An Emerging Trend for Wireless Power Amplifiers
By Richard J. Lisco, Vice-President-Engineering, Cigma Technologies, Inc. (firstname.lastname@example.org)
Power amplifiers are ubiquitous and indispensable in the world of wireless communications. What's more, despite the recent economic downturn, mobile communications continues to grow at a 15% per year rate for new subscribers. To keep pace with the resultant increase in network capacity and coverage demands, wireless service providers will be called upon to upgrade or augment base station equipment. And, with power amplifiers essential components for growth, innovative technologies must be empowered to accommodate the need for increased efficiency, enhanced performance, reduced cost and downsized real-space associated with third generation broadband wireless base stations.
Consequently, the new breed multi-carrier power amplifiers should meet these basic criteria: more economical operation with efficiency better than 15%; up to 50 % lower acquisition cost; high reliability; easier maintenance and the ability to generate broader instantaneous operating bandwidths, up to 60 MHz wide.
The criteria associated with growth demands placed on base station power amplifiers present a significant challenge for power amplifier designers and manufacturers reaching for technological advances. They must produce new power transistor technologies that support good gain flatness; broader bandwidth and higher efficiency. Although GaAs power transistor technology offers promising efficiency benefits, reliability issues and power conversion requirements have caused concerns for adopting the technology by the wireless base station power amplifier producers. A new generation of LDMOS power transistors was recently introduced into the market by a highly respected developer in the industry. The new technology provides increased gain, better IMD (Inter-Modulation Distortion) performance, and higher efficiency. Although these devices alone will not satisfy the linearity requirements for a 3G MCPA, they are essential elements for an optimum amplifier design.
Other critical technological advances for wireless power amplifiers rest with innovative linearization techniques. Due to high "Peak-to-Average Ratio" (PAR) for 3G signal formats (CDMA 2000, UMTS etc.), multi-carrier power amplifiers must utilize linearization techniques to minimize output distortions (IMD). Conventional linearization such as Feed-Forward provides good IMD performance but suffers significant power efficiency loss. In addition, the implementation complexity of the Feed-Forward design results in higher manufacturing costs and a decrease in overall system reliability.
Cutting edge pre-distortion technologies appear to be at the heart of the next generation of linear power amplifiers. Of these two different approaches for pre-distortion techniques are utilized. Digital pre-distortion designs employ advanced digital signal processing methods to limit the input signal crest factor and to pre-distort the input waveforms in the baseband. The other approach is a RF pre-distortion system which uses innovative RF linearization circuit topology to pre-distort input signal gain and phase characteristics. The resulting amplified signal becomes highly linear. Both digital and RF pre-distortion systems utilize feedback control mechanisms to ensure good amplifier linearity performance under environmental and aging conditions.
While digital pre-distortion provides more precision and flexibility than RF pre-distortion, the associated instantaneous bandwidth limitations are severe. The problem gets worst since recently some European network operators started to share core network resources because of financial difficulties. Base Stations operating in this mode require a much wider instantaneous bandwidth than the 15 to 20 MHz limit imposed by today's digital PD platforms. Another factor that hinders deployment of digital pre-distortion amplifiers is a lack of interface standards between radio outputs and power amplifier inputs. Although some progress has been made over the past few years, agreed-upon digital interface standards have yet to be developed. As such digital interface amplifiers are not compatible for retrofit or upgrade of currently existing Multi-Carrier Power Amplifiers (MCPA) or Single-Carrier Power Amplifiers (SCPA). Innovations based on the direct RF pre-distortion system have made possible high efficiency, low cost, reliable and broad instantaneous bandwidth MCPA.
Several benefits are evident when advanced RF pre-distortion technology is applied to wireless power amplifiers, wider operating bandwidth, increased efficiency, smaller physical size, and lower acquisition and operating cost are the most notable.