Handover

As a handover or handover (engl. Handover - delivery, handover) is called an operation in a mobile telecommunication network (e.g. GSM or UMTS ), in which the mobile terminal ( mobile station ) during a call or a data connection without interrupting the connection of a radio cell changes to another. In US parlance, the term handoff is more common.

General

Handover between two radio cells

During a call or a data connection, it may be necessary to transfer the connection to another radio cell . The most common reason for this is that the subscriber is moving out of the coverage area of the cell currently in use. However, the quality of the radio channel, the capacity utilization of the cell currently in use, the distance to the cell or maintenance work can make a handover necessary.

The end device continuously measures the signal strength and quality of the current cell as well as the field strength of the neighboring cells - with GSM it measures the received signal strength of the Broadcast Control Channel (BCCH). The measurement results are sent to the Base Station Controller (BSC) (every 480 ms with GSM). The decision on the need for a handover is then made in the BSC. Before the handover can be carried out, a suitable channel must be reserved by the BSC, which is responsible for the target cell. Only when this was successful can the BSC give the handover command to the mobile station-

In most mobile radio systems there is a wealth of system parameters that influence the start and course of a handover. The adjustments to be made differ according to the type of handover. In UMTS, for example, the quality of the radio connection ( call quality ) can be improved at the expense of network capacity by the mobile station more often in soft handover , i.e. more often connected to several base stations at the same time. A simultaneous improvement in the quality of the radio link and the network capacity can be achieved, for example, by optimizing the base station antenna parameters.

A handover generally only takes place during a call or a data connection. As long as the terminal does not have a connection (device in “standby”, idle mode ), the mobile station makes its own decision about changing cells based on the parameters specified by the network.

Reasons for handover

There are different so-called "triggers" for handover processes:

By moving, the subscriber comes into an area in which a neighboring cell is received with a better received signal strength or quality than that of the current cell.
The signal strength or quality falls below a defined threshold and at the same time the reception field strength of a cell (of another system) is above a defined threshold. Another parameter for the threshold value is a hysteresis, which the next base station only selects if it delivers at least one signal better than the hysteresis value .
The reception quality (measured using the bit error or frame error rate) falls below a defined threshold (quality handover).
For reasons of better traffic distribution (e.g. when a cell is overloaded), the network initiates a handover.
Conversations from fast-moving participants should be moved from small (hot-spot) cells to cells with a large cell area in order to reduce the signaling associated with frequent handovers and to reduce the likelihood of disconnections (velocity based handover).
Certain services (e.g. GPRS or HSCSD ) should preferably be handled in certain cells or time slots (service based handover).

Types of handovers

Differentiation according to the network elements involved

Intra-cell handover: There is a change to another frequency or another time slot in the same cell (GSM, GPRS).
Inter-Cell Handover (Intra-BSC Handover): There is a change to a neighboring cell that is connected to the same BSC.
Inter- BSC handover (Intra-MSC handover): During the handover, a change is made to a neighboring cell that is connected to a different BSC but to the same MSC .
Inter-MSC handover: During the handover, a change is made to a neighboring cell that is connected to another BSC, which in turn is connected to another MSC.
Inter- PLMN handover: During the handover, a cell in another cellular network is switched to.
Inter-system handover: A change is made to a cell that uses a different mobile radio technology (e.g. handover between GSM and UMTS).

Differentiation according to the controlling network element

Network Controlled Handover (NCHO): Both the channel measurement and the decision about a handover are made on the network side . This variant is (or was) used in analog systems.
Mobile Assisted Handover (MAHO): The channel measurement is performed on both sides of the link. The measurement results are transmitted from the mobile device to the network, which then makes a handover decision. This variant is used with GSM and UMTS.
Mobile Controlled Handover (MCHO): A channel measurement is only carried out on the end device. Based on the results, only the mobile device decides on a handover to be carried out. Use, for example, with DECT or wireless LANs according to IEEE 802.11 .

Differentiation according to the type of connection transition

Hard handover: The existing connection to the current cell is completely disconnected before the connection to the new cell is established. Since GSM neighboring cells use different radio frequencies, GSM handovers are always "hard handover" (mobile radio device must change the sending and receiving frequency during the "handover").
Soft handover: The connection to the new cell is established before the existing connection is disconnected. For a certain time there are two simultaneous connections to two cells. This method is used, for example, in UMTS (neighboring cells usually use the same radio frequencies here). If the cells between which the handover takes place belong to different NodeBs , one speaks of "soft handover", a handover between two cells of the same NodeB is called "soft handover".

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↑ 3GPP TS 23.009: Handover procedures, chap. 5: Handover initiation conditions ( English , ZIP / DOC; 2.4 MB) September 28, 2009. Accessed November 29, 2009.
↑ 3GPP TS 23.009: Handover procedures, chap. 6.1: Procedure for Intra-MSC Handovers ( English , ZIP / DOC; 2.4 MB) September 28, 2009. Accessed November 29, 2009.
↑ C. Brunner, A. Garavaglia, M. Mittal, M. Narang, and J. Vargas Bautista: Inter-System Handover Parameter Optimization. ( Memento from October 25, 2014 in the web archive archive.today ) In: Proceedings of IEEE Vehicular Technology Conf. (VTC case '06). Montreal, Canada, September 2006 (English)
^ C. Brunner, D. Flore: Generation of Pathloss and Interference Maps as SON Enabler in Deployed UMTS Networks. ( Memento from October 25, 2014 in the web archive archive.today ) In: Proceedings of IEEE Vehicular Technology Conf. (VTC Spring '09). Barcelona, Spain, April 2009 (English)

[3GPP23009-5-1] 3GPP TS 23.009: Handover procedures, chap. 5: Handover initiation conditions ( English , ZIP / DOC; 2.4 MB) September 28, 2009. Accessed November 29, 2009.

[3GPP23009-6-2] 3GPP TS 23.009: Handover procedures, chap. 6.1: Procedure for Intra-MSC Handovers ( English , ZIP / DOC; 2.4 MB) September 28, 2009. Accessed November 29, 2009.

[3] C. Brunner, A. Garavaglia, M. Mittal, M. Narang, and J. Vargas Bautista: Inter-System Handover Parameter Optimization. ( Memento from October 25, 2014 in the web archive archive.today ) In: Proceedings of IEEE Vehicular Technology Conf. (VTC case '06). Montreal, Canada, September 2006 (English)

[4] C. Brunner, D. Flore: Generation of Pathloss and Interference Maps as SON Enabler in Deployed UMTS Networks. ( Memento from October 25, 2014 in the web archive archive.today ) In: Proceedings of IEEE Vehicular Technology Conf. (VTC Spring '09). Barcelona, Spain, April 2009 (English)