IEEE Spectrum January, 2014 - 41

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-MHzwide 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 LTEAdvanced 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 LTEAdvanced toolbox.
Besides carrier aggregation, four other key features
distinguish LTE-Advanced
from its predecessors.
The first of these is called
multiple input, multiple
icon by

Greg Mably

fun fact: In South
Korea, LTE-Advanced
subscribers can download an 800-megabit
movie in as little as
43 seconds.
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. LTEAdvanced 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 LTEAdvanced 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

*HonoRable Mention*

buGatti's
DReaM Finally
takes FliGHt
the famed race car designer's illfated plane gets a second chance
EttorE bugatti was
known for his elegant
race cars of the 1920s
and '30s. Few people
realize that he also
designed an elegant-
and extraordinarily innovative-airplane.
That's because World War II interrupted its
construction, and the aircraft never flew.
But a copy of it will be completed this year.
Be on the lookout for Bugatti's Blue Dream
to finally take to the air. -DaviD schnEiDEr
Table of Contents for the Digital Edition of IEEE Spectrum January, 2014
