LTE Handover Concept

Types of Handover in LTE

Categorization of Handover (EPC Entities are Changed or not) in three kinds

  • Intra-LTE Handover
  • Inter-LTE Handover
  • Inter-RAT

Categorization of Handover (EPC Entities are Involved or not) in two types

  • X2 Handover
  • S1 Handover

Basic Concept of X2 and S1 Handover Selection

  • S1 handover is when the X2 procedure fails (due to unreachability/Error response etc.)
  • The MME and the SGW are involved during the handover procedure.

X2 and S1 Handover Selection

step 1
step 2
step 3
step 4

X2 Handover Call Flow

X2 LTE Handover Call Flow Description

  1. UE is in connected state.
  2. Source eNodeB send MEASUREMENT CONTROL REQ to the UE to send a measurement report to eNodeB.
  3. The Source eNodeB Based on the received RESOURCE STATUS RESPONSE,can procced for handover procedure using the X2 interface.
  4. The Source eNodeB sends the handover request to target eNodeB.
  5. The Target eNodeB further response source eNodeB with Handover Ack.
  6. The Source-eNodeB sents the RRC message (RRCCONNECTION RECONFIGURATION) to perform the handover.
  7. The Source-eNodeB sends the Target eNodeB a STATUS TRANSFER message to convey the PDCP and HFN status of the E-RABs.
  8. The Source-eNodeB starts forwarding the downlink data packets to the Target-eNodeB.
  9. After RACH If it succeeds in accessing the target cell, it sends the RRC CONNECTION RECONFIGURATION COMPLETE to the Target-eNodeB.
  10. The Target-eNB sends a PATH SWITCH REQUEST message to the MME.
  11. The MME sends a MODIFY BEARER REQUEST (eNodeB address and TEIDs for downlink user plane for the accepted EPS bearers) message to the SGW.
  12. After Path switch request ack from MME to Target NodeB, data can be used through downlink/uplink.
  13.  Target eNodeB will send UE Context Release to Source eNodeB.

S1 Handover Call Flow

There are 3 main things to focus for measurement related rrc reconfiguration message
A. Measurement object id
B. Report config id
C. Measurement id

Measurement id is the combination of

  • Measurement object id
  • Report config id

Measurement control

  • Its periodic or triggered by network
  • can be event based

Logs of Measurement object id, Report config id and Measurement id

measObjectId: 1
measObject: measObjectEUTRA (0)
measObjectEUTRA
carrierFreq: 2175
allowedMeasBandwidth: mbw50 (3)
…. 0… presenceAntennaPort1: False
neighCellConfig: No MBSFN subframes are present in all neighbour cells (1)
offsetFreq: dB0 (15)
Item 1
MeasObjectToAddMod
measObjectId: 2
measObject: measObjectEUTRA (0)
measObjectEUTRA
carrierFreq: 5790
allowedMeasBandwidth: mbw50 (3)
…. ..0. presenceAntennaPort1: False
neighCellConfig: No MBSFN subframes are present in all neighbour cells (1)
offsetFreq: dB0 (15)
reportConfigToAddModList: 1 item
Item 0
ReportConfigToAddMod

reportConfigId: 1
reportConfig: reportConfigEUTRA (0)
reportConfigEUTRA
triggerType: event (0)
event
eventId: eventA3 (2)
eventA3
a3-Offset: 0dB (0)
…. .0.. reportOnLeave: False
hysteresis: 1dB (2)
timeToTrigger: ms640 (11)
triggerQuantity: rsrp (0)
reportQuantity: both (1)
maxReportCells: 1
reportInterval: ms1024 (4)
reportAmount: r8 (0)

measIdToAddModList: 2 items
Item 0
MeasIdToAddMod
measId: 1
measObjectId: 1
reportConfigId: 1
Item 1
MeasIdToAddMod
measId: 2
measObjectId: 2
reportConfigId: 1
quantityConfig
quantityConfigEUTRA
measGapConfig: setup (1)
setup
gapOffset: gp0 (0)
gp0: 30

Information elements (I.E) of messages involved in Handover

X2AP Handover Request

  • Old eNB UE X2AP Id
  • Cause
  • Target Cell Id
  • UE Context Information
  • eRABs to be Setup List

X2AP Handover Request Acknowledge

  • Old eNB UE X2AP Id
  • New eNB UE X2AP Id
  • eRABs Admitted List

 X2AP SN Status Transfer

  • Old eNB UE X2AP Id
  • New eNB UE X2AP Id
  • eRABs Subject to Status Transfer (eRAB Id, For UL/DL -PDCP SN, HFN) 

These fields are needed for continuing ciphering and integrity protection after the handover

S1AP Path Switch Request

  • eNB UE S1AP ID
  • eRABs to be Switched DL List
  • TAI
  • UE Security Capabilities

Modify Bearer Request

  • Bearer Context to be Updated
  • TEID

Modify Bearer Response

  • Bearer Context Updated
  • TEID

S1AP Path Switch Acknowledge

  • MME UE S1AP ID
  • eNB UE S1AP ID
  • eRABs to be Switched UL List
  • Security Context

X2AP UE Context Release

  • Old eNB UE X2AP ID
  • New eNB UE X2AP ID

Measurement GAP

  • Measurement GAP is introduced to measure the different frequencies of neighbor cell and inter RAT from the current cell in that mentioned GAP, so when neighboring inter frequency Cell has better signal than serving Cell then UE will measure the RSRP in that GAP and informs to current cell in measurement report. During this gap ue will not listen or transmit from/to serving cell.
  • Ultimately the network makes the decision, but the gap provides the UE sufficient time to change frequency, make a measurement, and switch back to the active channel
  • There is two profile gp0 and gp1 ..
    Gp0 for fast moving traffic as it’s periodicity is 40 ms
    Gp1 for slow moving as it’s periodicity is 80 ms(like small cell application)


Categories: LTE,5G,TELECOMMUNICATION

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