Tuesday, August 7, 2012

Performance of a Bluetooth Piconet in the Presence of IEEE 802.11 WLANs

Due to its global availability, the 2.4 GHz ISM unlicensed band is a popular frequency band to low-cost radios. Bluetooth and the IEEE 802.11 WLAN both operate in this band. Therefore,
it is anticipated that some interference will result from both these systems operating in the
same environment. Interference may lead to significant performance degradation. In this section, we evaluate Bluetooth 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 [15 – 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 traffic time includes packet transmission time and a back off period.

To simplify the analysis, we make the following assumptions:

● The link is continuously established and the collocated systems are sufficiently close to each other such that the Bluetooth packet will be corrupted completely by the interference packets even if they overlap by a single bit.

● The desired Bluetooth packet will not be destroyed by another piconet if it is hit during the idle time.

● The desired Bluetooth packet will not be destroyed by an IEEE 802.11 network 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 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 DS, the data stream is converted into a symbol 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.

Source of Information : Elsevier Wireless Networking Complete

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