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Saturday, May 14, 2011

Coexistence of Bluetooth Piconets with Wireless LANS: The coexistence problem

With the upcoming pervasive deployment of wireless networks and devices on the unlicensed industrial, scientific, and medical (ISM) band, multiple (homogeneous and/or heterogeneous) networks using the same frequency band are likely to coexist in a physical environment. For example, it is common for users to be connected to the network from their IEEE 802.11b–enabled notebooks in the presence of Bluetooth devices or to form multiple Bluetooth piconets in a conference room. Without coordination among colocated networks, cochannel interference due to frequency collision has become a major performance limiting factor.

A Bluetooth piconet is formed when one master device and up to seven slave devices are connected via Bluetooth technology. Each piconet has a unique hop sequence that is determined by the clock and address of the master device. A time division duplex (TDD) scheme is used for the master and slave devices to transmit alternatively. In Bluetooth, different piconets have different frequency hop sequences, but they share the same 79 frequency channels on the 2.4-GHz band. Access technique in one piconet is time division multiple access (TDMA).

Bluetooth transmissions can be either symmetric or asymmetric. A symmetric link occurs when both the master node and slave node in a piconet transmit packets of the same size. An asymmetric link occurs when the master sends a packet of one size and receives a packet of a different size as a response from the slave. Real-time information such as voice is transmitted using a synchronous connection-oriented (SCO) link. The SCO link is a symmetric point-to-point link between a master and a single slave in the piconet. The master maintains the SCO link by using reserved slots at regular intervals. SCO packets are never retransmitted. Non-time-critical application such as data information is transmitted using an asynchronous connectionless (ACL) link. The ACL link is a point-to-multipoint link between the master and all the slaves participating in the piconet. Retransmission can be applied for most ACL packets.

The radio transmission technology adopted in Bluetooth is frequency-hopping spread spectrum (FHSS). Gaussian frequency shift keying (GFSK) is the modulation scheme used in Bluetooth. The Bluetooth transceiver operates in the unlicensed 2.4-GHz ISM band, where signals can hop among 79 frequency channels between 2.4 and 2.480GHz with 1-MHz channel spacing. The same 2.4-GHz band is also used by IEEE 802.11 WLAN, which usually adopts direct-sequence spread spectrum (DSSS). WLAN has a data rate up to 11 Mb/s and a transmission range up to 100 m. Using the carrier sense multiple access with collision avoidance protocol, several wireless stations form an ad hoc network where they can communicate with each other directly or communicate with a wired network through a centralized access point (AP). Consider the scenario illustrated below;

In this illustration two Bluetooth piconets are colocated with a WLAN. A Bluetooth packet may be destroyed if the transmission is overlapped by other transmissions from Bluetooth and/or WLAN both in time and frequency. Cochannel interference (CCI) caused by neighboring networks degrades performance significantly.

To reduce the CCI one well known scheme is the use of a Coordinated Colocated Access Point (CCAP).

The CCAP scheme reduces CCI in colocated Bluetooth devices by coordinating their hopping frequencies in a scatternet scenario. A group of piconets in which connections exist between different piconets is called a scatternet. The illustration below is an example of the scatternet consisting of three piconets, with master node denoted by a square and slave node denoted by a circle in each piconet.

In a scatternet, slaves can participate in different piconets on a time division multiplex basis. Slave E is an example for this case. In addition, a master in one piconet can be a slave in other piconets. For example, device B is the master of piconet B and also a slave in piconet A. In the Bluetooth specification, piconets are not frequency synchronized and each piconet has its own hopping sequence.

By breaking this rule and using CCAP, there can be a significant gain in capacity and throughput. The colocated devices forming a Bluetooth AP are essentially the masters of the piconets that they form. The CCAP technique operates by coordinating hop frequency selection between colocated master nodes. First, the hop timings of the master nodes are synchronized; then the same hopping sequence with different frequency offset is applied to each master node. Therefore no two devices use the same frequency at the same time, and CCI is eliminated between Bluetooth piconets.

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