• Background

Ericsson began work on the Bluetooth technology around 1994 and invited several other large companies (IBM, Toshiba, Intel and Nokia) to form the Bluetooth Special Interest Group in 1998.  Since that time the Bluetooth SIG has seen substantial growth in membership.  The Bluetooth SIG has the responsibility of providing Bluetooth hardware and software certification, although a move is now afoot to permit self certification of the Bluetooth protocol.

The IEEE 802.11.15  WPAN Working Group was formed to standardize Wireless PANS operating in the Unlicensed 2.4 GHz operating range.    

The general IEEE 802.15 (WPAN) Working Group can be found here:

http://grouper.ieee.org/groups/802/15/index.html

The Bluetooth Special Interest Group can be found here:

http://www.bluetooth.com/

• Bluetooth description

Bluetooth is a low power (0dBm) ad-hoc wireless technology that operates in the unlicensed 2.4 GHz. ISM band at data rates up to 1 Mbits/sec over a range of about 30 ft.  An optional higher power PA (20 dBm) has been specified that can increase the range to roughly 300 ft.  The Bluetooth range numbers are not as per specification, but are derived quantities based upon the use of 5 dBi antennas operating in a free space environment.

The Bluetooth standard uses GFSK modulation on a Frequency Hopped Spread Spectrum signal with Time Division Duplex access.  Frequency hopping takes place pseudo-randomly at 1,600 times per second across 78 channels, each channel being 1MHz. wide.  Each Bluetooth conversation uses a different pseudo-random sequence.

Bluetooth uses the frequencies between 2.4 GHz. to 2.4835 GHz.  With the 78, 1 MHz. channels, this provides a lower guard band of 2 MHz. and an upper guard band of 3.5 MHz.

• Bluetooth operation

More than one Bluetooth device in a wireless network forms a piconet, with one master and up to 255 slaves.  There can be at most 7 active slaves with the rest remaining idle.  In the absence of any connections, all devices are in standby mode.  Devices listen for pages every 1.28 seconds on 32 predefined hop frequencies.

If the device address is known apriori, then page messages can be sent.   Otherwise, an inquiry message is sent first, and then the page message is sent.

Collections of piconets form something called scatternets.   The standard for this is still evolving with much work to be done in the area of resource co-ordination in a device currently active in one piconet that wishes to listen for pages in another piconet.

• Receiver Sensitivity

Communication distance is predominantly governed by transmitter output power, large scale channel impairments like path loss, small scale channel impairments like fading, Tx/Rx antenna characteristics and receiver sensitivity.

The Bluetooth range numbers of 30 and 300 (as a function of output power) feet being talked about are heavily dependent upon the sensitivity of the Bluetooth transceiver.   The sensitivity of a Bluetooth transceiver is specified to be -70 dBm.  On the face of it, this is a relatively relaxed value.  However, when you consider that this sensitivity must be achieved and maintained in a very small package, with very low power consumption, at low cost, in an environment of high digital noise, some of the challenges become visible.  This difficulty becomes even more apparent if co-existence requirements dictate one or more standards on the same PC card for example.

Some of the constituent metrics that influence the receiver sensitivity are noise figure, gain, and bandwidth.  The sensitivity of a receiver can be approximately expressed as (see Modern Communication Circuits, by Smith, pp.82):

S_i = F * K * T * B * (S/N)_o in the linear domain

where:

S_i = Sensitivity

F = Noise Factor (linear)

K = Boltzmann's constant

T = temperature (in degrees Kelvin)

B = Bandwidth

Alternatively:

S_i (dB) = NF -174 + 10 Log (B) + 10 Log [(S/N)_o] in the logarithmic domain (at room temp.)

where:

NF = Noise Figure (in dB)

Now, assuming that the GFSK modulation scheme used in Bluetooth requires a signal to noise ratio of roughly 21 dB to achieve an acceptable BER, and a 1 MHz. bandwidth, the required Noise Figure can be derived to be about 23 dB.

This is a really relaxed Noise Figure, but even this value may be hard to achieve given the amount of digital noise present in wideband, dual standard radios.  See app note on digital modulation circuit impairments.

• Interference

Besides receiver sensitivity, the other major factor contributing to communication link performance is the ability to handle interference.

In the unlicensed 2.4 GHz. ISM band, interference can come from many sources including: Microwave oven interference as well as IEEE 802.11b WLAN interference.

The frequency hopping nature of the Bluetooth scheme will provide some degree of robustness against interference.  However, depending upon the magnitude of the interferer and antenna separations, this interference can manifest itself as a reduction in data rate of a Bluetooth transmission.  In more severe scenarios (where the interferer is extremely close to the Bluetooth antenna), complete loss of communication can occur.

There are several white papers on Bluetooth and IEEE 802.11 co-existence here:

http://www.mobilian.com/whitepaper_frame.htm

There is a white paper on the performance of IEEE 802.11 WLANs in a Bluetooth environment here:

http://www.wlana.org/learn/whitep.htm