LTE Hand Over

When the source node detects that a handover is required, it connects with the target eNB to commence the switching process. Once the tunnels have been moved across to the target eNB, the UE performs a handover and connects to the target node. A path switch request is made from the target eNB.

Description and Definition

  1. A data call is established between the UE, S-eNB (Source-eNB) and the network elements. Data packets are transferred to/from the UE to/from the network in both directions (Downlink as well as Uplink).

  2. The network sends the MEASUREMENT CONTROL REQ message to the UE to set the parameters to measure and set thresholds for those parameters. Its purpose is to instruct the UE to send a measurement report to the network as soon as it detects the thresholds.

  3. The UE sends the MEASUREMENT REPORT to the Serving eNB, which contains the RQRS from all the nearby eNBs. The Serving eNB makes the decision to hand off the UE to a T-eNB (Target-eNB) using the handover algorithm mentioned in the Introduction.

  4. The S-eNB issues a HANDOVER REQUEST message to the T-eNB passing necessary information to prepare the handover at the target side.

  5. The T-eNB sends back the HANDOVER REQUEST ACKNOWLEDGE message including a transparent container to be sent to the UE as an RRC message to perform the handover.

  6. The S-eNB generates the RRC (Radio resource control used for signaling transfer) message to perform the handover, i.e., RRC CONNECTION RECONFIGURATION message including the mobility Control Information.

  7. The S-eNB starts forwarding the downlink data packets to the T-eNB for all the data bearers which are being established in the T-eNB during the HANDOVER REQ message processing.

  8. The T-eNB now requests the S-eNB to release the resources. With this, the handover procedure is complete.

Analysis/Algorithm

NetSim handover algorithm utilizes the Reference Signal Received Quality (RSRQ) measurements, to trigger the handover. When the target eNB’s RSRQ crosses the serving eNB’s RSRQ by a factor know as margin of handover (equal to 3dB), hand over is triggered.

Open NetSim and Select Examples > LTE and LTE-A > LTE Handover then click on the tile in the middle panel to load the example as shown in below screenshot Figure 4‑5.

Figure 4‑5: List of scenarios for the example of LTE Handover

The following network diagram illustrates what the NetSim UI displays when you open the example configuration file as shown Figure 4‑6.

Chart, line chart Description automatically generated

Figure 4‑6: Network set up for studying the LTE-Handover

Network Settings

The following set of procedures were done to generate this sample:

Step 1: Environment Grid length: 5000m x 5000m.

Step 2: A network scenario is designed in NetSim GUI comprising of 2 ENBs, 1 EPC, and 2UEs in the “LTE/LTE-A” Network Library.

Step 3: The device positions are set as per the table given below.

ENB 2 ENB 3 UE 4 UE 5
X Co-ordinate 1000 4000 1000 4000
Y Co-ordinate 1500 1500 3000 3000

Table 4‑3: Device Position

Step 4: In the General Properties of UE 4 and UE 5, set Mobility Model as File Based Mobility.

Step 5: Right click on the eNB 2 and select Properties, the following is set Table.

Interface(LTE) Properties
CA_TYPE INTER_BAND_CA
CA_Configuration DL_2A-48A_UL_2A-48A_BCSO
CA_Count 2
Numerology 0
Channel Bandwidth (MHz) 5
PRB Count 25
MCS Table QAM64
CQI Table TABLE1
X_Overhead XOHO
DL UL Ratio 1:1
Outdoor Scenario URBAN_MACRO
LOS_NLOS_Selection USER_DEFINED
LOS Probability 1
Shadow Fading Model None
Fading and Beamforming NO_FADING
O2I and Building Penetration model NONE

Table 4‑4: eNB > Interface (LTE) Properties Setting

Similarly, it is set for eNB 3.

Step 6: Right click on the Application Flow App1 CBR and select Properties or click on the Application icon present in the top ribbon/toolbar.

A CBR Application is generated from UE 4 i.e., Source to UE 5 i.e., Destination with Packet Size remaining 1460Bytes and Inter Arrival Time remaining 20000µs. QOS is set to BE. Additionally, the “Start Time(s)” parameter is set to 15s, while configuring the application.

File Based Mobility

In File Based Mobility, users can write their own custom mobility models and define the movement of the mobile users. Create a mobility.txt file for UE’s involved in mobility with each step equal to 0.5 sec with distance 50 m.

The NetSim Mobility File (mobility.txt) format is as follows:

#Initial position of the UE 4

$node_(3) set X_ 1000.0

$node_(3) set Y_ 3000.0

$node_(3) set Z_ 0.0

#Initial position of the UE 5

$node_(4) set X_ 4000.0

$node_(4) set Y_ 3000.0

$node_(4) set Z_ 0.0

#Positions of the UE 4 at specific time

$time 0.0 "$node_(3) 1000.0 3000.0 0.0"

$time 0.5 "$node_(3) 1050.0 3000.0 0.0"

$time 1.0 "$node_(3) 1100.0 3000.0 0.0"

$time 1.5 "$node_(3) 1150.0 3000.0 0.0"

$time 2.0 "$node_(3) 1200.0 3000.0 0.0"

$time 2.5 "$node_(3) 1250.0 3000.0 0.0"

$time 3.0 "$node_(3) 1300.0 3000.0 0.0"

$time 3.5 "$node_(3) 1350.0 3000.0 0.0"

$time 4.0 "$node_(3) 1400.0 3000.0 0.0"

$time 4.5 "$node_(3) 1450.0 3000.0 0.0"

$time 5.0 "$node_(3) 1500.0 3000.0 0.0"

$time 5.5 "$node_(3) 1550.0 3000.0 0.0"

$time 6.0 "$node_(3) 1600.0 3000.0 0.0"

$time 6.5 "$node_(3) 1650.0 3000.0 0.0"

$time 7.0 "$node_(3) 1700.0 3000.0 0.0"

$time 7.5 "$node_(3) 1750.0 3000.0 0.0"

$time 8.0 "$node_(3) 1800.0 3000.0 0.0"

$time 8.5 "$node_(3) 1850.0 3000.0 0.0"

$time 9.0 "$node_(3) 1900.0 3000.0 0.0"

$time 9.5 "$node_(3) 1950.0 3000.0 0.0"

$time 10.0 "$node_(3) 2000.0 3000.0 0.0"

$time 10.5 "$node_(3) 2050.0 3000.0 0.0"

$time 11.0 "$node_(3) 2100.0 3000.0 0.0"

$time 11.5 "$node_(3) 2150.0 3000.0 0.0"

$time 12.0 "$node_(3) 2200.0 3000.0 0.0"

$time 12.5 "$node_(3) 2250.0 3000.0 0.0"

$time 13.0 "$node_(3) 2300.0 3000.0 0.0"

$time 13.5 "$node_(3) 2350.0 3000.0 0.0"

$time 14.0 "$node_(3) 2400.0 3000.0 0.0"

$time 14.5 "$node_(3) 2450.0 3000.0 0.0"

$time 15.0 "$node_(3) 2500.0 3000.0 0.0"

$time 15.5 "$node_(3) 2550.0 3000.0 0.0"

$time 16.0 "$node_(3) 2600.0 3000.0 0.0"

$time 16.5 "$node_(3) 2650.0 3000.0 0.0"

$time 17.0 "$node_(3) 2700.0 3000.0 0.0"

$time 17.5 "$node_(3) 2750.0 3000.0 0.0"

$time 18.0 "$node_(3) 2800.0 3000.0 0.0"

$time 18.5 "$node_(3) 2850.0 3000.0 0.0"

$time 19.0 "$node_(3) 2900.0 3000.0 0.0"

$time 19.5 "$node_(3) 2950.0 3000.0 0.0"

$time 20.0 "$node_(3) 3000.0 3000.0 0.0"

$time 20.5 "$node_(3) 3050.0 3000.0 0.0"

$time 21.0 "$node_(3) 3100.0 3000.0 0.0"

Step 7: Packet Trace is enabled in NetSim GUI. At the end of the simulation, a large .csv file is containing all the packet information is available for the users to perform packet level analysis. Plots is enabled in NetSim GUI.

Step 8: The log file can enable per the information provided in Section 3.18 5G-NR technology library document.

Step 9: Run the Simulation for 50 Seconds.

Results and Discussion

Handover Signaling

Figure 4‑7: Control packet flow in the LTE handover process

Note:

  • Handover Request and Handover Request Ack will be sent from the > serving eNB to Target eNB through MME.

  • Context Release and Context Release Ack will be sent from the > serving eNB and to Target eNB through MME.

The packet flow depicted above can be observed from the packet trace.

  1. UE will send the UE_MEASUREMENT_REPORT every 120ms to the connected eNB

  2. The initial UE- eNB connection, eNB will send the RRC_MIB packets to the UE every 40 ms and RRC_SIB1 every 80 ms.

  3. After the transmission of the RRC_MIB and RRC_SIB1 packets, the eNB will send RRC_SI packet to the UE.

  4. After reception of RRC_SI packet, UE will send RRC_Setup_Request to the eNB.

  5. On receiving the RRC_Setup_Request packet, the eNB will acknowledge the request by transmitting RRC_Setup packet to the UE.

  6. The UE will send back the RRC_Setup_Complete packet on the receipt of RRC_Setup message.

  7. As Per the configured file-based mobility, UE 4 moves towards eNB 3.

  8. After 18.00s eNB 2 sends the HANDOVER REQUEST to eNB 3.

  9. eNB 3 sends back HANDOVER REQUEST ACK to eNB 2.

  10. After receiving HANDOVER REQUEST ACK from eNB 3, eNB 2 sends the HANDOVER COMMAND to UE 4

  11. After the HANDOVER COMMAND packet is transferred to the UE, the target eNB will send the PATH SWITCH packet to the EPC_1.

  12. When the EPC_1 receives the PATH SWITCH packet, it sends PATH_SWICTH_ACK packet to the eNB 3.

  13. The target eNB sends CONTEXT RELEASE to source eNB, and the source eNB sends back CONTEXT RELEASE ACK to target eNB. The context release request and ack packets are sent between the source and target eNB via EPC 1.

  14. RRC Reconfiguration will take place between target eNB and UE 4.

Table Description automatically generated

Figure 4‑8: NetSim packet trace file showing the control packets involved in handover

  1. The UE 4 will start sending the UE MEASUREMENT REPORT to eNB 3

Plot of SNR vs.Time

Figure 4‑9: Plot of DL SNR (at UE_4 from eNB2 and eNB3) vs time

This plot can be got from the LTENRLog file. However, it would involve a fair amount of time and effort. Users can analyze the log file and see.

  • Time 15s when the SNR from eNB2 is 13.65dB and the SNR from eNB3 is > 13.65dB. This represents the point where the two curves intersect.

  • Time 18s when the SNR from eNB 2 is 11.93dB and the SNR from eNB 3 > is 15.38dB. This represents the point where Adj cell RSRP is > greater than serving cell RSRP by Hand-over margin (HOM) of 3dB.

Note: SNR value is available in LTENRLog file But Academic version does not support for code.