Mobile Telecommunication Networks
Principle of automatic reconnection

A mobile station communicates always with the nearest base station, more precisely with a station that provides the strongest signal for the mobile station. If the mobile station moves into a neighboring cell, it is automatically switched to the base station of the neighboring cell.

This operating mode is referred to as a handover.

The handover manages a change of a current serving station to a proper target station during user’s movement across the cells boundaries. The major purpose of handovers in mobile networks is to ensure continuous connection with high QoS or balance load in network.

Tracking of current position of mobile station in the network enables continuous automatic connection established between the mobile and base station. This indication is stored in the mobile network registers, which allows routing of connection to the called party directly into the area where the station is currently located.

On the basis of frequency planning communication channels with different frequencies in the adjacent cells are always used. When the mobile subscriber moves across the border lying between two different cells, it is always necessary to retune user’s mobile station.

Principle of handover

Basically, two types of handovers can be distinguished:

- Hard handover - if the hard handover is performed, a MS firstly closes all connections with the current serving station. As soon as the connections to the serving station are terminated, new connections with the target station are established. Therefore, this type of handover is also known as break-before-make since a short interruption in communication between the MS and the network is introduced. The duration of the handover interruption depends on the management message flow exchanged between the MS and the network. Thus the length of the interruption depends on several factors such as used wireless technology (e.g., LTE, or WiMAX), physical layer frame length, or network load. In general, the duration of the interruption varies from tens to hundreds of milliseconds in networks.
- Soft handover - enables simultaneous connection of a MS to several BSs. Consequently, no handover interruption is observed by users during communication. This handover is also known as make-before-break. The soft handover can be realized as a MDHO (Macro Diversity HandOver) or a FCS (Fast Cell Selection) also denoted as a FBSS (Fast Base Station Switching). In MDHO, the macro diversity combining of signals received from several BSs included in active set (in WiMAX denoted as diversity set) is performed. The significant drawback of this approach is high complexity and complicated implementation. In the case of FCS, the best frame simultaneously received from all stations included in active set is selected and processed. Even if the implementation is simpler comparing to MDHO, it is still essentially more complex than in the case of the hard handover. Therefore, the hard handover is considered as mandatory in mobile networks while other types of handovers are optional.