

AFH!It was introduced in the v1.2 Bluetooth specification developed by the Bluetooth Special Interest Group and provides an effective way for a Bluetooth radio to counteract normal interference. AFH identifies "bad" channels, where either other wireless devices are interfering with the Bluetooth signal or 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 will 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.

Channel skipping offers some of the benefits of AFH to Bluetooth v1.1-qualified devices, even though some sacrifice of Bluetooth bandwidth is 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.

AFH identifies 'bad' channels where other wireless devices interfere with the Bluetooth signal or vice versa.

TDM!This is a tool used against the front-end overload-type interference that AFH can't manage. This approach was originally introduced to protect 802.11b/g transmissions from Bluetooth interference rather than vice versa. It works by shutting down all Bluetooth transmissions except those that are high-priority when the 802.11b/g radio is active on the ISM band.

Like channel skipping, this approach sacrifices part of the Bluetooth bandwidth, but the amount of bandwidth sacrificed is proportional to the 802.11b/g duty cycle. Thus, if the 802.11b/g is idle, the link maintenance traffic may lead to a 2-3 percent bandwidth degradation, which is impossible for a user to detect.

Channel quality-driven data rate (CQDDR)!Two forms of data packets exist: the DH and DM, which use high and medium bandwidth, respectively. DH packets can transmit more data within the packets. But if a part of the packet is corrupted, the entire packet must be retransmitted to recover the data.

The DM packets include FEC code, which takes up a third of the payload!for every 10bits of data, a 5bit FEC code is added, allowing the correction of up to two bit errors in each 15bit data/FEC block. This data packet format may reduce maximum data rate, but is more robust than the DH packets, which have no error correction included. It allows a receiving device to negotiate with a transmitter to agree which packet format is used according to ambient interference.

CQDDR remains an optional addition to a Bluetooth link and is not required by any Bluetooth specification.

Extended SCO (eSCO) channels!These are error-checking voice channels that allow the retransmission of corrupted voice data. Each packet has a CRC, so the receiver can check if packets have been received correctly.

Packets that are received with errors or lost altogether are negatively acknowledged. Retransmission windows allow retransmission of unacknowledged packets. The eSCO channel was introduced with v1.2 of the Bluetooth specification.

Version 1.1 SCO used in earlier versions of Bluetooth only used single slot packets. On the other hand, eSCO allows the use of three slot packets for synchronous voice or data.

This means that it is possible to get >100Kbps connection compared with the fixed 64Kbps from version 1.1. This is possible due to link capacity being lost (in the case of single slot packets) and gaps between packets (while the radio changes frequencies).

Error-checking voice channels allow retransmission of corrupted voice data.

At each eSCO instant that the master transmits an eSCO packet, the slave responds using the normal SCO rules (the slave is allowed to respond even if it doesn't receive the master's packet). Then the differences from SCO become apparent: There is a retransmission window during which unacknowledged packets can be resent until acknowledged. The spacing of the eSCO instant is negotiable.

With version 1.1 SCO, there was a choice of three different packet spacings all giving the same 64Kbps. With eSCO, both packet length and intervals can be negotiated in both directions of the link allowing asymmetric traffic. Although eSCO channels do not actively handle or avoid interference, the retransmission of corrupt data packets ensures that audio quality is less affected by interference from other radio devices than before.

Proprietary techniques
Besides these four mechanisms, companies have also made enhancements through proprietary techniques. For example, CSR also manufactures an 802.11 b/g hardware solution for embedded applications (UniFi). CSR has been able to develop further optimization measures via priority and channel signaling. It has implemented these additional features because even when current protection techniques are used, there are still some co-existence issues.

Take the example of someone using a Bluetooth headset that is paired with a wireless VoIP phone for voice communication. The synchronous Bluetooth SCO connection can still be disrupted by the packet reception acknowledgements that Wi-Fi is forced to transmit. This results in bad voice quality for the Bluetooth link.

With TDM and CSR's proprietary measures built into the UniFi device (with UMA-compliant 17dBm radio frequency output power), synchronous Bluetooth HV3 packets do not cause interference.

- Simon Finch
VP of Wi-Fi Strategic Business Unit
CSR plc




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