Explain the hand-off mechanism

3. Explain the hand-off mechanism.

Hand-off Mechanism: Hand-off is the process of automatically changing the frequencies. When the mobile unit moves out of the coverage areas of a particular cell site, the reception becomes weak. At this instant the present cell site requests Hand-off, then system switches the call to a new frequency channel in a new cell site without interrupting either call or user. This phenomenon is known as “hand -off’ or ‘handover’. Hand -off processing scheme is an important task for any successful mobile system. This concept can he applied to one dimensional as well as two dimensional cellular configurations.

By the reception of weak signals from the mobile unit by the cell site, the Hand-off is required in the following two situations. They are

1. The level for requesting a Hand-off in a noise limited environment is at the cell boundary say-l00 dBm.

2. In a particular cell site, when the mobile unit is reaching the signal strength holes (gaps). Figure 4 shows the usage of frequency F1 in two co channel cells which arc separated by a distance D. Now, we have to provide a communication system in the whole area by filling other frequency channels F2,F3 and F4 between two co-channel cells.

cochannel interference reduction ratio

Depending on the same value of q the cells C2 ,C3 and C4 to which the above fill-in frequencies F2, F3 and F4 are assigned respectively as shown in figure.

Initially a mobile unit is starting a call in cell with fill-in frequency F1 and then moves to a cell with fillin frequency F2. The mobile unit moves from cell C1 to cell C2, meanwhile however the call being dropped and reinitiated in the frequency channel from F1 to F2. This process of changing frequencies can be done automatically by the system without the user’s intervention. In the cellular system the above mentioned Hand-off process is used.