Applicable Versions
NetSim StandardNetSim Pro


In 5G networks, maintaining seamless connectivity as user equipment (UE) moves is critical. This connectivity is ensured through handover, where an ongoing call or data session is transferred from one base station (gNB) to another. Handover is essential for preventing call drops and interruptions in data transmission as the UE transitions.


In NetSim, a handover event is triggered if the condition
SNR(t-gNB) - SNR(s-gNB) > HOM
holds true for a duration equal to TTT i.e., when the SNR from the neighbor (target) gNB exceeds the SNR from the serving gNB by a value equal to the hand-over-margin and this condition holds true for a duration equal to the time-to-trigger. In NetSim by default the HOM is set to 3 dB.
Readers can refer to page 137 onwards in https://tetcos.com/downloads/v14.1/5G-NR.pdf. The exact handover algorithm, including handover margin and time-to-trigger, is explained in detail. A helpful figure is on page 143.


What is Ping Pong handover?

A ping pong handover occurs when a device repeatedly switches between two base stations. This frequent switching can reduce the efficiency of the network.


Network Scenario

1. Position two gNBs and a UE so that the UE is equally distant from both gNBs.

2. Set the UE to move along a line, always staying at an equal distance from the gNBs, using file-based mobility.


Fig: Logical diagram of the scenario


3. Enable shadowing in the network settings.

4. Due to shadowing, the SNRs from gNB1 and gNB2 will vary randomly as the UE moves.

5. These variations in SNRs will cause the UE to switch between the gNBs based on the observed signal strength.

6. Generate full buffer downlink (DL) traffic for the simulation. 


Fig: Scenario in NetSim


The properties have been configured as follows:


gNB Interface 5G RAN Properties

CA configuration

n78

Numerology

1

Channel Bandwidth

100 MHz

Tx Antenna Count

1

Rx Antenna Count

1

MCS Table

QAM256

CQI Table

TABLE2

PathLoss Model

Log Distance

PathLoss Exponent (η)

3

Reference Distance (D0)

1

Shadowing Model

Log Normal

Standard Deviation (dB)

5

UE properties

Tx Antenna Count

1

Rx Antenna Count

1


Results

The Handover log can be acquired from the logs of the NetSim Results Dashboard, where it tracks handover events, noting the timestamp, serving cell ID, UE ID, target cell ID, and Handover Trigger time (when the handover condition was met) with TTT enabled. These logs help understand handover attempts and the effect of TTT on the process.



The overall throughput achieved and the number of handovers as the time-to-trigger changes 


Time-to-Trigger
 (ms)

Throughput
 (Mbps)

No. of
 Handovers

0

106.8

19

100

108.7

8

200

94.2

14

300

87.9

2

400

91.5

2

500

71.3

0


Low TTT leads to more handovers and resources will be utilized for exchange of handover control packets. This leads to lower throughput since no user-level data is transmitted during the handover procedure. 

High TTT means UE would be attached to gNB with lower SNR for a longer duration. The UE would therefore obtain a lower MCS and hence this also leads to lower throughput.

Simulation/analysis is therefore required to arrive at the optimal TTT.


Attached herewith is an experiment file associated with this network configuration, which can be downloaded and imported in NetSim. 


Useful Resources


1. https://tetcos.com/simulating-5G-HetNets.html Impact of handover margin and time-to-trigger on a 5G heterogenous network