GSM players eye Edge despite transmit woes
By Robert Keenan, CommsDesign.com
Mar 26, 2002 (11:34 AM)
URL: commsdesign.com
ORLANDO, Fla. — While debate continued at last week's CTIA Wireless 2002 show about spectrum allocation and when 3G will become a reality, Enhanced Data Rates for GSM Evolution (Edge) again appears to be the technology of choice for GSM operators in the U.S. and European markets. For Edge to catch on, however, handset makers must resolve engineering problems in the transmit section of their mobile phone architectures.
"There have definitely been hot and cold cycles with Edge," said Mohy Abdelgany, vice president of RF components and subsystems at Conexant Systems Inc. (Newport Beach, Calif.). "But for the past four to five months, Edge has been consistently hot." In fact, Abdelgany said, tier-one companies, such as Nokia, Siemens and Motorola, are demanding Edge capabilities.
"To counter 1x performance levels, carriers need to implement Edge," said Javed Patel, vice president of sales and marketing at Tropian Inc. (Cupertino, Calif.). According to Patel, Edge provides a capacity increase in the number of users a network can support. At the same time, Edge gives end users a jump in data rates. Now, most members of the industry see 56 kbits/second as the high-end rate for clients operating on a GPRS network. With Edge, Patel said, operators can deliver a practical bandwidth of 60 to 64 kbits/s.
Making the leap to Edge, however, requires that developers implement a modulation scheme that causes glitches in the RF front end. Traditional GSM systems use a constant envelope modulation scheme. Edge, on the other hand, employs 8-level phase-shift keying, separating amplitude and phase during transmission. This split wreaks havoc on traditional GSM transmit architectures.
Amplitude signals
In a typical GSM phone, Abdelgany said, designers use a translation loop architecture in the transmit section. In this architecture, an I/Q modulator is followed by a phase-locked loop, which in turn is followed by a voltage-controlled oscillator and power amplifier. According to Abdelgany, this architecture does fine in processing the phase components of the Edge architecture. But it falls short in handling the amplitude signals, thus creating timing and efficiency problems in the RF front-end architecture
"The translation loop approach is the best method for GSM designs, but does not work for Edge," Abdelgany said.
Chip vendors are working to solve the problem. Some are trying to perform I/Q modulation in the RF domain to pass signals to the power amplifier without signaling, Abdelgany said. This approach causes lower efficiency in the power amplifier, however; in fact, Abdelgany said, power amplifier efficiency can drop to as low as 35 percent in these architectures.
Conexant has decided on a different solution. It is using the same GSM architecture employed in designs today but is finding a path to pass amplitude information directly to the power amp. The Conexant architecture uses the traditional translation loop architecture to process the phase information, but also sets up a separate path that transfers the amplitude info to the power amplifier before it enters the translation loop path.
A polar move?
Elsewhere, TTPCom, Tropian, Philips Semiconductors and others are considering using polar-mode techniques to build Edge transmitter designs. Polar mode entails splitting phase and amplitude components within the transmit IC. The chips are able to maintain timing in the paths and handle both amplitude components, thus allowing the chip to handle signal to the power amplifier in sync.
"Using polar loops is the only sensible way to do it," said Mike Bradley, business development manager for TTPCom.
Tropian has chips on the market that support polar-mode operation for Edge designs. TTPCom is testing its polar loop architecture and, according to Bradley, the company is confident it will work well. Philips and Conexant are also developing their Edge transmit solutions.
Which solution will dominate? "It remains to be seen," Patel said, noting that most silicon vendors don't have silicon on the market.
Another uncertainty has to do with when system architectures using these solutions will become available. Widespread sentiment has the first Edge deployments occurring in the United States, where spectrum-allocation difficulties plague traditional GSM and TDMA operators.
Europe is also preparing for a shift to Edge. With so much money being spent on licenses, the European sector is clearly being pressured to roll out wideband CDMA systems. By rolling out Edge operation as well, European carriers can let users roam without worrying about huge drops in data rates, Patel said.
Patel expects that Edge-enabled wireless terminals will be available late this year or early next.
Copyright 2002 © CMP Media, LLC |
