Handover

As handover or handover (English handover - delivery, transfer ) refers to a process in a mobile telecommunications network (eg GSM or UMTS), in which the mobile terminal ( mobile station) during a call or data connection without interruption of this connection from one radio cell changes to another. In U.S. parlance, the term handoff is more common.

General

During a call or data connection can arise the need to hand over the connection to another radio cell. The most common reason for this is that the participant moves out of the coverage area of the cell currently being used. But also the quality of the radio channel, the occupancy of the cell currently being used, the distance to the cell, or maintenance requirements can make a handover is required.

The terminal performs ongoing measurements of the signal strength and quality of the current cell and the field strength of the neighboring cells - in GSM, it measures the received signal strength of the Broadcast Control Channel (BCCH ). The measurement results are sent to the Base Station Controller (BSC ) ( in GSM every 480 ms). In BSC, the decision on the need for a handover is then made. Before the handover can be performed, a suitable channel to be allocated by the BSC, which is responsible for the target cell. Only if this is successful, the BSC to the mobile station may enter the handover command.

In most wireless systems, there is a wealth of system parameters that influence the onset and course of a handover. The appropriate adjustments differ according to the type of handover. In UMTS as can the quality of the radio connection (call quality) improve at the expense of network capacity by the mobile station is often in soft handover, is so often associated with multiple base stations. A simultaneous improvement of the quality of the radio connection and the network capacity can be achieved, for example, by optimizing parameters of the base station antenna.

A handover is only performed during a call or data connection. As long as the device holds no connection (device in "Standby " idle mode ), but meet the mobile station based on the parameters specified by the network independently deciding on a cell change.

Reasons for Handover

There are different trigger events for handover operations:

• By moving the participant comes into an area where a neighboring cell with better reception signal strength or quality is received than the current cell.
• The signal strength or quality is below a defined threshold and at the same time, the reception field strength of a cell ( a different system ) is above a defined threshold. A further parameter to the threshold value is a hysteresis that only selecting a closest base station, if it provides at least a better signal to the hysteresis value.
• The reception quality ( measured in terms of bit error or frame error rate ) is below a defined threshold (quality handover).
• For the sake of better traffic distribution (eg in case of overload a cell) causes the network handover.
• Discussions of fast moving participants should be moved from the small ( hot-spot ) cells into cells with large cell surface to reduce the signaling that goes along with frequent handovers, and to reduce the probability of dropped connections (velocity based handover ).
• Certain services (eg GPRS or HSCSD) should preferably be carried out in certain cells or time slots (service based handover ).

Types of handover

Distinction according to the network elements involved

Distinction by steuerndem network element

Distinction on the type of connection transfer

Swell

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