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RRC(Radio ResourceControl)
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OUTLINE
RRC Introduction
DRX in RRC_Idle
Analysis
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Radio Resource Control (RRC) UE management and control is handled in the Radio
Resource Control (RRC)
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LTE Protocol Architecture Control Plane
NAS control protocol AS control protocol Between UE and EPC Between UE and eNB
PLMN selection Radio-specific functionality
Tracking area update Interaction with NAS
Paging RRC as main controlling function in AS
Authentication RRC as Layer 3 in AS protocol stack
EPS bearer establishment, modification, release
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RRC Main Functions Broadcast of system information
Including NAS common information
Information applicable for UEs in RRC_Idle and information applicable forUEs in RRC_Connected
Including ETWS(Earthquake and Tsunami Warning System) and
CMAS(Commercial Mobile Alert Service) notifications
RRC connection control Paging
Establishment/modification/release of RRC connection
Initial security activation
RRC connection mobility handover
Establishment/modification/release of RBs carrying user data (DRBs)
Radio configuration control
Assignment/modification of ARQ, HARQ, DRX configurations
QoS control Assignment/modification of semi-persistent scheduling configuration
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Recovery from radio link failure
Inter-RAT(Radio Access Technology) mobility
Measurement configuration and reporting
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RRC state
RRC protocol has two states RRC_Idle
RRC_Connected
When an RRC connection has been establishment, the UE is inRRC_Connected state
Otherwise, the UE is RRC_Idle state
RRC_Idle RRC_Connected releaseentestablishm
Connection
/
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RRC_IDLE:
A UE specific DRX may be configured by upper layers.
UE controlled mobility;
The UE: Monitors a Paging channel to detect incoming calls, system information change, for
ETWS capable UEs, ETWS notification, and for CMAS capable UEs, CMAS notification
Performs neighbouring cell measurements and cell (re-)selection
Acquires system information.
RRC_CONNECTED:
Transfer of unicast data to/from UE.
At lower layers, the UE may be configured with a UE specific DRX.
Network controlled mobility, i.e. handover
The UE: Monitors a Paging channel and/ or System Information Block Type 1 contents to detect
system information change, for ETWS capable UEs, ETWS notification, and for CMAScapable UEs, CMAS notification;
Monitors control channels associated with the shared data channel to determine if data isscheduled for it;
Provides channel quality and feedback information;
Performs neighbouring cell measurements and measurement reporting;
Acquires system information.
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RRC state characteristics
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Paging
The purpose of paging procedure Transmit paging information provided to upper layers which in
response may initiate RRC connection establishment
e.g. to receive an incoming call
Inform UEs in RRC_Idle and Ues in RRC_Connected about asystem information change
Inform an ETWS notification
Inform a CMAS notification
Initiation E-UTRAN initiates the paging procedure by transmitting the Paging
message at the UEs paging occasion
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RRC Connection Establishment-SRB1
SRB(Signalling Radio Bearer): defined as Radio Bearers (RB) that are used for thetransmission of RRC and NAS messages
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RRC Connection Establishment-SRB2
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DRB Establishment
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RRC Signal Low Layer KeyConfiguration for DRB (1)
MAC : scheduling in eNB Distributes the available radio resources
In one cell among the UEs
Among the bearers of each UEs
Accordance
Downlink data buffered in the eNB
Buffer Status Reports(BSRs), received from the eNB
QoS of each DRB
Scheduling algorithm left to the eNB implementation, signalling tosupport the scheduling standardized
Scheduling mode Dynamic scheduling
Persistent scheduling
Semi-persistent scheduling
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RRC Signal Low Layer KeyConfiguration for DRB (2)
MAC : Discontinuous Reception (DRX) Configured for RRC_Conneted UE
Short-Cycle and Long-Cycle configured tradeoff between batterysaving and latency
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RRC Connection Release
When the UE does not have packets to bereceived/transmitted for an extended period of time The eNB initiates the release of the UEs RRC connection
Request MME to release the UEs S1 connection
eNB removes the UE context MME and SGW only remove the eNB specific part of the UE
context
S1 release procedure
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DRX in RRC_Idle In RRC_Idle state, the discontinuous reception is focus on paging in
order to reduce power consumption
Paging Frame (PF) : One Radio Frame which may contain one or multiple Paging Occasion
Paging Occasion (PO) : A subframe where there may be P-RNTI transmitted on PDCCH addressing
the paging message
PF and PO is determined by using the DRX parameters provided inSystem Information
System Information DRX parameters stored in the UE shall beupdated locally in the UE whenever the DRX parameter values arechanged in SI
When DRX is used the UE needs only to monitor one PO per DRXcycle
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DRX parameters in Idle state DRX parameters
T : DRX cycle of the UE. T is determined by
UE specific DRX cycle if configured by upper layer
Default DRX cycle
nB: 4T, 2T, T, T/2, T/4, T/8, T/16, T/32
N: min(T,nB)
Ns: max(1,nB/T)
UE_ID: IMSI mod 1024
Default DRX cycle broadcast in System Information
UE specific DRX cycle is sent to MME by Attach Request orTracking Area Update Request (NAS control message), thenMME notifies the eNB
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PF is determined by
SFN mod T= (T div N)*(UE_ID mod N)
PO is determinde by
i_s = floor(UE_ID/N) mod Ns
FDD:
Ns PO when i_s = 0 PO when i_s = 1 PO when i_s = 3 PO when i_s = 4
1 9 N/A N/A N/A
2 4 9 N/A N/A
4 0 4 5 9
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Network Re-entry time The UE in RRC_Idle state may be paged by the network
when there is data addressed to that particular UE
Then the UE will returns to RRC_connected state
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Express the network re-entry time as
k
= t0
+ t1
+ t2
+ t3
+ 1
+ 2
+ krtx
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Analytical Result
For simplifying the analysis, assume that
1 (eNB pages UE) is uniform distribution with mean T/2
Paging messages are successfully received by the UE with aprobability of 0.9
The maximum page retransmissions are limited to 4
The preamble detection error rate is 3%
The maximum number of preamble retransmission is 5
UEs preparation time to send the RACH preamble is 2ms
The time to receive RACH response from the eNB is 15ms
RACH transmission backoff time is 6ms
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Multiple pages per paging frame improve the reentry time significant23
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Reference
LTE-Advanced Radio Layer 2 and RRC aspects MagnusLindstrm, Ericsson 3GPP TSG-RAN WG2
Long Term Evolution Protocol Overview freescale
DRX Mechanism forPowe Saving in LTE Chandra S.
Bontu, ED Illidge, IEEE Communication Magazine
3GPP TS 36.331 Radio Resource Control (RRC) Protocolspecification
3GPP TS 36.304 User Equipment (UE) procedures in idle
mode
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