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21 Wireless Local Area Networks
MAC layer performance in the presence of neighboring Bluetooth piconets and
neighboring IEEE 802.11 WLANs.
A packet collision occurs when a desired Bluetooth packet [11,12,17] overlaps
the interfering packets in time and frequency. In Bluetooth, the duration of a single
slot packet is 366 ms and the duration of the slot is 625 ms. The time between the
end of the transmission of the packet and the start of the next slot is the idle time.
Similarly, the duration of one 802.11 packet traffi c time includes packet transmis-
sion time and a backoff period.
To simplify the analysis, we make the following assumptions:
The link is continuously established and the collocated systems are suf-
fi ciently close to each other such that the Bluetooth packet will be cor-
rupted completely by the interference packets even if they overlap by a single
bit.
The desired Bluetooth packet won’t be destroyed by another piconet if it is
hit during the idle time.
The desired Bluetooth packet won’t be destroyed by an IEEE 802.11 net-
work during the IEEE 802.11 backoff period.
In Bluetooth, the hopping patterns are 100% uncorrelated.
For a long enough observation time, a given transmitter uses the 79 hopping
channels equally.
There are also 79 channels spaced 1 MHz apart in the IEEE 802.11 frequency
hopping (FH) system.
Each station’s signal hops from one modulating frequency to another in a
predetermined pseudo-random sequence.
The collision probability of Bluetooth to the IEEE 802.11 FH system is 1/79.
In the IEEE 802.11 direct sequence (DS), the data stream is converted into a sym-
bol stream which spreads over a relatively wide band channel of 22 MHz, so the
interference on a Bluetooth packet from IEEE 802.11 DS system is much higher
than that from the 802.11 FH system. It is because the bandwidth of a channel
in DS is 22 times as wide as Bluetooth one channel. The collision probability of
Bluetooth to the IEEE 802.11 DS system is 22/79.
