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Ltelocationandmobilitymanagement

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Ltelocationandmobilitymanagement

  1. 1. Irfan Ali Power Management and Mobility Management in LTE Irfan Ali October 2014 1
  2. 2. Ir Ifrafann AAlil i 2 2 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. 3. • Network controls UE’s movement through handover. • Location of the UE is known to the network at granularity of a cell. Ir Ifrafann AAlil i • Network does not control UE’s movement. UE • Network only knows the location of the UE to the 3 3 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. 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 Ir Ifrafann AAlil i 4 4 Data/Signal activity Yes No Connected Idle Connected -> Idle Inacitivity Timer Connected -> Idle Inacitivity Timer Time Time
  5. 5. UE’s Activity States for AS and NAS EMM State Time Ir Ifrafann AAlil i 5 5 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. 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. Ir Ifrafann AAlil i 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. 6 6
  7. 7. Ir Ifrafann AAlil i 7 7 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. 8. State in Network for Connected and Idle mode MME Ir Ifrafann AAlil i MME 8 8 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. 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 Ir Ifrafann AAlil i Release Access Bearer Req. (IMSI, TEIDs, ) GTPC Tunnel GTPC-1 Tunnel 9 9 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. 10. Irfan Ali Packet arrives at Serving GW for idle UE: Where to page the UE? 10
  11. 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 Ir Ifrafann AAlil i 11 11 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. 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; Ir Ifrafann AAlil i however larger the number of eNBs that the UE will need to be paged in. Perimeter-crossings where UE-1 performs TAU 12 12 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. 13. Ir Ifrafann AAlil i 3 digits MCC: Mobile Country Code 13 13 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. 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. 14
  15. 15. Frequency Ir Ifrafann AAlil i 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} 15 15 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. 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 Ir Ifrafann AAlil i 16 16 (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. 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 Ir Ifrafann AAlil i 17 17 • To receive pages from the network. ON Duration DRX Cycle UE Montiors PDCCH DRX Sleep
  18. 18. Formula to determine which radio frame number (SFN) and which subframe within the SFN for UE to monitor for page message Ir Ifrafann AAlil i 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 18 18 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. 19. Transition from Idle to Active: Network Triggered – Part 1 of 1 UE eNB HSS Ir Ifrafann AAlil i MME Internet GTPC Tunnel GTPC-1 Tunnel Downlink Data Notification 19 19 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. 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 Ir Ifrafann AAlil i MME Internet 20 20 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. 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 Ir Ifrafann AAlil i MME Internet Modify Bearer Req. (IMSI, eNB TEIDs…) S1-MME GTPC Tunnel 21 21 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. 22. Tracking Area Update, Inter-MME – Part 1 of 3 UE eNB HSS RACH Random Access Preamble Ir Ifrafann AAlil i MME-1 MME-2 Internet SGW MME-1 MME-2 22 22 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. 23. Tracking Area Update, Inter-MME – Part 2 of 3 UE eNB HSS Ir Ifrafann AAlil i MME-1 MME-2 Internet NAS MSG Cancel Location Request (IMSI,..) Cancel Location Resp (IMSI,..) 23 23 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. 24. Tracking Area Update, Inter-MME – Part 3 of 3 UE eNB HSS Ir Ifrafann AAlil i 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. 25. Irfan Ali Idle mode procedures in network: Selecting an MME and finding context of UE in MME 25
  26. 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. Ir Ifrafann AAlil i MME-1 MME-2 26 26 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. 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 Ir Ifrafann AAlil i • 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. 27 27 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. 28. MME Pool-1 MME Pool-2 MME MME Ir Ifrafann AAlil i Pool Area-2 28 28 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. 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 Ir Ifrafann AAlil i 29 29 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. 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. 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 Ir Ifrafann AAlil i 31 31 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.
  32. 32. Irfan Ali Cell-Selection and Cell-Reselection in Idle-mode: Which cell should UE “camp” on? 32
  33. 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 Ir Ifrafann AAlil i 33 33 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. 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. Ir Ifrafann AAlil i 34 34 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. 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 Ir Ifrafann AAlil i channel around the location of the reference symbol 35 35 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. 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 Ir Ifrafann AAlil i • 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. 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. 38. Irfan Ali Power savings in active state: DRX in connected mode in LTE 38
  39. 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. 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. 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. 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. 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
  44. 44. Irfan Ali Mobility Management in LTE 44 Irfan Ali
  45. 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
  46. 46. Irfan Ali Handovers, or Mobility Management in Connected Mode 46
  47. 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. 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. 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. 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.
  51. 51. Irfan Ali X2 HO with S-GW relocation 51
  52. 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. 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. 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. 55. Irfan Ali S1 HO with MME change and no SGW relocation 55
  56. 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. 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. 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. 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.

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