Meet Don Kimball
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1.31.03 - This article represents the beginning of a series of Web articles introducing Calit² staff members to the Calit² community and the world beyond.
Please welcome Don Kimball, UCSD Division, as the inaugural focus for this column. His role is to study and introduce high-efficiency, high-powered (defined as 20-60-watt) amplifiers for wireless infrastructure in Calit² research programs.
"Radio base stations use high-power radio frequency (RF) amplifiers to communicate with your cell phone," explains Kimball. "They help to provide coverage and capacity. My role in Calit² is to invent new hardware - high-efficiency, high-powered amplifiers for cellular base stations and wireless LANs."
Kimball is working with Larry Larson and Peter Asbeck, leading UCSD Jacobs School faculty members in Electrical and Computer Engineering, on Calit² and Center for Wireless Communications projects. In the near term, Kimball will be focusing on two projects: one for Intersil addressing transmitter power efficiency and multiple receive antennas for 5-GHz OFDM RF components for WLAN applications, and the other for Ericsson addressing base station power amplifier improvements for next-generation digital (WCDMA GPRS/CDMA 2000) cellular services.
"Ericsson, like many telecom businesses these days, is restructuring," says Kimball. "They had a large amount of unutilized equipment, and they saw the benefit of donating it to CWC's research activities related to power amplifiers. While Ericsson was transferring all engineering development and staff to Montreal, I chose to 'donate' myself to Calit²," says Kimball with a laugh.
Ericsson is now on its third year of partnership with Calit² totaling $12 M for sponsored research and gift support for Calit² fellowships, faculty endowed chairs, laboratory equipment, and support for academic professionals working on Calit² research projects. (Read: The ABCs of Connectivity: Always Best Connected)
Radio base station power amplifiers are traditionally very inefficient, something on the order of 10%, explains Kimball. For example, 60 watts of output power requires 600 watts of DC power, which requires an air-conditioning system for cooling and batteries to back it up.
"This 'translation,'" says Kimball, "has a multiplying impact on the initial demand for power, complexity, and reliability. That's where the interesting work comes in: Analyzing the tradeoffs and determining the best ones."
To address the manifest inefficiency, Kimball studies how various techniques might be implemented, which include:
- Envelope elimination and restoration (EER): This technique involves decomposing the RF signal into amplitude and phase components. The phase component is sent through the RF chain, while the amplitude component is sent through the DC power chain. "Doing this in production hasn't yet become possible," says Kimball. "But Ericsson was the first to implement this technique at high-power base stations."
- Envelope tracking: "This is a simplified version of the previous technique," explains Kimball. "You don't have to decompose the signal into amplitude and phase. It's less costly and complex, but not as efficient," he says with a sigh, pursuing the elusive Holy Grail of efficiency.
- Doherty method: "Larry Larson has been studying this technique in which you use a main amplifier to efficiently provide the low-to-medium signal components and an auxiliary amplifier to efficiently provide the medium-to-high signal components, then sum them together to re-create the complete signal ."
While theory is fundamental to most research and instructional programs at UCSD, Kimball, like other recent Calit² hires, is also a dyed-in-the-wool, hands-on engineer who excels in building real proofs of concept: "My work lies somewhere between the theory of academic science and industrial production systems," he says, pleased to have found his niche. (Look for a future article on another new staff member at UCSD, Ganz Chockalingam, for more on this theme.)
"We're implementing testbeds to test out these various techniques in our lab in 3710 in EBU 1," says Kimball, emphasizing the importance of Calit²'s living labs in enabling this development and prototyping. "Eventually," he adds, "we're going to build a pre-production amplifier with access to our own licensed frequency band. In this way, we can evaluate end-to-end connectivity and reliability under real-world, 'over-the-air' conditions."
Kimball also mentions interest in combining EER with digital amplification techniques - Class D, E, and F. "The reason," he says, "is that these switching implementations have difficulty with signals that lack a constant amplitude envelope or that are much better suited to a constant-amplitude envelope. Another way of saying this is that they have problems with 'variable envelope.'"
Others have solved this problem by increasing the switching rate and using novel forms of coding, but these techniques greatly increase the needed bandwidth, making hardware implementations impractical. The EER technique, however, can eliminate the excessive bandwidth requirement, resulting in more practical hardware implementations.
Kimball began his research in high-efficiency, high-power amplifiers related to wide-band CDMA, CDMA2000, and GSM/edge. He started with QUALCOMM in 1994. Then Ericsson purchased QUALCOMM's Infrastructure Division in 1999, which caused Kimball to transfer to Ericsson. "I consider myself a living symbol of the changing face of the telecom industry because I've actually lived it," says Kimball with a rueful smile.
Always one to produce a great, if perhaps unintended, quote, he further - and too humbly - explains: "I keep getting moved with the equipment."
That might make you think the equipment is what matters. Hardly.
Kimball goes on to explain how his engineering expertise provided a foundation for his interest and involvement with the larger realm of policy issues - not typical of your everyday engineer. "At QUALCOMM, I studied electromagnetic compatibility, safety, wireless physical interfaces, wired physical interfaces, and so forth," he says.
In the next breath he comes to regulatory issues, and his eyes light up: "There were no regulations for CDMA. By that, I mean there were no FCC regulations. So I helped design recommendations and reinterpret existing regulations to make them applicable to CMDA. It was fascinating because the government works by frequency band and analog modulation, not by coding or digital modulation techniques. So I had to develop techniques to demonstrate compliance, then put them to use. Compliance was important because we were adapting a digital form of communication to the analog world. That was the early 90s," he says, making that seem like a very long time ago.
Our conversation turned to the new Calit² facilities and what recommendations Kimball might have as an electrical engineer to enhance them.
"We need a compact, indoor 'antenna range,'" he says, "on the top floor of UCSD's new Calit² building. They're good and less expensive these days, and will complement the antenna farm already planned for the roof of the building We may also want a SAR (Specific Absorption Rate) test chamber to test the antenna's interaction with simulated human tissue. We need to make these capabilities available for industrial groups to utilize. They realize that their testing is not sufficient, and I expect they'll want to work with us to gain a different perspective and our advice. That's one of the unique things that Calit² can offer industry."