Electronic Design

Two-Chip Transceiver Set Will Eliminate 802.11b/Bluetooth Interference

Bluetooth and 802.11b Ethernet are popular wireless communication standards. Yet their growth is expected to cause plenty of interference problems in the license-free ISM band. The TrueRadio two-chip transceiver set, produced by startup Mobilian Corp. of Portland, Ore., should prevent this interference by permitting simultaneous operation of both standards.

The 2.45-GHz ISM band hosts a range of devices, including microwave ovens and cordless phones. Also, 802.11b Ethernet wireless LANs, Bluetooth wireless personal area networks (WPANs), and HomeRF networking devices share the same band. While the broadband modulation methods and low power generally ensure that these devices do not interfere with one another, interference may become a problem as the volume of users increases.

One of the most likely problems is the co-existence of a Bluetooth radio inside a device that also uses an 802.11b LAN card. PCs and notebook computers are good examples. The close proximity of these two transceivers almost guarantees that one will at least desensitize the other, if not actually interfere with it. The Mobilian TrueRadio chip set prevents this.

The first chip in the TrueRadio set contains the analog/RF circuits. It listens to the 2.4-GHz band and detects all nearby wireless networks. Then, it connects automatically to both 802.11b and Bluetooth if both are present. The second chip, the baseband chip, sorts them out and allows two-way data transfers in both protocols. For example, a notebook computer could simultaneously communicate via Bluetooth to a nearby printer while connecting to an Ethernet LAN for Internet access or e-mail. Neither signal will degrade the other.

A plot of bit rate versus time showing the effect of a Bluetooth radio nearby tells the story (see the figure). A notebook PC is connected to an Ethernet LAN whose access point is 15 m away. A Bluetooth node is 1 m away. With the Bluetooth off, the 802.11b modem connects at a data rate of 5 Mbits/s. When the Bluetooth radio kicks in, the data rate drops considerably. Using the TrueRadio chip set with the Bluetooth radio on, the data rate on the 802.11b modem remains at its normal level.

While details of the chips have not been revealed, complex DSP and some patented techniques must be involved. Both protocols operate in the same frequency range, but their operational details differ.

The 802.11b protocol specifies direct-sequence spread-spectrum (DSSS) transmission with BPSK modulation and a carrier-sense multiple-access/collision avoidance (CSMA/CA) access method. Its maximum data rate is 11 Mbits/s with QPSK/CCK modulation and fallback rates of 5.5, 2 (DQPSK) and 1 (DBPSK) Mbits/s, depending on distance, noise, and other factors. Distances up to 100 m are possible, depending on the environment.

Bluetooth, on the other hand, uses frequency-hopping spread-spectrum (FHSS) transmission. It has 79 hop frequencies from 2402 to 2480 MHz, with a rate of 1600 hops/s. The modulation is Gaussian frequency shift keying (GFSK). The maximum data rate is about 721 kbits/s, and the maximum range is typically 10 m. The chip set simultaneously detects, selects, rejects, and connects with both signals present.

While the TrueRadio chip set won't be formally available until midyear, PC and PDA manufacturers as well as network card and access point providers will be interested. Mobilian's white papers on the coexistence issue can be found at www.mobilian.com. The Bluetooth (www.bluetooth.com) and Wi-Fi 802.11b (www.wi-fi.com) sites also have some coexistence information.

TAGS: Digital ICs
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