Powering Up CWC’s Research Agenda

“The quest for enhanced spectral efficiency in wireless communications is becoming pervasive in the design of wireless systems.” —Larry Larson, CWC Director

Wireless base stations are only as good as the amplifiers that extend their range to provide coverage for wireless customers. Yet the efficiency of these amplifiers for third-generation (3G) cellular applications is typically only 10 percent, which means they use ten times more power than they should need to transmit.

But engineers affiliated with the Jacobs School 's Center for Wireless Communications (CWC) and Calit2 have achieved greater than 50 percent power added efficiency (PAE) in the latest generation of their power amplifiers, generating more power with less heat. That efficiency is the best reported for a single stage base station power amplifier and could foretell more powerful, smaller and lower cost base stations.

The early-November announcement came two weeks before CWC marked its 10th anniversary with a one-day symposium on spectrally efficient wireless communications.

"The past decade coincided with a quest for enhanced spectral efficiency in wireless communications," says CWC director Larry Larson. "These considerations are becoming pervasive in the design of wireless systems and a focus of economic and regulatory attention."

More than 20 professors and 50 Ph.D. students collaborate on $2.5 million in wireless research funded each year by CWC's 12 corporate members. More efficient use of wireless spectrum is a hallmark of its hottest new research areas, including coding for wireless networks, scalable video, and shared spectrum technologies such as ultra wideband and cognitive radios. The center also continues its work on smart antennas and multiple-input multiple-output (MIMO) systems, as well as speech, video, image compression—and novel power amplifier architectures.

"There are two big trends," explains Larson, who chaired the IEEE Topical Workshop on Power Amplifiers for Wireless Communications in January 2006 ."One is the increasing use of digital signal processing in power amplifier design."

The UCSD amplifier employs a novel architecture based on ‘envelope tracking,’ a technique to adjust voltage dynamically, instead of using the constant feed of DC voltage employed by today’s high-power amplifiers. In this diagram: the amplifier operates closer to saturation; the transistor maintains a lower temperature; and the dynamic peak voltage reaches higher values than can be used for constant drain bias voltages.

The second trend, observes Larson, is the emergence of gallium nitride (GaN) as a viable alternative to silicon in power amplifiers for 3G and 4G base stations. The composite semiconductor material was used to achieve the record PAE announced in the November paper coauthored by Larson and ECE professor Peter Asbeck, Calit2 engineer Don Kimball, their graduate students, and researchers from Nokia, QUALCOMM and Nitronex.

In a subsequent study, UCSD researchers also reported the highest combination of power density and efficiency in any WCDMA base station amplifier—with an amplifier less than one-tenth the size of existing commercial devices.