Three-Minute Tech: LTE
[In our Three-Minute Tech series, we tell you everything you really need to know about a technology in three minutes or less.]
The iPhone 5 is the latest smartphone to hop on-board the LTE (Long Term Evolution) bandwagon, and for good reason: The mobile broadband standard is fast, flexible, and designed for the future. Yet LTE is still a young technology, full of growing pains. Here’s an overview of where it came from, where it is now, and where it might go from here.
The evolution of ‘Long Term Evolution’
LTE is a mobile broadband standard developed by the 3GPP (3rd Generation Partnership Project), a group that has developed all GSM standards since 1999. (Though GSM and CDMA—the network Verizon and Sprint use in the United States—were at one time close competitors, GSM has emerged as the dominant worldwide mobile standard.)
Cell networks began as analog, circuit-switched systems nearly identical in function to the public switched telephone network (PSTN), which placed a finite limit on calls regardless of how many people were speaking on a line at one time.
The second-generation, GPRS, added data (at dial-up modem speed). GPRS led to EDGE, and then 3G, which treated both voice and data as bits passing simultaneously over the same network (allowing you to surf the web and talk on the phone at the same time).
GSM-evolved 3G (which brought faster speeds) started with UMTS, and then accelerated into faster and faster variants of 3G, 3G+, and “4G” networks (HSPA, HSDPA, HSUPA, HSPA+, and DC-HSPA).
Until now, the term “evolution” meant that no new standard broke or failed to work with the older ones. GSM, GPRS, UMTS, and so on all work simultaneously over the same frequency bands: They’re intercompatible, which made it easier for carriers to roll them out without losing customers on older equipment. But these networks were being held back by compatibility.
That’s where LTE comes in. The “long term” part means: “Hey, it’s time to make a big, big change that will break things for the better.”
LTE needs its own space, man
LTE has “evolved” beyond 3G networks by incorporating new radio technology and adopting new spectrum. It allows much higher speeds than GSM-compatible standards through better encoding and wider channels. (It’s more “spectrally efficient,” in the jargon.)
LTE is more flexible than earlier GSM-evolved flavors, too: Where GSM’s 3G variants use 5 megahertz (MHz) channels, LTE can use a channel size from 1.4 MHz to 20 MHz; this lets it work in markets where spectrum is scarce and sliced into tiny pieces, or broadly when there are wide swaths of unused or reassigned frequencies. In short, the wider the channel—everything else being equal—the higher the throughput.
Speeds are also boosted through MIMO (multiple input, multiple output), just as in 802.11n Wi-Fi. Multiple antennas allow two separate benefits: better reception, and multiple data streams on the same spectrum.
Unfortunately, in practice, LTE implementation gets sticky: There are 33 potential bands for LTE, based on a carrier’s local regulatory domain. In contrast, GSM has just 14 bands, and only five of those are widely used. (In broad usage, a band is two sets of paired frequencies, one devoted to upstream traffic and the other committed to downstream. They can be a few MHz apart or hundreds of MHz apart.)
And while LTE allows voice, no standard has yet been agreed upon; different carriers could ultimately choose different approaches, leaving it to handset makers to build multiple methods into a single phone, though they’re trying to avoid that. In the meantime, in the U.S., Verizon and AT&T use the older CDMA and GSM networks for voice calls, and LTE for data.
LTE in the United States
Of the four major U.S. carriers, AT&T, Verizon, and Sprint have LTE networks, with T-Mobile set to start supporting LTE in the next year. But that doesn’t mean they’re set to play nice. We said earlier that current LTE frequencies are divided up into 33 spectrum bands: With the exception of AT&T and T-Mobile, which share frequencies in band 4, each of the major U.S. carriers has its own band. Verizon uses band 13; Sprint has spectrum in band 26; and AT&T holds band 17 in addition to some crossover in band 4.
In addition, smaller U.S. carriers, like C Spire, U.S. Cellular, and Clearwire, all have their own separate piece of the spectrum pie: C Spire and U.S. Cellular use band 12, while Clearwire uses band 41.
As such, for a manufacturer to support LTE networks in the United States alone, it would need to build a receiver that could tune into seven different LTE bands—let alone the various flavors of GSM-evolved or CDMA networks.
With the iPhone, Apple tried to cut through the current Gordian Knot by releasing two separate models, the A1428 and A1429, which cover a limited number of different frequencies depending on where they’re activated. (Apple has kindly released a list of countries that support its three iPhone 5 models.) Other companies have chosen to restrict devices to certain frequencies, or to make numerous models of the same phone.
Other solutions are coming. Qualcomm made a regulatory filing in June regarding a seven-band LTE chip, which could be in shipping devices before the end of 2012 and could allow a future iPhone to be activated in different fashions. Within a year or so, we should see most-of-the-world phones, tablets, and other LTE mobile devices that work on the majority of large-scale LTE networks.
That will be just in time for the next big thing: LTE-Advanced, the true fulfillment of what was once called 4G networking, with rates that could hit 1 Gbps in the best possible cases of wide channels and short distances. By then, perhaps the chip, handset, and carrier worlds will have converged to make it all work neatly together.
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