ltelocationandmobilitymanagement
TRANSCRIPT
Irfan Ali 1
Power Management and
Mobility Management in LTE
Irfan Ali
October 2014
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Overview
• Power Conservation in UE Ø High Power: “Connected Mode” when UE has both its
transmitter and receiver always on. Ø Low Power: “Idle Mode” when UE turns off it transmitter. It
turns on its receiver periodically • Transition between the states • Mobility in Idle Mode
Ø Cell Selection and Re-selection Ø Tracking Area Update
• Mobility in Connected Mode: Handovers
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Power Management in LTE High Power Mode Connected Mode
Low Power Mode Idle Mode
• UE’s radio is in ON state. • UE is constantly communicating with the network.
• Network controls UE’s movement through handover.
• Location of the UE is known to the network at granularity of a cell.
Mobility Mobility
Tracking Area 1 Tracking Area 2
• Network does not control UE’s movement. UE autonomously selects new cell as it moves.
• Network only knows the location of the UE to the granularity of a tracking-area.
• UE’s radio is in low-power state. UE’s transmitter is off.
• UE only listens periodically to control channel. If UE enters a new location area, based on hearing information from base-station, the UE informs the network of the new tracking area it has entered.
UE is like a dog on a leash J UE is like a dog without a leash enclosed in an electronic fence
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Activity State Management
• A UE in LTE can be in two states: Ø Connected Mode: The UE is transmitting and receiving data from the
network. Ø Idle Mode: The UE is only monitoring the paging and broadcast channel.
• After the UE stops transmitting/receiving data/signal for a period of time, called inactivity period, the network moves the UE’s state to idle-state
Data/Signal activity
Yes
No
Connected
Idle
UE’s State
Connected -> Idle Inacitivity Timer
Connected -> Idle Inacitivity Timer
Time
Time
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UE’s Activity States for AS and NAS
AS
NAS EMM-IDLE EMM-
Connected
RRC connection Established
RRC connection Released
RRC-IDLE RRC-Connected
RRC connection Established
RRC connection Released
EMM-IDLE EMM-Connected
NAS connection Established
NAS connection Released
S1-MME
UE eNB MME
EMM Enhanced Mobility Management NAS Non Access Stratum AS Access Stratum RRC Radio Resource Control
RRC State
RRC-Connected
RRC-Idle
EMM-Connected
EMM-Idle
EMM State Time
Time
Time MME Request S1 connection to be torn down
eNB tears down RRC Connection
UE has a packet to send UE sets up RRC Connection
MME Request eNB to setup data radio bearers
eNB sets up data radio bearers
Events
UE’s State Machine
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Key Points about UE’s AS (RRC) and NAS State Machines
• The RRC state machine transitions are very clear Ø When the RRC Connection is setup, the UE transitions from RRC-Idle
to RRC-Connected, and vice-versa • The NAS state machine transitions are based on RRC events
Ø The NAS specification (TS 24.301), does not have EMM-Connected and EMM-Idle shown in a state-transition diagram. TS 24.301 has more detailed NAS state machine diagrams, with states such as EMM-Registered, EMM-Deregistered, etc. The following two statements buried deep in 24.301 provide details of state-transitions:
• In S1 mode, when the RRC connection has been released, the UE shall enter EMM-IDLE mode and consider the NAS signalling connection released
• In S1 mode, when the RRC connection has been established successfully, the UE shall enter EMM-CONNECTED mode and consider the NAS signalling connection established.
Ø Details of NAS specifications for MME are not explicitly provided in TS 24.301. One needs to infer these from TS 24.301, which is written from a UE implementation point of view.
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Activity States of UE
EMM-IDLE RRC connection established
RRC connection released
EMM-IDLE EMM-CONNECTED NAS Connection established
NAS Connection released
UE
MME
Idle Mode • UE monitors paging channels periodically (DRX cycle)
and some System Information channel • No NAS signalling connection between UE and MME • UE (independently) performs cell selection/re-selection
based on broadcast information • No UE information in the eNB • Location of UE is known to the MME at granularity of
Tracking Area. • UE performs TAU when UE enters a new TAI or when the
periodic TAU timer expires. • UE enters connected mode when RRC signaling
connection is established. • For MME there is no clear indication when the
UE’s state transitions to EMM-Connected. Typically this happens when the S1-MME connection is established for the UE.
Connect Mode • UE monitors System Information channel and control
channels associated with shared data channels. • NAS signalling connection between UE and MME x • Network (eNB) controls UE’s movement through
handover. • UE context in the eNB • Location of the UE is known to the MME at granularity
of eNB. • UE performs TAU when UE enters a new TAI broadcast • UE enters idle mode when RRC connection is
released. • For MME there is no clear indication when the
UE’s state transitions to EMM-Idle. Typically this happens when the S1-MME connection is released for the UE.
EMM Enhanced Mobility Management NAS Non Access Stratum RRC Radio Resource Control
Both the UE and MME keep track of the state of the UE
EMM-CONNECTED
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State in Network for Connected and Idle mode
UE
MME
eNB
SGW
PGW
NAS (logical)
S1-MME
S1-u S11
S5
DRB SRB
UE
MME
eNB
SGW
PGW
S11
S5
No UE Context
UE Context
No S1-U tunnel
Connected Mode Idle Mode DRB Data Radio Bearer SRB Signaling Radio Bearer
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Transition from Connected to Idle State – S1 Release Procedure
SGW HSS eNB UE
PGW
MME
Internet
Data Radio Bearer-10 GTP-U-10 Tunnel GTPU-10 Tunnel
GTPC Tunnel GTPC Tunnel S1-MME SRB-2
SRB-1
SRB-0
UE reamains inactive for sometime
S1 UE Context Release Request
EMM-Connected
GTPC
Release Access Bearer Resp. (IMSI, TEIDs)
Release Access Bearer Req. (IMSI, TEIDs, )
S1 UE Context Release Command DL-SCH:DCH SRB1
RRC Connection Release
S1 UE Context Release Complete
EMM-Idle
RRC-Idle
EMM-Idle No UE Context in eNB
GTP-U-10 Tunnel
GTPC Tunnel GTPC-1 Tunnel
SGW does not have DL S1-U TEIDs for UE
RRC-Idle
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Packet arrives at Serving GW for idle UE: Where to page the UE?
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Concept of Tracking Area-1 • Tracking Area consists of a set of eNBs. • The concept of tracking area is introduced to reduce the amount of location
reporting (Tracking Area Update TAU) signaling that a UE does when in idle-state Ø The UE only signals to the network (MME) when the UE enters a TA to which it is not
admitted. Ø The MME knows the location of the UE to the granularity of TAs.
• Tracking areas are non-overlapping in LTE. • The identity of each tracking area is called Tracking Area Identity (TAI). • Each cell in a eNB can belong to only one TAI. • Each cell advertises in broadcast message the TAI to which it belongs. • The MME tells the UE which Tracking areas the UE is registered in.
Ø This is done in EMM-Connected mode.
TA-1
TA-2
TA-3
TA-4
TA-5
TA-6
TA-7
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Concept of Tracking Area-2 • A UE in LTE can be admitted to multiple tracking areas. The list of tracking areas
to which the UE is admitted is called the tracking area list (TAI List) is provided to the UE.
• When a UE is idle and the MME needs to locate the UE, the MME pages the UEs in the set of eNB which belong to the TAI that the UE is registered in. Ø Larger the tracking area, less frequent will be the UE’s need to signal to the network;
however larger the number of eNBs that the UE will need to be paged in.
TA-1
TA-2
TA-3
TA-4
TA-5
TA-6
TA-7
UE-1 is admitted to TAI-1 UE-2 is admitted to {TAI-2, TAI-4}
Perimeter-crossing where UE-2 performsTAU Area to page UE-2
Area to page UE-1 Perimeter-crossings where UE-1 performs TAU
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Tracking Area Identity (TAI)
World
US Turkey India
Turkcell Vodafone Avea
Izmir Istanbul Antalya
MCC
MCC MNC
MCC MNC TAC
MCC: Mobile Country Code 3 digits
2-3 digits
MNC: Mobile Network Code
2 Octets
TAC: Tracking Area Code
310 286 404
01 02 03 Uniquely identifies an operator
TAI: Tracking Area Identifier
Source for MCC and MNC codes: www.wikipedia.org
1-400 401-2000 3000-3500
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Idle-mode: When to page the UE?
In the next few set of slides we figure out when the UE turns on its receiver to figure out if the network is paging the UE.
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DL Frame Structure – Type 1 (FDD)
• 1 subframe = 1ms • 10 subframes make up Radio Frame • Each subframe consists of 14 symbols • DL control signalling is in the first 1-3 symbols
Ø The rest of the symbols (11-13) are used for data and dedicated control channels.
#0 #1 #9 … … …
Sub-frame (1 ms)
One Radio Frame (10 ms)
CCH 1
CCH 2
CCH 3
CCH 4
RB 0 RB 1 RB 2
RB n-1
.
.
.
CCH 1
CCH 2
CCH 3
CCH 4
RB 0 RB 1 RB 2
RB n-1
.
.
.
CCH 1
CCH 2
CCH 3
CCH 4
RB 0 RB 1 RB 2
RB n-1
.
.
.
Time
Frequency
Indication of page message for UE will be contained in the Common Control Channel (CCH) Pages may only be present in the subframe {0, 4, 5, 9}
CCH Common Control Channel RB Resource Block
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UEs DRX cycle in idle mode: Paging DRX • The UE’s paging DRX cycle period is one of the following:
Ø {32, 64, 128, 256} frames (each frame is 10 msec), i.e Ø {0.32, 0.64,1.28, 2.56} seconds
• The UE determines its idle-mode DRX paging cycle either Ø From the information in System Information Block (SIB) Ø Or is provided to the UE via dedicated signal before UE goes idle.
• Not all radio frames contain page messages. Ø Paging Occasion (PO) is a subframe that contains paging message Ø Paging Frame (PF) is a radio frame that contains one or more paging occasions.
• The UE needs to monitor only one paging occasion per DRX cycle. • Changes in the system information are indicated by the network using a
Paging message. Ø Hence UE only monitors PDCCH. Ø If there is a page message, the ID in the PDCCH is P-RNTI. All UEs share the same P-
RNTI (FFFE). Ø Once the UE finds PRNTI, it looks at the appropriate Resource Block in the PDSCH
pointed to by the PDCCH message. If it finds its P-TMSI in the PDSCH, then page is destined for the UE.
Ø When the Paging message indicates system information changes then UE shall re-acquire all system information.
PDCCH Physical Downlink Common Control Channel DRX Discontinuous Reception P-RNTI Paging Radio Network Temporary Identity S-TMSI S Temporary Mobile Service Identity
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Low Power (Idle Mode)
• UE’s radio is in low-power state. UE’s transmitter is off. • UE listens periodically to control channel.
• To receive pages from the network.
DRX Cycle
ON Duration UE Montiors
PDCCH
DRX Sleep
UE’s Receiver
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Formula to determine which radio frame number (SFN) and which subframe within the SFN for UE to monitor for page message
SFN mod T = (T/N) X (UE_ID mod N) i_s = floor(UE_ID/N) mod Ns
T = min (Tc, Tue) N = min (T, number of paging subframes per frame X T)
Ns = max (1, number of paging subframes per frame(Nf) )
where, Tc cell specific paging cycle {32,64,128,256} radio frames Tue UE specific paging cycle {32,64,128,256} radio frames N number of paging frames within the paging cycle of the UE UE_ID IMSI mod 1024 i_s index to a table containing the subframes with a radio frame used for paging N_f number of paging subframes in a radio frame that is used for paging. {4, 2, 1, 1/2, 1/4, 1/8, 1/16,1/32}
SFN System Frame Number
Ns PO when i_s=0 PO when i_s=1 PO when i_s=2 PO when i_s=3 1 9 N/A N/A N/A 2 4 9 N/A N/A 4 0 4 5 9
Table to determine the subframe within a radio frame that is used for paging
Source: 36.304 Section 7.1
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Transition from Idle to Active: Network Triggered – Part 1 of 1
SGW HSS eNB UE
PGW
Internet MME
RRC-Idle
GTP-U-10 Tunnel
GTPC Tunnel GTPC-1 Tunnel
SGW does not have DL S1-U TEIDs for UE.
EMM-Idle
GTPC
Downlink Data Nofic. Ack
Downlink Data Notification
eNB eNB
S1AP Page (S-TMSI)
DL-SCH: Common CC: SRB0
RRC Paging (S-TIMSI)
UE Trigerred Service Request Procedure
IP Packet
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Transition from Idle to Active: UE Triggered (1 of 2)
DL-SCH: Common CC
Random Access Preamble RACH
Random Access Preamble
UL-SCH: SRB0 RRC Connection Request
DL-SCH: Common CC: SRB0 RRC Connection Setup
UL-SCH: SRB1 RRC Connection Complete
NAS MSG
SGW HSS eNB UE
PGW
Internet MME
Random Access Procedure
RRC Setup Procedure
RRC-Idle
RRC-Connected
UE needs to send data GTPC Tunnel GTPC-1 Tunnel
SGW does not have DL S1-U TEIDs for UE.
EMM-Idle
GTP-U-10 Tunnel
EMM-Connected
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Transition from Idle to Active – UE Triggered (2 of 2)
SGW eNB UE
PGW
Internet MME
Initial UE Message NAS MSG: Service Request, GUTI, UE Network Capability
MME looks up EMM Context based on GUTI
S1-MME
Initial Context Setup Request (UE Context Info: UE Security Capability, KeNB
DL-SCH:CCH SRB1 RRC Security Mode Command, AS Algorithm
UL-SCH: SRB1 RRC Security Mode Complete
Initial Context Setup Complete
AS Security
Data Radio Bearer-10 GTPU-10 Tunnel
GTPC Tunnel S1-MME
DL-SCH:CCH SRB1 RRC Connection Reconfig
UL-SCH: SRB1 RRC Reconfig Complete
SRB-2
SRB-1
SRB-0
GTPC
Modify Bearer Resp (IMSI, TEID)
Modify Bearer Req. (IMSI, eNB TEIDs…)
EMM-Connected
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Tracking Area Update, Inter-MME – Part 1 of 3
DL-SCH: Common CC
Random Access Preamble RACH
Random Access Preamble
UL-SCH: SRB0 RRC Connection Request
DL-SCH: Common CC: SRB0 RRC Connection Setup
UL-SCH: SRB1 RRC Connection Complete
NAS MSG
SGW HSS eNB UE
PGW
Internet MME-1
Random Access Procedure
RRC Setup Procedure
RRC_Idle
RRC-Connected
TA-3 TA-5
MME-1 MME-2
SGW
MME-2
UE reads the TAI advertised by eNB and realizes that it is in a new TA.
PGW
EMM-Connected
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Tracking Area Update, Inter-MME – Part 2 of 3
SGW HSS eNB UE
PGW
Internet MME-1 MME-2
Initial UE Message NAS MSG: TAU Request, GUTI, UE Network Capability
MME-2 does DNS lookup based on GUTI
Context Req (GUTI)
Context Resp (IMSI, MM Cntxt, SM Cntx)
NAS MSG
GTPC Tunnel
GTPC
Modify Bearer Resp (IMSI, S1U TEID)
Modify Bearer Req. (IMSI, TEIDs…)
Location Update Request IMSI, …
Location Update Response Subscription Data
Cancel Location Request (IMSI,..)
Cancel Location Resp (IMSI,..)
MME-1 checks msg integrity
Downlink NAS transport NAS: TAU Accept( new GUTI, TAI,..)
DL-SCH: Dedicated CC: SRB1 DL Information Transfer
MME-2 allocates new GUTI to UE
NAS: TAU Accept
S1-MME
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Tracking Area Update, Inter-MME – Part 3 of 3
SGW HSS eNB UE
PGW
Internet MME-1 MME-2
UL-SCH: SRB1 UL Information Transfer
NAS: TAU Accept Complete
UL NAS Transport
S1-MME
S1 UE Context Release Command DL-SCH:DCH SRB1
RRC Connection Release
S1 UE Context Release Complete
RRC-Idle
EMM-Idle EMM-Idle No UE Context in eNB
NAS Msg
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Idle mode procedures in network: Selecting an MME and finding context of UE in MME
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Tracking Area and MME Service Area
• MME Service Area is defined as the set of TAIs served by the MME. Ø MME Service Area consists of complete TAI(s).
• The Service Area of two MMEs can be overlapping.
TA-1
TA-2
TA-3
TA-4
TA-5
TA-6
TA-7
MME-1 MME-2
Service Area of MME-1 {TA-1, TA2, TA-3, TA-4}
Service Area of MME-2 {TA-3, TA-4, TA-5, TA-6, TA-7}
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?
UE performing Tracking Area Update
• UE in idle-mode informs the MME about its current location by performing Tracking Area Update either Ø When the UE enters a new Tracking area (not in the UE’s TAI List), or
• The new tracking area may be under the same MME serving the UE current TAI, or served by a new MME.
Ø When the periodic Tracking Area Update timer expires (to let the network know that it is alive)
• Routing to get to the old MME. Ø For periodic TAU, the UE should provide sufficient information to the eNB to route the UE’s
message to the MME that currently holds the UE’s context.
Ø For normal tracking area to a new MME, the new MME should be able to identify the old MME inorder to get the UE’s context from the old MME.
• The identity used to perform routing is the UE’s temporary identity, called Globally Unique Temporary identity (GUTI) Ø The next few slides provides and overview of how GUTI is used to route to the MME that contains
the UE’s context.
MME-1 MME-2 MME-3
(Periodic) TAU message
MME-1
MME-2 MME-3
MME-4
?
(normal) TAU message
MME-5
TAI-1 TAI-2
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MME Pooling Concept
S-GW
eNB eNB eNB Cell Cell Cell Cell Cell
eNB Cell Cell
eNB Cell Cell
PDN GW
TA1 TA2
S-GW
eNB Cell Cell
MME MME
S-GW
MME
MME Pool-1 MME Pool-2
• Pool areas can be overlapping.
• A cell in an eNB belongs to only one TA.
• A eNB (single cell) can be connected to multiple MMEs (belonging to more than one MME pools).
TA3 TA4 Pool Area-1
Pool Area-2
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MME Identification in a pool
S-GW
eNB eNB eNB Cell Cell Cell Cell Cell
eNB Cell Cell
eNB Cell Cell
PDN GW
TA1 TA2
S-GW
eNB Cell Cell
MME MME
S-GW
MME
MME Pool-1 MME Pool-2 MME Group ID = 1 MME Group ID = 2
MMEC=1 MMEC=2 MMEC=3
MMEGI (MME Group ID)
MMEC (MME Color) Code
16 bits 8 bits
MME Pool # MME # within Pool
TA3 TA4
Pool Area-1 Pool Area-2
MMEC cannot be 1 or 2 due to overlapping pool area
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UE’s NAS Temporary ID in LTE
3 BCD digits
2 or 3 BCD digits MMEGI
(MME Group ID) MMEC (MME Code)
16 bits 8 bits 32 bits
Globally Unique Temporary ID
S-TMSI M-TMSI MMEC
8 bits
40 bits
• An M-TMSI is the unique part of a GUTI within the domain of one MME.
• A GUTI is globally unique. • A GUTI is allocated to each UE by the serving MME. • An M-TMSI is the uniqueness part of a GUTI within the domain of one MME.
• An S-TMSI is unique within the domain of an MME Pool. • A UE is paged with its S-TMSI • The UE identifies itself in a service request with the S-TMSI
S-TMSI
MME Pool # MME # within Pool
GUTI
MNC MCC MMEI (MME ID) M-TMSI GUMMEI
UEs ID used for Paging
UEs ID used In Signaling
1 1
24 bits 8 bits
1
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Routing parameters provided by UE and used by eNB for Selecting MME
UE eNB MME
RRC Connection Setup Complete ( selectedPLMN-Identity, registeredMME: plmn-Identity, mmegi, mmec dedicatedInfoNAS )
S1-MME for UE
Select MME: Service request/periodic TAU: based on S-TMSI Attach w GUTI or TAU in new TA: MME ID+PLMN Attach w/o GUTI: selected PLMN-ID
RRC Connection Request ( UE Identity: S-TMSI or rand,..)
Signaling channel- SRB0
RRC Connection Setup
Signaling channel- SRB1
S-TMSI is only provided by upper layer if the cell belongs to UE’s registered TA. If S-TMSI is not provided UE generates random number
The “registered MME” ID is not provided by upper layer if the cell is in a TA the UE is already registered to, i.e in service request or periodic TA. [Ref: Section 5.3.1.1 TS24.301]
Attach request
MME Code: uniquely identifies an MME in case of over-lapping pools. Selected PLMN is used for MOCN to get to the right MME.
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Cell-Selection and Cell-Reselection in Idle-mode: Which cell should UE “camp” on?
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Low Power (Idle Mode)
Mobility
Location Area 1 Location Area 2
• Network does not control UE’s movement. UE autonomously selects new cell as it moves.
• Network only knows the location of the UE to the granularity of a location-area.
• UE’s radio is in low-power state. UE’s transmitter is off. • UE only listens periodically to control channel. If UE enters a new location area, based on
hearing information (SIB) from base-station, the UE informs the network of the new location area it has entered.
DRX Cycle
ON Duration UE Montiors
PDCCH
DRX Sleep
UE’s Receiver
Cell Reselection Instances
TAU TAU
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Cell Selection vs Cell Reselection
• Cell selection or cell-reselection is the process of UE choosing a cell.
• Camped on a cell: UE has completed the cell selection/reselection process and has chosen a cell. The UE monitors system information and (in most cases) paging information.
Power-on
Return from Out-of-Coverage
RRC-Connected to RRC-Idle
Camped on a Cell
Camped on a different
Cell
Cell Selection Cell Re-Selection
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What does the UE measure to determine if it can camp on a cell? (1 of 3) • Reference Symbols
Ø In order for receiver to estimate the channel, known reference symbols also referred to as pilot symbols are inserted at regular intervals within the OFDM time-frequency grid.
Ø Using knowledge of the reference symbols the receiver can estimate the frequency- domain channel around the location of the reference symbol
Ø The reference symbols should have sufficient high density in time and frequency to provide estimates of the entire time/frequency grid.
Ø There are four resource elements per resource block that are dedicated to Reference Symbols.
Ø The location of Reference Symbols depends on the Physical layer cell identity of the cell.
Ø Once the UE has decoded the Primary and Secondary Synchronization Signals and consequently identified the Physical Layer Cell Identity, the UE is able to deduce the resource elements allocated to the Reference Signal.
7 symbols = 0.5 ms (Slot)
12 s
ubca
rrie
rs =
180
kH
z
Resource Block
Resource Elements used for Reference Symbols
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What does the UE measure to determine if it can camp on a cell? (2 of 3) • Reference Signal Received Power (RSRP)
Ø The RSRP is the average power (in watts) received from a single Reference Signal resource element • The power measurement is based upon the energy received during the useful part of the OFDMA symbol and
excludes the energy of the cyclic prefix.
Ø Knowledge of absolute RSRP provides the UE with essential information about the strength of cells from which path loss can be calculated for power-control calculations.
• Reference Signal Received Quality (RSRQ) Ø RSRP on its own it gives no indication of signal quality. Ø The Received Signal Strength Indicator RSSI parameter represents the entire received power
including the wanted power from the serving cell as well as all co-channel power and other sources of noise.
Ø where N is the number of Resource blocks over which the RSSI is measured
Ø RSRQ is always less than 1 (< 0 dB, actually < -3dB)
7 symbols = 0.5 ms (Slot)
12 s
ubca
rrie
rs =
180
kH
z
Resource Block
Resource Elements used for Reference Signals
RSRP= Energy in one Reference Signal Resource Element
RSSI = Total energy in OFDMA symbol containing Reference Signal RE
OFDMA Symbol
RSRQ = RSRP RSSI / N
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What does the UE measure to determine if it can camp on a cell? (3 of 3) • Cell Selection Criteria
Cell is selected if: Srx > 0, and Sq > 0
Measured Rx Level (dBm)
Time (s)
Measured Cell Quality (dB)
Time (s)
Srx = Rx_measured – P_comp – Rx_min
Srx
P_compensation Rx_min
Sq = Q_measured – Q_min
Q_min
Sq
Cell Selected Cell Not Selected
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Power savings in active state: DRX in connected mode in LTE
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Overveiw • DRX allows UE to not continuously monitor the PDCCH
Ø Leads to power-savings for UE in active state.
Ø Configured using RRC signaling by the eNB
Ø Per UE mechanism
Ø The eNB keep track of UE’s DRX cycle, so that it transmits DL data to the UE only during the subframe when the UE is listening to PDCCH.
• DRX Cycle: Specifies the periodic repetition of the On Duration followed by a period of sleep Ø Two types of DRX cycles: Long DRX cycle, and (optional) Short DRX cycle. The Long DRX cycle is a multiple of
short DRX cycle.
• On Duration Timer: Specifies the number of consecutive PDCCH-subframe(s) at the beginning of a DRX Cycle
Long DRX Cycle
Short DRX Cycle
ON Duration
UE Montiors PDCCH
PDCCH Physical Downlink Common CHannel Source: 36.300 (Section 12), 36.321 (Section 5.7)
DRX Sleep
UE Montiors PDCCH
DRX Sleep
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RRC State Transition in LTE with Connected Mode DRX
DRX
Continuous Reception
Short DRX Long DRX Inactivity Timer
Data Transfer
RRC-CONNECTED RRC-IDLE
DRX Inactivity Timer
DRX Short Cycle Timer
Timer Expiration Data Transfer
Source: A Close Examination of Performance and Power Characteristics of 4G LTE Networks, Junxian Huang, et al, 2012
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Entering DRX operation
• Inactivity Timer: Duration in downlink subframes that the UE waits from the last successful decoding of a PDCCH which contained data for UE, till entering DRX.
• On-duration: Duration in downlink subframes that the UE waits for, after waking up from DRX, to receive PDCCHs. If the UE receives PDCCH with data for UE, the UE stays awake and starts the inactivity timer.
Inactivity timer DRX Start Offset On Duration
Long DRX Cycle DRX Short Cycle Timer
• All DRX parameters are signalled by eNB during RRC Connection Setup message.
• The frame-number, x, and the subframe number, y, to start the On-duration is computed as follows: Æ [x * 10 + y] mod (Short_DRX_Cycle) = DRX_Start_Offset mod (Short_DRX_Cycle), for Short DRX cycle Æ [x * 10 + y] mod (Long_DRX_Cycle) = DRX_Start_Offset, for Long DRX Cycle
• DRX Start Offset: Number of subframes.
• Short DRX Cycle: Value in number of subframes. 2,5, 8, 10,…, 320,512,640
• DRX Short Cycle Timer: Number of short cycles before the UE enters Long DRX Cycle
• Long DRX Cycle: Value in number of subrame.10, 20, .. 2560 (2.56s)
DRX Sleep
UE Montiors PDCCH
PDCCH contains DL data for UE
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Exiting and re-entering DRX operation
• Inactivity Timer: Duration in downlink subframes that the UE waits from the last successful decoding of a PDCCH which contained data for UE, till entering DRX.
• On-duration: Duration in downlink subframes that the UE waits for, after waking up from DRX, to receive PDCCHs. If the UE receives PDCCH with data for UE, the UE stays awake and starts the inactivity timer.
Inactivity timer DRX Start Offset
On Duration
Long DRX Cycle DRX Short Cycle Timer
• All DRX parameters are signalled by eNB during RRC Connection Setup message.
• The frame-number, x, and the subframe number, y, to start the On-duration is computed as follows: Æ [x * 10 + y] mod (Short_DRX_Cycle) = DRX_Start_Offset mod (Short_DRX_Cycle), for Short DRX cycle Æ [x * 10 + y] mod (Long_DRX_Cycle) = DRX_Start_Offset, for Long DRX Cycle
• DRX Short Cycle Timer: Number of short cycles before the UE enters Long DRX Cycle
• Short DRX Cycle: Value in number of subframes. 2,5, 8, 10,…, 320,512,640
• DRX Start Offset: Number of subframes.
DRX Sleep
UE Montiors PDCCH
PDCCH contains DL data for UE
Active Time
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Difference between Connected mode DRX and Idle mode DRX
• Typically the DRX period of connected mode DRX is shorter than that of idle mode DRX Ø In connected mode, there is a higher probability of data activity from
UE. Longer connected mode DRX would mean higher delay in sending the first packet to the UE.
• Power consumption for UE in connected-mode DRX is typically greater than that during idle-mode DRX. Ø For more details, please refer to: A Close Examination of Performance
and Power Characteristics of 4G LTE Networks, Junxian Huang, et al, 2012
• Since smart phones generate constant dribble of traffic, with several background processes doing keep-alives, and there is too much signaling overhead in transitioning the UE to idle and then back to connected state, operators keep smartphones in connected mode for long duration of time using connected mode DRX in LTE.
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Mobility Management in LTE
Irfan Ali
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Overview
Mobility Mangement in LTE
Mobility Management in Idle-Mode
Mobility Management in Connected Mode
Cell selection/reselection Covered in previous slides
Handovers Covered next
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Handovers, or Mobility Management in Connected Mode
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Overview of Handovers
• All handovers in LTE are prepared handovers Ø Resources are prepared in the target eNB, before the UE
connects to the target eNB • All handovers in LTE are UE assisted network controlled
Ø The UE is asked to make measurements of neighbouring cells by the source eNB and report back to the source eNB.
Ø The source eNB decides as to which target eNB the UE should be handed over to and directs the UE to that particular target eNB.
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Measurement (1 of 2)
• There is no need to indicate neighbouring cell IDs to enable the UE to search and measure a cell i.e. E-UTRAN relies on the UE to detect the neighbouring cells
• For the search and measurement of inter-frequency neighbouring cells, at least the carrier frequencies need to be indicated
• eNB signals reporting criteria for event-triggered and periodical reporting Ø Events can be defined eg to be low Rx threshold on current cell, etc.
• An NCL (network cell list) can be provided by the serving cell by RRC dedicated signalling to handle specific cases for intra- and inter-frequency neighbouring cells. This NCL contains cell specific measurement parameters for specific neighbouring cells;
• Black lists can be provided to prevent the UE from measuring specific neighbouring cells.
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Measurement (2 of 2) • Depending on whether the UE needs transmission/reception gaps to perform the
relevant measurements, measurements are classified as gap assisted or non-gap assisted. Ø Gap patterns (as opposed to individual gaps) are configured and activated by RRC. Ø Intra-frequency cell measurements are non-gap assisted. Ø Inter-frequency cell measurements may be gap-assisted based on UE’s capabilities
and the current operating frequency. The UE determines whether a particular cell measurement needs to be performed in a transmission/reception gap and the scheduler needs to know whether gaps are needed
current cell UE target cell
fcfc
Scenario C
current cell UE target cell
fcfc
Scenario A
current cell UE target cell
fcfc
Scenario B
current cell UE target cell
fcfc
current cell UE target cell
fcfc
Scenario D Scenario E
current cell UE target cell
fc
fc
Scenario F
Non-Gap Assisted Measurement
Gap Assisted Measurement
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Types of handovers
IMS Internet
eNB-1 eNB-2 eNB-3
MME-A
S-GW-1
S-GW-2
P-GW HSS
S1-MME
S11
S1-U
S6a S5
X2 X2 eNB-4 eNB-5 eNB-6 X2
S-GW-3
MME-B
MME-C
1 2 3 4 5
1 X2 Handover with no SGW relocation
2 X2 Handover with SGW relocation
3 S1 Handover with MME and SGW relocation
S10 S10
4 X2 Handover with no SGW relocation
X2 Handovers cannot have an MME change, i.e for an X2 HO, both the source-eNB and target-eNB have to be under the control of the same MME.
5 S1 Handover with MME and no SGW relocation
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X2 HO with S-GW relocation
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X2 HO Basics • X2 Handovers cannot have an MME change.
Ø Both the source-eNB and target-eNB have to be under the control of the same MME. • X2 Handovers with S-GW relocation assumes that there is connectivity
between the Source S-GW and the target eNB. Ø The reason being that in X2 handover the MME is informed after the X2 HO is
complete, i.e the UE has already moved to the target eNB. If the target eNB has no connectivity to the source SGW, then packet in UL and DL will be dropped untill the MME moves the SGW.
Ø In case the target eNB is not connected to the SGW to which the source eNB is connected, only S1-HO is allowed. In S1-HO, the MME in handover preparation tells the target SGW to be ready to accept packets from the target eNB. Thus there is no interruption in traffic from the target eNB.
eNB-2 eNB-3 X2 eNB-2 eNB-3 X2
PGW
SGW
SGW
MME
SGW SGW
PGW
MME
X2 handover not allowed; only S1 HO in this case X2 handover allowed
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X2-HO with Serving GW change (1 of 2) S-SGW S-eNB UE PGW MME T-SGW T-eNB
1. MME provides area restrictions to eNB for UE 2. eNB Configures measurement reporting
3. Measurement Reports
4. HO Decision
5. Handover Request
6. Admission Control
7. Handover Request Ack
Transparent Container RRCConnReconfig (CRNTI, RACH preamble) 8. RRC Connection
Reconfig
DL-SCH:CCH SRB1
9. Detach from old Cell Synch to new Cell
DL-SCH: Common CC
10. Random Access Preamble RACH
11. Random Access Preamble
Random Access Procedure (Handover)
GTP-U UL Frwd
GTP-U DL Frwd
One per EPS Bearer
X2 AP
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X2 AP
X2-HO with Serving GW change (2 of 2) S-SGW S-eNB UE PGW MME T-SGW T-eNB
UL-SCH: SRB0 12. RRC Connection Request
DL-SCH: Common CC 13. RRC Connection Setup
UL-SCH: SRB1 14. RRC Connection Complete
RRC Setup Procedure
15. Path Switch Req (UE S1AP ID, TAI)
16. Selects new SGW
GTPC
20. Create Session Response(IMSI, TEIDs)
17. Create Session Request (IMSI, TEIDs, PGW IP,…)
GTPC 18.Modify Bearer Req (IMSI, TEIDs)
19.Modify Bearer Rsp (IMSI, TEIDs)
S5 Bearer Setup
GTP-U-10 Tunnel
GTPC Tunnel GTPC Tunnel
GTP-U-10 Tunnel
21. Path Switch Req Ack (S1 TEID) 22. UE Cntxt
Release
23.Releases UE resources
GTPC
25. Delete Session Response(IMSI)
24. Delete Session Request (IMSI)
S1 MME
Target eNB forwards UL packets to the Source SGW
Target eNB forwards UL packets to the Target SGW
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S1 HO with MME change and no SGW relocation
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S1-HO without Serving GW change (1 of 3)
T-MME S-eNB UE PGW S-MME SGW T-eNB 1. MME provides area restrictions to eNB for UE 2. eNB Configures
measurement reporting
3. Measurement Reports
4. HO Decision
5. Handover Required (Target eNB, target TAI)
6. Target MME chosen
Transparent Src to Target Container
12. RRC Connection Reconfig
DL-SCH:CCH SRB2
13. Detach from old Cell Synch to new Cell
X2 AP
Transparent Source to Target Container
7. Frwd Reloc Req (IMSI, target eNB)
8. Handover Request
9. Handover Request Ack
Admission Control
Transparent Target to Src Container
10. Frwd Reloc Rsp (IMSI)
Transparent Target to Src Container
11. Handover Command Transparent Target to Src Container
S10
S10
X2 AP
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S1-HO without Serving GW change (2 of 3)
T-MME S-eNB UE PGW S-MME SGW T-eNB
DL-SCH: Common CC
14. Random Access Preamble RACH
15. Random Access Preamble
Random Access Procedure (Handover)
UL-SCH: SRB0 16. RRC Connection Request
DL-SCH: Common CC 17. RRC Connection Setup
UL-SCH: SRB1 18. RRC Connection Complete
RRC Setup Procedure
19. Handover Notify
20. Forward Reloc Complete
21. Forward Reloc Complete Ack
GTPC 23. Modify Bearer Req (IMSI, TEIDs)
24. Modify Bearer Rsp (IMSI, TEIDs)
22. Start timer to release resources
S1 MME
S10
GTPC Tunnel
GTP-U-10 Tunnel
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S1-HO without Serving GW change (3 of 3)
T-MME S-eNB UE PGW S-MME SGW T-eNB
UL-SCH: SRB2 25. UL Info Transport
26. Uplink NAS Transport S1-MME
HSS
27. Location Update Req. IMSI, …
30. Location Update Response Subscription Data
28. Cancel Location Request (IMSI,..)
29. Cancel Location Resp (IMSI,..)
32. Downlink NAS transport NAS: TAU Accept( new GUTI, TAI,..)
DL-SCH: Common CC: SRB1 33. DL Information Transfer
31. T-MME allocates new GUTI to UE
NAS: TAU Accept
NAS Msg NAS: TAU Request
35. UE Context Release Command
36. UE Context Release Complete
34. Timer from 22. Expires
S1-MME
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Summary of LTE handover
• All handovers are prepared and network controlled. Ø The UE is provided the slot to attempt random access also during the
preparation phase from the target eNB. Ø “Transparent Target to Source Container” is used by the target eNB to
provide preparation information to the UE.
• The SGW in UL direction is expected to receive packets from target eNB for the UE and forward it to the PGW before receiving path switch message from MME Ø So the UL GTP TEID allocated for the UE by SGW for S1-U should be
unique across all eNBs connected to the SGW. Ø The same is true for PGW from SGW.