Mobile consumers are now demanding bandwidths comparable to those that can be achieved in a fixed location. 3G networks based on W-CDMA, and other new wireless technologies such as WiMAX, provide mobile consumers with this high-bandwidth connectivity, but all these newer systems have one major weakness. Specifically, as spectrum efficiency improves through the use of more complex modulation systems, the energy efficiency of the power amplifier (PA) in the base station is dramatically reduced, with significant implications for overall power consumption.
The high power consumption of 3G infrastructure is proving to be one of the most significant problems facing the wireless industry. This is not only because of the rising cost of energy, but also because the heat dissipation of the PA is a limiting factor to making smaller, lighter and cheaper base stations. Furthermore, the environmental impact of deploying large numbers of power-hungry base stations is becoming a major concern. For example, operators such as Vodafone are beginning to set challenging targets on energy efficiency for OEMs to meet, in support of their efforts to reduce the energy costs and environmental impact of their networks.
The crest factor determines the efficiency of a PA or peak/average power ratio (PAPR). In a W-CDMA system, the PA is usually operating far below peak power, with a crest factor of approximately 6.5 dB to 7.0 dB. OFDM systems such as 3GPP long-term evolution (LTE) and WiMAX use even higher crest factors (approximately 9.0 dB to 9.5 dB) to improve spectral efficiency, resulting in even lower PA efficiencies.
It is possible, however, to achieve a significant improvement in PA efficiency, and to the energy efficiency of the entire network, by means of changes to the base station architecture. A technique known as envelope tracking (ET), where the bias voltage of the PA is changed dynamically to ensure that the output power transistors remain in the optimum part of their operating curve, can make a significant contribution to the energy efficiency of 3G and WiMAX networks, and to DVB transmitters.
In a traditional amplifier, the main output amplifier stage is operated at a constant drain voltage, and is only efficient when driven close to compression. An ET amplifier operates close to compression at all envelope levels and hence always operates at maximum efficiency. While the principle of ET has been known for several years[1, 2, 3, 4], it has not been commercialized until now because of the difficulty in resolving the critical performance issues. These issues include accuracy, bandwidth (for multicarrier support), stability, compliance with spurious and noise specifications, and modulator efficiency.
Nujira is the first company that has been able to apply ET commercially with its groundbreaking high-accuracy tracking (HAT) technology, which involves the addition of a supply-voltage modulator and drive software to the DSP in the PA. The implementation of the modulator itself is critical, and involves some challenging design issues that Nujira has spent much of the past five years in solving.
The use of HAT has been shown to be able to improve the PA efficiency of linear amplifiers, such as those used for W-CDMA and OFDM systems, from a typical figure of around 15% to more than 50%. HAT is the only technology that enables high PA efficiencies to be achieved for OFDM systems. As well as reducing power consumption, this allows the Pas to be smaller in size, with increased reliability.
Figure 1 shows the PA configuration without the HAT modulator, while Figure 2 shows the PA configuration with the HAT modulator. Figures 3 and 4 respectively compare the power dissipated without and with the HAT modulator dynamically controlling the supply voltage. The modulator and its associated drive software are an addition to an existing PA, requiring some minor redesign of the PA itself to ensure that the optimum matching and performance are obtained. The implementation of the modulator is critical, introducing some challenging design issues. For example, the efficiency of the modulator itself must be very high, and it must track the envelope of the signal accurately in order to retain compliance with the demanding spurious and noise specifications.
The introduction of HAT to an amplifier design, therefore, requires a close partnership between the PA designer and the Nujira design team, who can provide evaluation platforms (Figure 5) and reference designs as well as support to the PA design engineer.
The HAT technology can be used in any frequency band or with any modulation scheme, with the greatest efficiency improvements occurring at high PAPRs. The technique is also suited to use with broadband Pas, in contrast to some of the other techniques, which achieve improvements in efficiency but only over a relatively narrow bandwidth. It is also applicable to all major RF device technologies and semiconductor materials, working equally well with Pas based on GaAs FETs, GaAs HBTs, Si LDMOS or GaN devices.
The benefits of using HAT can be capitalized on in a number of ways. For example, a HAT-enabled PA design can be used to achieve greater RF power output, such as a 40 W amplifier with the same space and weight characteristics as a 15 W conventional PA. Alternatively, a smaller and lighter PA can be produced having the same RF power output, which is particularly useful for remote radio heads and for reducing the overall size of node Bs. Finally, the amplifiers can be combined for multicarrier operation, with the potential for amplifying three carriers using an amplifier of only the same size and power dissipation as a single-carrier conventional design.
This improvement in PA efficiency has a significant effect on the overall energy efficiency of base stations and, therefore, on the network as a whole. A more efficient PA generates far less heat and can greatly simplify the design of the base station, in many cases eliminating the need for cooling fans or air-conditioning systems. The cost of adding a modulator and installing the drive control software onto the DSP is more than offset by the savings achieved by the removal of the cooling hardware. This has the dual benefit of lowering operating costs (through reduced energy consumption) and lowering maintenance costs (through increased equipment reliability). An additional benefit of reduced energy consumption is the reduced demands placed on the power system, including the requirements for battery back up, power supply and other ancillary costs.
The extremely power-efficient base stations that are enabled by the use of HAT will open up new possibilities for network base station deployment in situations where it might otherwise be difficult or uneconomical. For example, in some emerging markets, grid power is not readily available. However, the opportunity exists with HAT to run a base station on alternative forms of energy, such as localized wind or solar power.
Another example of a situation where base station deployment might be difficult is the case of installing additional capacity for 3G networks in urban areas. Where a traditional base station solution might require too much power, or produce too much noise through the operation of its cooling fans or air conditioning, a base station using HAT could avoid these problems.
Envelope tracking defines a new standard
There is no longer any need or justification for 3G operators to settle for less than 50% efficiency from their base station Pas. Envelope-tracking technology, in the form of the HAT modulator, is available, and OEMs will find themselves under increasing pressure from the operators to adopt the technology as rapidly as possible. This technology ensures that stringent energy reduction targets can be met. It can also help base station vendors to be in accord with the environmental business policies of their customers.
Khan, L.R., “Single Sideband Transmission by Envelope Elimination and Restoration,” Proc. IRE, vol. 40, July 1952, pp. 803-806.
Koch, M.J., Fisher, R.E., “A High Efficiency Linear Power Amplifier for Digital Cellular Telephony,” Proc. 39th IEEE Conference on Vehicular Technology, 1989, pp. 17-19.
Langridge, R., et. al., “A Power Reuse Technique for Improved Efficiency of Outphasing Microwave Power Amplifier,” IEEE Trans. Microwave Theory and Technology, vol. MTT-47, No. 8, August 1999, pp. 1467-71.
Cripps, S.C., “Advanced Techniques in RF Power Amplifier Design,” Artech House, 2002.
ABOUT THE AUTHOR
Tim Haynes is CEO and founder of Nujira. He brings more than 14 years of senior management experience to Nujira. Most recently, Haynes was UK operations director at Symbionics/Tality. Haynes holds a first class honors degree in electrical & electronic engineering.