Handoff and its Types
Handoff is known to be an important concept of cellular mobile communication which is associated with mobility of mobile users during communication using the phone. Handoff is defined as the process of varying the channel characteristics (like frequency, time, code or combination of them) during a phone call without disturbing the connection.
It is also known as Handover and is generally abbreviated as HO.
Basics of Handoff
In wireless cellular communication system, mobility is regarded as one of the crucial features. The continuity in services can only be obtained when handoff from one cell to another is supported.
In simplest terms, handoff is known to be the process through which ongoing call or connectivity can be shifted from one base station to another. We are aware of the fact that there are multiple base stations but single mobile switching centre and therefore, when a mobile phone moves at the time of communication then the mobile switching centre i.e., MSC shifts the call from one channel to another channel. At the time of this transferring, the call connectivity must be maintained at the best level as much as possible.
Let us understand the same by the pictorial representation shown below:
As it is clearly shown above that there is a user i.e., mobile user P and we have two base stations namely BS 1 and BS 2. When the mobile user moves P moves from BS 1 to BS 2 then the signal strength relative to connectivity provided by BS1 becomes weak while that provided by BS2 becomes strong this helps in maintaining the data connectivity of the ongoing calls without deteriorating the quality of conversation.
Concept of Frequency Reuse
Have a look at the pictorial representation shown below:
It is known to all of us that one of the biggest advantageous feature of mobile phones is that these provides the facility of moving around while communicating using it. This is due to the fact that even when there is mobility associated with the user, there is continuous access to the network. When the motion is within the same cell then no network issue arises however, handoff becomes necessary when the mobile user moves from one cell to another.
Thus, it can be said that handoff helps in transferring an active call from one cell to a different cell when the user moves in between the two and all this takes place while maintaining the integrity of the call that is taking place.
Here the mobile switching centre i.e., the MSC plays the crucial role as it is responsible for automatically transferring the call to a different base station BS when required. When handoff takes place then identification of new base station and allocation of voice as well as control signals altogether takes place.
Let us understand the stepwise procedure of how handoff takes place.
- BS1 is transmitting and receiving signals via mobile unit.
- Now, consider that the mobile user moves from BS1 to BS2.
- The base station 1 now experiencing that the signal is getting weak from this specific mobile unit.
- While the base station 2 is noticing that there is an improved signal strength with regards to that mobile unit.
- Handoff immediately occurs when the user’s signal from one mobile station deteriorates while improves at another.
In this way, communication from BS1 is shifted to BS2 for the sake of user’s communication.
Types of Handoff
The two main categories in which handoff is categorized are:
- Hard Handoff (HHO)
- Soft Handoff (SHO)
Let us now understand each one in detail