Preventing Bluetooth Interference

With the imminent arrival of the Bluetooth specification, attention is turning to the real details of how to successfully engineer Bluetooth solutions that work - and work with each other. The radio solutions must be like good orthodontics: a durable combination of skill and art - not brute force - that results in beautiful alignment.

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By Terry Bourk Ph.D,Senior Director, Advanced Products,Silicon Wave.

The largest potential market for Bluetooth solutions appears to be cellular telephones. Indeed, Bluetooth's roots are in the GSM world and forecasts predict fast growth of Bluetooth in the GSM markets. However, putting a powerful cellular radio next to a low-power Bluetooth radio in a cellular phone requires careful design because of the possibility of RF transmit and receive interference between the two radios.

Cellular Phone Transmitter Blocking a Bluetooth Receiver

The biggest problem in adding Bluetooth to cellular telephones is the potential for the powerful cellular phone transmitter to block the Bluetooth receiver during transmission.

This problem affects half-duplex cellular systems (the cellular radio either transmits or receives, but does not do both simultaneously) such as the TDMA-based GSM standard, as well as full-duplex systems (the cellular radio can simultaneously transmit and receive) such as CDMA or analogue FM.

This cellular phone interference is caused by wide-band noise generated by the cellular transmitter. Basically, the cellular phone will transmit not only the required data signal but also a certain level of noise. Some of this noise will appear in the Bluetooth band. The level of this noise may be sufficient to interfere with or block an incoming Bluetooth signal.

To illustrate, consider the situation of a GSM-based phone containing a Bluetooth module. In the GSM standard there are three possible bands: GSM900, PCS1900 and DCS1800. The output power for each band is shown in Table 1.

While GSM handheld transmitters produce 1 to 3 watts, the Bluetooth receiver is intended to operate effectively with signals as low as 10 picowatts, or 1/100,000,000,000 of the power, and thus the Bluetooth receiver can be overwhelmed by its overbearing cellular phone neighbour. In addition, in GSM conventional transmitter architectures, the radio up-converter can generate significant noise.

Therefore, engineers must develop Bluetooth systems using special radio filters that can function despite noise from the GSM internal and spurious radio signal interference.

Wide-band noise is noise generated by the transmitting signal which is beyond the limits for legal transmission. In all three types of GSM systems, for example, the wide-band noise that falls in certain bands is restricted.

The cellular specifications do not address these issues because all radio standards assume one kind of radio transmission in a given device.

However, the rules are just catching up with the Bluetooth application of a cellular transceiver and a Bluetooth transceiver in the same device.

The FCC/ETSI requirements do not yet consider the case of having a 2.4-GHz device inside a 900-MHz, 1800-MHz or 1900-MHz device.

To ensure that the Bluetooth radio module will operate effectively inside a cellular phone, the level of noise from the phone's transmitter must be measured and controlled. This is particularly true if the phone uses a filter at the output. It is important that this filter does not have a spurious response in the 2.4-GHz band.

In addition, designers need to build in a defence against interference. In radio engineering terms, the wide-band noise, measured in a 1-MHz wide band in the 2.4-GHz band, should be less than -100 dBm. If this is not the case, a trap should be placed at the output of the transmitter to attenuate energy in the 2.4-GHz band.

This will be relatively easy to add to today's phone designs. The new third-generation (3G) systems being planned will challenge Bluetooth developers even further because their signals (usually 2.1 GHz) are so close to the 2.4- GHz band used by Bluetooth.

Bluetooth Transmitter Blocking a Cellular Phone Receiver

The second problem in the cellular phone application is the Bluetooth transmitter noise blocking the receiver of the cellular phone - the "David and Goliath" problem. The aim is to keep the transmitted noise from the Bluetooth transmitter into the receiver of the cellular phone less than or equal to the channel noise.

Each cellular standard presents  specific challenges to Bluetooth. Overall, Bluetooth has an innovative and well thought-out architecture to survive in this harsh radio environment, but extensive testing of real radios is the only way to insure compatible high performance solutions.

Silicon Wave, Inc. develops and manufactures innovative RF-enabled systems-on-a chip for broadband communications products. Taking advantage of an advanced silicon-on-insulator (SOI)BiCMOS , software and systems design technology, Silicon Wave is developing wireless IC products with significant advantages in size, performance and power consumption. High levels of integration make the company's products especially attractive in Bluetooth and W-CDMA applications. Silicon Wave was founded in 1997 and is based in San Diego, California, USA.

For more information, please see the Silicon Wave web-site .

This article first appeared in Incisor, the Bluetooth newsletter.  Contact Vince Holton at ; Telephone: +44 (0)1256 701646

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