Publication date: 25 November 2009
If Napoleon’s armies marched on their stomachs, 21st century infantry marches on batteries. No less than 30% of the 70kg kit carried by today’s soldiers can be power related, and a platoon can be burdened with 25-40kgs of communications equipment. There is high level realisation that the benefits of new communications standards need to be realised while reducing, rather than adding to this burden. Western armed forces are giving focus to the power density of their systems, defined as watt hours per kilogram.
For example, the UK Ministry of Defence has initiated a program called ‘The Energy Efficient Soldier’ with a goal to reduce the weight burden of portable power by 75% by 2011. This program looks at both improvements in power sources like batteries, and better use of power so that communications systems can run for longer from the same batteries without reducing functionality. Although battery technology is improving all of the time, the rate of progress is comparatively slow and the target is unlikely to be achieved from this direction alone in the short time allowed. There is substantial interest therefore in addressing the power equation from the other end – by reducing the energy consumption of the systems that troops use.
One of the most significant of these are communications systems. New Handheld, Manpack and Small Form Fit (HMS) radios under the US Joint Tactical Radio System (JTRS) offer tactical vehicles and dismounted units with reliable, good quality connectivity over a wide bandwidth even in rugged and urban environments. Such secure, reliable high bandwidth data, video and voice communications technology is essential to deliver the required speed of command in today’s conflict environment. It is the two-way backbone that connects the whole chain of command from the top down to the lowest level, giving critical and immediate situational awareness and maximising combat effectiveness. Vehicles and dismounted units are also being provided with jamming systems as a countermeasure against Improvised Explosive Devices (IEDs). In both communications and jamming electronics, the radio transmitter can use half or more of the total power budget – mostly in the Power Amplifier (PA). This area is therefore seeing significant focus.
Networked battlefield communications protocols are usually OFDM or QAM based and support frequency hopping and adaptive signal to noise encoding schemes, and sometimes use technology such as W-CDMA developed commercially for existing mobile phone networks. RF PAs are classic AB class amplifiers, which offer most efficient operation when the RF envelope waveform is closest to peak power. Complex modulation schemes such as OFDM and QAM mean that a typical amplifier rarely runs up to its saturated output power capability resulting in lower efficiency. There are a number of techniques available that could address the efficiency challenges, such as linearization, DPD or Doherty, yet these approaches become untenable for the wide transmit bandwidths required.
Envelope Tracking as a technique for improving power efficiency of RF Power Amplifiers was first described by Bell Labs in 1937. Instead of optimising a final RF stage power transistor supplied by constant voltage, the supply voltage to the Power Amplifier output transistor is adjusted dynamically, in synchronism with the envelope of the modulated RF signal passing through the device. This ensures that the output device remains in its most efficient operating region (i.e. in saturation) dramatically reducing the energy dissipated. An evolution of this principle, High Accuracy Tracking (HATTM) is showing an impressive improvement in efficiency, going from typically a low 30 percent for standard a class AB amplifier to beyond 60 percent with HAT.
Nujira, who developed and patented HAT technology, has run demonstrations of its Coolteq™modules on a GaN PA. With QAM based waveforms similar to SRW (Soldier Radio Wave), they have shown a potential 30% less power consumption and 42% more battery life for a manpack radio based on SRW. This was achieved over a wide frequency range and across multiple modulation modes. The power dissipation of the PA transistor itself is reduced by two-thirds, leading to a significant reduction in device thermal management requirements. Significant reduction in device temperature also leads to increased PA device reliability. Also, for handset applications, demonstrations have shown an improvement in linearity with Coolteq™ High Accuracy Tracking, eliminating the requirement for DPD altogether.
HAT implementation is relatively straightforward, involving the addition of a HATTM Modulator module, plus the addition of a digital connector allowing the envelope signal to be routed from the digital baseband and passed to the HAT module. The module can be a small box (70mm x 70mm x 18mm for the commercial units supporting 40W cellular base station transmitters), or can be a silicon chip for lower power handheld transmitters. In the future, the HAT™ algorithm may be integrated into power management chips already used in a radio transmitter. In addition, some minor redesign of the PA layout is needed to ensure optimal matching and hence efficiency.
The communications protocols and waveforms that are being proposed for tomorrow’s networked battlefield are increasingly being drawn from or based on commercial technologies used in 3G and 4G cellular communications. As a result, power optimisation technologies developed in the cellular network domain can translate readily to the field of military communications.
