Can envelope tracking save LTE? Well, perhaps.
We’re a couple of weeks away from Mobile World Congress, when the great and the good of the mobile world gather in Barcelona to talk up the industry. This year they’ll be talking up LTE, which is being rolled out in the US and elsewhere and is supposed to be The Next Big Thing for operators looking to make a buck. Think of all that juicy early-adopter pricing, those premium data tariffs and the 24-month contracts just to access the hardware. With rumours circulating that the iPad 3 will include an LTE modem and be supported in the US by Verizon and AT&T, MWC 2012 could mark the point at which LTE starts getting mass consumer attention.
This attention could prove it downfall, at least in the short term. As is traditional with new cellular technologies, the performance of the first couple of generations of equipment tends to fall short of expectations. All the components are new, the systemic optimisations leave something to be desired, the modulation schemes behave unpredictably in the brutal real-world conditions of crowded downtowns and the overall experience, which in this case is being punted as wireline browsing speeds from wireless devices, ends up just a bit blah. Result? Disgruntled consumers and a reset of public perceptions.
One particular bugbear for LTE is likely to be power consumption: no point bragging about owning the best mobile browsing device in the world if its battery goes flat after a couple of hours of demonstrating it to weary friends.
Why is LTE particularly burdened in terms of power consumption? One reason is that it uses a more complex modulation scheme than previous mobile standards in order to squeeze more information-carrying capacity from the channel, which means more work encoding and decoding the signal. The second issue is the large and rapid variation in the RF power used to transmit those encoded bits. As Jeremy Hendy, vice president of sales and marketing for envelope-tracking technology company Nujira puts it, if the 2:1 peak to average power ratio (PAPR) of a 3G w-CDMA signal produces an output “like thrash metal – just constant noise”, and the 5:1 PAPR of HSUPA is “like jazz”, then the 7:1 PAPR of an LTE signal is “like classical music – very dynamic”.
With a fixed supply voltage, this means that the RF power amplifier spends a lot of its time shedding unused energy as heat just so that when it needs to accommodate the far excursions of the input signal, it can. Envelope tracking replaces the DC:DC converter supplying the PA with a power supply whose voltage is adjusted to match the instantaneous amplitude of the RF signal. Result? Much greater energy efficiency in the RF stage.
Envelope tracking is not a new technique – the broadcast industry has been working with similar schemes since the 1930s. Nujira’s secret sauce is to have built a power modulator that can track the rapid changes in RF signal amplitude of modern modulation schemes in a way that is cost-effective enough to be used in broadcast equipment, basestations and, they hope, from the third quarter of this year, handsets.
So will envelope tracking save LTE? On its own, probably not. But it looks likely to be one of a host of techniques that will have to be pressed into service to turn first-generation LTE basestations and mobile devices into the second- or third-generation equipment that actually delivers the promise of the new standard.
Over the next few weeks, we hope to bring you more details about the impact of envelope tracking in LTE. We’re also hoping that Nujira will be able to detail the modelling strategy it used to persuade baseband chip makers that they needed to use higher-resolution digital to analogue converters than they at first thought necessary to create the low noise envelope-tracking signal used to drive their RF power modulator.