Electronic Design

5-GHz Chip Sets Use OFDM To Break Wireless Ethernet LAN Speed Barrier

Two fabless semiconductor companies, Atheros Communications Inc. of Sunnyvale, Calif., and Radiata of Sydney, Australia, have announced chip sets to implement complete wireless Ethernet LAN interfaces with previously unattained data rates. Both sets implement the IEEE-802.11a wireless Ethernet LAN standard. This development is possible thanks to the use of orthogonal frequency-division multiplexing (OFDM). This technique takes advantage of recent fast DSP chips that make it practical (see the figure).

The 802.11a standard specifies speeds as high as 54 Mbits/s using OFDM in the 5-GHz industrial, scientific, and medical (ISM) band. It offers benefits over the older 802.11b standard, which spells out a data rate up to 11 Mbits/s using direct-sequence spread-spectrum (DSSS) techniques in the 2.4-GHz ISM band. In fact, Apple's AirPort wireless LAN already uses this technology. These chip sets also can be used to implement wireless LANs that conform to the ETSI's HyperLAN2 standards in Europe.

Overcoming Hurdles
Wireless LANs have never become wildly popular because their data rates have typically been less than 2 Mbits/s. Also, they're overly sensitive to noise and multipath interference common to transmission of signals indoors. They've been too expensive as well. All of these problems are about to be minimized with the new chip sets.

The 5-GHz band was selected for the 802.11a standard because it is a cleaner, less-used band than the 2.4-GHz ISM band. Many consider the 2.4-GHz band a "junk" or "garbage" band because of the potential interference from microwave ovens (2.45 GHz), cordless telephones, forthcoming Bluetooth products, 802.11b interfaces, and a variety of unlicensed wireless services.

Additionally, the U.S. Federal Communications Commission (FCC) set aside 300 MHz of spectrum space in the 5-GHz band in 1998 to help implement the Unlicensed National Information Infrastructure (U-NII). This initiative is designed to give schools, hospitals, and other institutions greater access to the Internet and other networks.

One chip in Atheros' two-chip set, the AR5000, is based on the radio-on-a-chip (RoC) idea the company was founded on in 1998. It contains a receiver, a low-noise amplifier, a downconverter, IF stages with filters, an AGC, a transmitter upconverter, a power amplifier, a frequency synthesizer, and an entire OFDM modem. The second chip contains all of the baseband and media-access control (MAC) circuits, including the analog-to-digital converter (ADC) and digital-to-analog converter (DAC), the host interface, and RAM. Together, these chips form the complete wireless LAN interface.

External parts needed include the crystal oscillator for the synthesizer, an RF switch, RF bandpass and low-pass filters, voltage regulators, an EEPROM, some discrete components, and an antenna. For higher receiver sensitivity, an external low-noise amplifier can be included. Designers who want to achieve a higher output power can add an external low-noise amplifier to the power amplifier.

The Atheros chip set permits up to 1 W in some segments of the 5-GHz band. Its total parts count is considerably lower than the parts count in a typical 802.11b product. The resulting product can achieve data rates up to 72 Mbits/s with an operating radius of 100 to 300 feet.

Radiata's chip set consists of the R-RF5 radio transceiver and the R-M11a wireless LAN modem. All circuits are on-chip, including the receiver low-noise amplifier with a bypass switch, synthesizers, IF filters, and the transmitter power amplifier. The signals are all differential. Also, the receiver gain is variable over a 70-dB range. The receiver's noise figure is 6 dB.

Like the Atheros chip set, an external low-noise amplifier can be accommodated to improve sensitivity, as can an external power amplifier to boost output power. The transmitter's nominal output power of 0 dBm can be controlled over a 60-dB range.

The R-M11a modem implements the complete 802.11a modem functions. It uses a 64-channel OFDM system with BPSK, QPSK, 16-QAM, or 64-QAM modulation. It can reach up to 54 Mbits/s, and it connects directly to the R-RF5 transceiver via 20-MHz differential baseband links. The modem is designed to work with one of several existing third-party 802.11a MAC interface chips.

An Array Of Applications
The new CMOS chip sets address the cost issue, as their price point should translate into a very affordable wireless network interface card (NIC). As a result, they're expected to find a home in many applications. The companies are quoting a price of $35 per set in 100,000-unit quantities. They expect the chips to be available early next year.

Their primary use will be in wireless LAN network-interface cards for enterprise PCs and networks. Wireless LANs offer the undeniable advantages of installation flexibility, scalability, speed, and simplicity over traditional hard-wired LANs.

A secondary application is the emerging home-networking market. Many different technologies—such as HomeRF, Bluetooth, power-line modems, and HomePNA—are vying for adoption in the home market. Yet there is no clear leader so far. These chip sets may change all that.

A third potential market includes the companies that provide wireless connectivity for travelers who need a reliable Internet connection in public places, like airports and hotels. The sets will help these services grow and achieve broader acceptance.

Both Atheros and Radiata have managed to build a complete 5-GHz radio transceiver and modem with standard 0.18- and 0.25µm CMOS processes. Getting CMOS to work at these frequencies is tough. But with small geometries, careful layout, and obviously some proprietary techniques, it has been achieved. Previously, it was necessary to use GaAs or SiGe devices to achieve satisfactory performance at 5 GHz, making the resulting chips and end products very expensive.

For more information, contact Atheros at (408) 773-5200 or go to www.atheros.com. To contact Radiata, call (408) 938-5740 or point your browser to www.radiata.com.

TAGS: Digital ICs
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