Handover (Handoff) is one of the basic and essential functions of the cellular system. Without it, the use of a mobile phone would be burdensome. It is hard to imagine a situation in which a voice call would be broken every time the user moves out of range of the currently serving base station.
Engineers developing the standards upon which the operation of the current cellular systems is based have introduced a mechanism called the handover. It allows transferring an active call, for example during movement of the mobile terminal, to another base station whose signal is stronger. The most important is that handoffs should be imperceptible to the user.
Based on the example of a UMTS network (3G), different types of handovers should be distinguished (marked as HO) such as:
- Intra-system HO
- Hard Handover (HHO)
- Soft Handover (SHO)
- Softer Handover
- Inter-system HO
- Hard Handover (HHO)
Inter-system HO is a transfer between two different access technologies, eg. UMTS and GSM, LTE and GSM or UMTS and LTE. Changeover between two different operating modes, for example 3G, falls also within this category: UTRA FDD and TDD. However, in Poland 3G is used only for FDD. Operators have been given frequency band for TDD transmission but the band is not currently in use.
Hard Handover (HHO) is present in all the above-described Inter-system HO type handovers. Intra-system HO occurs when switching between a base station/sector during simultaneous change of frequency. It is characterized by the fact that the connection between the first base station/sector and terminal is broken and only in the next step the new connection with the second base station/sector is established.
Soft Handover (SHO) uses, in contrast to the Hard Handover, an intermediate state comprising of maintaining the radio connection of the mobile terminal with two base stations simultaneously. In order for SHO to occur, the base stations connecting to the mobile terminal must operate on the same frequency.
This connection at the border of two cells, where signals of both transmitters have a relatively low level, allows obtaining, using RAKE receiver, improved signal to noise ratio, and thus a better quality of service. However, this solution has its drawbacks – one terminal during the handover uses resources of both cells, which reduces the capacity of the system.
Softer handover is a special case of Soft Handover. It is a handover between sectors belonging to one Node B base station. The handover takes place at the intermediate phase of reception of signal from two sectors at the same time and is the result of signal interference from neighboring sectors or multipath propagation, i.e. signal reflections from other objects, for example, buildings.
The decision to perform a handover is made by the network on the basis of measurements made by base stations and terminals. The difference between the threshold of the signal received, at which handover is initiated, and the minimum usable signal level, should be adjusted optimally. If the difference if too big, there will be many handovers, for example on the border between two cells. If the difference is too little, there will be many broken connections. This value depends on the propagation environment, the expected speed of the terminal and the time required for handover.
In summary, the operation of the network is aimed at the increase in mobile phone use comfort. Possible disadvantages related to changes in cell or deteriorating signal level of the base station have been solved by the use of various types of handovers. In 3G technology, in contrast to other technologies, soft handover was introduced allowing maintaining a better quality of connections at the border of the cells.