OPEN COMMUNICATION
THE RADIO
TECHNOLOGY
LTE is a mobile broadband
wireless system that features high
data rates, low latency, improved
subscriber capacity, and coverage. It
is based on the modulation method
known as orthogonal frequency
division multiplexing (OFDM). This is
a multi-carrier system that divides the
useable spectrum into many narrow
band subchannels or subcarriers.
Several hundred or several thousand
subcarriers are used. The high speed
serial data representing the data to
be transmitted is subdivided into
many slower data streams; these are
then used to modulate each
subcarrier in parallel. This method
spreads the signal over a wide
bandwidth, making it less affected by
fading and multipath signals created
by reflections. The overall effect is a
more solid wireless link.
OFDM is the modulation method
of choice for most new wireless (and
wired) systems. It is used in TV data
transmission worldwide (not in the
US), HD radio, wireless LANs using
Wi-Fi, and wireless broadband
systems. Even power line
communications (PLC) that send high
speed data riding on the AC power
lines use OFDM. Despite its
popularity, it is an amazingly complex
technology. The only way to achieve
OFDM practically is to create it using
mathematical techniques
implemented with digital signal
processing (DSP). Separate DSP
microprocessors or FPGAs are
commonly used to
create OFDM.
LTE is so good that practically
all cellular carriers are
adopting it as a replacement
for all previous forms of 2G
or 3G technologies.
What is 3GPP?
The 3rd Generation Partnership Project (3GPP) unites [six] telecommunicationsstandardsdevelopmentorganizations(ARIB,ATIS,
CCSA, ETSI, TTA, TTC), and provides their members with a stable
environment to produce the Reports and Specifications that define 3GPP
technologies.
The four Technical Specification Groups (TSG) in 3GPP are Radio
Access Networks (RAN), Service & Systems Aspects (SA), Core Network &
Terminals (CT), and GSM EDGE Radio Access Networks (GERAN).
Each of the four TSGs has a set of Working Groups, which meet
regularly four to six times a year. Each TSG has its own quarterly plenary
meeting where the work from its WGs is presented for information,
discussion, and approval. Each TSG has a particular area of responsibility
for the Reports and Specifications within its own Terms of Reference.
The 3GPP technologies from these groups are constantly evolving
through generations of commercial cellular/mobile systems. Since the
completion of the first LTE and the Evolved Packet Core specifications,
3GPP has become the focal point for mobile systems beyond 3G.
For more information about the 3GPP, visit
www.3GPP.org
The LTE standard supports
bandwidths of 1.4, 3, 5, 10, 15, and
20 MHz. This flexibility allows each
wireless carrier to match the desired
data rate and capacity to their
available spectrum holdings. The
modulation can be QPSK, 16QAM,
or 64QAM. The maximum typical
data rates are 100 Mb/s downlink
and 50 Mb/s uplink. The latency is
less than 10 ms.
A key feature of LTE is the use of
multiple input multiple output
(MIMO) antenna technology. MIMO
uses multiple transmitters, receivers,
and antennas to increase data speeds
and provide mitigation of fading and
multipath conditions that let the
carrier implement a more reliable
connection.
What MIMO does is divide up
the fast serial data into multiple
streams, and transmits these different
parallel signals simultaneously over
the same frequency bandwidth. With
special coding and taking advantage
of the spatial differences in each
received signal, the parallel data
streams can be recovered and the
original fast serial data can be
reconstructed. MIMO allows the
overall data rate to be multiplied by
the number of streams used. Some of
the MIMO formats supported are
1x2, 2x1, 2x2, and 4x4, where the
first number means the amount of
transmit antennas and the second
number is the amount of receive
antennas. Using a 20 MHz
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