Handoff in Cellular Communication

Handoff and its Types

Handoff is known to be an important concept of cellular mobile communication which is associated with the 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 systems, 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. 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:

call handoff process

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

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:

  1. Hard Handoff (HHO)
  2. Soft Handoff (SHO)

Let us now understand each one in detail.

Hard Handoff (HHO): It is based on ‘make before break’ approach.

hard handoff

The connection with the prior base station is terminated first and then the call is transferred to the base station of the new cell. This means that in hard handoff the mobile station is linked to only one base station at a time.

It generally shows suitability with FDMA and TDMA based systems as in these two approaches adjacent channels are allotted different frequency ranges so as to lessen the channel interference at the time of communication.
Due to this reason, when there is movement of the mobile unit from one base station to another then it becomes impossible to communicate with two base stations as both are using different frequencies.

Soft Handoff:  This technique is based on ‘make before break’ approach.soft handoff

This follows quite a different approach than hard handoff as here multiple connections can be made with neighboring cell sites in the region. Soft handoff is prominently used in CDMA systems where same frequency band is used by multiple cells but code words are different.

It is comparatively costly approach than the former one. This basically improves the chances of call reception as the mobile unit gets connected to multiple cells at the same time. Whenever the mobile station moves from one base station to a different base station then the call continues without interruption as the MS is forwarding from weaker connection to a stronger one.

Prioritization Schemes

Basically in mobile communication if we talk about calling then there can be two calls first one is new call while the other one is handoff call. When talking about non-prioritization scheme new and handoff calls are treated similarly. This means in case of idle channel, it does not matter whether the call is new or handoff, the channel assignment is done on first come first serve basis.

Forced termination of any active call is not at all good over blocking any new call. Due to this reason, prioritization scheme is necessary as this lowers the probability of forced termination. There are two methods of prioritization which are guard channel and queuing handoff calls.

If we talk about guard channels technique some channels are kept fixed for handoff calls only. By doing so, the handoff calls are treated in a better way and there this decreases the probability of forced termination. However, as every coin has two sides, its drawback is that the call blocking probability increases and total carried traffic decreases. The overall guard channels determination is decided by the neighboring base stations.

As against, the other scheme is queuing of handoff calls where the handoff calls are queued at the time of unavailable channels at the base station. Once any channel gets free then it is assigned to the queued handoff call. In case there is no queued handoff calls then the available channel is assigned to a new call. In order to decrease the call blocking probability, sometimes new calls are queued as well.

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