LTE RADIO PROTOCOLS

Presented by :
KAVYASHREE B R
LTE Radio Protocols

CONTENTS
• Radio Resource Control (RRC)
• UE States and State Transitions Including Inter-RAT
• RRC Functions and Signaling Procedures
• Self Optimization - Minimization of Drive Tests
• X2 Interface Protocols
• Handover on X2 Interface
• Load Management
• Understanding the RRC ASN.1 Protocol Definition
• Early UE Handling in LTE

Radio Resource Control (RRC)
• Radio Resource Control messages are a major part of control information
exchanged between UE and EUTRAN.
• In EUTRAN was simplified significantly by reducing the number of
messages and by removing the redundancies in the messages.
• In WCDMA – Abstract Syntax Notation One (ASN.1) – as it has been
proven to facilitate evolution efficiently between different release versions
with the extension capability inside ASN.1.

UE States and State Transitions Including Inter-RAT
UE states in EUTRAN are simplified significantly and there are only two states:
i. RRC_CONNECTED
ii. RRC_IDLE
• In the RRC_IDLE state - UE monitors a paging channel to detect incoming calls,
acquires system information and performs neighboring cell measurement and cell
(re-)selection.
• In the RRC_CONNECTED state -UE transfers/receives data to/from the network
• UE monitors control channels, which are associated with the shared data channel to
determine if data are scheduled.

E-UTRAN States and Inter-RAT State Mobility
Figure 1. E-UTRAN RRC states and state transitions among 3GPP systems

Signaling Radio Bearers (SRB)
• Signaling Radio Bearers (SRBs) are special radio bearers that convey only RRC
messages and NAS messages.
• Three SRBs are defined.
i) SRB0 : Used for RRC messages using CCCH, for example during RRC
Connection setup or during radio link failure.
ii) SRB1: Used to transfer both RRC messages using DCCH and NAS messages
until the security is activated during establishment of RRC connection.
iii) SRB2 : SRB2 is setup, once the security is successfully activated.

RRC Functions and Signaling Procedures
The following functions are provided by RRC protocol layer:
• Broadcast of system information
• Paging
• Establishment, maintenance and release of an rrc connection between the ue and e-utran
• Security functions including key management
• Establishment, configuration, maintenance and release of point to point radio bearers
• UE measurement reporting and control of the reporting
• Handover
• UE cell selection and reselection and control of cell selection and reselection
• Context transfer between enodeBs
• NAS direct message transfer between network and UE
• UE capability transfer
• Generic protocol error handling
• Support of self-configuration and self-optimization

Broadcast of System Information
• System information contains both non-access stratum-related (NAS) and access stratum -
related(AS) information.
• Based on the characteristics and uses of the information, the system information elements are
grouped together into a master information block (MIB) and different system information
blocks (SIBs).
• Figure 2. shows how UE can find each SI message to read actual SIBs in it.
• SIB2 - Radio resource configuration information common to all UEs.
• SIB3 - Cell-reselection information, which is common for intra-frequency, inter-frequency
and/or inter-RAT cell re-selection.
• SIB4 - Neighbor cell-related information only for intra-frequency cell re-selection.
• SIB5 - Information relevant only for inter-frequency cell re-selection.

Figure 2. Acquisition of SI message

• SIB6 - Cell re-selection to the UTRAN.
• SIB7 - Cell re-selection to the GERAN like GERAN neighboring frequency list.
• SIB8 - Cell re-selection to the CDMA2000 system.
• SIB9 - Home eNodeB identifier, with maximum 48 octets.
• SIB10 - Earthquake and Tsunami Warning System (ETWS) primary notification.
• SIB 11 - ETWS secondary notification.

• In case the system information is modified, the SI messages may be repeated during
the modification period.
• The modification period starts at SFN mod modification period = 0.
• The modification period is calculated as follows:
Modification period (in number of radio frames)
= modificationPeriodCoeff∗ defaultPagingCycle DIV 10 ms
where modificationPeriodCoeff : signaled in BCCH-Configuration in SIB2.

PAGING
• The main purpose of the paging message is to page a UE in RRC_IDLE mode for a
mobile terminated call.
• The paging message can also be used to inform UEs, which are in RRC_IDLE as
well as in RRC_CONNECTED, that system information will be changed or that the
ETWS notification is posted in SIB10 or SIB11.

Establishment, Maintenance and Release of an RRC Connection
between the UE and e-UTRAN
Figure 3. RRC Connection Setup procedure

Security Functions Including Key Management
From the eNode B perspective, the following keys are necessary :
1. KeNodeB is derived by the UE and MME from the ‘master key’ (KASME) and provided by
MME to eNodeB. KeNodeB is used to derive the necessary keys for AS traffic and also for
the derivation of KeNodeB∗ during handover.
2. KeNodeB∗ is derived by the UE and source eNodeB from either KeNodeB∗ or from a valid
NH, from KeNodeB∗, the UE and the target eNodeB will derive a new KeNodeB for AS traffic
at handover.
3. KUPenc is used for the user plane traffic ciphering and derived from KeNodeB.
4. KRRCint is derived from KeNodeB to be used for RRC message integrity handling.
5. KRRCenc is derived from KeNodeB to be used for RRC message ciphering.

Establishment, Maintenance and Release of Point-to-Point
Radio Bearers
Figure 4. RRC connection reconfiguration procedure

UE Measurement Reporting and Control of the Reporting
The measurement configuration consists of the following parameters.
1. Measurement objects : The objects on which the UE performs the measurements.
E-UTRAN configures only a single measurement object for a given frequency.
2 Reporting configuration: Contains reporting criteria and reporting format.
3 Measurement identities: Measurement identity binds one measurement object with one
reporting configuration. It is used as a reference number in the measurement report.
4 Quantity configurations: One quantity configuration can be configured per RAT and contains
the filter coefficient for the corresponding measurement type.
5 Measurement gaps: Periods where UE may use to perform measurement. Measurement gaps
are used for inter-frequency and inter-RAT measurements.

HANDOVER
A handover can be performed inside EUTRAN or to EUTRAN from an other RAT or from
EUTRAN to an other RAT and is classified as
1. Intra-frequency intra-LTE handover
2. Inter-frequency intra-LTE handover
3. Inter-rat towards LTE
4. Inter-rat towards UTRAN handover
5. Inter-rat towards GERAN handover
6. Inter-rat towards cdma 2000 system handover

Intra-LTE handover
Figure 5 Inter-eNodeB Handover procedure

Inter-radio access technology handover to other Radio
Access Technology(RAT)
• To perform the inter-RAT handover to another RAT, the actual handover command
message is built by the target RAT and is sent to the source eNodeB transparently.
• As the LTE system supports the packet service only, if a multi-mode UE wants to start a
CS service, CS fallback can be performed.
• For a CS fallback to the GERAN system, either a handover procedure or a cell-change
order procedure are used. For a CS fallback to the CDMA 2000 system, the RRC
Connection release procedure is used.

CS Fallback
Figure 6 CSFB architecture

CS Fallback
• SGs interface between MSC server and MME is required .
• Via SGs interface, the UE performs combined EPC and IMSI attachment and
detachment procedure.
• The SGs interface is also used to deliver MO SMS and MT SMS and in this case
CS fallback does not occur.

CS fallback with Cell Change Order

CS fallback to UMTS
Figure7 CS fall-back to UTRAN with RRC Connection Release with redirection
information

CS fallback to UMTS
• RRC Connection Release with redirection information is the least-effort solution because all
the network and UE will support RRC Connection Release procedure.
• UE starts a CS call setup request via an extended service request, or receives a CS paging.
• UE moves to the target RAT and searches for a suitable cell by utilizing the frequency
information in the RRC Connection Release.
• Once the UE finds a suitable cell, it starts from IDLE and acquires necessary system
information blocks to access the cell.

CS fallback to GSM
Figure 8 CS fall-back to GSM with RRC Connection Release with redirection information with sys info

CS fallback to GSM
• The UE RRC Connection in the LTE system is released and the UE is redirected to the GSM
system to set up the CS service like CS fallback to UMTS.
• In this redirection information, the eNodeB can indicate a target frequency.
• The UE moves to the target RAT and searches for a suitable cell by using the frequency
information in the RRC Connection Release.
• With the Release 9 enhancement, the eNodeB can include a selected set of system information
blocks for maximum 32 cells for GSM.

CS fallback to the cdma2000 system
• The CS fallback mechanisms to cdma2000 system, 1xRTT system, was defined as well
and this is called 1xCSFB.
• The eNodeB executes one of the following CS fallback to 1xRTT procedures depending
on network support and UE capabilities:
i. The RRC Connection Release : with redirection to 1xRTT, is the default procedure
and the eNodeB can solicit 1xRTT measurements from the UE before performing RRC
Connection Release with redirection.
ii. Enhanced 1xCSFB: 1xRTT handover signaling is tunneled between the UE and the
1xRTT network.
iii. The dual receiver 1xCSFB: RRC connection release without redirection information.

UE Capability Transfer
Figure 9 UE capability transfer procedure

Generic Protocol Error Handling
Figure 10 RRC connection re-establishment procedure

Self Optimization – Minimization of Drive Tests
The Release 10 functionalities are as follows
1. MDT-logged measurements
2. MDT immediate measurement
Figure 11 Logged mode MDT in idle mode

X2 Interface Protocols
Figure 12 X2 interface User and Control Plane protocol stacks

The X2 Application Protocol (X2AP) functionalities are:
• Mobility management for Intra LTE mobility .The handover message between
eNodeBs is transmitted on the X2 interface.
• Load management to enable inter-cell interference coordination by providing
information of the resource status, overload and traffic situation between different
eNodeBs.
• Setting up and resetting of the X2 interface.
• Error handling for covering specific or general error cases.

Handover on X2 Interface
Figure 13 X2 Handover preparation over X2 interface

Load Management
Figure 14 Interference level reporting over X2 interface

Load Management
Figure 15 Transmission power versus threshold reporting over X2 interface

RRC Protocol
Table 1 First code fragment form RRC ASN.1 protocol specification

High-level Structure
Table 2 Rel9 DL-CCCH logical channel message type

RRC Extension Mechanisms
• Within the RRC protocol specification there are two distinct extension mechanisms
used for this protocol evolution:
1. Critical extensions : The existing definition is in effect completely replaced with a
new definition.
2. Non-critical extensions : Only a part of the existing definition is replaced or the
definition is simply extended.

Critical extensions
Table 3 Possible future definition for DL-CCCH-MessageType

Non-critical extensions
 Non-critical extensions are a way to add new elements or information to the
existing specification.
 Non-critical extension at the end of a message or of a field contained within a BIT
or OCTET STRING then an optional empty SEQUENCE construct is used.
 In all other cases the place holder for non-critical extension is provided by the built-
in ASN.1 extension marker ‘. . .’.
 An example of this construct can be seen in the RRCConnectionReestablishment-
v8a0-IEs definition in Table 6.5 .

Non-critical extensions
Table 4 Definition of Rel9 RRC connection re-establishment message

Early UE Handling in LTE
Figure 16 Signaling of the device capability at connection set-up

LTE RADIO PROTOCOLS

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