Bluetooth
Adaptive Frequency Hopping
Bluetooth operates within the same frequency band as
other devices so there will be interference to and from other devices.
Some of common types of devices that operate at 2.4 GHz include cordless
telephones, microwave ovens, wireless cameras, and 802.11 wireless LAN.
Some 2.4 GHz cordless telephones use simple FM
modulation and they continuously transmit when they operate (continuous
interference). These cordless telephones have a relatively narrow
bandwidth so only a small portion of the Bluetooth radio channels will be
affected.
Inexpensive (residential) microwave ovens transmit
over a wider frequency range but they only transmit for short cycles of 16
to 20 msec on and 16 to 20 msec off. This interference will only occur
when the microwave is operating and when Bluetooth is transmitting, and
the interference is relatively small.
Wireless cameras (such as a security monitoring
system) can use wide 6 MHz channels and continuously operate, which can
interfere with 6-8 Bluetooth RF channels.
One of the most likely interference sources with
Bluetooth is 802.11 Wireless LAN (WLAN). The most popular version of
802.11 (802.11b) uses 25 MHz wide channels that are very sensitive to
radio distortion. This means
that 802.11b is more sensitive to Bluetooth than Bluetooth is sensitive to
802.11b.
Because Bluetooth transmits over a wide frequency
bandwidth of 79 channels, much of the Bluetooth packets will get through
even the toughest interference sources. Because Bluetooth transmits at
such a low power, the amount of interference Bluetooth causes to other
systems is low.
This figure shows some of the types of devices that
interact with Bluetooth and how they may interfere with each other. This
diagram shows that a 2.4 GHz cordless telephone only uses a relatively
narrow radio channel and it only interferes with a fairly small amount of
Bluetooth packets. The microwave interferes with a larger number of
packets, yet the burst nature of microwave ovens still allow many
Bluetooth packets to get through. The interference of a wireless security
camera is constant and can interfere with a small percentage of Bluetooth
packets. This example shows that the most significant interference occurs
with 802.11 WLAN systems because they use a relatively high-power 25 MHz
radio channel that is also sensitive to even small amounts of radio
distortion.
Bluetooth Radio
Channel Interference Diagram
Bluetooth specification 1.2 introduced adaptive
frequency hopping (AFH) that can reduce the effects of interference
between Bluetooth and other types of devices. AFH adapts the access
channel sharing method so that the transmission does not occur on channels
that have significant interference. By using interference avoidance,
devices that operate within the same frequency band and within the same
physical area can detect the presence of each other and adjust their
communication systems to reduce the amount of overlap (interference)
caused by each other.
The adaptive frequency hopping process reassigns the
transmission of packets on frequency channels that have interference to
other channels that have lesser interference levels. This reduced level of
interference increases the amount of successful transmissions therefore
increasing the overall efficiency and increased overall data transmission
rates for the Bluetooth device and reduces the effects of interference
from the Bluetooth transmitter to other devices.
This figure shows how the Bluetooth specification 1.2
allows the Bluetooth device to change its hopping pattern to avoid
interference to and from other devices that operate within its frequency
band. This example shows a video camera that is using multiple frequency
channels. After the Bluetooth device detects the presence of a continuous
signal being transmitted by the video camera in the 2.4 GHz frequency
band, it automatically changes its frequency hopping patter to avoid
transmitting on the frequency band that is used by the video camera signal
transmission. This results in more packets being successfully sent by the
Bluetooth device and reduced interference from the Bluetooth device to the
transmitted video signal.
Bluetooth Adaptive
Frequency Hopping Operation
Diagram
Bluetooth Books
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more
details
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Introduction
to Bluetooth, 2nd Edition
This
book explains how Bluetooth devices can locate, authenticate, discover the
capabilities, and setup connections with other Bluetooth devices. Radio packet
structures, modulation types, and protocols are described and explained along
with Bluetooth security pairing (authentication) and privacy (encryption)
processes.
$1 9.99
Printed, $16.99 eBook9.99
Printed, $16.99 eBook9.99
Printed, $16.99 eBook
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