bandwidth, a downlink speed of up
to 300 Mb/s can be achieved with
4x4 MIMO, and up to 150 Mb/s is
possible with 2x2 MIMO.
To implement full duplexing —
meaning simultaneous transmit and
receive — LTE uses frequency division
duplexing (FDD). This is the process
of using two separate chunks of
spectrum: one for the downlink and
the other for the uplink. The
spectrum segments have to be
separated from one another; enough
so that with filtering, the transmitter
won’t interfere with the companion
receiver. This is the system currently
used by all cellular systems in the US
and in most other countries. It works
great but requires lots of spectrum.
Another LTE duplexing technique
is time division duplexing (TDD). This
system uses different time segments
in which to transmit and receive. The
timing is so fast a user cannot tell
that alternating talk-listen time
segments are occurring. Therefore,
this method requires precise timing.
The big benefit is that only one
segment of spectrum is needed. This
arrangement is a more economical
use of spectrum. TDD-LTE is used by
China and a few other nations.
LTE operates in the same cellular
bands currently used by 2G and 3G
systems. However, most LTE systems
; FIGURE 1.
These new smartphones use LTE. The
iPhone 5 is the first Apple phone to use
LTE. The Nokia Lumia 920 also incorporates
LTE and uses Microsoft’s new Windows
Mobile 8 operating system.
use the higher cellular bands
between 1,800 and 2,600 MHz,
depending on the country of origin
and the carrier’s spectrum
assignments. LTE also uses the newer
700 MHz spectrum available to some
carriers. There are a few dozen
specific LTE bands, and each carrier
is assigned certain ones. This means
that the cell phones supplied by the
carrier will work mainly in the
assigned bands and may not operate
in other bands of different carriers,
making roaming a problem.
VOICE ON LTE
LTE is a packed-based system that
uses the Internet Protocol. For that
reason, it is a data only technology; it
does not carry voice traffic. That
means LTE phones also incorporate
older GSM or CDMA technology for
phone calls. Soon, LTE systems will
deploy a new voice technology
called voice over LTE (or VoLTE) that
will use digital voice in the LTE
spectrum. It has been deployed on a
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