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802.11, bluetooth, Spectrum Stuff

802.11 Co-existence: Bluetooth 4.1, 5.x

Bluetooth 5.x consists of three main modes.  Those are Low Energy (LE), Basic Rate or Enhanced Data Rate (BR / EDR), and Alternative MAC/PHY (AMP).

Both LE and BR / EDR modes utilise frequency hopping spread spectrum for interference protection, whilst AMP uses the 802.11 phy and mac to enable higher data rates.  A device which utilises AMP will only use this feature for times where it needs to rely on higher data rates.  The rest of the time the communication is done over BR or EDR.

BR / EDR utilises 79 channels, 1 MHz wide, whilst LE uses 40 channels, 2MHz wide.

When presented with an option between BR and EDR, it is better to select EDR because it presents less interference to wireless or other Bluetooth devices.  This is because the time spent transmitting data will be reduced due to the higher data rates.

Bluetooth takes advantage of adaptive frequency hopping (AFH).  AFH defines two types of channels, those ‘used’, and those ‘unused’.  Unused channels are replaced with used channels when required using a random selection method.  A slave device will report channel information back to its master device to help in defining the channels used in channel hopping.  When Bluetooth is operating with 802.11 WiFi, AFH is used along with other proprietary protocols to negotiate which channels to avoid.

This can be seen working effectively in a lab setup:

Train Nudging was introduced for dealing with interference from other wireless protocols on the ISM 2.4GHz band in version 4.1.  Train Nudging provides a method to improve page and inquiry success rates when channels are periodically unavailable.  It will adjust the channel scan activity for slots which were unavailable during the initial process until a timeout is reached.  Train Nudging is only applied for inquiry and page activities, which are used to discover and form a connection between two devices.

Bluetooth is exceptionally resilient, as can be seen by injecting 802.11 packets into the ISM 2.4GHz band creating a high duty cycle, and playing music via a Bluetooth speaker.  The process can be repeated, introducing different forms of interference into the ISM 2.4GHz band.  The protocol will detect that the channel is unavailable and mark it as such.

Whilst Bluetooth AMP requires some further analysis, it seems that the techniques implemented within the protocol will allow both 802.11 and Bluetooth to co-exist with each other without major impact to the respective applications

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