3G LTE Basics Tutorial - Long Term Evolution
- information, overview, or tutorial about the basics of the 3G LTE, the
long term evolution plans for the next generation of cellular telecommunications
services
With services such as WiMAX offering very high data speeds,
work on developing the next generation of cellular technology has started. The
UMTS cellular technology upgrade has been dubbed LTE - Long Term Evolution. The
idea is that LTE will enable much higher speeds to be achieved along with much
lower packet latency (a growing requirement for many services these days), and
that LTE will enable cellular communications services to move forward to meet
the needs for cellular technology to 2017 and well beyond.
With HSPA (High Speed Packet Access), a combination of HSDPA
and HSUPA, now being deployed, the 3G LTE is being dubbed 3.99G as it is not a
full 4G standard, although in reality there are many similarities with the
cellular technologies being touted for the use of 4G. However, regardless of the
terminology, it is certain that LTE will offer significant improvements in
performance over the basic 3G standards and also HSPA.
3G LTE beginnings
3GPP, the Third Generation Partnership Project that oversaw
the development of the UMTS 3G system started the work on the evolution of the
3G cellular technology with a workshop that was held in Toronto Canada in
November 2004.
The workshop set down a number of high level requirements for
3G LTE:
- Reduced cost per bit
- Increased service provisioning - more services at lower cost with better
user experience
- Flexibility of use of existing and new frequency bands
- Simplified architecture, Open interfaces
- Allow for reasonable terminal power consumption
In terms of actual figures, targets for LTE include download
rates of 100Mbps, and upload rates of 50Mbps for every 20MHz of spectrum. In
addition to this LTE must be able to support at least 200 active users in every
5MHz cell. (i.e. 200 active phone calls). Targets have also been set for the
latency in IP packet delivery. With the growing use of services including VoIP
where latency is of concern, figures need to be set for this. As a result a
figure of sub-5ms latency for small IP packets has been set.
LTE technologies
Although the work on 3G LTE is not completed, the basic
technologies have been agreed along with their modes of implementation. Some of
the main technologies and changes included are:
- OFDM (Orthogonal Frequency Division Mulplex)
- MIMO (Multiple Input Multiple Output)
- SAE (System Architecture Evolution)
OFDM: The modulation format for LTE will be OFDM
(Orthogonal Frequency Division Multiplex) for the signal bearer and the access
scheme will be OFDMA (Orthogonal Frequency Division Multiple Access).
The actual implementation of the technology will be different
between the downlink (i.e. from base station to mobile) and the uplink (i.e.
mobile to the base station) as a result of the different requirements between
the two directions and the equipment at either end. However OFDM was chosen as
the signal bearer format because it is very resilient to interference. Also in
recent years a considerable level of experience has been gained in its use from
the various forms of broadcasting that use it along with Wi-Fi and WiMAX. OFDM
is also a modulation format that is very suitable for carrying high data rates -
one of the key requirements for LTE.
Note on OFDM:
Orthogonal Frequency Division Multiplex (OFDM) is a
form of transmission that uses a large number of close spaced carriers
that are modulated with low rate data. Normally these signals would be
expected to interfere with each other, but by making the signals
orthogonal to each another there is no mutual interference. This is
achieved by having the carrier spacing equal to the reciprocal of the
symbol period. This means that when the signals are demodulated they
will have a whole number of cycles in the symbol period and their
contribution will sum to zero - in other words there is no interference
contribution. The data to be transmitted is split across all the
carriers and this means that by using error correction techniques, if
some of the carriers are lost due to multi-path effects, then the data
can be reconstructed. Additionally having data carried at a low rate
across all the carriers means that the effects of reflections and
inter-symbol interference can be overcome. It also means that single
frequency networks, where all transmitters can transmit on the same
channel can be implemented.
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The OFDM signal used in LTE comprises a maximum of 2048
different sub-carriers having a spacing of 15 kHz. Although it is mandatory for
the mobiles to have capability to be able to receive all 2048 sub-carriers, not
all need to be transmitted by the base station which only needs to be able to
support the transmission of 72 sub-carriers. In this way all mobiles will be
able to talk to any base station. For the modulation format used within the OFDM
signal there is a choice of: QPSK, 16QAM, and 64QAM. Tthe exact format is chosen
according to the prevailing conditions. The whole signal is received by all
mobiles and the required data extracted.
For the LTE uplink, a different concept is used for the
access technique. Although still using a form of OFDMA technology, the
implementation is called Single Carrier Frequency Division Multiple Access
(SC-FDMA). In essence a mobile is allocated a sub-carrier or sub-carriers for
its link to the base station and it uses these to establish the uplink. This
form of modulation overcomes one of the major problems encountered with the 3G
systems using CDMA. Using CDMA the peak to average power ratio was high, and
this considerably reduced the efficiency available from the transmitter power
amplifiers which in turn reduced the battery life. Using SC-FDMA, this can be
overcome and much greater levels of power amplifier efficiency can be attained.
MIMO: Another of the LTE major technology
innovations is the use of MIMO or Multiple Input Multiple Output. This
technology provides LTE with the ability to further improve its data throughput
and spectral efficiency above that obtained by the use of OFDM.
Note on MIMO:
Two major limitations in communications channels can
be multipath interference, and the data throughput limitations as a
result of Shannon's Law. MIMO provides a way of utilising the multiple
signal paths that exist between a transmitter and receiver to
significantly improve the data throughput available on a given channel
with its defined bandwidth. By using multiple antennas at the
transmitter and receiver along with some complex digital signal
processing, MIMO technology enables the system to set up multiple data
streams on the same channel, thereby increasing the data capacity of a
channel.
For more information on MIMO click
here |
MIMO is being used increasingly in many high data rate
technologies including Wi-Fi and other wireless and cellular technologies to
provide improved levels of efficiency. Essentially MIMO employs multiple
antennas on the receiver and transmitter to utilise the multi-path effects that
always exist to transmit additional data, rather than causing interference.
The schemes employed in LTE again vary slightly between the
uplink and downlink. The reason for this is to keep the terminal cost low as
there are far more terminals than base stations and as a result terminal works
cost price is far more sensitive.
For the downlink, a configuration of two transmit antennas at
the base station and two receive antennas on the mobile terminal is used as
baseline, although configurations with four antennas are also being considered.
For the uplink from the mobile terminal to the base station,
a scheme called MU-MIMO (Multi-User MIMO) is to be employed. Using this, even
though the base station requires multiple antennas, the mobiles only have one
transmit antenna and this considerably reduces the cost of the mobile. In
operation, multiple mobile terminals may transmit simultaneously on the same
channel or channels, but they do not cause interference to each other because
mutually orthogonal pilot patterns are used. This techniques is also referred to
as spatial domain multiple access (SDMA).
System Architecture Upgrades: These are naturally
only some of the very basic features as a completely new cellular system will
involve many changes. One of the major other areas of work is the infrastructure
technology or System Architecture Evolution (SAE) which is required to simplify
the system, particularly as IP data is more widely used. To handle not only the
speeds required, but also the levels of data likely to be carried by 3G LTE a
totally new infrastructure technology is needed.
3G LTE Specification overview
It is worth summarizing the key parameters of the 3G LTE
specification. In view of the fact that there are a number of differences
between the operation of the uplink and downlink, these naturally differ in the
performance they can offer.
| Link |
Modulation |
Access
scheme |
Duplex
UL / DL |
Channel
bandwidth |
Peak data rate |
| Downlink |
QPSK, 16QAM, 64QAM |
OFDMA |
TDD / FDD |
Scaleable to 20 MHz |
100 Mbps |
| Uplink |
QPSK, 16QAM, 64QAM |
SC-DFMA |
TDD / FDD |
|
50 Mbps |
3G LTE Summary
The basic work on LTE has not all been completed, although
the initial drafts were released in September 2007 and the parallel work on the
infrastructure technology known as LTE System Architecture Evolution (SAE)
followed shortly afterwards. In terms of the deployments of real systems some
anticipate that the first deployments may be seen in 2010 although it is
possibly to early to judge the actual first deployment dates. Nevertheless 3G
LTE is sure to happen and cellular technology will be in a position to offer
much higher data rates than is possible today.
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