ENERGY EFFICIENT COMMUNICATIONS FOR THE BATTLEFIELD
By Simon Whittle, Design Centre Manager, Nujira
New technologies give the military fast, secure communications on a tight power budget.
A platoon these days carries typically 25-40kg of communications equipment, which compares unfavourably with the 27kg average weight of armour worn by medieval knights. All this equipment needs feeding with portable electrical power. As a result, as much as 30% of the average 70kg pack carried by an individual soldier today can be power related.
However, there is a tension between the drive to realise the benefits of new communications standards and that of reducing the soldier’s burden in order to maintain agility and vigilance; which is why Western armed forces are increasingly turning their focus to the energy density of their systems.
The UK Ministry of Defence programme called ‘The Energy Efficient Soldier’ set a goal to reduce the weight of portable power by 75% by 2011. While this specific programme has fallen foul of spending cuts, the need remains; both to improve the energy density of sources like batteries, and simultaneously to make 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 the time, the rate of progress is comparatively slow and targets are unlikely to be achieved from this direction alone – at least in the short term. 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 is communications systems. For example, the latest Handheld, Manpack and Small Form Fit (HMS) radios under the US Joint Tactical Radio System (JTRS) provide tactical vehicles and dismounted units with reliable, good quality connectivity over a wide bandwidth, even when deployed 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 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 – and most of this taken by the power amplifier (PA).
It is here that new technologies aimed at improving power efficiency can deliver substantial benefits. A technique called high accuracy tracking (HAT™) has proven particularly successful in the communications sector.
Here’s the science bit
The Radio Frequency Power Amplifiers (RF PAs), which are the last part in the transmit chain, use energy from the battery to create the high power transmit signal at the antenna. The RF PAs used in battlefield communications are traditional class AB amplifiers, which offer efficient operation when the RF envelope waveform is closest to peak power. However, the complexity of signals used in today’s military radios limits the time that typical PAs are operating in this zone: the result is a substantial reduction in overall efficiency.
Whilst a number of techniques have been tried in efforts to address the efficiency challenges, these have proven untenable for the wide operating frequency ranges. For example, the Doherty amplifier structure, commonly used in cellular base stations, is restricted to around 10% bandwidth and is unusable for the octave(or wider) frequency range requirements of many military systems. A more successful approach is to adjust the supply voltage itself in synchronism with the envelope of the modulated radio signal p