LTE System Architecture and
interfaces
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Contents
• LTE Network Architecture
• Network elements and Interfaces
• LTE Layer Protocols
• LTE Channels
• Multiple Access Techniques
• Radio Frame
• Radio Bearers
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Challenges
After Completion of the PPT you should be able to answer below questions
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Which channels send paging indicator and paging message
What the information Contained in MIB Message
What the different between S5 interface and S8 interface
What the Mean of the following .. Bearers, Radio Bearers, EPS Bearers
What different between HARQ and ARQ
What is main function of the eNB & MME & S-GW
Why use OFDMA in DL and SC-FDMA in Uplink
List advantages of LTE
What the function of X2 interface
The LTE scheduler locate in which layer
What difference between User Plane and Control Plane
Mention LTE Voice Policies
Mention at least 3 Function of RLC and PDCP
What is the USE of PCFICH
What is the CRS .. What its Main function
What is the Ranges of PCI .. What is the use of PSS and SSS
PUCCH Responsible for what, does we can send PUCCH and PUSCH at same time
What is the use of Cyclic prefix .. Mention its types and when to being used
What the different between AS and NAS signaling
What is the radio frame duration
Define EPS bearer .. What the factors which add characteristics to the E-RAB
Determine which QCI Used for the following services as well the QCI Specifications
(Conversational Voice, IMS Signaling, Conversational Video, Real Time Gaming )
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Why LTE ?
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Provide high Data Rates (Target 1 Gbps)
Lower latency than legacy network
Merging CS services into PS Core ( VOLTE Provide HD Call and shortest call setup time)
Flexible bandwidth deployment (six different bandwidths)
Flexible Spectrum options
Increase Spectrum efficiency
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LTE System Architecture
SAE System Architecture Evolution
SGSN
EPS (Evolved Packet System)
Gb
Control plane
GPRS
BTS
Iu
User plane
BSC/PCU
S3
S6d
HSS
PCRF
S6a
S10
S9
UMTS
NodeB
RNC
MME
S12
Operator Service
Network
S4
Gx
S11
S1-MME
S5/8
S1-U
E-UTRAN
eNodeB
Serving GW
A10/A11
SGi
PDN GW
Corporate
Internet
S2a
cdma2000
BTS
BSC
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LTE Voice Policies
During the UE attach and tracking area update (TAU) period, the MME selects a voice policy based on the
UE capability and configuration on the MME side.
In case UE Support both CSFB and VOLTE .. voice policies specified by operators during UE registration will
be one of the following.
• CS Voice only
• IMS PS Voice only
• Prefer CS Voice with IMS PS Voice as secondary
• Prefer IMS PS Voice with CS Voice as secondary
Voice Call Based on VOLTE
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Voice Call based on CSFB
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LTE Network Elements
eNode-B
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Radio Resources Management which include
• Radio Bearer Control
• Radio admission Control
• Resources allocation for the UEs
(Scheduling)
Routing of user Plane data towards serving
gateway
Connection Mobility Control (H.O decision)
Scheduling and transmission of broadcast
information ( SI & NAS)
measurement reporting configuration for
mobility and scheduling
MME ( Mobility Management Entity)
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NAS Signaling & NAS Security
Idle state Mobility Handling (Paging)
EPS Bearer Control
Mobility Management, Authentication
PDN & SGW Selection
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S-GW Function (Serving Gate Way)
• Deal with User Plane (Data Routing from the
EPC to )
• The Serving GW is the point of interconnect
between the radio-side and the EPC
PDN-GW (Packet Data Network Gate Way)
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The PDN GW is the point of interconnect
between the EPC and the external IP networks.
IP Address allocation
HSS (Home Subscriber Server)
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in charge of storing and updating when necessary
the database containing all the user subscription
information
User identification and addressing Using user
IMSI
The AUC part of the HSS is in charge of generating
security information from user identity keys
PCRF (Policy and Charging Rules Function)
• Provide QoS information to packet gateway
• For VoIP session, PCRF will initiate dedicated
bearer dynamically
• Accounting rule information.
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E-UTRAN Interfaces
LTE-Uu interface
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Air interface of the E-UTRAN , Access Technique based on OFDMA on Downlink and SC-FDMA on
Uplink, Duplex Techniques available for FDD and TDD
Scalable Bandwidth from 1.4 to 20 MHZ .. MIMO is the Major Component and the key of enhancement
Carry information for both Control Plane and User Plane
X2 Interface
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Inter eNB interface
Handover Coordination without involving EPC in the process ( less signaling, Improve latency)
Provide inter-eNB Cooperation for a features such like ICIC
Carry information for both Control Plane and User Plane
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EPC Interfaces
S1-MME Interface
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Control Plane Interface between eNB and MME
Involved in signaling procedures such like TAU (tracking area update), attach, detach
S1-U interface
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User Plane interface between eNB and S-GW
Carry user data
S10 Interface
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Interconnection between different MMEs .. It is a pure Control interface
Used during inter-MME Tracking area Update
S6a Interface
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Interconnection between the MME and the HSS providing related signaling for user subscription data and registration
S11 Interface
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Interconnection between a MME and the serving Gate Way
A single MME Can handle Multiple serving gate way each one with its own S11 Interface
S5 Interface
• Interface between S-GW and PDN-GW .. Both of them belong to the H-PLMN (Non Roaming Scenario)
S8 interface
• Interface between S-GW and PDN-GW .. Where PDN- GW belong to the H-PLMN and S-GW belong to (Roaming Scenario)
SGs Interface
• Interconnection between MSC and MME for CSFB procedure
SGi interface
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Interconnection between P-GW and the external packet data networks
RX Interface
• Interconnection between PCRF and Operator IP Services
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LTE Planes and Protocols
The E-UTRA interface provides connectivity between
the User Equipment and the eNB. It can be logically
split into a control plane and a user plane.
User Plane
Packets in the core network (EPC) are encapsulated
in a specific EPC protocol and tunneled between the
P-GW and the eNB. Different tunneling protocols
are used depending on the interface. GPRS
Tunneling Protocol (GTP) is used on the S1 interface
between the eNB and S-GW and on the S5/S8
interface between the S-GW and P-GW
GTP-U (GPRS- Tunneling Protocol User
Plane)
GTP-U Tunnels are used to carry
encapsulated T-PDUs and signaling
messages between a given pair of GTPU Tunnel Endpoints
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Control Plane
we can Split Control Plane into two types
AS (Access Stratum) : Provided by RRC and carries Signaling
between the UE and the eNB.
NAS (Non Access Stratum) : the Key Control between the UE
and the MME .. eNB just forwarding the NAS Signaling between
them Such as Paging, Extended service Request
S1AP (Application protocol)
S1AP provides the signaling service between E-UTRAN and the
evolved packet core (EPC) and has following functions
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E-RAB management function
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Initial Context Transfer function
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UE Capability Info Indication function
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Mobility Functions
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S1 interface management functions
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NAS Signaling transport function
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S1 UE context Release function
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UE Context Modification function
SCTP (Stream Control Transmission Protocol)
This protocol guarantees delivery of signaling messages
between MME and eNB
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NAS Signaling
EMM Procedures
The non-access stratum (NAS)
protocols form the highest stratum of
the control plane between the user
equipment (UE) and MME.
NAS Procedures divided into two Main
Categories
EMM : EPS Mobility Management
ESM : EPS Session Management
ESM Procedures
Attach , Detach
Default EPS Bearer Context Activation
Tracking Area Update
Dedicated EPS Bearer Context Activation
Service Request
EPS Bearer Context Modification
Extended Service Request
EPS Bearer Context Deactivation
GUTI Reallocation
UE Requested PDN Connectivity
Authentication
UE Requested PDN Disconnect
Identification
UE Requested Bearer Resource Allocation
Security Mode Control
UE Requested Bearer Resource Allocation
EMM Status
UE Requested Bearer Resource Modification
EMM Information
ESM Information Request
NAS Transport
ESM Status
Paging
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RRC - Radio Resource Control
The main air interface control protocol is RRC (Radio Resource Control).
• For RRC messages to be transferred between the UE and the eNB it uses the services of PDCP, RLC,
MAC and PHY.
• RRC provides the main configuration and parameters to the lower layers.
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PDCP – Packet Data Convergence Protocol
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RLC – Radio Link Control
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TM (Transparent Mode) - This is utilized for some of the air interface channels, e.g. broadcast and
paging. It provides a connectionless service for signaling.
•
UM (Unacknowledged Mode) - This is like Transparent Mode, in that it is a connectionless service
however it has the additional features of sequencing, segmentation and concatenation.
•
AM (Acknowledged Mode) - This offers an ARQ (Automatic Repeat Request) service. As such,
retransmissions can be used.
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MAC – Medium Access Control
Provides the interface between the E-UTRA protocols and the E-UTRA Physical Layer.
HARQ (Hybrid Automatic Repeat Request) - MAC utilizes HARQ to provide error correction services
across the air. HARQ is a feature which requires the MAC and Physical Layers to work closely together.
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PHY – Physical Layer
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LTE Channels - DL Channels
Carry System
information
Carry Paging
Logical
channels
BCCH
PCCH
Transport
Channels
BCH
PCH
Physical
Channels
PBCH
PHICH
Logical channels
BCCH : Broadcast Control Channel
PCCH : Paging Control Channel
CCCH : Common Control Channel
DCCH: Dedicated Control Channel
DTCH : Dedicated Traffic Channel
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Carry Control
when RRC Idle
CCCH
Carry Control when
RRC Connected
DCCH
Carry
Traffic
DTCH
DL-SCH
PCFICH
PDCCH
Transport channels
BCH : Broadcast Channel
PCH : Paging Channel
DL-SCH : Downlink Shared Channel
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PDSCH
LTE Channels - DL Physical Channels
PBCH : Physical Broadcast Control Channel
Carry MIB (Master Information Block) Message which Contain
1 - B.W in terms of Resource Block
2 - PHICH Configurations
3 - System frame Number
PHICH : Physical HARQ Indicator channel
Carry ACK/NACK indicators Regarding the UL-Transmission
PCFICH : Physical Control Frame Indicator Channel
Carry CFI element which indicate the size of PDCCH(1,2,3 or 4 )
PDCCH : Physical Downlink Control Channel - Used for Resource allocation such likes
• Assignment Indication for RACH Response
• Assignment Indication for Paging Messages
• Assignment Indication for SIB
• Downlink Grants for User Traffic
• Uplink Grants for UL Transmission
• Power Control Commands for PUSCH & PUCCH
PDSCH : Physical Downlink Shared Channel – Carry the following
• RACH Response, Paging Message, System Information
• DL User Traffic
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Resources allocation
Check the PCFICH it will
tell you how Many
symbols allocated for the
PDCCH (1,2,3 or 4) at the
Start of each Sub frame
Check the PDCCH to
Know SIBs location on
the PDSCH and to know
how they look
PCFICH
PDCCH
PDCCH
PDSCH
I would like to read the
SIBs from the PDSCH but I
don’t know which
resources are allocated
for them and how they
look
I would like to read the
PDCCH .. But where is it
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Downlink Control Indicator – (DCI)
The LTE system uses a set of DCI (Downlink Control Information) messages to convey control and
scheduling information to devices. The set of Downlink Control Information messages is defined LTE
Release 8 as below
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Downlink Physical Signals
1- PCI : Physical Cell Identity (PCI = 3*SSS + PSS) (from 0
to 504)
• In order for the UE to identify the cell and
synchronize with the downlink transmission, the
eNB sends synchronization signals over the center
72 sub-carriers around the DC
• PSS (Ranges from 0 to 2) : Perform Slot
Synchronization / locate in #6 symbol of slot 0 and
slot 10
• SSS (Ranges from 0 to 167) : Perform Frame
Synchronization / locate in #5 symbol of slot 0 and
slot 10
• It Must avoiding PCI Collision/Confusion/Mod 3 CoCoverage during the PCI Planning
2- CRS : Cell-Specific Reference Signal
• Used for DL Channel Quality estimation
• Its location depending on
o type of Cyclic prefix (Normal/extended)
o Number of antenna Ports
o PCI Value
• Its power uses as reference for the fixed power assignment
• CRS uses QPSK Modulation
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LTE Channels - UL Channels
Carry Control
when RRC Idle
Physical Channels
PUCCH : Physical Uplink Control Channel
• As the name implies, the PUCCH carries uplink
control information
• It is never transmitted simultaneously with
PUSCH data.
• PUCCH conveys control information including
channel quality indication (CQI), ACK/NACK,
HARQ and uplink scheduling requests (SR)
PRACH : Physical Random Access Channel
• The random access procedure is used in various
scenarios, Such as initial access , handover, or
re-establishment
• A guard period Required when Sending
Preamble
• The preamble sequence may be repeated to
enable the eNB to decode the preamble when
link conditions are poor.
• A Cell has 64 Preamble Sequence
PUSCH : Physical Uplink Shared Channel
• Carry User Traffic
• Carry CQI Feedback in case it performing UL
Transmission
• Carry SRS (Sounding Reference signal) which
keep the UE Ul-Synchronized and uses for ULChannel Quality estimation
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Carry Control when
RRC Connected
Logical
channels
CCCH
DCCH
Transport
Channels
RACH
UL-SCH
Physical
Channels
PRACH
PUCCH
Logical channels
CCCH : Common Control Channel
DCCH: Dedicated Control Channel
DTCH : Dedicated Traffic Channel
Transport Channels
RACH : Random Access Channel
UL-SCH : Uplink Shared Channel
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Carry
Traffic
DTCH
PUSCH
Channel Mapping VS Layers
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LTE Multiple Access Technique – ( OFDMA / SC-FDMA )
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OFDM Technique increase spectral efficiency by reducing spacing between Subcarriers
The subcarriers are mathematically Orthogonal to each other. As such, when a subcarrier is at its maximum the two
adjacent subcarriers are passing through zero
LTE Utilize OFDMA in DL and SC-FDMA in UL .. Why? .. For transmission SC-FDMA requires low PARP (Peak Average
Power Ratio) which will be suitable for the Mobile Battery .. In Other hand OFDMA has a high peak to average ratio.
While this is not a problem for the base station where power is not a particular problem
SC-FDMA has Similar Structure and performance to OFDMA
Advantages of OFDM
• More resistance to Multipath interference
• Higher spectral efficiency for wideband channels
• Flexible Spectrum utilization
• Simple Implementation
Disadvantages of OFDM
• Doppler shift impact subcarriers orthogonally
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LTE FDD Radio Frame Structure
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One frame is 10ms and it consists of 10 subframes.
One LTE sub frame is 1ms and contains 2 slots
One slot is 0.5ms in time domain and each 0.5ms
assignment can contain N resource.
One resource block is 0.5ms and contains 12
subcarriers for each OFDM symbol in frequency
domain.
There are 7 symbols (normal cyclic prefix) per
time slot in the time domain or 6 symbols in long
cyclic prefix for LTE.
Cyclic prefix is nothing but guard timer. It is used
to make successful receiving of OFDM symbol at
receiver side. There are two types cyclic prefix
has been introduced in lte namely "Normal cyclic
prefix" and "Extended cyclic prefix". Usually
normal cyclic prefix is sufficient to suppress the
multi-path delay spread. But some places(hilly
areas / Rural areas ) multi-path delay is
significantly high. So to suppress multi-path
delay in hilly area's extended cyclic prefix is
being used.
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Bearer Service Architecture
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Bearer mean the tunnel where the User data encapsulated to be delivered for/From the authorized UE
Radio Bearer transports the packet s of an EPS bearer between a UE and an eNB .. There are one to one
Mapping between EPS Bearer and a radio bearer
S1 Bearer transports the packets of an EPS Bearer between an eNB and serving gateway
eNB is the binding point between the radio bearer and S1 bearer to Obtain the EPS Bearer in both DL and UL
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Bearer level Quality of Service
Bearers can be classified into two categories
• GBR (Guaranteed bit rate Bearers )
• Non-GBR (Non Guaranteed bit rate Bearers )
Each bearer has associated Quality of service class indicator (QCI) and an Allocation and Retention priority
(ARP)
• ARP .. Used for admission Control
• QCI .. Add the Characteristics to the EPS Bearer in terms off
• Priority
• Packet delay budget
• Acceptable packet loss rate
• GBR Bearers could have additional associated parameters Which are
• Guaranteed bit rate (GBR)
• Maximum bit rate (MBR)
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