LTE Network Architecture and Protocols

figure : Basic EPS entities and interfaces

The LTE network was then introduced as a flat architecture, with user plane direct tunneling
between the core and access networks. The EPS system is similar to the flat architecture option
in HSPA+. Similar to the 3G system, the LTE system consists of core and access networks,
but with different elements and operations.
EPS consists of an E-UTRAN access network and EPC CN. EPS can also interconnect with
other RAN; 3GPP (GERAN (GSM/EDGE radio access network), UTRAN) and non-3GPP
(CDMA, WiFi, WiMAX).
Though the CS domain is not part of the EPS architecture, 3GPP defines features to allow
interworking between EPS and CS entities. This interworking allows traditional services, CS
voice speech call, to be set up directly via traditional or evolved CS domain calls, known as
CS fallback.
Figure shows the basic EPS entities and interfaces. Table summarizes the functions
of the EPS core and access networks.



Evolved Packet Core (EPC)
EPC includes an MME (mobility management entity), an S-GW (serving sateway), and an
P-GW (packet gateway) entities. They are responsible for different functionalities during the
call or registration process. EPC and E-UTRAN interconnects with the S1 interface. The S1
interface supports amany-to-many relation between MMEs, S-GWs, and eNBs (eNodeBs) .
MME connects to E-UTRAN by means of an S1 interface. This interface is referred to as
S1-C or S1-MME. When a UE attaches to an LTE network, UE-specific logical S1-MME
connections are established. This bearer, known as an EPS bearer, is used to exchange UE
specific signaling messages needed between UE and EPC.
Each UE is then assigned a unique pair of eNB and MME identifications during S1-MME
control connection. The identifications are used by MME to send the UE-specific S1 control
messages and by E-UTRAN to send themessages to MME. The identification is released when
the UE transitions to idle state where the dedicated connection with the EPC is also released.
This process may take place repetitively when the UE sets up a signaling connection for any
type of LTE call.
MME and E-UTRAN handles signaling for control plane procedures established for the UE
on the S1-MME interface including:
• Initial context set-up/UE context release,
• E-RAB (EPS-radio access bearer) set-up/release/modify,
• Handover preparation/notification,
• eNB/MME status transfer,
• Paging,
• UE capability information indication.
MMEs can also periodically send the MME loading information to E-UTRAN for mobility
management procedures. This is not UE-specific information.
S-GW are connected to E-UTRAN by means of an S1-U interface. After the EPS bearer
is established for control plane information, the user data packets start flowing between the
EPC and UE through this interface.
Inside the EPC architecture, MME and S-GW interconnects through the S11 interface. The
S11 links the MME with the S-GW in order to support control plane signaling [6]. The S5
interface links the S-GW with the PDN-GW (packet data network-gateway) and supports both
a control and user planes. This interface is used when these elements reside within the same
PLMN (public land mobile network). In the case of an inter-PLMN connection, the interface
between these elements becomes S8.
The details of all the interfaces in EPC and E-UTRAN are further discussed .

Evolved Universal Terrestrial Radio Access Network (E-UTRAN)
E-UTRAN consists of the eNB. The eNB typically consists of three cells [8]. eNB can,
optionally, interconnect to each other via the X2 interface. The interface utilizes functions for
mobility and load exchange information .
eNB connects with the UE on the LTE-Uu interface. This interface, referred to as the air
interface, is based on OFDMA.
E-UTRAN provides the UE with control and user planes. Each is responsible for functions
related to call establishment or data transfer. The exchange of such information takes place
over a protocol stack defined in UE and eNB. Over the interface between the UE and the EPS,
the protocol stack is split into the access stratum (AS) and the non-access stratum (NAS).

LTE User Equipment
Like that of UMTS, the mobile device in LTE is termed the user equipment and is comprised
of two distinct elements; the USIM (universal subscriber identity module) and theME (mobile
equipment).
The ME supports a number of functional entities and protocols including:
• RR (radio resource) – this supports both the control and user planes. It is responsible for all
low level protocols including RRC (radio resource control), PDCP (packet data convergence
protocol), RLC, MAC (medium access control), and PHY layers. The layers are similar to
those in the eNB protocol layer.
• EMM(EPS mobilitymanagement) – is a control plane entity whichmanages themobility
states of the UE: LTE idle, LTE active, and LTE detached. Transactions within these states
include procedures such as TAU (tracking area update) and handovers.
• ESM (EPS session management) – is a control plane activity which manages the activation,
modification, and deactivation of EPS bearer contexts. These can either be default or
dedicated EPS bearer contexts.
The PHY layer capabilities of the UE may be defined in terms of the frequency bands and
data rates supported. Devices may also be capable of supporting adaptivemodulation including
QPSK, 16QAM, and 64QAM. Modulation capabilities are defined separately in 3GPP for
uplink and downlink.
The UE is able to support several scalable channels, including 1.4, 3, 5, 10, 15, and 20 MHz,
while operating in FDD and/or TDD. The UE may also support advanced antenna features
such as MIMO with a different number of antenna configurations.
The PHY layer and radio capabilities of the UE are advertized to EPS at the initiation
of the connection with the eNB in order to adjust the radio resources accordingly. An LTE
capable device advertizes one of the categories listed in Table according to its software
and hardware capabilities. Categories 6, 7, and 8 are considered part of LTE-advanced
UE’s capabilities.