1. Irfan Ali
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
3. • Network controls UE’s movement through
handover.
• Location of the UE is known to the network at
granularity of a cell.
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• Network does not control UE’s movement. UE
• Network only knows the location of the UE to the
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Power Management in LTE
High Power Mode
Connected Mode
Low Power Mode
Idle Mode
Mobility
• UE’s radio is in ON state.
• UE is constantly communicating with the network.
autonomously selects new cell as it moves.
granularity of a tracking-area.
Tracking Area 1 Tracking Area 2
Mobility
• 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
4. 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
UE’s
State
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Data/Signal
activity
Yes
No
Connected
Idle
Connected -> Idle
Inacitivity Timer
Connected -> Idle
Inacitivity Timer
Time
Time
5. UE’s Activity States for AS and NAS
EMM State Time
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AS
NAS
RRC connection
Established
EMM-IDLE EMM-Connected
RRC connection
Released
RRC connection
Established
RRC-IDLE RRC-Connected
RRC connection
Released
EMM Enhanced Mobility Management
NAS Non Access Stratum
AS Access Stratum
RRC Radio Resource Control
NAS connection
Established
EMM-IDLE EMM-Connected
NAS connection
Released
S1-MME
UE
eNB
MME
RRC State
RRC-Connected
RRC-Idle
EMM-Connected
EMM-Idle
Time
MME Request S1 connection Time
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
6. 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,
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
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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EMM-Deregistered, etc. The following two statements
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.
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Activity States of UE
EMM-IDLE
RRC connection
released
RRC connection
established
NAS Connection
released
EMM-IDLE EMM-CONNECTED
NAS
Connection
established
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.
Both the UE and MME keep track
of the state of 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
EMM-CONNECTED
8. State in Network for Connected and Idle mode
MME
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MME
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UE
PGW
SGW
eNB
NAS
(logical)
S1-MME
S1-u
S11
S5
SRB DRB
UE
PGW
SGW
eNB
S11
S5
No S1-U
tunnel
UE Context
No UE
Context
Connected Mode Idle Mode DRB Data Radio Bearer
SRB Signaling Radio Bearer
9. Transition from Connected to Idle State – S1 Release Procedure
UE eNB HSS
SRB-0
SRB-1
SRB-2 S1-MME GTPC Tunnel GTPC Tunnel
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Release Access Bearer
Req. (IMSI, TEIDs, )
GTPC Tunnel GTPC-1 Tunnel
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SGW
PGW
MME
Internet
Data Radio Bearer-10 GTPU-10 Tunnel GTP-U-10 Tunnel
UE reamains
inactive for
sometime
S1 UE Context
Release Request
EMM-Connected
GTPC
Release Access Bearer
Resp. (IMSI, TEIDs)
S1 UE Context
DL-SCH:DCH SRB1 Release Command
RRC Connection Release
S1 UE Context
Release Complete
EMM-Idle
RRC-Idle
EMM-Idle No UE
Context in
eNB
GTP-U-10 Tunnel
SGW does not have DL
S1-U TEIDs for UE
RRC-Idle
10. Irfan Ali
Packet arrives at Serving GW for idle
UE: Where to page the UE?
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11. 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
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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
12. 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;
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however larger the number of eNBs that the UE will need to be paged in.
Perimeter-crossings where UE-1 performs TAU
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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}
Area to page UE-1
Area to page UE-2 Perimeter-crossing where UE-2 performsTAU
13. Ir Ifrafann AAlil i
3 digits MCC: Mobile Country Code
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Tracking Area Identity (TAI)
World
US Turkey India
Turkcell Vodafone Avea
Izmir Istanbul Antalya
MCC
2-3 digits
MCC MNC
MCC MNC TAC
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
14. Irfan Ali
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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15. Frequency
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Sub-frame
(1 ms)
One Radio Frame (10 ms)
#0 #1 … … … #9
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}
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DL Frame Structure – Type 1 (FDD)
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
• 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
CCH 1
CCH 2
CCH 3
CCH 4
RB 0
RB 1
RB 2
.
.
.
RB n-1
Time
CCH Common Control Channel
RB Resource Block
Ø The rest of the symbols (11-13) are used for data and dedicated control channels.
16. 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
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(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
17. Low Power (Idle Mode)
• UE’s radio is in low-power state. UE’s transmitter is off.
• UE listens periodically to control channel.
UE’s Receiver
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• To receive pages from the network.
ON Duration
DRX Cycle
UE Montiors
PDCCH
DRX Sleep
18. Formula to determine which radio frame number (SFN) and which
subframe within the SFN for UE to monitor for page message
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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) )
Table to determine the subframe within a radio frame that is used for paging
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
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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
Source: 36.304 Section 7.1
19. Transition from Idle to Active: Network Triggered – Part 1 of 1
UE eNB HSS
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MME Internet
GTPC Tunnel GTPC-1 Tunnel
Downlink Data Notification
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SGW
PGW
RRC-Idle
GTP-U-10 Tunnel
SGW does not
have DL S1-U
TEIDs for UE.
EMM-Idle
GTPC
Downlink Data Nofic. Ack
eNeBN B
S1AP Page (S-TMSI)
DL-SCH: Common CC: SRB0
RRC Paging (S-TIMSI)
UE Trigerred Service Request Procedure
IP Packet
20. Transition from Idle to Active: UE Triggered (1 of 2)
UE eNB HSS
UE needs to send data GTPC Tunnel GTPC-1 Tunnel
RACH
Random Access Preamble
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MME Internet
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DL-SCH: Common CC
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
PGW
Random
Access
Procedure
RRC Setup
Procedure
RRC-Idle
RRC-Connected
SGW does not
have DL S1-U
TEIDs for UE.
EMM-Idle
GTP-U-10 Tunnel
EMM-Connected
21. Transition from Idle to Active – UE Triggered (2 of 2)
UE eNB SGW
DL-SCH:CCH SRB1
RRC Connection Reconfig
UL-SCH: SRB1
RRC Reconfig Complete
SRB-0
SRB-1
SRB-2
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MME Internet
Modify Bearer Req.
(IMSI, eNB TEIDs…)
S1-MME GTPC Tunnel
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PGW
S1-MME
Initial UE Message
NAS MSG: Service
Request, GUTI, UE
Network Capability
MME looks up EMM Context
based on GUTI
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
Modify Bearer Resp
(IMSI, TEID)
EMM-Connected
22. Tracking Area Update, Inter-MME – Part 1 of 3
UE eNB HSS
RACH
Random Access Preamble
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MME-1 MME-2
Internet
SGW
MME-1 MME-2
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DL-SCH: Common CC
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
PGW
Random
Access
Procedure
RRC Setup
Procedure
RRC_Idle
RRC-Connected
TA-3 TA-5
UE reads the TAI
advertised by eNB
and realizes that it is
in a new TA.
PGW
EMM-Connected
23. Tracking Area Update, Inter-MME – Part 2 of 3
UE eNB HSS
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MME-1 MME-2 Internet
NAS MSG
Cancel Location Request (IMSI,..)
Cancel Location Resp (IMSI,..)
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SGW
PGW
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)
GTPC
Modify Bearer Req.
(IMSI, TEIDs…)
Modify Bearer Resp
(IMSI, S1U TEID)
GTPC Tunnel
Location Update Request
IMSI, …
Location Update Response
Subscription Data
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
24. Tracking Area Update, Inter-MME – Part 3 of 3
UE eNB HSS
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MME-1 MME-2 Internet
S1-MME
UL NAS Transport
24 24
SGW
PGW
UL-SCH: SRB1
UL Information Transfer
NAS: TAU Accept
Complete
S1 UE Context
NAS Msg
DL-SCH:DCH SRB1 Release Command
RRC Connection Release
S1 UE Context
Release Complete
RRC-Idle
No UE
Context in
eNB
EMM-Idle EMM-Idle
25. Irfan Ali
Idle mode procedures in network:
Selecting an MME and finding context of
UE in MME
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26. 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.
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MME-1 MME-2
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TA-1
TA-2
TA-3
TA-4
TA-5
TA-6
TA-7
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}
27. 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
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
Ø For normal tracking area to a new MME, the new MME should be able to identify 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
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• The new tracking area may be under the same MME serving the UE current TAI, or served by a new MME.
message to the MME that currently holds the UE’s context.
inorder to get the UE’s context from the old MME.
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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
28. MME Pool-1 MME Pool-2
MME MME
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Pool Area-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
S-GW
MME
TA3 TA4
Pool Area-1
• 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).
29. 16 bits 8 bits
MME Pool-1 MME Pool-2
MME Group ID = 1 MME Group ID = 2
MMEC=1 MMEC=2
MME MME
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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
S-GW
MME
MMEC=3
MMEGI
(MME Group ID)
MMEC
(MME Color)
Code
MME Pool # MME #
within Pool
TA3 TA4
Pool Area-1
Pool Area-2
MMEC cannot be 1 or 2
due to overlapping pool
area
30. Ir Ifrafann AAlil i
GUTI
MCC MNC MMEI (MME ID) M-TMSI
16 bits 8 bits 32 bits
30 30
UE’s NAS Temporary ID in LTE
3 BCD
digits
2 or 3
BCD
digits MMEGI
(MME Group ID)
MMEC
(MME Code)
Globally Unique Temporary ID
S-TMSI
• An M-TMSI is the unique part of
a GUTI within the domain of one
MME.
40 bits
MMEC M-TMSI
8 bits
• 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
GUMMEI
UEs ID used
for Paging
UEs ID used
In Signaling
1 1
24 bits 8 bits
1
31. Routing parameters provided by UE and used by eNB for Selecting MME
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
UE eNB MME
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RRC Connection Setup Complete (
selectedPLMN-Identity,
registeredMME: plmn-Identity, mmegi, mmec
dedicatedInfoNAS )
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
S1-MME for UE
RRC Connection Request (
UE Identity: S-TMSI or rand,..)
Signaling channel- SRB0
RRC Connection Setup
Signaling channel- SRB1
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.
33. • 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 Receiver
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Low Power (Idle Mode)
Mobility
Location Area 1 Location Area 2
TAU TAU
• 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.
ON Duration
DRX Cycle
UE Montiors
PDCCH
DRX Sleep
Cell Reselection Instances
34. 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.
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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
35. 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
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
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channel around the location of the reference symbol
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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 subcarriers = 180 kHz
Resource Block
Resource Elements
used for Reference
Symbols
36. 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
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• The power measurement is based upon the energy received during the useful part of the OFDMA symbol and
36 36
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 subcarriers = 180 kHz
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
37. What does the UE measure to determine if it can
camp on a cell? (3 of 3)
• Cell Selection Criteria
Ir Ifrafann AAlil i
Srx = Rx_measured – P_comp – Rx_min
37 37
Cell is selected if:
Srx > 0, and
Sq > 0
Measured Rx
Level (dBm)
Time (s)
Measured Cell
Quality (dB)
Time (s)
Srx
P_compensation
Rx_min
Sq = Q_measured – Q_min
Q_min
Sq
Cell Selected
Cell Not Selected
38. Irfan Ali
Power savings in active state: DRX in
connected mode in LTE
38
39. 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
Ir Ifrafann AAlil i
39 39
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
Short DRX Cycle
ON Duration
Long DRX Cycle
UE Montiors
PDCCH
Source: 36.300 (Section 12), 36.321 (Section 5.7)
PDCCH Physical Downlink Common CHannel
DRX Sleep
UE Montiors
PDCCH
DRX Sleep
40. RRC State Transition in LTE with Connected Mode DRX
Ir Ifrafann AAlil i
Data Transfer
40 40
DRX
Continuous
Reception
Short DRX
Long DRX
Inactivity Timer
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
41. UE Montiors
PDCCH
DRX Sleep
PDCCH contains DL data for UE
Ir Ifrafann AAlil i
• 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)
41 41
Entering DRX operation
Inactivity timer DRX Start Offset
On Duration
• 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.
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
42. Exiting and re-entering DRX operation
UE Montiors
PDCCH
DRX Sleep
Ir Ifrafann AAlil i
Inactivity timer DRX Start Offset
• 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.
42 42
Active Time
PDCCH contains DL data for UE
• 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.
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
43. 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.
Ir Ifrafann AAlil i
43 43
45. Ir Ifrafann AAlil i
45 45
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
47. Ir Ifrafann AAlil i
47 47
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.
48. Ir Ifrafann AAlil i
48 48
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.
49. 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
Scenario A
Scenario D Scenario E
Ir Ifrafann AAlil i
Scenario C
Scenario F
49 49
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
fc fc
current cell UE target cell
fc fc
Scenario B
current cell UE target cell
fc fc
current cell UE target cell
fc fc
current cell UE target cell
fc fc
current cell UE target cell
fc
fc
Non-Gap Assisted Measurement
Gap Assisted Measurement
50. Ir Ifrafann AAlil i
4 X2 Handover with no SGW relocation
5 S1 Handover with MME and no SGW relocation
S10 S10
MME-B
50 50
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
1 X2 Handover with no SGW relocation
2 X2 Handover with SGW relocation
3 S1 Handover with MME and SGW relocation
S-GW-3
X2 X2 eNB-4X 2 eNB-5 eNB-6
MME-C
1
2
3
4
5
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.
52. 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
Ø In case the target eNB is not connected to the SGW to which the source eNB is
Ir Ifrafann AAlil i
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.
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 X2 eNB-3 eNB-2 X2 eNB-3
52 52
PGW
SGW
SGW
MME
SGW
SGW
PGW
MME
X2 handover allowed X2 handover not allowed; only S1 HO in this case
53. X2-HO with Serving GW change (1 of 2)
UE S-eNB T-eNB MME S-SGW T-SGW PGW
2. eNB Configures 1. MME provides area restrictions to eNB for UE
measurement reporting
DL-SCH:CCH SRB1
Ir Ifrafann AAlil i
53 53
3. Measurement Reports
4. HO Decision
5. Handover
Request
6. Admission Control
7. Handover
Request Ack
Transparent Container
RRCConnReconfig
8. RRC Connection (CRNTI, RACH preamble)
Reconfig
9. Detach from old Cell
Synch to new Cell
RACH
10. Random Access Preamble
DL-SCH: Common CC
11. Random Access Preamble
Random Access
Procedure (Handover)
GTP-U UL Frwd
GTP-U DL Frwd
One per EPS
Bearer
X2 AP
54. X2-HO with Serving GW change (2 of 2)
UE S-eNB T-eNB MME S-SGW T-SGW PGW
Ir Ifrafann AAlil i
GTPC Tunnel GTPC Tunnel
22. UE Cntxt Req Ack (S1 TEID)
Release
24. Delete Session
Request (IMSI)
54 54
X2 AP
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
17. Create Session Request
(IMSI, TEIDs, PGW IP,…)
20. Create Session
Response(IMSI, TEIDs)
GTPC
18.Modify Bearer
Req (IMSI, TEIDs)
19.Modify Bearer
Rsp (IMSI, TEIDs)
S5
Bearer
Setup
GTP-U-10 Tunnel
21. Path Switch
GTP-U-10 Tunnel
23.Releases UE resources
GTPC
25. Delete Session
Response(IMSI)
S1 MME
Target eNB forwards UL
packets to the Source SGW
Target eNB forwards UL
packets to the Target SGW
55. Irfan Ali
S1 HO with MME change and no SGW
relocation
55
56. S1-HO without Serving GW change (1 of 3)
UE S-eNB T-eNB S-MME T-MME SGW PGW
2. eNB Configures 1. MME provides area restrictions to eNB for UE
measurement reporting
DL-SCH:CCH SRB2
Ir Ifrafann AAlil i
56 56
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
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
Admission Control
9. Handover Request Ack
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
57. S1-HO without Serving GW change (2 of 3)
UE S-eNB T-eNB S-MME T-MME SGW PGW
Ir Ifrafann AAlil i
S1 MME
57 57
RACH
14. Random Access Preamble
DL-SCH: Common CC
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
S10
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
GTPC Tunnel
GTP-U-10 Tunnel
58. S1-HO without Serving GW change (3 of 3)
UE S-eNB T-eNB S-MME T-MME SGW PGW
Ir Ifrafann AAlil i
S1-MME
34. Timer from 22. Expires
58 58
UL-SCH: SRB2
25. UL Info Transport
26. Uplink NAS Transport
HSS
27. Location Update Req.
IMSI, …
28. Cancel Location Request (IMSI,..)
29. Cancel Location Resp (IMSI,..)
30. Location Update Response
Subscription Data
32. Downlink NAS transport
NAS: TAU Accept( new
GUTI, TAI,..)
NAS Msg
DL-SCH: Common CC: SRB1
33. DL Information Transfer
31. T-MME allocates new GUTI to UE
NAS: TAU Accept
NAS: TAU Request
S1-MME
35. UE Context Release Command
36. UE Context Release Complete
59. Ir Ifrafann AAlil i
59 59
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.
