Have you ever called your cellphone carrier to report poor signal strength? Sure you have. And did that carrier do anything significant to fix the problem? Of course it didn't—unless you live in South Korea.
"I guarantee you—if I call my carrier tonight and complain about not getting a good signal in my bathroom, they will send someone to install a repeater first thing tomorrow morning," said Wonil Roh during an interview in Suwon last October.
Full disclosure: Roh heads the Advanced Communications Laboratory at Samsung Electronics Co. But he doesn't need the lofty title to get that kind of attention in South Korea's intensely competitive wireless arena. Home to Samsung and LG Corp., the world's first- and fourth-largest smartphone makers, the country boasts some of the most advanced wireless networks on earth. Last June, for instance, SK Telecom Co. launched what it called the "world's first publicly available LTE-Advanced network." Short for Long Term Evolution, LTE is the globally embraced standard behind today's top-of-the-line 4G smartphones and tablets. For the same price as an LTE plan, LTE-Advanced subscribers could now get twice the data rates, SK claimed. Not to be outdone, its competitors LG Uplus Corp. and KT Corp. began offering their own LTE-Advanced services in July. By October, a million people had signed up for SK's service alone.
What's happening in South Korea will soon come to pass in other parts of the world. Operators everywhere face a universal and unremitting predicament: Customers want more data at faster speeds to run ever more sophisticated applications. Today it's video calls and sports broadcasts; tomorrow it'll be telemedicine and virtual shopping sprees. Each year, according to Cisco Systems, global mobile traffic more than doubles. And that exponential growth is showing no signs of waning.
So now, four years after the first networks using LTE went live, operators are looking to its successor. Already, more than a dozen carriers outside of South Korea, including AT&T, Australia's Telstra, Japan's NTT DoCoMo, and Telenor Sweden have reported that they are testing LTE-Advanced technologies, and analysts expect commercial rollouts to start this year. By 2018, according to ABI Research, global LTE-Advanced connections will approach 500 million—about five times as many as LTE can claim today.
"There's no way around it—LTE has to evolve," says Lingjia Liu, a wireless expert at the University of Kansas. "LTE-Advanced will become the dominant standard."
Wireless specialists are calling LTE-Advanced "true 4G" because unlike ordinary 4G LTE, it actually meets the International Telecommunication Union's specifications for fourth-generation wireless systems.
One of these criteria is speed. LTE-Advanced can theoretically achieve data download rates as high as 3 gigabits per second and upload rates as high as 1.5 Gb/s. By comparison, LTE tops out around 300 Mb/s for downloads and 75 Mb/s for uploads. And LTE-Advanced isn't just about faster rates. It also includes new transmission protocols and multiple-antenna schemes that enable smoother handoffs between cells, increase throughput at cell edges, and stuff more bits per second into each hertz of spectrum. The result will be higher network capacity, more consistent connections, and cheaper data.
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Illustration: James Provost
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As its name implies, LTE-Advanced is meant to enhance LTE. The two standards are mutually compatible, which is great for consumers. New LTE-Advanced phones will still work on LTE networks, and old LTE phones will connect to LTE-Advanced networks. Operators will benefit as well. Those wishing to upgrade to LTE-Advanced won't need to scrape together new radio spectrum or build out new infrastructure as they did to make the leap from 3G to LTE.
But here's the catch: Carriers won't roll out all of LTE-Advanced's capabilities at once. Like LTE before it, the new standard isn't a single technology but rather a grab bag of many technologies, and operators will pick and choose items as they're needed. For instance, the South Korean telcos that now claim to have LTE-Advanced networks are really talking about just one of LTE-Advanced's capabilities, known as carrier aggregation.
This feature increases the bandwidth available to a mobile device by stitching together frequency channels, or carriers, that reside in different parts of the radio spectrum. Ordinary LTE can deliver data using a contiguous block of frequencies up to 20 megahertz wide. But as more and more companies and devices vie for radio spectrum, such wide swaths are increasingly scarce. Most operators, having bought bits and pieces of spectrum wherever they could, have fragmented collections.
Carrier aggregation solves that problem. It allows operators to combine their narrow, disjointed channels into "one very big pipe," says Sang-min Lee, a senior manager at SK's R&D center in Seoul. To deliver its LTE-Advanced service, for example, the company combined two separate 10-MHz-wide channels, at 800 MHz and 1.8 gigahertz, into a single 20-MHz-wide channel, essentially doubling the data rate available to each user.
"We can get a huge performance gain," Lee says, pointing out that a connection on SK's new network can support downloads up to 150 Mb/s versus the maximum 75 Mb/s available through its LTE service. The LTE-Advanced standard allows operators to combine up to five carriers as wide as 20 MHz each for a maximum bandwidth of 100 MHz—five times as much bandwidth as conventional LTE offers.
Following SK's lead, most early LTE-Advanced adopters will likely focus on carrier aggregation because the higher data rates are an easy sell. "From a marketing standpoint, it's a slam dunk," says Peter Jarich at Current Analysis, in Washington, D.C. But, he adds, that's just the beginning. To keep their networks running smoothly, operators will need to reach deeper into the LTE-Advanced toolbox.
Besides carrier aggregation, four other key features distinguish LTE-Advanced from its predecessors. The first of these is called multiple input, multiple output (MIMO), which allows base stations and mobile units to send and receive data using multiple antennas. LTE already supports some MIMO, but only for the download stream. And it limits the number of antennas to four transmitters in the base station and four receivers in the handset. LTE-Advanced allows for up to eight antenna pairs for the download link and up to four pairs for the upload link.
MIMO serves two functions. In noisy radio environments—such as at the edge of a cell or inside a moving vehicle—the multiple transmitters and receivers work together to focus the radio signals in one particular direction. This "beamforming" boosts the strength of the received signal without upping transmission power.
If signals are strong and noise is low, however—such as when stationary users are close to a base station—MIMO can be used to increase data rates, or the number of users, for a given amount of spectrum. The technique, called spatial multiplexing, permits multiple data streams to travel over the same frequencies at the same time. A base station with eight transmitters, for instance, can send eight streams simultaneously to a smartphone with eight receivers. Because each stream arrives at each receiver at a slightly different angle, strength, and time, processing algorithms in the smartphone can combine these inputs and use the differences to sort out the original streams.
As a rule of thumb, spatial multiplexing can multiply data rates proportionately to the number of antenna pairs available. So under the best circumstances, eight pairs could increase data rates roughly eightfold.
Another important LTE-Advanced technology is relaying, which extends coverage to places where reception is poor. Wireless network architects have long used relays to extend a tower's reach, such as into a train tunnel or a remote area. But traditional relays, or repeaters, are relatively simple. They receive signals, amplify them, and then retransmit them.
LTE-Advanced supports more advanced relays, which first decode the transmissions and then forward only those destined for the mobile units that each relay is serving. This scheme reduces interference and lets more users link with the relay. LTE-Advanced also allows a relay to communicate with the base station and with devices using the same spectrum and protocols as the base station itself. This has the advantage of letting regular LTE handsets connect to the relay as if it were a traditional tower. The relay avoids interfering with the base station by scheduling its transmissions during certain times when the base station is silent.
Yet another principal LTE-Advanced tool will help alleviate network congestion. Known as enhanced inter-cell interference coordination, or eICIC, it will be used for so-called heterogeneous networks, in which low-power base stations, or small cells, overlay the "macro" network of traditional towers. Many carriers have already begun using variously sized small cells (also called metro-, micro-, pico-, or femtocells) to expand data capacity in busy urban centers. These compact boxes are cheaper, less obtrusive, and easier to install, and analysts see a bright future for them. But as operators cram more and more cells into the same spaces, they will have to find ways to lessen the inevitable crosstalk.
The eICIC protocol builds on the LTE protocol ICIC, which helps reduce interference between two macrocells. Using ICIC, a base station can reduce its transmission power at certain frequencies at certain times when a neighboring station is using those resources to talk to mobile users near the edge of its coverage area. But this spectrum-sharing scheme works only for delivering data streams. To communicate with a mobile device and help it make sense of the data, a base station must also send control signals, which carry housekeeping information such as scheduling decisions, retransmission requests, and decoding instructions. And because the device expects these messages to arrive on predictable frequencies at predictable times, the base station can't simply lend those resources to a neighbor whenever it needs them. LTE resolves this predicament by making control signals robust enough to withstand relatively high amounts of interference.
Small cells, however, make things more complex. For some devices trying to link with a small cell, which sits inside a macrocell, control signals from the macro tower can overwhelm the signals from the small cell. The eICIC protocol handles this scenario in one of two ways. If the network is using carrier aggregation to combine two or more different frequency channels, the macrocell and small cell can simply use separate channels to send control signals. Meanwhile, both cells use all the channels to deliver data so that mobile users still benefit from the combined bandwidth. The two cells share this spectrum, as with ICIC, by coordinating their use of different time-frequency resources.
For networks using only one frequency channel, eICIC offers a different solution. It permits the macrocell to mute data traffic and reduce the power of control signals at particular 1-millisecond time intervals, called subframes. A small cell can then schedule both control and data transmissions during these times, allowing it to expand its coverage range. This technique lets more users link to the small cell, which provides more data capacity.
The last major item on LTE-Advanced's broad menu helps further improve signals and increase data rates at a cell's edge, where it can be tough to get a good connection. The technique is called coordinated multipoint, or CoMP. Essentially, it enables a mobile device to exchange data between multiple base stations at the same time. For example, two neighboring base stations could send the same data to the device simultaneously, increasing its chance of getting a decent signal. Likewise, the device could upload data to both base stations at once, and the stations, acting as a virtual antenna array, would jointly process the signals to eliminate errors. Or the device might instead choose to upload to a nearby small cell, saving transmission power while still receiving a strong download signal from a larger tower.
It will take years for operators to make use of everything that LTE-Advanced has to offer. Many carriers have yet to deploy some of the more sophisticated features of LTE, such as voice services and "self-organizing" software, which would let base stations adapt to new network conditions on their own or heal themselves after a disruption.
Meanwhile, the evolution of LTE won't end with LTE-Advanced. The 3rd Generation Partnership Project, the international body behind the standard, plans to release the next iteration later this year. Some companies are calling it LTE-B, although the 3GPP disapproves, asserting that all future breeds of LTE will still be officially titled LTE-Advanced. Whatever its name, the new variant will offer operators even more radical options, including protocols for three-dimensional antennas, more energy-efficient transmissions, and direct communication between mobile devices and other smart sensors and machines. Together, such breakthroughs could give networks some 30 times as much capacity as LTE-Advanced. Now that's something worth waiting for.
This article originally appeared in print as "4G Gets Real."
