3 g lte tutorial

3G LTE Tutorial - 3GPP Long Term
Evolution
- information, overview, or tutorial about the basics of the 3GPP / 3G LTE, the
long term evolution plans for the next generation of cellular telecommunications
services

3G LTE technology tutorial includes:
• Tutorial Introduction
• OFDM and OFDMA / SC-FDMA
• MIMO
• TDD and FDD duplex schemes
• Frame and subframe structure
• Physical logical & transport channels
• Frequency bands and spectrum
• UE category definitions
• SAE system architecture evolution
• Voice over LTE, VoLTE
• Security
See also: 4G LTE Advanced

3G LTE is now being deployed and is the way forwards for high speed cellular services.

There has been a rapid increase in the use of data carried by cellular services, and this increase
will only become larger in what has been termed the "data explosion". To cater for this and the
increased demands for increased data transmission speeds and lower latency, further
development of cellular technology have been required.

The UMTS cellular technology upgrade has been dubbed LTE - Long Term Evolution. The idea
is that 3G 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 3GPP LTE will enable
cellular communications services to move forward to meet the needs for cellular technology to
2017 and well beyond.

Many operators have not yet upgraded their basic 3G networks, and 3GPP LTE is seen as the
next logical step for many operators, who will leapfrog straight from basic 3G straight to LTE as
this will avoid providing several stages of upgrade. The use of LTE will also provide the data
capabilities that will be required for many years and until the full launch of the full 4G standards
known as LTE Advanced.

3G LTE evolution
Although there are major step changes between LTE and its 3G predecessors, it is nevertheless
looked upon as an evolution of the UMTS / 3GPP 3G standards. Although it uses a different
form of radio interface, using OFDMA / SC-FDMA instead of CDMA, there are many
similarities with the earlier forms of 3G architecture and there is scope for much re-use.

LTE can be seen for provide a further evolution of functionality, increased speeds and general
improved performance.

HSPA
WCDMA
HSDPA / HSPA+ LTE
(UMTS)
HSUPA
Max downlink speed
384 k 14 M 28 M 100M
bps
Max uplink speed
128 k 5.7 M 11 M 50 M
bps
Latency
50ms
round trip time 150 ms 100 ms ~10 ms
(max)
approx
3GPP releases Rel 99/4 Rel 5 / 6 Rel 7 Rel 8
Approx years of initial roll 2005 / 6 HSDPA
2003 / 4 2008 / 9 2009 / 10
out 2007 / 8 HSUPA
OFDMA / SC-
Access methodology CDMA CDMA CDMA
FDMA

In addition to this, LTE is an all IP based network, supporting both IPv4 and IPv6. There is also
no basic provision for voice, although this can be carried as VoIP.

3GPP LTE technologies
LTE has introduced a number of new technologies when compared to the previous cellular
systems. They enable LTE to be able to operate more efficiently with respect to the use of
spectrum, and also to provide the much higher data rates that are being required.

• OFDM (Orthogonal Frequency Division Multiplex): OFDM technology has been
incorporated into LTE because it enables high data bandwidths to be transmitted
efficiently while still providing a high degree of resilience to reflections and interference.
The access schemes differ between the uplink and downlink: OFDMA (Orthogonal
Frequency Division Multiple Access is used in the downlink; while SC-FDMA(Single
Carrier - Frequency Division Multiple Access) is used in the uplink. SC-FDMA is used in
view of the fact that its peak to average power ratio is small and the more constant power

enables high RF power amplifier efficiency in the mobile handsets - an important factor
for battery power equipment. Read more about LTE OFDM / OFDMA / SCFMDA
• MIMO (Multiple Input Multiple Output): One of the main problems that previous
telecommunications systems has encountered is that of multiple signals arising from the
many reflections that are encountered. By using MIMO, these additional signal paths can
be used to advantage and are able to be used to increase the throughput.

When using MIMO, it is necessary to use multiple antennas to enable the different paths
to be distinguished. Accordingly schemes using 2 x 2, 4 x 2, or 4 x 4 antenna matrices
can be used. While it is relatively easy to add further antennas to a base station, the same
is not true of mobile handsets, where the dimensions of the user equipment limit the
number of antennas which should be place at least a half wavelength apart. Read more
about LTE MIMO
• SAE (System Architecture Evolution): With the very high data rate and low latency
requirements for 3G LTE, it is necessary to evolve the system architecture to enable the
improved performance to be achieved. One change is that a number of the functions
previously handled by the core network have been transferred out to the periphery.
Essentially this provides a much "flatter" form of network architecture. In this way
latency times can be reduced and data can be routed more directly to its destination. Read
more about LTE SAE

These technologies are addressed in much greater detail in the following pages of this tutorial.

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.

Parameter Details
Peak downlink speed
64QAM 100 (SISO), 172 (2x2 MIMO), 326 (4x4 MIMO)
(Mbps)
Peak uplink speeds
50 (QPSK), 57 (16QAM), 86 (64QAM)
(Mbps)
All packet switched data (voice and data). No
Data type
circuit switched.
Channel bandwidths
1.4, 3, 5, 10, 15, 20
(MHz)
Duplex schemes FDD and TDD
0 - 15 km/h (optimised),
Mobility
15 - 120 km/h (high performance)
Idle to active less than 100ms
Latency
Small packets ~10 ms

Parameter Details
Downlink: 3 - 4 times Rel 6 HSDPA
Spectral efficiency
Uplink: 2 -3 x Rel 6 HSUPA
OFDMA (Downlink)
Access schemes
SC-FDMA (Uplink)
Modulation types
QPSK, 16QAM, 64QAM (Uplink and downlink)
supported

These highlight specifications give an overall view of the performance that LTE will offer. It
meets the requirements of industry for high data download speeds as well as reduced latency - a
factor important for many applications from VoIP to gaming and interactive use of data. It also
provides significant improvements in the use of the available spectrum.

LTE OFDM, OFDMA and SC-FDMA
- overview, information, tutorial about the basics of LTE OFDM, OFDMA and
SC-FDMA including cyclic prefix, CP.

• MIMO
• Security

One of the key elements of LTE is the use of OFDM (Orthogonal Frequency Division Multiplex)
as the signal bearer and the associated access schemes, OFDMA (Orthogonal Frequency
Division Multiplex) and SC-FDMA (Single Frequency Division Multiple Access).

OFDM is used in a number of other of systems from WLAN, WiMAX to broadcast technologies
including DVB and DAB. OFDM has many advantages including its robustness to multipath
fading and interference. In addition to this, even though, it may appear to be a particularly
complicated form of modulation, it lends itself to digital signal processing techniques.

In view of its advantages, the use of ODFM and the associated access technologies, OFDMA and
SC-FDMA are natural choices for the new LTE cellular standard.

OFDM basics
The use of OFDM is a natural choice for LTE. While the basic concepts of OFDM are used, it
has naturally been tailored to meet the exact requirements for LTE. However its use of multiple
carrier each carrying a low data rate remains the same.

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.

Click on the link for an OFDM tutorial

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.

In addition to this, OFDM can be used in both FDD and TDD formats. This becomes an
additional advantage.

LTE channel bandwidths and characteristics
One of the key parameters associated with the use of OFDM within LTE is the choice of
bandwidth. The available bandwidth influences a variety of decisions including the number of
carriers that can be accommodated in the OFDM signal and in turn this influences elements
including the symbol length and so forth.

LTE defines a number of channel bandwidths. Obviously the greater the bandwidth, the greater
the channel capacity.

The channel bandwidths that have been chosen for LTE are:

1. 1.4 MHz
2. 3 MHz
3. 5 MHz
4. 10 MHz
5. 15 MHz
6. 20 MHz

In addition to this the subcarriers are spaced 15 kHz apart from each other. To maintain
orthogonality, this gives a symbol rate of 1 / 15 kHz = of 66.7 µs.

Each subcarrier is able to carry data at a maximum rate of 15 ksps (kilosymbols per second).
This gives a 20 MHz bandwidth system a raw symbol rate of 18 Msps. In turn this is able to
provide a raw data rate of 108 Mbps as each symbol using 64QAM is able to represent six bits.

It may appear that these rates do not align with the headline figures given in the LTE
specifications. The reason for this is that actual peak data rates are derived by first subtracting
the coding and control overheads. Then there are gains arising from elements such as the spatial
multiplexing, etc.

LTE OFDM cyclic prefix, CP
One of the primary reasons for using OFDM as a modulation format within LTE (and many
other wireless systems for that matter) is its resilience to multipath delays and spread. However it
is still necessary to implement methods of adding resilience to the system. This helps overcome
the inter-symbol interference (ISI) that results from this.

In areas where inter-symbol interference is expected, it can be avoided by inserting a guard
period into the timing at the beginning of each data symbol. It is then possible to copy a section
from the end of the symbol to the beginning. This is known as the cyclic prefix, CP. The receiver
can then sample the waveform at the optimum time and avoid any inter-symbol interference
caused by reflections that are delayed by times up to the length of the cyclic prefix, CP.

The length of the cyclic prefix, CP is important. If it is not long enough then it will not
counteract the multipath reflection delay spread. If it is too long, then it will reduce the data
throughput capacity. For LTE, the standard length of the cyclic prefix has been chosen to be 4.69
µs. This enables the system to accommodate path variations of up to 1.4 km. With the symbol
length in LTE set to 66.7 µs.

The symbol length is defined by the fact that for OFDM systems the symbol length is equal to
the reciprocal of the carrier spacing so that orthogonality is achieved. With a carrier spacing of
15 kHz, this gives the symbol length of 66.7 µs.

LTE OFDMA in the downlink
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.

Within the OFDM signal it is possible to choose between three types of modulation:

1. QPSK (= 4QAM) 2 bits per symbol
2. 16QAM 4 bits per symbol
3. 64QAM 6 bits per symbol

The exact format is chosen depending upon the prevailing conditions. The lower forms of
modulation, (QPSK) do not require such a large signal to noise ratio but are not able to send the
data as fast. Only when there is a sufficient signal to noise ratio can the higher order modulation
format be used.

Downlink carriers and resource blocks
In the downlink, the subcarriers are split into resource blocks. This enables the system to be able
to compartmentalise the data across standard numbers of subcarriers.

Resource blocks comprise 12 subcarriers, regardless of the overall LTE signal bandwidth. They
also cover one slot in the time frame. This means that different LTE signal bandwidths will have
different numbers of resource blocks.

Channel bandwidth
1.4 3 5 10 15 20
(MHz)

Number of resource blocks 6 15 25 50 75 100

LTE SC-FDMA in the uplink
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).

One of the key parameters that affects all mobiles is that of battery life. Even though battery
performance is improving all the time, it is still necessary to ensure that the mobiles use as little
battery power as possible. With the RF power amplifier that transmits the radio frequency signal
via the antenna to the base station being the highest power item within the mobile, it is necessary
that it operates in as efficient mode as possible. This can be significantly affected by the form of
radio frequency modulation and signal format. Signals that have a high peak to average ratio and
require linear amplification do not lend themselves to the use of efficient RF power amplifiers.
As a result it is necessary to employ a mode of transmission that has as near a constant power
level when operating. Unfortunately OFDM has a high peak to average ratio. While this is not a
problem for the base station where power is not a particular problem, it is unacceptable for the
mobile. As a result, LTE uses a modulation scheme known as SC-FDMA - Single Carrier
Frequency Division Multiplex which is a hybrid format. This combines the low peak to average
ratio offered by single-carrier systems with the multipath interference resilience and flexible
subcarrier frequency allocation that OFDM provides.

LTE MIMO
- overview, information, tutorial about the basics of how MIMO is used within
3G LTE.

• MIMO

• Security

MIMO, Multiple Input Multiple Output is another of the LTE major technology innovations used
to improve the performance of the system. This technology provides LTE with the ability to
further improve its data throughput and spectral efficiency above that obtained by the use of
OFDM.

Although MIMO adds complexity to the system in terms of processing and the number of
antennas required, it enables far high data rates to be achieved along with much improved
spectral efficiency. As a result, MIMO has been included as an integral part of LTE.

LTE MIMO basics
The basic concept of MIMO utilises the multipath signal propagation that is present in all
terrestrial communications. Rather than providing interference, these paths can be used to
advantage.

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.

Click on the link for a MIMO tutorial

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).

LTE FDD, TDD, TD-LTE Duplex Schemes
- information, overview, or tutorial about the LTE TDD and LTE FDD duplex
schemes used with LTE and including TD-LTE.

• MIMO
• Security

LTE has been defined to accommodate both paired spectrum for Frequency Division Duplex,
FDD and unpaired spectrum for Time Division Duplex, TDD operation. It is anticipated that
both LTE TDD and LTE FDD will be widely deployed as each form of the LTE standard has its
own advantages and disadvantages and decisions can be made about which format to adopt
dependent upon the particular application.

LTE FDD using the paired spectrum is anticipated to form the migration path for the current 3G
services being used around the globe, most of which use FDD paired spectrum. However there

has been an additional emphasis on including TDD LTE using unpaired spectrum. TDD LTE
which is also known as TD-LTE is seen as providing the evolution or upgrade path for TD-
SCDMA.

In view of the increased level of importance being placed upon LTE TDD or TD-LTE, it is
planned that user equipments will be designed to accommodate both FDD and TDD modes. With
TDD having an increased level of importance placed upon it, it means that TDD operations will
be able to benefit from the economies of scale that were previously only open to FDD
operations.

Duplex schemes
It is essential that any cellular communications system must be able to transmit in both directions
simultaneously. This enables conversations to be made, with either end being able to talk and
listen as required. Additionally when exchanging data it is necessary to be able to undertake
virtually simultaneous or completely simultaneous communications in both directions.

It is necessary to be able to specify the different direction of transmission so that it is possible to
easily identify in which direction the transmission is being made. There are a variety of
differences between the two links ranging from the amount of data carried to the transmission
format, and the channels implemented. The two links are defined:

• Uplink: the transmission from the UE or user equipment to the eNodeB or base station.
• Downlink the transmission from the eNodeB or base station to the UE or user
equipment.

Uplink and downlink transmission directions

In order to be able to be able to transmit in both directions, a user equipment or base station must
have a duplex scheme. There are two forms of duplex that are commonly used, namely FDD,
frequency division duplex and TDD time division duplex..

Note on TDD and FDD duplex schemes:

In order for radio communications systems to be able to communicate in both directions it is
necessary to have what is termed a duplex scheme. A duplex scheme provides a way of
organizing the transmitter and receiver so that they can transmit and receive. There are several
methods that can be adopted. For applications including wireless and cellular
telecommunications, where it is required that the transmitter and receiver are able to operate
simultaneously, two schemes are in use. One known as FDD or frequency division duplex uses
two channels, one for transmit and the other for receiver. Another scheme known as TDD, time
division duplex uses one frequency, but allocates different time slots for transmission and
reception.

Click on the link for more information on TDD FDD duplex schemes

Both FDD and TDD have their own advantages and disadvantages. Accordingly they may be
used for different applications, or where the bias of the communications is different.

Advantages / disadvantages of LTE TDD and LTE FDD for
cellular communications
There are a number of the advantages and disadvantages of TDD and FDD that are of particular
interest to mobile or cellular telecommunications operators. These are naturally reflected into
LTE.

Parameter LTE-TDD LTE-FDD
Does not require paired Requires paired spectrum with
Paired spectrum as both transmit and sufficient frequency separation to
spectrum receive occur on the same allow simultaneous transmission
channel and reception
Lower cost as no diplexer is
needed to isolate the
transmitter and receiver. As
Diplexer is needed and cost is
Hardware cost cost of the UEs is of major
higher.
importance because of the vast
numbers that are produced,
this is a key aspect.
Channel propagation is the
Channel characteristics different
Channel same in both directions which
in both directions as a result of
reciprocity enables transmit and receive to
the use of different frequencies
use on set of parameters
UL / DL It is possible to dynamically UL / DL capacity determined by
asymmetry change the UL and DL frequency allocation set out by

Parameter LTE-TDD LTE-FDD
capacity ratio to match the regulatory authorities. It is
demand therefore not possible to make
dynamic changes to match
capacity. Regulatory changes
would normally be required and
capacity is normally allocated so
that it is the same in either
direction.
Guard period required to
ensure uplink and downlink
Guard band required to provide
transmissions do not clash.
sufficient isolation between
Guard period / Large guard period will limit
uplink and downlink. Large
guard band capacity. Larger guard period
guard band does not impact
normally required if distances
capacity.
are increased to accommodate
larger propagation times.
Discontinuous transmission is
required to allow both uplink
Discontinuous and downlink transmissions. Continuous transmission is
transmission This can degrade the required.
performance of the RF power
amplifier in the transmitter.
Base stations need to be
synchronised with respect to
the uplink and downlink
transmission times. If
Cross slot neighbouring base stations use
Not applicable
interference different uplink and downlink
assignments and share the
same channel, then
interference may occur
between cells.

LTE TDD / TD-LTE and TD-SCDMA
Apart from the technical reasons and advantages for using LTE TDD / TD-LTE, there are market
drivers as well. With TD-SCDMA now well established in China, there needs to be a 3.9G and
later a 4G successor to the technology. With unpaired spectrum allocated for TD-SCDMA as
well as UMTS TDD, it is natural to see many operators wanting an upgrade path for their
technologies to benefit from the vastly increased speeds and improved facilities of LTE.
Accordingly there is a considerable interest in the development of LTE TDD, which is also
known in China as TD-LTE.

With the considerable interest from the supporters of TD-SCDMA, a number of features to make
the mode of operation of TD-LTE more of an upgrade path for TD-SCDMA have been
incorporated. One example of this is the subframe structure that has been adopted within LTE
TDD / TD-LTE.

While both LTE TDD (TD-LTE) and LTE FDD will be widely used, it is anticipated that LTE
FDD will be the more widespread, although LTE TDD has a number of significant advantages,
especially in terms of higher spectrum efficiency that can be used by many operators. It is also
anticipated that phones will be able to operate using either the LTE FDD or LTE-TDD (TD-
LTE) modes. In this way the LTE UEs or user equipments will be dual standard phones, and able
to operate in countries regardless of the flavour of LTE that is used - the main problem will then
be the frequency bands that the phone can cover.

LTE Frame and Subframe Structure
- information, overview, or tutorial about the LTE frame and subframe structure
including LTE Type 1 and LTE Type 2 frames.

• MIMO
• Security

In order that the 3G LTE system can maintain synchronisation and the system is able to manage
the different types of information that need to be carried between the base-station or eNodeB and
the User Equipment, UE, 3G LTE system has a defined LTE frame and subframe structure for
the E-UTRA or Evolved UMTS Terrestrial Radio Access, i.e. the air interface for 3G LTE.

The frame structures for LTE differ between the Time Division Duplex, TDD and the Frequency
Division Duplex, FDD modes as there are different requirements on segregating the transmitted
data.

There are two types of LTE frame structure:

1. Type 1: used for the LTE FDD mode systems.

2. Type 2: used for the LTE TDD systems.

Type 1 LTE Frame Structure
The basic type 1 LTE frame has an overall length of 10 ms. This is then divided into a total of 20
individual slots. LTE Subframes then consist of two slots - in other words there are ten LTE
subframes within a frame.


The frame structure for the type 2 frames used on LTE TDD is somewhat different. The 10 ms
frame comprises two half frames, each 5 ms long. The LTE half-frames are further split into five
subframes, each 1ms long.

(shown for 5ms switch point periodicity).

The subframes may be divided into standard subframes of special subframes. The special
subframes consist of three fields;

• DwPTS - Downlink Pilot Time Slot
• GP - Guard Period
• UpPTS - Uplink Pilot Time Stot.

These three fields are also used within TD-SCDMA and they have been carried over into LTE
TDD (TD-LTE) and thereby help the upgrade path. The fields are individually configurable in
terms of length, although the total length of all three together must be 1ms.

LTE TDD / TD-LTE subframe allocations
One of the advantages of using LTE TDD is that it is possible to dynamically change the up and
downlink balance and characteristics to meet the load conditions. In order that this can be
achieved in an ordered fashion, a number of standard configurations have been set within the
LTE standards.

A total of seven up / downlink configurations have been set, and these use either 5 ms or 10 ms
switch periodicities. In the case of the 5ms switch point periodicity, a special subframe exists in
both half frames. In the case of the 10 ms periodicity, the special subframe exists in the first half
frame only. It can be seen from the table below that the subframes 0 and 5 as well as DwPTS are
always reserved for the downlink. It can also be seen that UpPTS and the subframe immediately
following the special subframe are always reserved for the uplink transmission.

Uplink- Downlink to
downlink uplink switch Subframe number
configuration periodicity
0 1 2 3 4 5 6 7 8 9

Uplink- Downlink to
downlink uplink switch Subframe number
configuration periodicity
0 5 ms D S U U U D S U U U
1 5 ms D S U U D D S U U D
2 5 ms D S U D D D S U D D
3 10 ms D S U U U D D D D D
4 10 ms D S U U D D D D D D
5 10 ms D S U D D D D D D D
6 5 ms D S U U U D S U U D

Where:
D is a subframe for downlink transmission
S is a "special" subframe used for a guard time
U is a subframe for uplink transmission

Uplink / Downlink subframe configurations for LTE TDD (TD-LTE)

LTE Physical, Logical and Transport
Channels
- overview, information, tutorial about the physical, logical, control and
transport channels used within 3GPP, 3G LTE and the LTE channel mapping.

• MIMO

• Security

In order that data can be transported across the LTE radio interface, various "channels" are used.
These are used to segregate the different types of data and allow them to be transported across
the radio access network in an orderly fashion.

Effectively the different channels provide interfaces to the higher layers within the LTE protocol
structure and enable an orderly and defined segregation of the data.

3G LTE channel types
There are three categories into which the various data channels may be grouped.

• Physical channels: These are transmission channels that carry user data and control
messages.
• Transport channels: The physical layer transport channels offer information transfer to
Medium Access Control (MAC) and higher layers.
• Logical channels: Provide services for the Medium Access Control (MAC) layer within
the LTE protocol structure.

3G LTE physical channels
The LTE physical channels vary between the uplink and the downlink as each has different
requirements and operates in a different manner.

• Downlink:
o Physical Broadcast Channel (PBCH): This physical channel carries system
information for UEs requiring to access the network.
o Physical Control Format Indicator Channel (PCFICH) :
o Physical Downlink Control Channel (PDCCH) : The main purpose of this
physical channel is to carry mainly scheduling information.
o Physical Hybrid ARQ Indicator Channel (PHICH) : As the name implies, this
channel is used to report the Hybrid ARQ status.
o Physical Downlink Shared Channel (PDSCH) : This channel is used for unicast
and paging functions.
o Physical Multicast Channel (PMCH) : This physical channel carries system
information for multicast purposes.
o Physical Control Format Indicator Channel (PCFICH) : This provides
information to enable the UEs to decode the PDSCH.
• Uplink:

o Physical Uplink Control Channel (PUCCH) : Sends Hybrid ARQ
acknowledgement
o Physical Uplink Shared Channel (PUSCH) : This physical channel found on the
LTE uplink is the Uplink counterpart of PDSCH
o Physical Random Access Channel (PRACH) : This uplink physical channel is
used for random access functions.

LTE transport channels
The LTE transport channels vary between the uplink and the downlink as each has different
requirements and operates in a different manner. Physical layer transport channels offer
information transfer to medium access control (MAC) and higher layers.

• Downlink:
o Broadcast Channel (BCH) : The LTE transport channel maps to Broadcast
Control Channel (BCCH)
o Downlink Shared Channel (DL-SCH) : This transport channel is the main
channel for downlink data transfer. It is used by many logical channels.
o Paging Channel (PCH) : To convey the PCCH
o Multicast Channel (MCH) : This transport channel is used to transmit MCCH
information to set up multicast transmissions.

• Uplink:
o Uplink Shared Channel (UL-SCH) : This transport channel is the main channel
for uplink data transfer. It is used by many logical channels.
o Random Access Channel (RACH) : This is used for random access
requirements.

LTE logical channels
• Control channels:
o Broadcast Control Channel (BCCH) : This control channel provides system
information to all mobile terminals connected to the eNodeB.
o Paging Control Channel (PCCH) : This control channel is used for paging
information when searching a unit on a network.
o Common Control Channel (CCCH) : This channel is used for random access
information, e.g. for actions including setting up a connection.
o Multicast Control Channel (MCCH) : This control channel is used for
Information needed for multicast reception.

o Dedicated Control Channel (DCCH) : This control channel is used for carrying
user-specific control information, e.g. for controlling actions including power
control, handover, etc..

• Traffic channels:
o Dedicated Traffic Channel (DTCH) : This traffic channel is used for the
transmission of user data.
o Multicast Traffic Channel (MTCH) : This channel is used for the transmission of
multicast data.

LTE Frequency Bands & Spectrum
Allocations
- a summary and tables of the LTE frequency band spectrum allocations for 3G
& 4G LTE - TDD and FDD.

• MIMO
• Security

There is a growing number of LTE frequency bands that are being designated as possibilities for
use with LTE. Many of the LTE frequency bands are already in use for other cellular systems,
whereas other LTE bands are new and being introduced as other users are re-allocated spectrum
elsewhere.

FDD and TDD LTE frequency bands
FDD spectrum requires pair bands, one of the uplink and one for the downlink, and TDD
requires a single band as uplink and downlink are on the same frequency but time separated. As a
result, there are different LTE band allocations for TDD and FDD. In some cases these bands
may overlap, and it is therefore feasible, although unlikely that both TDD and FDD
transmissions could be present on a particular LTE frequency band.

The greater likelihood is that a single UE or mobile will need to detect whether a TDD or FDD
transmission should be made on a given band. UEs that roam may encounter both types on the
same band. They will therefore need to detect what type of transmission is being made on that
particular LTE band in its current location.

The different LTE frequency allocations or LTE frequency bands are allocated numbers.
Currently the LTE bands between 1 & 22 are for paired spectrum, i.e. FDD, and LTE bands
between 33 & 41 are for unpaired spectrum, i.e. TDD.

LTE frequency band definitions

FDD LTE frequency band allocations
There is a large number of allocations or radio spectrum that has been reserved for FDD,
frequency division duplex, LTE use.

The FDDLTE frequency bands are paired to allow simultaneous transmission on two
frequencies. The bands also have a sufficient separation to enable the transmitted signals not to
unduly impair the receiver performance. If the signals are too close then the receiver may be
"blocked" and the sensitivity impaired. The separation must be sufficient to enable the roll-off of
the antenna filtering to give sufficient attenuation of the transmitted signal within the receive
band.

Width
LTE Duplex Band
Uplink Downlink of
Band Spacing Gap
(MHz) (MHz) Band
Number (MHz) (MHz)
(MHz)
1 1920 - 1980 2110 - 2170 60 190 130
2 1850 - 1910 1930 - 1990 60 80 20
3 1710 - 1785 1805 -1880 75 95 20
4 1710 - 1755 2110 - 2155 45 400 355
5 824 - 849 869 - 894 25 45 20
6 830 - 840 875 - 885 10 35 25
7 2500 - 2570 2620 - 2690 70 120 50
8 880 - 915 925 - 960 35 45 10
9 1749.9 - 1784.9 1844.9 - 1879.9 35 95 60
10 1710 - 1770 2110 - 2170 60 400 340
11 1427.9 - 1452.9 1475.9 - 1500.9 20 48 28
12 698 - 716 728 - 746 18 30 12
13 777 - 787 746 - 756 10 -31 41
14 788 - 798 758 - 768 10 -30 40
15 1900 - 1920 2600 - 2620 20 700 680
16 2010 - 2025 2585 - 2600 15 575 560
17 704 - 716 734 - 746 12 30 18
18 815 - 830 860 - 875 15 45 30
19 830 - 845 875 - 890 15 45 30
20 832 - 862 791 - 821 30 -41 71
21 1447.9 - 1462.9 1495.5 - 1510.9 15 48 33
22 3410 - 3500 3510 - 3600 90 100 10
23 2000 - 2020 2180 - 2200 20 180 160
24 1625.5 - 1660.5 1525 - 1559 34 -101.5 135.5
25 1850 - 1915 1930 - 1995 65 80 15

TDD LTE frequency band allocations
With the interest in TDD LTE, there are several unpaired frequency allocations that are being
prepared for LTR TDD use. The TDD LTE allocations are unpaired because the uplink and
downlink share the same frequency, being time multiplexed.

LTE Band
Allocation (MHz) Width of Band (MHz)
Number

33 1900 - 1920 20
34 2010 - 2025 15
35 1850 - 1910 60
36 1930 - 1990 60
37 1910 - 1930 20
38 2570 - 2620 50
39 1880 - 1920 40
40 2300 - 2400 100
41 2496 - 2690 194
42 3400 - 3600 200
43 3600 - 3800 200

There are regular additions to the LTE frequency bands / LTE spectrum allocations as a result of
negotiations at the ITU regulatory meetings. These LTE allocations are resulting in part from the
digital dividend, and also from the pressure caused by the ever growing need for mobile
communications. Many of the new LTE spectrum allocations are relatively small, often 10 -
20MHz in bandwidth, and this is a cause for concern. With LTE-Advanced needing bandwidths
of 100 MHz, channel aggregation over a wide set of frequencies many be needed, and this has
been recognised as a significant technological problem. . . . . . . . .

Additional information on LTE frequency bands.

LTE UE Category and Class Definitions
- summary and overview of the 3G LTE UE or User Equipment categories and
the performance specifications of these LTE categories.

• MIMO

• Security

In the same way that a variety of other systems adopted different categories for the handsets or
user equipments, so too there are 3G LTE UE categories. These LTE categories define the
standards to which a particular handset, dongle or other equipment will operate.

LTE UE category rationale
The LTE UE categories or UE classes are needed to ensure that the base station, or eNodeB,
eNB can communicate correctly with the user equipment. By relaying the LTE UE category
information to the base station, it is able to determine the performance of the UE and
communicate with it accordingly.

As the LTE category defines the overall performance and the capabilities of the UE, it is possible
for the eNB to communicate using capabilities that it knows the UE possesses. Accordingly the
eNB will not communicate beyond the performance of the UE.

LTE UE category definitions
there are five different LTE UE categories that are defined. As can be seen in the table below, the
different LTE UE categories have a wide range in the supported parameters and performance.
LTE category 1, for example does not support MIMO, but LTE UE category five supports 4x4
MIMO.

It is also worth noting that UE class 1 does not offer the performance offered by that of the
highest performance HSPA category. Additionally all LTE UE categories are capable of
receiving transmissions from up to four antenna ports.

A summary of the different LTE UE category parameters provided by the 3GPP Rel 8 standard is
given in the tables below.

Category 1 2 3 4 5
Downlink 10 50 100 150 300
Uplink 5 25 50 50 75
LTE UE category data rates

Category 1 2 3 4 5

Category 1 2 3 4 5
Downlink QPSK, 16QAM, 64QAM
QPSK,
Uplink QPSK, 16QAM 16QAM,
64QAM
LTE UE category modulation formats supported

Category 1 2 3 4 5
2 Rx Assumed in performance requirements across all LTE UE
diversity categories
Not
2 x 2 MIMO Mandatory
supported
4 x 4 MIMO Not supported Mandatory
LTE UE category MIMO antenna configurations

Note: Bandwidth for all categories is 20 MHz.

LTE UE category summary
In the same way that category information is used for virtually all cellular systems from GPRS
onwards, so the LTE UE category information is of great importance. While users may not be
particularly aware of the category of their UE, it will match the performance an allow the eNB to
communicate effectively with all the UEs that are connected to it.

LTE SAE System Architecture Evolution
- information, overview, or tutorial about the basics of the 3G LTE SAE, system
architecture evolution and the LTE Network

• MIMO

• Security

Along with 3G LTE - Long Term Evolution that applies more to the radio access technology of
the cellular telecommunications system, there is also an evolution of the core network. Known as
SAE - System Architecture Evolution. This new architecture has been developed to provide a
considerably higher level of performance that is in line with the requirements of LTE.

As a result it is anticipated that operators will commence introducing hardware conforming to the
new System Architecture Evolution standards so that the anticipated data levels can be handled
when 3G LTE is introduced.

The new SAE, System Architecture Evolution has also been developed so that it is fully
compatible with LTE Advanced, the new 4G technology. Therefore when LTE Advanced is
introduced, the network will be able to handle the further data increases with little change.

Reason for SAE System Architecture Evolution
The SAE System Architecture Evolution offers many advantages over previous topologies and
systems used for cellular core networks. As a result it is anticipated that it will be wide adopted
by the cellular operators.

SAE System Architecture Evolution will offer a number of key advantages:

1. Improved data capacity: With 3G LTE offering data download rates of 100 Mbps, and
the focus of the system being on mobile broadband, it will be necessary for the network
to be able to handle much greater levels of data. To achieve this it is necessary to adopt a
system architecture that lends itself to much grater levels of data transfer.
2. All IP architecture: When 3G was first developed, voice was still carried as circuit
switched data. Since then there has been a relentless move to IP data. Accordingly the
new SAE, System Architecture Evolution schemes have adopted an all IP network
configuration.
3. Reduced latency: With increased levels of interaction being required and much faster
responses, the new SAE concepts have been evolved to ensure that the levels of latency
have been reduced to around 10 ms. This will ensure that applications using 3G LTE will
be sufficiently responsive.
4. Reduced OPEX and CAPEX: A key element for any operator is to reduce costs. It is
therefore essential that any new design reduces both the capital expenditure (CAPEX)and
the operational expenditure (OPEX). The new flat architecture used for SAE System
Architecture Evolution means that only two node types are used. In addition to this a high

level of automatic configuration is introduced and this reduces the set-up and
commissioning time.

SAE System Architecture Evolution basics
The new SAE network is based upon the GSM / WCDMA core networks to enable simplified
operations and easy deployment. Despite this, the SAE network brings in some major changes,
and allows far more efficient and effect transfer of data.

There are several common principles used in the development of the LTE SAE network:

• a common gateway node and anchor point for all technologies.
• an optimised architecture for the user plane with only two node types.
• an all IP based system with IP based protocols used on all interfaces.
• a split in the control / user plane between the MME, mobility management entity and the
gateway.
• a radio access network / core network functional split similar to that used on WCDMA /
HSPA.
• integration of non-3GPP access technologies (e.g. cdma2000, WiMAX, etc) using client
as well as network based mobile-IP.

The main element of the LTE SAE network is what is termed the Evolved Packet Core or EPC.
This connects to the eNodeBs as shown in the diagram below.

LTE SAE Evolved Packet Core

As seen within the diagram, the LTE SAE Evolved Packet Core, EPC consists of four main
elements as listed below:

• Mobility Management Entity, MME: The MME is the main control node for the LTE
SAE access network, handling a number of features:
o Idle mode UE tracking
o Bearer activation / de-activation
o Choice of SGW for a UE

o Intra-LTE handover involving core network node location
o Interacting with HSS to authenticate user on attachment and implements roaming
restrictions
o It acts as a termination for the Non-Access Stratum (NAS)
o Provides temporary identities for UEs
o The SAE MME acts the termination point for ciphering protection for NAS
signaling. As part of this it also handles the security key management.
Accordingly the MME is the point at which lawful interception of signalling may
be made.
o Paging procedure
o The S3 interface terminates in the MME thereby providing the control plane
function for mobility between LTE and 2G/3G access networks.
o The SAE MME also terminates the S6a interface for the home HSS for roaming
UEs.

It can therefore be seen that the SAE MME provides a considerable level of overall
control functionality.

• Serving Gateway, SGW: The Serving Gateway, SGW, is a data plane element within
the LTE SAE. Its main purpose is to manage the user plane mobility and it also acts as
the main border between the Radio Access Network, RAN and the core network. The
SGW also maintains the data paths between the eNodeBs and the PDN Gateways. In this
way the SGW forms a interface for the data packet network at the E-UTRAN.

Also when UEs move across areas served by different eNodeBs, the SGW serves as a
mobility anchor ensuring that the data path is maintained.
• PDN Gateway, PGW: The LTE SAE PDN gateway provides connectivity for the UE to
external packet data networks, fulfilling the function of entry and exit point for UE data.
The UE may have connectivity with more than one PGW for accessing multiple PDNs.
• Policy and Charging Rules Function, PCRF: This is the generic name for the entity
within the LTE SAE EPC which detects the service flow, enforces charging policy. For
applications that require dynamic policy or charging control, a network element entitled
the Applications Function, AF is used.

LTE SAE PCRF Interfaces

LTE SAE Distributed intelligence
In order that requirements for increased data capacity and reduced latency can be met, along with
the move to an all-IP network, it is necessary to adopt a new approach to the network structure.

For 3G UMTS / WCDMA the UTRAN (UMTS Terrestrial Radio Access Network, comprising
the Node B's or basestations and Radio Network Controllers) employed low levels of autonomy.
The Node Bs were connected in a star formation to the Radio Network Controllers (RNCs)
which carried out the majority of the management of the radio resource. In turn the RNCs
connected to the core network and connect in turn to the Core Network.

To provide the required functionality within LTE SAE, the basic system architecture sees the
removal of a layer of management. The RNC is removed and the radio resource management is
devolved to the base-stations. The new style base-stations are called eNodeBs or eNBs.

The eNBs are connected directly to the core network gateway via a newly defined "S1 interface".
In addition to this the new eNBs also connect to adjacent eNBs in a mesh via an "X2 interface".
This provides a much greater level of direct interconnectivity. It also enables many calls to be
routed very directly as a large number of calls and connections are to other mobiles in the same
or adjacent cells. The new structure allows many calls to be routed far more directly and with
only minimum interaction with the core network.

In addition to the new Layer 1 and Layer 2 functionality, eNBs handle several other functions.
This includes the radio resource control including admission control, load balancing and radio
mobility control including handover decisions for the mobile or user equipment (UE).

The additional levels of flexibility and functionality given to the new eNBs mean that they are
more complex than the UMTS and previous generations of base-station. However the new 3G
LTE SAE network structure enables far higher levels of performance. In addition to this their
flexibility enables them to be updated to handle new upgrades to the system including the
transition from �G LTE to 4G LTE Advanced.

The new System Architecture Evolution, SAE for LTE provides a new approach for the core
network, enabling far higher levels of data to be transported to enable it to support the much
higher data rates that will be possible with LTE. In addition to this, other features that enable the
CAPEX and OPEX to be reduced when compared to existing systems, thereby enabling higher
levels of efficiency to be achieved.

Voice over LTE - VoLTE
- operation of Voice over LTE VoLTE system for providing a unified format of
voice traffic on LTE, and other systems including CSFB, and SV-LTE.

• MIMO
• Security

The Voice over LTE, VoLTE scheme was devised as a result of operators seeking a standardised
system for transferring voice traffic over LTE. Originally LTE was seen as a completely IP
cellular system just for carrying data, and operators would be able to carry voice either by
reverting to 2G / 3G systems or by using VoIP.

Operators, however saw the fact that a voice format was not defined as a major omission for the
system. It was seen that the lack of standardisation may provide problems with scenarios
including roaming. In addition to this, SMS is a key requirement. It is not often realised, that

SMS is used to set-up many mobile broadband connections, and a lack of SMS is seen as a
show-stopper by many.

As mobile operators receive over 80% of their revenues from voice and SMS traffic, it is
necessary to have a viable and standardized scheme to provide these services and protect this
revenue.

Options for Voice over LTE
When looking at the options for ways of carrying voice over LTE, a number of possible solutions
were investigated. A number of alliances were set up to promote different ways of providing the
service. A number of systems were prosed as outlined below:

• CSFB, Circuit Switched Fall Back: The circuit switched fallback, CSFB option for
providing voice over LTE has been standardised under 3GPP specification 23.272.
Essentially LTE CSFB uses a variety of processes and network elements to enable the
circuit to fall back to the 2G or 3G connection (GSM, UMTS, CDMA2000 1x) before a
circuit switched call is initiated.

The specification also allows for SMS to be carried as this is essential for very many set-
up procedures for cellular telecommunications. To achieve this the handset uses an
interface known as SGs which allows messages to be sent over an LTE channel.

In addition to this CSFB requires modification to elements within the network, in
particular the MSCs as well as support, obviously on new devices. MSC modifications
are also required for the SMS over SGs facilities. For CSFB, this is required from the
initial launch of CSFB in view of the criticality of SMS for many procedures.
• SV-LTE - simultaneous voice LTE: SV-LTE allows to run packet switched LTE
services simultaneously with a circuit switched voice service. SV-LTE facility provides
the facilities of CSFB at the same time as running a packet switched data service. This is
an option that many operators will opt for. However it has the disadvantage that it
requires two radios to run at the same time within the handset. This has a serious impact
on battery life.
• VoLGA, Voice over LTE via GAN: The VoLGA standard was based on the existing
3GPP Generic Access Network (GAN) standard, and the aim was to enable LTE users to
receive a consistent set of voice, SMS (and other circuit-switched) services as they
transition between GSM, UMTS and LTE access networks.

For mobile operators, the aim of VoLGA was to provide a low-cost and low-risk
approach for bringing their primary revenue generating services (voice and SMS) onto
the new LTE network deployments.
• One Voice / later called Voice over LTE, VoLTE: The Voice over LTE, VoLTE schem
for providing voice over an LTE system utilises IMS enabling it to become part of a rich
media solution.

Issues for Voice services over LTE
Unlike previous cellular telecommunications standards including GSM, LTE does not have
dedicated channels for circuit switched telephony. Instead LTE is an all-IP system providing an
end-to-end IP connection from the mobile equipment to the core network and out again.

In order to provide some form of voice connection over a standard LTE bearer, some form of
Voice over IP, VoIP must be used.

The aim for any voice service is to utilise the low latency and QoS features available within LTE
to ensure that any voice service offers an improvement over the standards available on the 2G
and 3G networks.

However to achieve a full VoIP offering on LTE poses some significant problems which will
take time to resolve. With the first deployments having taken place in 2010, it is necessary that a
solution for voice is available within a short timescale.

Voice over LTE, VoLTE basics
The One Voice profile for Voice over LTE was developed by a collaboration between over forty
operators including: AT&T, Verizon Wireless, Nokia and Alcatel-Lucent.

At the 2010 GSMA Mobile World Congress, GSMA announced that they were supporting the
One Voice solution to provide Voice over LTE.

VoLTE, Voice over LTE is an IMS-based specification. Adopting this approach, it enables the
system to be integrated with the suite of applications that will become available on LTE.

Note on IMS:

The IP Multimedia Subsystem or IP Multimedia Core Network Subsystem, IMS is an
architectural framework for delivering Internet Protocol, IP multimedia services. It enables a
variety of services to be run seemlessly rather than having several disparate applications
operating concurrently.

Click for a IMS tutorial

To provide the VoLTE service, three interfaces are being defined:

• User Network interface, UNI: This interface is located between the user's equipment
and the operators network.
• Roaming Network Network Interface, R-NNI: The R-NNI is an interface located
between the Home and Visited Network. This is used for a user that is not attached to
their Home network, i.e. roaming.
• Interconnect Network Network Interface, I-NNI: The I-NNI is the interface located
between the networks of the two parties making a call.

Work on the definition of VoLTE, Voice over LTE is ongoing. It will include a variety of
elements including some of the following:

• It will be necessary to ensure the continuity of Voice calls when a user moves from an
LTE coverage area to another where a fallback to another technology is required. This
form of handover will be achieved using Single Radio Voice Call Continuity, or SR-
VCC).
• It will be important to provide the optimal routing of bearers for voice calls when
customers are roaming.
• Another area of importance will be to establish commercial frameworks for roaming and
interconnect for services implemented using VoLTE definitions. This will enable
roaming agreements to be set up.
• Provision of capabilities associated with the model of roaming hubbing.
• For any services, including LTE, it is necessary to undertake a thorough security and
fraud threat audit to prevent hacking and un-authorised entry into any area within the
network..

In many ways the implementation of VoLTE at a high level is straightforward. The handset or
phone needs to have software loaded to provide the VoLTE functionality. This can be in the
form of an App.

The network then requires to be IMS compatible.

While this may appear straightforward, there are many issues for this to be made operational,
especially via the vagaries of the radio access network where time delays and propagation
anomalies add considerably to the complexity.

LTE Security
- overview, about the basics of LTE security including the techniques used for
LTE authentication, ciphering, encryption, and identity protection.

• MIMO
• Security

LTE security is an issue that is of paramount importance. It is necessary to ensure that LTE
security measures provide the level of security required without impacting the user as this could
drive users away.

Nevertheless with the level of sophistication of security attacks growing, it is necessary to ensure
that LTE security allows users to operate freely and without fear of attack from hackers.
Additionally the network must also be organised in such a way that it is secure against a variety
of attacks.

LTE security basics
When developing the LTE security elements there were several main requirements that were
borne in mind:

• LTE security had to provide at least the same level of security that was provided by 3G
services.
• The LTE security measures should not affect user convenience.
• The LTE security measures taken should provide defence from attacks from the Internet.
• The security functions provided by LTE should not affect the transition from existing 3G
services to LTE.
• The USIM currently used for 3G services should still be used.

To ensure these requirements for LTE security are met, it has been necessary to add further
measures into all areas of the system from the UE through to the core network.

The main changes that have been required to implement the required level of LTE security are
summarised below:

• A new hierarchical key system has been introduced in which keys can be changed for
different purposes.
• The LTE security functions for the Non-Access Stratum, NAS, and Access Stratum, AS
have been separated. The NAS functions are those functions for which the processing is
accomplished between the core network and the mobile terminal or UE. The AS
functions encompass the communications between the network edge, i.e. the Evolved
Node B, eNB and the UE.
• The concept of forward security has been introduced for LTE security.
• LTE security functions have been introduced between the existing 3G network and the
LTE network.

LTE USIM
One of the key elements within the security of GSM, UMTS and now LTE was the concept of
the subscriber identity module, SIM. This card carried the identity of the subscriber in an
encrypted fashion and this could allow the subscriber to keep their identity while transferring or
upgrading phones.

With the transition form 2G - GSM to 3G - UMTS, the idea of the SIM was upgraded and a
USIM - UMTS Subscriber Identity Module, was used. This gave more functionality, had a larger
memory, etc.

For LTE, only the USIM may be used - the older SIM cards are not compatible and may not be
used.

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