A theoretical technique that has been known about for years has finally been applied in practice to provide much more efficient power amplification, Julian Hildersley, vp strategy and marketing and Nujira, says
Radio networks account for some 80% of the total electricity used by an operator – and in current 3G base stations the linear RF power amplifiers (PAs) used in the RF transmission circuit account for approximately half of the total power used. 3G signals present such a challenge in terms of RF transmission efficiency that ABI Research is forecasting that "Energy consumption arising from mobile broadband service delivery <…will…> grow from 42.8 billion kilowatt hours (KWh) in 2006 to 124.4 billion KWh in 2011".
On average, each fully-loaded 3G cell site using 'traditional' PA technology may require some 3.0 kW of power. For a typical European operator with a network of 20,000 base stations, the total energy consumption is approximately 51.7 MW (equivalent to a large wind farm) resulting in annual electricity costs of $60M, and annual CO2 emissions of 194,000 tonnes a year.
Linear RF power amplifiers are classic Class AB amplifiers, and operate most efficiently over a narrow range of power levels, when the RF envelope waveform is close to peak power. Whilst 2G GSM signals operate at constant amplitude and hence enable high efficiency UMTS, WiMAX and further evolved standards such as Long Term Evolution (LTE), are generating new, more challenging technical requirements. These include the need to support a variety of different channel coding and modulation techniques (CDMA, OFDM, etc.), broader channel bandwidths, and high peak-to-average power ratio (PAPR) complex modulation schemes.
In addition, factors such as deregulation and the growth of new networks, particularly in parts of the world not yet well equipped with cellular networks, have created a requirement for basestations to support a wider range of frequency bands as new spectrum is released to meet urgent capacity demands.
Environmental or 'green' issues, such as the desire to reduce both direct and indirect CO2 emissions, and equipment size, are also moving up the agenda. Several operators have pledged to work with suppliers to increase the energy efficiency of their networks, because of the impact on the bottom line and environmental concerns. For example, Vodafone has announced that it will target a 33% improvement in energy efficiency of new network equipment by March 2008 (compared to the 2006 baseline). The savings, in both cost and carbon emissions, which could be made by realizing these PA efficiency improvements are huge. Vodafone has stated its intention to remove air conditioning units from sites where possible, which alone account for about 25% of network energy usage, and use remote radio heads where possible. But the removal of air conditioning units and the deployment of remote radio heads are only feasible if PA efficiency is greatly improved. Improving PA efficiency with high peak-to-average power (PAPR) systems is particularly challenging due to the need to use linear PAs to meet the critical RF performance criteria.
The new generation of Node Bs now starting to be deployed use techniques such as Doherty and Digital Pre Distortion (see panel) to approximately halve the figures quoted above, but the overall power consumption of 3G radio networks, and their carbon footprint, will remain unacceptably high, and bolder solutions are required.
Envelope tracking as a technique for improving power efficiency of RF Power Amplifiers was first described by Bell Labs in 1937, and has featured in RF design textbooks since. What has been lacking is an effective commercial implementation, because of the difficulty of making a power supply modulator capable of achieving the accuracy, bandwidth and noise specifications necessary at a level of conversion efficiency that delivers a significant energy saving for the system as a whole. The threat of ballooning power consumption in 3G cellular base stations has lead to renewed focus on this area. The first practical implementation is Nujira's High Accuracy Tracking (HATTM) technology. The only addition required to the standard PA architecture is an output from the DPD/Linearisation function to drive the HAT Power Modulator with a digital representation of the modulation envelope. First released for the mobile basestation market, these new modulators are already seeing interest from digital broadcasters, and the possibilities of implementation in the handset are also being explored.
HAT(tm) envelope tracking (table 1) can make a significant contribution to the power efficiency of the PA, improving this from the 15% of 'traditional' PAs to 45% and more. A typical European national cellular network with 20,000 base stations could save 28MW per year – saving the operator $30 million in energy costs, reducing CO2 emissions by 110,000 tonnes.
Poor PA efficiency has a direct impact on the base stations cooling requirements. With a PA efficiency of 20%, 80% of the electrical power supplied is wasted as heat, requiring careful cooling design to reduce equipment temperatures and ensure reliability.
Cooling is a major concern for network operators for a number of reasons. Firstly, all forms of forced cooling involve moving parts in one form or another (fans, pumps, etc.) and other items such as filters that require regular maintenance. Secondly, mechanical noise can be a concern in many urban transmitter locations: some countries are already starting pass legislation to control noise levels at such sites. Finally, the cooling plant increases the equipment required on-site and hence site rental costs.
Another impact of poor PA efficiency is on the on-site backup power supply – higher transmitter power consumption necessitates either larger battery packs or larger diesel backup facilities, both of which again push up equipment and site costs.
It can be seen, therefore, that poor PA efficiency has a significant impact not just on the design of the transmitter itself, but also the cooling and backup facilities required on site, impacting both initial costs (CAPEX) and also long-term operational costs (OPEX).
The potential savings available from High Accuracy Tracking have proved so compelling that RF designers from fields as diverse as digital broadcasting and cellular handset design have shown an interest. Digital Broadcast Transmitters operate at about 2KW transmitted power, typically using a bank of parallel PAs, but there is no reason in principle why the envelope tracking technology applied successfully in 40W base station transmitters cannot be upscaled to work in this environment. Cellular handsets represent a very different challenge again, with a transmitted power of just 0.5W or so – but very tight constraints in terms of power consumption and space. Here the potential is not only to extend the battery life, but also to reduce the Bill of Materials cost involved in creating an RF front end that supports all the world-wide bands by having one wideband PA instead of several narrowband devices.
Power Amplifiers are commonly used in a huge range of electronic designs from military radios to satellite equipment to medical systems. Having proved the High Accuracy Tracking concept in the cellular network environment, there is no reason why it can't be extended in the future to other applications.