When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. This process of transferring the channels between two cells is called handoff.
This handoff operation not only involves identifying a new base station,but also requires that the voice and control signals be allocated to channels associated with the new base station.
In order to meet these requirements,system designers must specify an optimum signal level at which to initiate a handoff.
Once a particular signal level is specified as the minimum usable signal for acceptable voice quality at the base station receiver, a slightly stronger signal level is used as a threshold at which handoff is made.
This margin given by-
Δ= Pr(handoff)-Pr(min usable)
cannot be too large or too small. If Δ is larger unnecessary handoffs which will burden the MSC may occur, and if Δ is too small, there may be insufficient time to complete a handoff before a call is lost due to weak signal conditions.
Factors on which handoff depends-
- Transmitted signal strength-If the signal strength is fluctuating very rapidly then number of handoffs increases.
- Vehicle Speed– The length of the time needed to decide if a handoff is necessary, depends on the speed at which vehicle is moving.If the slope of the short term average received signal level in a given time interval is steep, the handoff should be made quickly.
Types of handoff–
- In deciding when to handoff, it is important to ensure that the drop in the measured signal level is not due to momentary fading and the mobile is actually moving away from the serving BTS.
- But when there is a drop in the measured signal level due to momentary fading and not due to the actual moving mobile away from the BTS, in that situation if the handoff of a call takes place then such handoff is known as forced handoff.
- In Mobile Assisted Handoff (MAHO) every mobile station measures the received power from surrounding BTS and continually reports the result of these measurements of signals to the serving BTS.
- A handoff is initiated when the power received from the BTS of a neighboring cell begins to exceed the power received from the current BTS by a certain period of time.
- The MAHO method enables the call to be handed over between the BTS at a much faster rate than in first generation analog systems since handoff measurements are made by each mobile and the MSC no longer constantly monitors signal strength.
- The soft handoff is applied to one kind of digital cellular system named CDMA. In CDMA systems,all cells can use the same radio carrier. Therefore, the frequency reuse factor K approaches to one.
- Since the operating radio carriers of all cells are the same, no need to change from one frequency to another frequency but change from one code to another. Thus there is no hard handoff. We call this kind of handoff a soft handoff.
- If sometimes there are more than one CDMA radio carrier operating in a cell,and if the soft handoff from one cell to another is not possible for some reason,the intra-cell hard handoff may take place first,then go to the inter-cell soft handoff.