Manhasset, N.Y. While direct-conversion radios are not new, no one had done an implementation for two radio interfaces in different bands. Startup Quorum Systems Inc. (San Diego) leveraged its staff's expertise in converter, filter and synthesizer design in developing a Wi-Fi and quad-band GSM/GPRS/Edge transceiver that it says overcomes the space, power and cost barriers to internetwork communication for mobile handsets.
Using a proprietary scheduling scheme, the chip is said to allow pseudo-simultaneous operation of wireless and cellular radios, without the bulky, expensive RF isolation and shielding techniques that have dogged integration attempts to date.
Quorum was able to tap the experience its team acquired while at InnoComm Wireless, a wireless-LAN chip developer bought by National Semiconductor in 2001. At the core of Quorum's intellectual property are a direct-conversion, multimode down- and up-converter; an adaptive filter arrangement; and a fast-settling fractional-N synthesizer.
"We do a fast charge on the loop filter by injecting a current across the charge-pump capacitor," said founder and CEO Bernard Xavier. That has reduced the settling time when switching from one wireless interface to another from 150 microseconds to about 120 µs. "There are ways to get to 50 microseconds, but we haven't figured them out yet," he said. The eventual goal is 10 µs.
Quorum "had to solve a lot of problems that we never faced before" in developing the integrated transceiver, said Xavier. Those included handling the second-order intercept point (IP2) and dc offsets, as well as the in-phase and quadrature-phase imbalances that occur in a radio that "has to change itself such that it looks like a different radio in 10 microseconds." This was done using signal processing in the analog domain.
The adaptive filter has a bandwidth of 200 kHz (GSM) to 11 MHz (Wi-Fi), Xavier said, "and can change its shape, order, bandwidth and even its own power consumption, all dynamically and all over a single antenna." External roof filters are required, as are coupling capacitors and a 13-MHz crystal. The VCO is on board.
To allow the two radios to run seemingly simultaneously, the company developed a software layer, called the Quorum Multi-Access Technology, that interleaves Wi-Fi packets into unused GSM slots while still ensuring that GSM calls receive priority. This sequencing eliminates the shielding and isolation requirements, rated at 80 dB, that have stymied integrated radio to date. The software is implemented on an FPGA, but Xavier said it would best reside in the GSM protocol stack in the baseband.
Though the scheme does require some back-off for Wi-Fi, Xavier said that by injecting Barker codes into the baseband, "we can educate the MAC [media access control] as to the back-off it would need to move to, so we do it more intelligently and can squeeze in an extra packet or two." Missed beacons would amount to less than 50 percent.
The chip is implemented in a 0.18-micron IBM SiGe process and is sampling now. Key specs include receive sensitivity on the GSM side of - 110 dBm, IP2 of 72 dB, die footprint of about 10 mm2 and power consumption in receive mode of 80 mA for both radios.