GMSK-Modulated Magnetic Link Spurs PANs

July 23, 2001
Magnetic induction provides a low-cost alternative to traditional RF communications for personal-area networks in the 1- to 3-meter range.

For over 50 years, engineers have exploited magnetic induction and coupling to serve a multitude of applications. Voice and data communications, though, have eluded inductive technology for many reasons. One major factor is its limited range. Unlike RF communications, magnetic induction-based communication is restricted to a distance of about 3 meters.

But, the emergence of wireless headsets for cellular phones and personal-area networks (PANs) has made inductive communications an attractive alternative. This technology features lower cost, smaller size, reduced power consumption, and lack of interference and FCC compliance issues, as well as higher security. Motivated by these advantages, fabless semiconductor startup Aura Communications has begun to tap this technology and extend its reach into the communications arena. Aura is ready to go head-on against traditional RF communication systems and IR point-to-point links.

By implementing the principles of magnetic induction, the company is creating an infrastructure for wireless voice, audio, and data communications in PANs. Aura has crafted a system-on-a-chip (SoC) solution. Called LibertyLink, it enables point-to-point and point-to-multipoint voice and data transmission/reception in close-proximity applications.

LibertyLink permits multiple wireless devices to communicate with a single base transceiver. Offering maximum data rates of 204.8 kbits/s for point-to-point communications and 51.2 kbits/s for point-to-multipoint communications with one master and four slaves, it transmits a Gaussian minimum-shift-keyed (GMSK) modulated magnetic field between the base and remote units. The same chip is used in both the base and remote implementations, with only minor circuit and firmware changes.

The chip, via firmware, can also be configured for voice communication. In this mode, a 64-kbit/s continuously variable-slope delta (CVSD) encoded voice can be transmitted and received. Concurrently in the voice mode, the chip also permits data transmission at 21 kbits/s. Made on a 0.25-µm CMOS process, the SoC integrates all functions needed to complete a wireless communications system, including an 8051 microprocessor core (Fig. 1). The only external components required are a 64-kbit EEPROM, ferrite coils for antennas, and a few passives.

For firmware development, Aura provides application programming interfaces (APIs) and a library of function calls, or a dynamic link library. The developer has crafted a number of APIs targeting headset, keyboard, mouse, joystick, PDA sync, PDA voice, game controller, and Internet devices. Additionally, Aura's firmware team has generated most of the applications code, which resides in an external EEPROM. To differentiate their products, users can add their own hooks to the standard code written by the developer.

Magnetic Coupling In Aura's scheme, signals are coupled over a short distance using magnetic fields. "An important aspect of the field is that its electric field component is close to zero, thereby eliminating EMI interference," says Alan Swahn, vice president of marketing at Aura. "The absence of an electric field eases FCC requirements. But the magnetic field component drops off very rapidly, limiting its range."

The system sets up a quasi-static magnetic field around the transmitting coil. A receiver coil introduced into that field acts as an antenna and senses the magnetic flux. Associated circuitry around the antenna further detects the modulated current and processes the information to recover the voice or data.

Because magnetics are limited by the orthogonal properties of the fields, Aura has developed a novel antenna design. Taking the orthogonal properties into consideration, the system employs three antennas mounted 90° to each other, or one in each of the X, Y, and Z planes (Fig. 2). The magnetic field produced by an antenna oriented on the Y axis of a three-dimensional coordinate system will produce a field pattern such that a second antenna oriented in the same Y direction will receive the maximum field strength at a particular distance. At the same time, antennas oriented in the X and Z planes will receive minimum signal. According to Aura, this tri-axial antenna diversity scheme will require about 24 by 10 mm of spacing on one side of the pc board.

To minimize battery drain on the remote units in a PAN, the antenna diversity scheme is only used in the base transceiver. The remote units employ a single antenna. The LibertyLink incorporates a clever measurement technique on-chip that samples the incoming power from each antenna and detects the strongest signal.

Typically for a 2.2-V supply, the chip consumes 5 mA for voice and 4 mA for data communications across a 1-meter link. According to Chris Bunszel, Aura's marketing manager, LibertyLink consumes 10 times less power than competing RF solutions. Also, the cost to build a complete system is substantially lower using the SoC, Bunszel maintains.

Price & AvailabilitySampling now, the LibertyLink SoC is slated to go into production later this year. Housed in a 48-lead LQFP, it's priced at $6.50 in OEM quantities. An evaluation kit, which includes a board and a reference design, is available at no cost to qualified customers.

Aura Communications Inc., 187 Ballardvale St., Wilmington, MA 01887; (978) 988-0088;


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