Image Processing Reference
In-Depth Information
It was found that IEEE .b requires a CIR of about  dB to cope with a (narrowband) Bluetooth
hop into its (wide) passband. IEEE .b has the disadvantage that its back-off procedure was
designed to optimize the IEEE .b WLAN performance but not to handle external interferers:
Each loss of a packet due to a collision with Bluetooth will increase the back-off window size by
factor two (causing an unnecessary throughput reduction). Furthermore, the protocols overlaying
WLAN often incorporate TCP, which includes the risk that packet losses on the air link are mistaken
for network congestion, which then might initiate a slow start. In contrast, the main disadvantage
of Bluetooth is that its transmit power is  dB below that of IEEE .b. Another one is that the
BT reverse link packet which contains the ACK is transmitted on a different frequency than the for-
ward link packet. his increases the packet loss probability in case of (frequency static) IEEE .b
interference. he packet loss rate for Bluetooth PLR BT
then yields to
PLR BT
=
PLR forward
+(
PLR header
PLR reverse
PLR forward
.
(.)
It would otherwise—were the forward and reverse link to use same hop frequency—be PLR BT
PLR forward (depending on the IEEE .b system load).
In [PRML], the coexistence between IEEE . system and IEEE .. has been investigated.
Here it was found that, owing to the (same) difference in transmit power, IEEE .. can be heavily
affected by ., but that vice versa effect can hardly be noticed. In contrast to Bluetooth, IEEE
.. does not hop. If IEEE .. is being used in the same frequency band as an interferer, the
performance is impaired for as long as the interferer transmits. Bluetooth's FH represents a principle
advantage here; also in the range performance. While the Bluetooth performance can be expected
to degrade smoothly with increasing range [Haa], IEEE .. has been found to enter a “gray
zone” (owing to the frequency selective channel), in which the performance is unpredictable, before
the connections is lost [PRML].
There are in principle three different approaches to assist otherwise interfering systems to coexist:
separation in time, separation in frequency and separation in space.
25.6.1 Separation in the Frequency Domain
. IEEE .b can be used with an improved transmit filter that reduces the interference
power on the side lobe frequencies and thus enhances the separation on those carriers.
. Bluetooth can perform AFH, i.e., exclude the most heavily interfered frequencies from
its hop sequence. Note that in a lot of realistic situations, where the IEEE .b and
Bluetooth units are not in the same device, AFH is sufficient to combat the interference
effects. AFH has thus become part of the Bluetooth Specification . [Blu].
. For IEEE .. the channel allocation needs to be done carefully (ideally with an algo-
rithm that automatically selects the least interfered channel). [PRML] found that the
center frequency of IEEE .. should be shifted by at least  MHz compared to the
center frequency of IEEE ..
∗The approaches “separation through code” (keyword “CDMA”), through the channel (keyword “MIMO”), or through
the modulation (“I versus Q”) allow to unlink several users of the same system. It is not obvious though to apply any of
these latter methods to improve the coexistence of different systems, which is what is needed here.
 
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