Written by: Bo Berglund, Jan Johansson and Thomas Lejon
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The never-ending quest for greater capacity and higher data rates in radio access networks puts increasing demands on the nominal output power from radio base stations. These demands must be met within the existing RBS footprint where cooling capacity and size are limited. At the same time, operators want to cut their radio network operating expenses (OPEX), of which energy consumption is a significant factor.
Linear radio frequency (RF) power amplifiers (PA) play a major role in terms of base station energy consumption and heat dissipation. More efficient base station power amplifiers are thus a crucial factor in the context of mobile system evolution. Reducing the energy consumed by radio base stations will also reduce the environmental impact of the radio access network.
The authors describe the critical aspects and limitations of highly efficient PA technology and indicate where new technology is needed to reach the industry’s ambitious goals for greater efficiency.
Background
Linear RF power amplifiers consume large amounts of energy, dissipate heat, and take up space in base stations. Significantly more efficient PA technology will be instrumental to the evolution of mobile systems. The main requirements for future power amplifier technology are
- high linearity, to satisfy higher-order modulation schemes;
- greater average output power levels;
- broader operating bandwidths (more than twice today’s typical 20MHz);
- reduced OPEX by decreasing RBS energy consumption; and
- reduced environmental impact by decreasing radio network energy consumption.
New technologies with greater PA drain efficiency were introduced in the past 12 months, significantly increasing radio unit (RU) power efficiency. Notwithstanding, new technologies with even greater drain efficiencies are needed to meet the challenges of the next three to five years. (Drain efficiency is the ratio of delivered output power divided by applied DC power in the PA.)
The main objective of PA research is to increase efficiency while maintaining linearity and broadening the operating bandwidth. Present-day power amplifiers with Doherty efficiency-enhancement technology will continue to evolve in coming years, but to increase efficiency significantly, other technologies must also be considered. Switch-mode PA technology has been identified as a way of achieving high PA efficiency. This technology is being used successfully with pulse width modulation (PWM) for audio and digital subscriber line (DSL) driver applications. In this role, switch-mode PAs are 80% to 90% efficient but their application is limited to operating frequencies of around a few megahertz. The challenge is thus to apply the technology to operating frequencies for mobile systems in the 1 - 4GHz range.
Because straightforward frequency scaling is not feasible due to the physical properties of the components, suitable methods must be found to mitigate the fundamental limitations of high-frequency operation. The major areas of research in this area are
- switch-mode PA concepts with potentially high drain efficiency (greater than 60%);
- architectures that retain efficiency over a large, dynamic range of output power (typically 20dB) while meeting linearity requirements; and
- RF power transistor technology with performance that is suitable for switch-mode applications in the 1 - 4GHz frequency range.
