Handover Basics

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Handover Basics:


Before starting handover in detail let us take an overview of LTE handover. We all are aware that one of the greatest advantages of a mobile device or wireless device is that one can travel (user) while using the service on it.

Because of this feature many users has outvoted the use of wired devices.  This feature of mobility has allowed users to comfortably use their mobile devices at any place, anywhere, anytime, weather at home or on the go.

Mobile (cell phone) users can use services while on the go due to the fact that mobile networks provide handovers. So this feature (handover) helps UE (user equipment) to switch from one cell or base station (eNB- in the case of LTE) (gNB- in the Case of NR) to other cell or base station without losing any information and communicate with the network without interruption. Thus handover ensures that user is served seamlessly no matter which cell the user is connected to.

In this section of handover we will see what kinds of actions need to be taken by LTE UE (DUT) and LTE entities in order for handover to be conducted. Also briefly explain different procedures related to those actions. Following is the main procedures related to handover.




1) Procedure: Measurement Configuration (Using RRC reconfiguration message)
    Direction: eNB =è UE
    Description: Specifies measurements like periodic or event base to be perform by UE.
 
2) Procedure: Measurement report
    Direction: UE =è eNB
    Description: indicates measurement result of neighbor Cell (mainly PCI, RSRP, RSRQ)

3) Procedure: Handover Decision
    Related Entities: Source Cell (Source eNB) (Source gNB- in the case of NR)
  Description: Source cell makes decision or select target cells, which is previously UE report. Here Source cell also take a decision like what kind of Handover it will X2 or S1.

4) Procedure: Handover Preparation, Execution and Completion.  
    Related Entities: UE (DUT) ç è Source eNB çè Target eNB (Changes depending on a HO type)
    Description: Prepares Forwarding path, Forwards data, Switches data path.

We know that UE has an antenna that can locate multiple frequency channels over multiple bands. Thus after checking multiple neighbor cells, it accesses the one with highest received signal strength, unless no access is allowed due to access restrictions.

Then when the received signal strength from the current cell is becoming weak due to UEs travel or shadowing etc., and the same time signal from neighboring cell is becoming strong, a hand over is initiated.

This allows the UE to enter the neighboring cell, thus establishing a new RRC connection there. Also, to this end as soon as the UE establishes an RRC connection with eNB, the eNB informs the UE in which event the received signal strength should be reported.

This is done by sending a configuration message known as RCC connection Reconfiguration message. It is UE then who keeps track of the received signal strength of both serving and neighboring cells. After this when one of the events specified occurs, it informs the received signal strength to the eNB through a measurement report message.



After receiving the message the eNB decides whether to initiate handover or not by analyzing the reported strength information and the overload status of the neighbor cells. And after deciding it performs a handover to a newly selected target cell.
In next document we will learn more about Handover.…….. so stay tune and happy reading.

--Pinal Dobariya(your feedbacks are always welcome)






Reference:
1)3gpp TS 38.300
2)3gpp TS 38.331
3)https://www.netmanias.com/en/post/techdocs/6224/emm-handover-lte/emm-procedure-6-handover-without-tau-part-1-overview-of-lte-handover 

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