Handover principle and concepts

Zahid Ghadialy

Last Updated: 07/02/2005


By definition, handover means transfer of user connection from one radio channel to other. This definition was formed before the advent of UMTS. When UMTS came this definition was no longer valid. In order not to confuse the jargon, the definition was kept as it is and new definitions were added. These new definitions called the soft and softer handover will be discussed later in this tutorial.

As most of us would be aware, the main purpose of handover is to maintain an ongoing call. This is necessary as the user might be moving (maybe with high speed) and it would be annoying if the call keeps dropping when the user changes to another cell/area. Also it is possible that the number of users in an area changes while the call for a user is ongoing and for the call to continue the network needs to change the frequency of an ongoing call. Finally the user might enter an area where the UMTS network coverage ends and the user might be handed over to a GSM/GPRS network.

It is very difficult to explain everything about handover in a tutorial like this but we will try our best. Interested users can refer to the documents specified in the references section. Also one thing that should be noted is that all these procedures come into effect only when the call is ongoing (RRC Connection is established).

SRNS Relocation, even though not strictly classified as handover will be explained in this tutorial.

Types of Handover(s)

Before we start discussing the handovers in detail we would like to list all of them for convenience of the reader

  1. Softer Handover
  2. Soft Handover
  3. Intra-frequency hard handover
  4. Inter-frequency hard handover
  5. SRNS Relocation
  6. Combined Hard handover and SRNS Relocation
  7. Inter-RAT hard handover

Softer Handover

Fig 1: Softer Handover

Strictly speaking softer handover is not really a handover. In this case the UE combines more than one radio link to improve the reception quality. On the other hand the Node B combines the data from more than one cell to obtain good quality data from the UE. [1] Specifies the maximum number of Radio Links that a UE can simultaneously support as 8. In practice this would be limited to 4 as it is very difficult to make the receiver with 8 fingers.

Generally speaking when RRC connection is established, it would always be established on one cell. The network initiates Intra-Frequency measurements to check if there are any other cells the UE can connect simultaneously to improve the quality of the data being transferred between the RNC and the UE. If a suitable cell is found then Active Set Update procedure is initiated. Using this Active Set Update message, the network adds or deletes more than one radio link to the UE. The only requirement is that from the start till the end of this Active Set Update procedure, one Radio Link should remain common.

Soft Handover

Fig 2: Soft Handover

Soft Handover is the same as softer handover but in this case the cells belong to more than one node B. In this case the combining is done in the RNC. It is possible to simultaneously have soft and softer handovers.

Fig 3: Soft Handover with Iur connection

A more complicated soft handover would include a cell that belongs to a Node B in different RNC. In this case an Iur connection is established with the drift RNC (RNC 2) and the data would be transferred to the Serving RNC (RNC 1) via Iur connection.

In a typical UMTS system, the UE is in soft/softer handover around 50% of the time. One of the very important requirements for the soft/softer handover is that the frames from different cells should be within 50ms of each other or this would not work.

The last thing one needs to remember is that the soft/softer handover is initiated from the RNC and the core network is not involved in this procedure.

Hard Handover

Hard handover occurs when the radio links for UE change and there are no radio links that are common before the procedure is initiated and after the procedure is completed. There are two types of hard handover. First is Intra-frequency hard handover and the second is Inter-frequency hard handover.

Intra-frequency hard handover will not occur for the FDD system. It would happen in TDD system. In this case the code spreading/scrambling code for UE will change but the frequency remains the same.

Inter-frequency hard handover generally occurs when hierarchical cells are present. In this case the frequency at which the UE is working changes. This happens when the current cell is congested, etc. Have a look at the Inter-Frequency Measurement primer for more information.

Hard handover procedure can be initiated by the network or by the UE. Generally it would be initiated by the network using one of the Radio Bearer Control messages. In case of UE initiated, it would happen if the UE performs a Cell Update procedure and that Cell Update reaches the RNC on a different frequency. The Core Network is not involved in this procedure.

SRNS Relocation

SRNS Relocation procedure is not strictly speaking a handover procedure but it can be used in combination with the handover procedure. A simple SRNS Relocation can be explained with the help of figures present in [9].

Fig 4: Data flow before SRNS Relocation procedure ([9], Fig 37)

Fig 5: Data flow after SRNS Relocation procedure ([9], Fig 38)

The UE is active on a cell that belongs to a different RNC (than the one on which call was initiated) and a different MSC/SGSN. This arrangement causes unnecessary signalling between two RNC's. Hence the relocation procedure is initiated.

In this, the CN negotiated the relocation procedure between the two RNS's. Once the procedure is completed the connection towards the old domain is released as shown in Fig. 5.

The relocation procedures will generally be used for UE in Packet Switched mode. This procedure is time consuming and is not really suitable for real time services.

Combined Hard handover and SRNS Relocation

Fig 6: Before Combined hard handover and SRNS Relocation Procedure([9], Fig. 40)

Fig 7: After Combined hard handover and SRNS Relocation Procedure([9], Fig. 41)

The combined procedure is a combination of hard handover and SRNS Relocation. Fig. 6 and 7 explain the procedure.

Inter-RAT hard handover

When UE reaches end of coverage area for UMTS services, it can handover to a 2G service like GSM (if the UE supports multiple RAT). Inter-RAT handover procedure can be initiated in variety of ways. RNS might send a Handover From UTRAN command explicitly telling the UE to move to a different RAT or the UE might select a cell that belongs to a different RAT or the Network may ask UE to perform Cell Change Order from UTRAN.

Inter-RAT hard handover using Handover from UTRAN command can be performed when there are no RAB's or when there is atleast one CS domain RAB. The state of the UE is CELL_DCH.

Inter-RAT hard handover using Cell change order from UTRAN can be performed when UE is either in CELL_DCH or CELL_FACH state. The only requirement is that there should be atleast a PS signalling connection and no CS signalling connection.


[1] 3GPP TS 25.331: RRC Protocol Specification

[2] 3GPP TS 25.304: UE Procedures in Idle Mode and Procedures for Cell Reselection in Connected mode

[3] 3GPP TS 22.129: Handover requirements between UTRAN and GERAN or other radio systems

[4] 3GPP TS 23.009: Handover Procedures

[5] 3GPP TS 25.413: UTRAN Iu interface RANAP Signalling

[6] 3GPP TS 25.401: UTRAN Overal Description

[7] 3GPP TR 25.832: Manifestations of Handover and SRNS Relocation

[8] 3GPP TR 25.936: Handovers for real time services from PS domain

[9] 3GPP TS 23.060: General Packet Radio Service (GPRS); Service description; Stage 2

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