Lte training an introduction-to-lte-basics

LTE Introduction
Basic Level Training material
5/22/2017 Fundarc Communication (xgnlab) 1

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What is LTE ?
 In Nov. 2004, 3GPP (3rd Generation Partnership Project) began a project
to define the Long-Term Evolution (LTE) of Universal Mobile
Telecommunications System (UMTS) cellular technology
 With the following prime objectives
 Higher performance (Latency, Speed, QoS etc)
 Spectrum efficiency (more bits per hertz, reusability)
 Backwards compatible (Evolution of GSM/UMTS)
 Wide applications (data network)

History of LTE
 LTE is a standard for wireless data communications technology and an evolution of the
GSM/UMTS standards.
 LTE uses many of the advance techniques from HSPA network like adaptive modulation,
MIMO, and Harq.
 OFDM technique was introduced in 3GPP world through LTE.
 It was paradigm shift, in network architecture, to an IP-based system with significantly
reduced transfer latency compared to the 3G architecture.
 LTE separated the control plane from data plane, thanks to all IP Paradigm.
 The goal of LTE was to increase the capacity and speed of wireless data networks using
new DSP ( Simple digital signal processing techniques) and modulations scheme.
 The LTE wireless interface is incompatible with 2G and 3G networks, so that it must be
operated on a separate wireless spectrum.
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History of LTE(Cont’d)
 LTE was first proposed by NTT DoCoMo of Japan in 2004, and
studies on the new standard officially commenced in 2005.
 The LTE standard was finalized in December 2008, and the first
publicly available LTE service was launched by TeliaSonera in Oslo
and Stockholm on December 14, 2009 as a data connection with a
USB modem.
 Samsung Galaxy Indulge being the world’s first LTE smartphone
starting on February 10, 2011.
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History of LTE(Cont’d)
 LTE grown at fastest pace ever any 3GPP technology.
 Initially, CDMA operators planned to upgrade to rival standards called
UMB and WiMAX.
 But all the major CDMA operators (such as Verizon, Sprint and MetroPCS
in the United States, Bell and Telus in Canada, au by KDDI in Japan, SK
Telecom in South Korea and China Telecom/China Unicom in China) have
announced that they intend to migrate to LTE after all.
 The evolution of LTE is LTE Advanced, which was standardized in March
2011.
 LTE Advance fit within the criteria of ITU-T 4G networks.
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Evolution of Radio Access Technologies
• LTE (3.9G) :
3GPP release 8~9
• LTE-Advanced :
3GPP release 10+
802.16d/e
802.16m

22-05-2017 Fundarc Communication 7

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LTE Basic Concepts
• LTE employs Orthogonal Frequency Division Multiple Access
(OFDMA) for downlink data transmission and Single Carrier FDMA
(SC-FDMA) for uplink transmission
• SC-FDMA is a new single carrier multiple access technique which has
similar structure and performance to OFDMA
• A salient advantage of SC-FDMA over OFDM is the low Peak to
Average Power (PAP) ratio : Increasing battery life

LTE EPS
In the 3GPP mobile network evolution path, 4G networks have come across significant network
architecture overhauling as compared to the change from 2G to 2.5G and 3G. This change is referred to as
system architecture evolution (SAE) with a paradigm of all IP network concepts, and in LTE networks finally
described as EPS (evolved packet system), which comprises evolved packet core (EPC) and EUTRAN.

EUTRAN
EUTRAN comprises eNodeBs, and provides UEs (user Equipment’s)--radio access to a mobile
core network, i.e. EPC. The complete system is also visualized as access stratum (AS) and non-
access stratum (NAS) where EUTRAN covers AS part of the system. This terminology is very
useful while discriminating the signaling over the radio network and core network.
For access to the core network, UE makes RRC connections with eNodeB as part of AS
signaling, which subsequently connects to MME as part of the NAS signaling connection with
a mobile core. UE is allocated a temporary identity at each connection, like C-RNTI and S-TMSI
respectively, for identification at the respective reference point.

EPC
EPC comprises mainly MME and SGW/PGW and provides the UE connectivity to the external world. It authenticates the
UE for network access, and keeps the registration and location track of registered UEs. For the data connection, EPC
maintains the context for connection, sets up the end-to-end bearer connectivity, applies the necessary security
measures, and enforces the policy per the policy and charging control architecture. The figure below depicts the end-to-
end bearer connectivity for UE.

The figure below depicts the functionality at access stratum and non-access stratum
distributed to individual nodes.

RRC connection
In LTE, the RRC connection has two states idle and connected; at EPC they are also referred to in
the connection management state as ECM_idle and ECM_connected. The UE first makes an RRC
connection to register with the network, and the network maintains the UE as EMM_registered or
EMM_unregistered mobility management states. To remain registered with the network, the UE
shall have to periodically update its location with the network. Based on the RRC connection
states, UE activities are defined.

This state transitions diagram shows the coordination of connection
management and mobility management states in EUTRAN.

EUTRA
EUTRA refers to radio connectivity of the UE to eNodeB. Every UE makes an RRC connection to eNodeB for
accessing the radio network. For each RRC connection, the UE is allocated with SRBs for setting the signaling
with the core network, either for registration or for data connection, and accordingly provided DRBs.
Each of the SRBs and DRBs are mapped to different logical channels provided by the lower layers of the LTE
protocol.

These logical channels are further mapped to transport channels, and they are again
imposed on the physical channels.

Radio Technology
The radio technology used for LTE is OFDMA for downlink and SC-FDMA for uplink (the use of
SC-FDMA in the uplink is due to the fact that it provides low PAPR (peak average power ratio).
OFDM is being utilized by multiple next-generation wireless technologies apart from LTE, like
WiMAX 802.16, WLAN and UMB, due to better spectrum efficiency and more importantly,
less complex signal processing, as the signals are represented in the frequency domain, not in
the time domain. That results in less complexity in functionalities such as channel
equalization and channel estimations.

The other important reason for using OFDM as a modulation format within LTE and other wireless systems is, its
resilience to multipath delays and spread (overlapping of frequency between carrier or subcarrier).
To avoid the inter symbol interference, a gap period is introduced between symbols. 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.

Bandwidth
LTE is flexible in terms of the bandwidth. The table below describes the various denomination of baseband
and respective data rates.

Mode of LTE
LTE works in two modes, known as FDD and TDD, and also called TD-LTE. Each mode has its own frame structure. For
example, FDD has a Type1 frame structure and TDD has a Type 2 frame structure.
Frame Structure
LTE has a 10-millisecond-long frame with 20 time slots of 0.5 milliseconds each. Consecutive two-time slots make a
sub-frame and constitute one TTI (transmit time interval) of 1 millisecond.
Type 1 Frame
A Type 1 Frame is used in the FDD mode. In FDD-LTE, every downlink subframe can be associated with an uplink
subframe.

Type 2 Frame
A Type 2 Frame is used in the TDD mode.
In TDD, the guard periods are used between the downlink and uplink transmissions for synchronicity of uplink and
downlink transmission.
In TD-LTE, the number of downlink and uplink subframes is different, and such association is not possible.
Both modes have their own pros and cons, and are selected by the operators by their own choices. Otherwise, both
modes of LTE are substantially similar -- they differ only in the physical layer, and as a result, are transparent to the
higher layers.

Resource Block
Each time slot of a frame contains a resource block of 180 KHz in frequency domain, which is
further divided into 12 subcarriers of 15 KHz. Each subcarrier is modulated with 6 (long CP) or 7
(short CP) symbols based on the CP (cyclic prefix) length.
Resource Element
A single symbol on a single subcarrier is known as a resource element, and may have a size from
2 to 6 bits, based on the order of modulation, i.e. for 64QPSK it is 6 bits, and for BPSK it is 2 bits.
The figure below depicts the resource block and resource elements.

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LTE Spectrum (Bandwidth and Duplex) Flexibility

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Generic Frame Structure
• Allocation of physical resource blocks (PRBs) is handled by a
scheduling function at the 3GPP base station: Evolved Node
B (eNodeB)
Frame 0 and frame 5 (always downlink)

Generic Frame Structure (Cont’d)
• DwPTS field: This is the downlink part of the special subframe and its
length can be varied from three up to twelve OFDM symbols.
• The UpPTS field: This is the uplink part of the special subframe and has a
short duration with one or two OFDM symbols.
• The GP field: The remaining symbols in the special subframe that have
not been allocated to DwPTS or UpPTS are allocated to the GP field,
which is used to provide the guard period for the downlink-to-uplink and
the uplink-to-downlink switch.
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Resource Blocks for OFDMA
• One frame is 10 ms consisting of 10 subframes
• One subframe is 1ms with 2 slots
• One slot contains N Resource Blocks (6 < N < 110)
 The number of downlink resource blocks depends on the transmission bandwidth.
• One Resource Block contains M subcarriers for each OFDM
symbol
 The number of subcarriers in each resource block depends on the subcarrier spacing
Δf
• The number of OFDM symbols in each block depends on both
the CP length and the subcarrier spacing.
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LTE Downlink Channels
• 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.
• 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.
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LTE Downlink Channels
Paging Channel
Paging Control Channel
Physical Downlink Shared Channel

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LTE Downlink Logical Channels

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LTE Downlink Logical Channels

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LTE Downlink Transport Channel

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LTE Downlink Transport Channel

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LTE Downlink Physical Channels

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LTE Downlink Physical Channels

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LTE Uplink Channels
Random Access Channel
Physical Radio Access Channel
Physical Uplink Shared Channel
CQI report

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LTE Uplink Logical Channels

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LTE Uplink Transport Channel

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LTE Uplink Physical Channels

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