Standardisation boosts momentum for Envelope Tracking
The new generation of data-oriented digital communications networks being rolled out in the cellular, broadcast, military and other environments carry major challenges for RF designers. In order to achieve high data rates, these new standards use advanced modulation techniques such as OFDM which significantly increase the variations in amplitude of the transmitted signal, reducing the efficiency of the RF transmission system. The frequency bands used for these new networks are allocated as a result of the “digital dividend” without consideration for the practicalities of designing a cost effective radio system, making the design job even harder.
Envelope Tracking is coming to the fore as the most effective wideband power optimisation technology to address this challenge. Not only does it offer attractive transmitter efficiency of 50% plus with high peak to average ratio signals, but it maintains this performance over a very wide bandwidth. As a result, an efficient 4G cellular handset design covering all fourteen frequency bands defined in the 3GPP for LTE and all of the operating modes (GSM, EDGE, WCDMA, HSUPA and LTE) can be implemented using just two Power Amplifiers (PAs) instead of the seven or more needed using alternative approaches. In a Base Station just a handful of PA subsystems are required to cover all the power level, frequency and modes required, instead of the more than 60 separate designs required by alternative narrow-band approaches. ET therefore enables significant cost reduction as well as power savings in cellular equipment.
World’s first openET interface.
These considerations have led to adoption of ET in both cellular terminal and infrastructure equipment. A Tier 1 semiconductor vendor in the handset ecosystem has already adopted ET technology, and several others are looking at it closely. On the network side, Sumitomo was first to market with an ET radio head, and reference designs for ET PA platforms have been developed by RFMD, Triquint and Nujira. ET can be combined with many of the other approaches to create an ‘efficiency multiplier’, as has been demonstrated by both Texas Instruments and Xilinx who have integrated ET with their Digital Pre Distortion (DPD) platforms.
Xilinx is the latest semiconductor vendor to support ET, and Dave Hawke, its senior product marketing manager, wireless communications, believes the cellular network industry is motivated by both economic and environmental concerns. “Xilinx is fully committed to supporting the cellular industry in minimising the environmental impact and operating cost of the next generation of 3G and 4G infrastructure, and is adding an Envelope Tracking port to its DPD solution,” he said.
David Brubaker, Product Line Manager, Wireless Infrastructure Radio Products at TI, says, “We see Texas Instruments digital predistortion technology and Envelope Tracking as key enablers for energy-efficient basestations. OEMs can now rapidly deploy these technologies to produce high-performance, power-efficient 3.5G and 4G basestation platforms that reduce operating costs for network operators worldwide.”
Envelope Tracking as seen on a scope.
Implementing ET requires a connection between the RF Transceiver or Baseband and the Envelope Tracking power supply modulator, allowing it to track the transmitted waveform and ensure that the optimal amount of power is available to the PA to satisfy transmission demand at any point.
To encourage faster adoption, the market leader in ET technology, Nujira, has assisted in the setting up the OpenET Alliance to develop and standardise these interface specifications to make them accessible to any manufacturer who wants to use the technology.
The OpenET Terminal Interface specification for Envelope Tracking ports enables ET capability to be added to baseband and RF Transceiver chipsets for mobile terminals, cellular base stations and other applications with minimal impact on the cost and power consumption of the chipset itself. The OpenET Alliance is a non-profit corporation that operates as an open membership organisation, and welcomes members from companies in the mobile industry, including Universities, Press and Analysts. ET is also attracting growing interest in other industries facing similar challenges such as broadcast and military communications, and the OpenET Alliance is welcoming members from these industries too. The Alliance aims to be not only a centre of excellence on Envelope Tracking technology and power optimisation of wireless transmission, but also to become the non-proprietary body that will define and develop Envelope Tracking and power reduction standards, leading to wider adoption for the benefit of all.
By acting as a focal point for promoting and developing best practice in efficient wireless transmitter design, the OpenET Alliance will not only minimise carbon emissions, but also promote the development of attractive wireless products that are cheaper, smaller, more reliable and with a longer battery life.
The author, Nigel Dixon is chairman of the OpenET Alliance. For further information visit: www.open-et.com.