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At 58% Power Added Efficiency (PAE), 50W of output power would require only 86 W of DC power, rather than the 500 W required by today's high power RF amplifiers, which operate at 10% efficiency.
Q: How can we improve Power Added Efficiency (PAE) and what role will high power RF amplifiers play in achieving this goal?
By Ray Pengelly, Strategic Business Development Manager, Cree, Inc.
Improvements in DC to RF conversion efficiencies of high power solid-state amplifiers are essential for next generation base station transceivers, and they have benefits beyond just reduced DC power consumption.
High efficiency power amplifiers can be designed using a variety of techniques. Some of these approaches, such as Chireaux out-phasing, can be made to work quite well, but they are difficult to manufacture. Other techniques, such as Doherty, envelope tracking (ET) and envelope elimination and restoration (EER), have been shown to work well with silicon LDMOS and GaAs (such as the recent UCSD result), but those technologies have been pushed to the limit with little further improvement likely.
With the advent of linear gallium nitride (GaN) HEMT power transistors, such as those currently in production at Cree, Inc., intrinsic device efficiencies are considerably improved over older technologies. For example, Class A/B GaN amplifiers routinely achieve 70% peak efficiency and greater than 35% when the power is backed-off to achieve the required linearity. Using a Doherty amplifier architecture, Cree has demonstrated 52% efficiency under W-CDMA modulation at an average power of 80 W over the 2.11 to 2.17 GHz UMTS band, exceeding the spectral mask requirements. This is almost twice the RF output power demonstrated by UCSD, and it uses a simple, easily manufacturable approach. The EER/ET approach does have additional flexibility, however, as it can be applied to a variety of amplifier configurations that will be well-suited in a GaN implementation. It is predicted that before the end of 2009, the 60% barrier will be broken by employing a GaN HEMT amplifier with a state-of-the-art ET modulator, each having efficiencies over 80%.
By Paul Misar, Director OSP Wireless and Alternative Energy for the Energy Systems business of Emerson Network Power
We like to look at energy consumption at the radio base station (RBS), and across the network, holistically. More than 60% of the power used by the typical RBS is consumed by the radio equipment and amplifiers, 11% is consumed by the DC power system and 25% by the cooling equipment, an air conditioning unit typical of many such sites.
Under these conditions, it takes 10.3 kW of electricity to produce only 120 W of transmitted radio signals, and to process the incoming signals from the subscriber cell phones. From a system efficiency perspective (output power/input power), this translates into an efficiency of 1.2%.
Clearly, there are opportunities for improvement, and they exist all along the energy path inside the RBS. Because of these inefficiencies, any Watt saved near the antenna will yield cascading benefits by avoiding the associated losses upstream. That cascade effect maximizes the ultimate energy savings at the source. The benefit of 1 W saved at the RF load is multiplied by the system block efficiencies, so the accumulated benefits are much higher than the original 1 W reduction.
In a typical model, saving 1 W in the feeder cables saves 17.3 W of modulation and amplification losses, 3.3 W of rectification losses and 7.1 W of associated cooling energy. In aggregate, this represents a 28X cascading benefit, with smaller benefits also occurring in signal processing and DC power. For these reasons, efforts must start closer to the antenna where they yield greater benefits and enable reduction in cooling and power requirements. Simply put, energy consumption at the RBS is a major industry issue, but opportunities for reductions of more than 50 percent are readily available.
Wireless Design & Development
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