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LTE phones are fast, but they can also suck a battery dry in a few hours. Nokia Siemens Networks(s nok)(s si) did some preliminary studies on LTE phone’s power drain versus their HSPA (3G) counterparts and found that LTE devices consume from 5 percent to 20 percent more than previous-generation phones, depending on the application used. Those numbers, however, seem conservative compared to the anecdotal evidence. In its review of the Samsung Galaxy Nexus(s goog), Engadget found that the Google Navigation running over the LTE network ate battery power faster than the Nexus’ car charger could restore it.

Why is LTE so greedy? For starters, the radio in your LTE device is doing a lot more than it ever did in your old 3G handset. The radio is the single biggest source of power drain in any device apart from the LED screen, but unlike the display, the radio is always on. And LTE is particularly hungry. The next time your new Galaxy Nexus or HTC Vivid conks out right after lunch, here are five reasons why:

Your phone has rabbit ears. All LTE devices sold today use a technology called MIMO, which doesn’t just send or receive a single signal, but rather multiple parallel transmissions. Today’s devices support two such paths – future devices will support more — which means each phone has two antennas, each of which requires its own power amplifier. It’s not quite as bad as the running two phones off of a single battery, but you get the idea.

Look at all those networks! Verizon Wireless(s vz)(s vod) and MetroPCS(s pcs) 4G phones aren’t just maintaining two LTE links, they’re running an additional radio to boot. Devices from these carriers have to remain in constant contact with both the CDMA network – to receive phone calls and texts – and the LTE network.

LTE devices are co-dependent. Your screen may be dark, but your phone is constantly pining for the network. That means its periodically scans the airwaves around it to determine which tower it should tether itself to. The more networks there are to choose from the more scans it must make. With the typical operator sporting some combination of GSM, HSPA, CDMA and EV-DO systems — often multiple version of each in different frequency bands — there are a lot of other networks for an LTE device to flip between.

Is that a tower on the horizon? Operators haven’t built out their new LTE footprints densely yet. With cells spaced much further apart, devices have to reach further – and thus boost their transmission power — to latch onto a tower. And since there are still plenty of coverage holes in these networks, phones are dropping in and out of LTE coverage quite often, initiating new rounds of scans and taxing the battery further.

This ain’t no FM radio. There is a reason each generation of wireless technology is faster and more efficient than its predecessors. RF engineers are finding more and better ways to wrestle more bits into a radio wave. But, the more complex the waveform, the more computing power phone processors use up modulating and demodulating that radio wave. The 64-state quadrature amplitude modulation (QAM) and orthogonal frequency division multiplexing (OFDM) techniques used in LTE are as complicated as they sound. Consequently, the faster and more spectrally efficient networks become, the more power phones will consume trying to make sense of their signals.

So are we doomed forever to a life of constantly reaching for our battery chargers? On Monday, we’ll explore what handset makers and network suppliers are doing to improve LTE battery life, but also how they may be fighting a losing battle.

Tesla coil image courtesy of Flickr user caseyyee