Amir ahmadian tlt-6507-seminar report final version-
An Overview of the LTE
Amir Mehdi Ahmadian
Electronics and Communications Department, Tampere University of Technology
Abstract— In this report we consider the advent of the future
cellular networks known as Long-Term Evolution (LTE) radio
interface architecture. Taking into account the variety of system
architectures and access networks, the functionality of radio
protocols in order to provide reliable, efficient data flow along
with deploying comprehensive channel protocol model,
scheduling mechanism and QoS plan must be accounted as well.
Keywords— E-UTRAN, LTE, EPC, SAE, eNodeB
Developments behind radio interface in Wideband Code
Division Multiple access (WCDMA) successor, known as the
Long-Term Evolution (LTE) soon became clear that the
system architecture would also need to be evolved. Since the
LTE is the first cellular communication system optimized to
support packet-switched services on the one hand, and it had
been shown the co-located efficient functionality performed in
NodeB on the other hand, the System Evolution Architecture
(SAE) followed the radio interface development and it was
agreed to schedule the completion of work in Release 8.
According to , the following lists some of aims shaped the
Optimization for packet switched services in general,
when there is longer a need to support the circuit
switched mode of operation
Optimized support for higher throughput required for
higher end user bit rate
Improvement in the response times for activation and
Improvement in the packet delivery delays
Optimized inter-networking with other 3GPP access
Optimized inter-networking with other wireless
In this work the overall architecture for both Radio Access
Network (RAN) and the Core Network (CN) were considered
separately from functionality issues point of view led to flatter
RAN architecture and the Evolved Packet Core (EPC).
All the radio related functionality of the overall network like
scheduling, radio resource handling and retransmission
protocols are controlled and planned in RAN. Whereas, all
issues regarded to providing a complete mobile-broadband
network like authentication or end-to-end setup connections
are handled separately by the EPC having several radio-access
Driving different architecture developments has indicated the
need to introduce a set of new functions and maybe new
interfaces to support specific protocols for each one of them.
Considering the target of keeping architecture simple and
supporting all potential inter-networking scenarios, the 3GPP
architecture specifications were split into three deployment
Basic system architecture with only E-UTRAN
(Evolved Universal Terrestrial Radio Access
Legacy 3GPP basic system architecture with existing
E-UTRAN basic system architecture with non-3GPP
II. SYSTEM ARCHITECTURE
A. Basic Architecture with Only E-UTRAN
An overall overview of simplest architecture and network
elements proposed by 3GPP TS 23.401 is depicts in
Figure1. The new architectural development comparing to
those in existing 3GPP is limited to EPC and E-UTRAN(or
As it can be seen UE,E-UTRAN and EPC represent the
main function provides the IP connectivity turns out this part
to be called Evolved Packet System (EPS). Apart from EPC
and E-UTRAN as main new network elements, it is seen an
element called SAE GW which the combination of the two
gateways ,Serving Gateway (S-GW) and Packet Data Network
(P-GW) defined for unit protocol handling in EPC. Following
we have a brief explanation of each network element is
User Equipment (UE): typically it is a hand held device
such as a smart phone or a data card such as those used
currently in 2G and 3G. Functionally the UE is a platform for
communication applications, which signal with the network
for setting up, maintaining and removing the communication
links the end user needs.
Fig. 1 Basic System Architecture with Only E-UTRAN
E-UTRAN NodeB (eNodeB): the main developments in E
UTRAN is concentrated on this complex logical node which
Is the termination point for all radio related protocols. As
Figure2 represents the eNodeB is connected to the EPC by
means of the S1 interface, more specifically to the S-GW by
means of the S1 user-plane part, S1-u, and to the MME by
means of the S1 control-plane part, S1-c to play important
role in Mobility Management specially includes exchanging
handover signalling between other eNodeBs and MME.. One
eNodeB can be connected to multiple MMEs/S-GWs for the
purpose of load sharing and redundancy. As a network, EUTRAN is simply a mesh of eNodeBs connected to
neighboring eNodeBs with the X2 interface. Moreover, the X2
interface is used for active-mode support mobility and multicell radio Radio Recourse Management (RRM) functions like
Inter-cell interference Coordination (ICIC). Figure3
summarizes all eNodeB responsibilities in a nutshell. Apart
from user management or ensuring QoS like latency and
minimum bandwidth requirements for real-time bearers,
eNodeB serves a set of both MMEs and S-GW being pooled
to a single eNodeB and it has to keep track of this association.
Evolved Packet Core (EPC): as a dedicatedly packetswitched support core network structured represented in
Figure 1 & 4 , contains 5 following main elements:
Mobility Management Entity (MME) : as the
Main control element in the EPC is responsible for
connection/release of bearers to a terminal and has a
logically direct CP connection to the UE to provide
functions like authentication, handover support,
establishment of bearers, Internetworking with other
radio networks and the mobility of functionality
operating between the EPC and the terminal known
as Non-Access Stratum (NAS).
Serving Gateway(S-GW) : as a high level operator
of unit protocol management and switching
functionality, it mainly acts as mobility anchor when
terminals move between eNodeBs as well as mobility
anchor between other technologies (GSM/GPRS and
HSPA ) made by MME. Moreover, it handles the
collection information and statistics necessary for
charging. Along with configuring in a one-to-many
fashion , One S-GW may be serving only a particular
geographical area with a limited set of eNodeBs and
likewise there may be a limited set of MMEs that
control that area.
Packet Data Network Gateway (P-GW) or (PDNGW): acting as the highest level mobility anchor in
the system, it is the edge router between the EPS and
external packet data networks. Simply put, it
provides IP address allocation to the UE.
Policy and Charging Resource Function (PCRF):
all the operations regarding policies and procedures
is charged by a server usually located in operator
switching centres in RNC.
Home Subscription Server (HSS) : records the
location of the user in the level of visited network
control node such as MME along with subscribing
data repository for all permanent user. Briefly, in all
authentication HSS interacts with MMS.
Fig. 2 Evolved UTRAN Architecture
B. Basic Architecture with E-UTRAN and Legacy 3GPPP
As one of the common 3GPP Inter-working system
architecture configurations, provides the similar connectivity
services from the end user point of view via different
functionalities (Figure5).As it can be seen, all defined 3GPP
access networks are connected to EPC. Main issues concerned
in this system are to how the bearers are managed in the EPS
compared to the existing networks with UTRAN or GERAN
access along with optimized connectivity between GERAN
(GSM/EDGE Radio Access Network) and UTRAN as before
by assuming the S-GW as role of GGSN (Gateway GPRS
Support Node). Therefore, a few new interfaces are needed as
it is shown in EPC, UTRAN and GERAN. Moreover,
optimized inter-working is achieved when the network is in
control of mobility events, such handovers and minimizing
interruptions in services.
optimizations, and the same procedures are applicable in both
connected and idle mode. Whereas, the second category
includes a specific solution for cdma 2000 HRPD (High Rate
Packet Data). Considering the only the first category (Figure6)
in this report, it relies only on loose coupling with generic
interfacing means, and without AN level interfaces, there are
so many different kinds of ANs, they have been categorized to
two groups, the trusted and un-trusted non-3GPP ANs,
depending on whether it can be safely assumed that 3GPP
deﬁned authentication can be run by the network, which
makes it trusted, or if authentication has to be done in overlay
fashion and the AN is un-trusted. depending on which
access network is un-trusted or trusted, the non-3GPP access
networks are connected to it either via the S2a or the S2b
interface. In addition, the UE may register in any non-3GPP
AN, receive an IP address from there, and register that to the
Home Agent in P-GW.
Fig. 3. eNodeB connections and main functions
Fig. 5. System architecture for 3GPP access networks
Fig. 4. EPC Overall Architecture
This solution addresses the mobility as an overlay function.
While the UE is served by one of the 3GPP ANs, the UE is
considered to bein home link, and thus the overhead caused by
additional MIP headers is avoided. Thus, this system
architecture requires additional interfaces and updated logical
elements supporting different scenarios described in 3GPP
C. Basic Architecture with E-UTRAN and Non-3GPPP
According to , the Inter-working with non-3GPP access
networks was one of the key design goals for SAE, and to
support it a completely different architecture designed in
3GPP.The non-3GPP Inter-working System Architecture
includes a set of solutions in two categories. The ﬁrst category
contains a set of generic and loose inter-working solutions that
can be used with any other non-3GPP AN. Mobility solutions
deﬁned in this category are also called handovers wi thout
Fig. 6. System architecture for 3GPP and non-3GPP access networks
III. RADIO PROTOCOL ARCHITECTURE
Generally speaking, protocols entities are common to the user
and control planes. However, the protocol stack split into two
parts.& as Figure7 illustrates the control plane protocols
are shown in the left side starts with NAS layer used for
mobility management and other purposes between the mobile
device and the MME. NAS messages are tunneled through the
radio network, and the eNodeB
just forwards them
Radio Resource Control (RRC) managing the air interface
connection used, for example, for handover along with
encapsulating NAS messages. In fact, The main difference on
the user data plane shown on the right of Figure 7 is that RRC
message does not necessarily have to include a NAS message.
Therefore, IP packets are always transporting user data and
are sent only if an application wants to transfer data.
attachment by physical layer to the transport block would be
the final for the relevant operations.
IV. LTE DOWNLINK & UPLINK CHANNEL MODEL
All higher layer signaling and user data traffic are
organized in channels . All protocols regarding transport,
physical and logical layers for downlink and uplink channels
are demonstrated in Figure10. This layered structure is to
separate the logical data flows from the properties of physical
Briefly speaking, downlink channel model elements have
Downlink Logical Channles:
Paging Control Channel (PCCH) : used for paging
of terminals whose location on a cell level is not
known to the network .The paging message therefore
needs to be transmitted in multiple cells
Broadcast Control Channel (BCCH): used for
transmission of system information from the network
to all terminals in a cell .
Dedicated Traffic Channel (DTCH): used for
transmission of user data to/from a terminal and for
transmission of all uplink user data.
Dedicated Control Channel (DCCH): used for
individual configuration of terminals such as
different handover messages.
Fig. 7. Radio interface protocol stack and main functions
Packet Data Convergence Protocol (PDCP) is in charge of
encapsulating as the first unifying transport layer IP packets
for ciphering to reduce the number of bits to transmit over the
radio interface. One layer below is RLC (Radio Link Control)
which is responsible for is responsible for segmentation
/concatenation , retransmission handling of higher layer
packets to adapt them to a packet size that can be sent over the
air interface. MAC layer simply speaking, multiplexes data
fromdifferent radio bearers and ensures QoS and also
responsible for HARQ packet retransmission functionality and
address field ,power management and time advance control.
Keep all radio protocol layers in mind, Figure 8 illustrates the
architecture for downlink in LTE . remember that not all
functionalities pointed out can be applicable in all scenarios.
As example, hybrid ARQ with soft combining is not used for
broadcast of the basic system information .
Simply put, the LTE data flow represents in Figure 9 in order
to distinguish each
radio protocol layer functionality
precisely. After adding RLC header the RLC PDU (Packet
Data Unit) is forwarded to MAC layer. After multiplexing the
MAC layer would be added to transport block e formed. CRC
Fig.8. Downlink Radio interface protocols in LTE
Downlink Transport Channles:
Transport format (TF) : Specifying how the
transport block is to be transmitted over the radio
interface includes information about transport-block
size , modulation and coding scheme and antenna
Broadcast Channel (BCH): has a fixed transport
format used for transmission of parts of the BCCH
system information, more specifically Master
Information Block (MIB).
Paging channel (PCH) : is used for transmission of
paging information from the PCCH logical channel .
Downlink Shared Channel (DL-SCH) : the main
transport channel used for transmission of downlink
data in LTE supporting features like dynamic rate
adaption and channel dependent scheduling , hybrid
ARQ with soft combining ,possibility of transmitting
of BCCH system not mapped to the BCH.
Downlink Physical Channels:
Physical Downlink Shared Channel (PDSCH) :
The main physical channel used for unicast data
transmission and transmission of paging
Physical Broadcast Channel (PBCH) : Carries part
of system information , required by terminal in
order to access the network.
Physical Downlink Control Protocol (PDCCH):
Used for downlink scheduling information
required for reception of PDSCH and aslo
enabling transmission on the PUSCH.
Fig.10. LTE Downlink Channels
by how and with what characteristics the information is
transmitted over the radio interface.
In uplink channel a similar channel model is used as in the
downlink direction as we would have transport, logical and
physical channel (Figure11) . The most important channel
here is Physical Uplink Shared Channel (PUSCH) which
carries the user data in addition to signaling information and
signal quality feedback. Other channels have been explained
briefly by the followings:
Common Control Channel (CCCH) : transports
Physical Random Access Channel (PRACH) :
synchronizing and requesting initial uplink resources
Ensures the contention based procedure is performed
when the connection establishment is repeated
Fig.11. LTE Uplink Channels
Fig.9. LTE Data Flow Example
As it is shown in Figure10, a transport channel is defined
Data transmissions in LTE in both the uplink and the
downlink directions are controlled by the network 
Which is eNodeB as high level radio network control in LTE.
Therefore, in LTE we would have network-based scheduling
being advantageous in network reaction to changing radio
conditions of each user and ensuring the QoS for each user.
As it is shown in Figure12, the eNode-B’s scheduler is
responsible for forwarding the data that it receives from the
network, for all users it serves in both downlink and uplink
over the air interface. Since the transmission buffer is not
always quiet, the scheduler has to decide which users and
bearers are given an assignment grant for the next sub frame
and how much capacity is allocated to each. Therefore, a
dynamic scheduling should is taken into account &:
For each eNodeB decides the number of users wants
to schedule and the number of resource blocks that
are assigned that are assigned to each user.
Required number of symbols on the time axis in each
sub frame for the control region.
Depending on the system configuration and the
number of users to schedule, one to four symbols are
used across the complete bandwidth for control
The downlink scheduler is responsible for (dynamically)
controlling which terminal(s) to transmit to and, for each of
these terminals, the set of resource blocks upon which the
terminal’s DL-SCH should be transmitted.
Whereas, The uplink scheduler serves per terminal a similar
purpose, namely to (dynamically) control which terminals are
to transmit on their respective UL-SCH and on which uplink
time–frequency resources (component carrier). Thus,the
eNodeB scheduler controls the transport format and the
terminal controls the logical-channel multiplexing.
Fig. 12. Transport format selection in downlink and uplink
The development of the SAE bearer model and the QoS
concept started with the assumption that improvements
compared to the existing 3GPP systems with, e.g. UTRAN
access, should be made, and the existing model should not be
taken for granted.
There has been many QoS parameters introduced , however
the most common ones are:
Priority : used to define the priority for packet
scheduling of the radio interface
Delay Budget: Helps the packet scheduler to
maintain sufficient scheduling rate to meet the
delay requirements for the bearer
Loss Rate :Helps to use appropriate RLC settings
number of re-transmission
Considering accuracy and integrity in QoS plan following
problems might be encountered:
It had not seen easy for operators to use Qos
in legacy 3GPP systems
It may Only reduced set of parameters for
Network resource management is the solely
Network decides how the parameters are set
and main bearer set-up logic consists of only
one signaling transaction from the network to
To summarize the discussion, we could list up the
the overall system architecture of LTE including EUTRAN and EPC were revisited in 3GPP but
separately unlike previous systems even though it
is mainly focused on E-UTRAN as the most
complex logical node.
Radio interface architecture in LTE adds different
features to facilitate configuration with different
access networks although not all architectures are
applicable in all scenarios.
Radio access network functionalities in LTE has
been efficiently compressed and colocated in
order to compensate redundancy and increase
integrity and accuracy .
Scheduling plays an important role in both uplink
and downlink channel to dedicate shared resources
through sub frames in MAC layer.
The overall goal for network orientation in bearer
set-up is to minimize the need for QoS knowledge
and configuration in the UE.
Apart from different effective parameters , the QoS
plan in LTE must be comprehensive enough to
cover all legacy 3GPP scenarios as before
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