The 5G handover procedure implemented in NetSim is based on the Strongest Adjacent Cell Handover Algorithm (Ref: Handover within 3GPP LTE: Design Principles and Performance. Konstantinos Dimou. Ericcson Research). 

The algorithm enables each UE to connect to that gNB which provides the highest Reference Signal Received Power (RSRP). Therefore a handover occurs the moment a better gNB (the adjacent cell has offset stronger RSRP, measured as SNR in NetSim) is detected.


This algorithm is similar to 38.331, 5.5.4.4 Event A3 wherein Neighbor cell’s RSRP becomes Offset better than serving cell’s RSRP. Note that in NetSim report-type is periodical and not eventTrigerred since NetSim is a discrete event simulator and not a continuous-time simulator.


This algorithm is susceptible to ping-pong handovers; continuous handovers between the serving and adjacent cells on account of changes in RSRP due to mobility and shadow-fading. At one instant the adjacent cell's RSRP could be higher and at the very next it could be the original serving cell's RSRP, and so on.

To solve this problem the algorithm uses:

  1. Hysteresis (Hand-over-margin, HOM) which adds an RSRP threshold (Adjacent_cell_RSRP - Serving_cell_RSRP > Hand-over-margin or hysteresis), and
  2. Time-to-trigger (TTT) adds a time threshold

This HOM is part of NetSim implementation while TTT can be implemented as a custom project in NetSim. 


Network Scenario:


The RAN part consists of 50 UEs which are connected to a gNB and the distance between the gNB and all the UEs are 100m. The gNB is in turn connected to an EPC which is connected to the server, which consists of a Router and a Wired Node.

The UEs are subjected to move in the downward direction using File-Based Mobility so that the control is handed over from one gNB to the other. (The scenario has a UE connected to gNB 55, because NetSim requires every gNB to be connected to atleast one UE)

The network traffic is modeled in such a way that all the UEs can handle both Upload and Download simultaneously. The total upload traffic has a Generation Rate of 500 Mbps and the total download traffic has a Generation rate of 400 Mbps

(Experiment files attached- Refer: How to import NetSim Experiments and Configuration files?  to import the experiment file in NetSim)




Properties configured in the network scenario:


The following parameters were configured in the Interface LTE_NR of gNB:


Parameter

 Value

gNB Height

10 m

Tx Power

40 dBm

DL MIMO Layer Count

2

UL MIMO Layer Count

2

CA Type

Single Band

CA Configuration

n78

DL: UL

1:4

Numerology

1

Channel Bandwidth

100 MHz

MCS Table

QAM64

CQI Table

Table 1

Outdoor Scenario

Urban Macro

Channel Characteristics

Pathloss Only

LOS Mode

User-Defined

LOS Probability

0

Simulation Parameter
Packet Trace
Enabled
Plots
Enabled
Simulation Time25 s


The following values were set in the Application configuration window:


Download Application Properties

(App51_CBR to App100_CBR)

Source Id

2

Destination Id

UE_Id

Packet Size

1460 Bytes

IAT

1460 µs

Generation Rate

8 Mbps each

Start Time

1 s

Transport Protocol

 UDP


Upload Application Properties

(App1_CBR to App50_CBR)

Source Id

UE_Id

Destination Id

2

Packet Size

1460 Bytes

IAT

1168 µs

Generation Rate

10 Mbps each

Start Time

1 s

Transport Protocol

 UDP


Results and Observations:


The time at which the UEs were handed over from the Serving gNB to the Target gNB can be observed using the NetSim Packet Trace file.

The Packet Trace and Plots can be opened from the Results Dashboard window


In Packet Trace, filter Control Packet Type to Handover Request. The Handover Request is sent from gNB3 to gNB55 (for each UE) for 8 seconds. The time at which each UE is handed over to the second gNB is different due to the movement of the UEs and also the position of the UEs

Note: Excel imposes a limit of 1 million rows. Hence, users can either simulate a scenario with a lesser number of applications or use a DB/tool which supports > 1 million rows. NetSim saves the packet trace in .csv format. 

Refer to the following article Opening large Netsim packet event trace files and generating pivot tables plots to view csv file with rows > 1 million.


The control of the UEs is passed to the Target gNB when the SNR of the Target gNB exceeds the SNR of the Serving gNB by the Handover Margin(3 dB by default).


The drop in Application throughput during the Handover can be observed in the Application Throughput Plot. As the UEs moves away from the Serving gNB, there is a drop in the Application throughput. As the UEs move further towards the Target gNB, they get handed over to the Target gNB and the throughput starts increasing after this.


Throughput vs Time Plot for Upload Application:


Throughput vs Time Plot for Download Application:


The Handover Procedure can be further understood with the help of the SNR plot. The SNR vs Time plot shown below represents the variation in SNR with respect to time between UE_4 and the connected gNB.

As the UE moves away from the Serving gNB, there will be a decrease in the SNR between the serving gNB and the UE. The decrease in SNR results in a decrease in throughput. As the UE moves further away, it gets handed over to the target gNB after which there will be an increase in SNR as the UE moves towards the target gNB.

Related articles:

How can I plot SNR measured by UE?

How is the cell selection and UE attach procedure modeled in NetSim 5G ?

How can I get plots for SNR in NetSim?

Where can I find packet trace and event trace csv files after simulation?

How does one start understanding the packet trace ?