SAN FRANCISCO — Broadcom Corp. this week at ISSCC offered the first peek at a universal quadrature-amplitude modulation transceiver for xDSL. The processor can handle QAM constellations ranging from four to 256 points, allowing its downstream speed to hit 51 Mbits/second. Broadcom said the chip can be used for either asymmetric or very-high-bit-rate digital subscriber line networks.

Asked whether a silicon layout optimized for VDSL was appropriate for simpler ADSL systems, Broadcom research director Robindra Joshi stressed that the key factor was not real estate but power dissipation. Broadcom has been able to implement a million-transistor, 100-pin transceiver chip that dissipates 850 mW in a central-office line card, Joshi said, and 1.25 W in a subscriber modem. The device is a true single-chip modulator/demodulator, integrating an ATM Utopia interface, packet-format logic and a Reed-Solomon forward-error-correction (FEC) block within a 0.35-micron four-level-metal CMOS process.

Elsewhere at ISSCC, Alcatel Bell (Antwerp, Belgium) gave a preview of what might be expected from the Alcatel Microelectronics team working on DynaMite ADSL chip sets. Daniel Veithen of the systems-research group described a VDSL processor using discrete-multitone (DMT) coding that hits 70 Mbits/s by performing 512-point fast Fourier transforms in less than 20 microseconds. Like the Broadcom device, the Alcatel modem integrates an FEC block and ATM Utopia interface in 0.35-micron CMOS. It uses 680,000 gates and 208 pins.

Texas Instruments Inc. gave its own analysis of new VDSL systems, specifically the synchronized DMT line codes that it has worked on with its new subsidiary, Amati Communications Inc. But where others described digital modems, TI envisions an analog front end integrating such functions as 12-bit pipelined A/D converter, line driver, programmable-gain amp and oscillator. Meanwhile, another group at TI described a codec used for filtering in ADSL environments.

Siemens AG, which has been focusing on multiple-rate symmetrical DSL services, presented two papers on new DSL devices. A group under Michael Moyal has developed a single-chip CMOS 768-kbit/s transceiver capable of carrying data on single-pair copper loops out to 25,000 feet. Another group at Siemens has developed a hybrid device that uses an independent analog adaptive-balancing filter for more effective echo cancellation.