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Mobile WiMAX's hidden problem

If you take the industry's word for it, mobile WiMAX -- 802.16e -- is serenely sailing on towards deployment.
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If you take the industry's word for it, mobile WiMAX -- 802.16e -- is serenely sailing on towards deployment. Networks are being built, laptops sprout adaptors, Powerpoint slides are being churned out in self-assured profusion. Money is being spent, and lots of it.

What nobody's talking about, though, is a technical drawback that has stubbornly refused to go away. Mobile WiMAX may be fast and the devices suitably small and capable - but the transmitters are hideously inefficient. For every milliwatt of radio power radiated, up to nine will be lost as heat, and that has frightening implications for battery life.

To give you some idea of how that measures up to alternatives, a WiMAX power amplifier - the part of the transmitter which boosts the synthesised signal to a level that can make it to the base station - is between ten and twenty percent efficient. 3G amplifiers are between 40 and 45 percent efficient, and good old-fashioned GSM can nudge 55 percent. That puts WiMAX at up to a 5:1 disadvantage; considering that the maximum power output is around 250mW, up to 2.5 watts will be consumed.

Given that an Eee, for example, takes around 7 or 8 watts in total – and that's without the low-power technology that's coming up – that means getting on for a third less battery life. Not a very palatable idea.

What's more, there's not much that can be done. While there are plenty of ways to reduce power consumption with digital systems, the power amplifier in a WiMAX transmitter is resolutely analogue. It has to reproduce the complex waveform of WiMAX's OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme, which has multiple simultaneous carrier frequencies that rely on a high degree of accuracy and very low distortion in the transmitter.

Moreover, if modulation peaks correspond on many different carriers, the OFDM signal can suddenly demand much more power from the transmitter than the average – and the only way to make an linear amplifier capable of providing a lot of power very quickly is to have it running with a large overhead all the time. That's inefficient. It's also basic physics. If you can't meet that need, you'll distort the output signal – leading to signal degradation, interference with adjacent channels and generally poor performance for you and others in the area.

This issue has been known about for a long time, and came up frequently during the 802.16e standardisation process – but in the end, the importance of getting the standard finished quickly led to the decision that the problem would best be solved by manufacturers. Unfortunately, nobody's found a way of doing this yet: various companies have promised new techniques that could cope, but so far there's no real evidence that they work. The short-term solutions may include reducing the output power from the specified maximum, but that will affect range, speed and reliability.

Mobile WiMAX's closest future competitor, LTE, has decided not to use ODFM for the uplink for precisely these reasons. And, while it'll turn up later than 802.16e, it may have an easier time of it if WiMAX spends its early years in working out this fundamental problem.

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