Q.Explain call hand-off and frequency re-use used in cell phone system.

As a cell-phone user moves around and as spectrum is limited, two key techniques keep calls connected and let many users share the same frequencies.

[!ANSWER]

Call hand-off is the automatic switching of a call by the MTSO from one cell to another (from a weaker to a stronger cell) as the user moves, based on the signal strength monitored in each cell, so the call is not dropped. Frequency reuse is the sharing of the same set of channel frequencies by cells that are far enough apart (in different clusters) so they do not interfere, letting the limited spectrum serve many users.

[!TLDR]

Call hand-off automatically transfers a moving user's call from a weaker to a stronger cell; frequency reuse lets distant cells use the same channels without interfering.

Call hand-off: When a person carrying a cell phone passes through various cells, the receiver in each cell continuously monitors the signal strength of the mobile unit. The MTSO (Mobile Telephone Switching Office) computer compares these strengths and automatically switches the call from the cell where the signal is becoming weaker to the cell where it is stronger. This transfer is called call hand-off. It happens in a very short time and is unnoticeable to the user, so the conversation continues without interruption as the person moves.

Frequency reuse: The number of available radio channels is limited, so individual frequency bands are shared by many base stations and users. To do this without interference, cells are grouped into clusters, and each cluster is allotted the same number of cell-phone channels. Cells marked with the same letter use the same set of channel frequencies. Because cells using the same frequencies are placed far apart in different clusters, they do not interfere with each other, and so the same frequencies can be reused many times over the whole service area — greatly increasing the number of users that can be served.

[!ANSWER]

Call hand-off is the MTSO's automatic switching of a call from a weaker cell to a stronger cell (based on monitored signal strength) as the user moves, so the call is not dropped; frequency reuse is the sharing of the same set of channel frequencies by cells in different clusters that are far enough apart not to interfere, allowing the limited spectrum to serve many subscribers.