Handoff

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 is called handoff.

          Handoffs are broadly classified into two categories—hard and soft handoffs. 

Usually, the hard handoff can be further divided into two different types—intra- and intercell handoffs. 

The soft handoff can also be divided into two different types—multiway soft handoffs and softer handoffs.

          The 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.  Handoff depends on cell size, boundary length, signal strength, fading, reflection and refraction of signals, and man-made noise.

          Handoff can be initiated either by the BS or the MS, and it could be due to i) The radio link ii) Network management iii) Service issues.

          The cells must not interfere with each other. This is accomplished by giving each cell a slightly different chunk of the frequency spectrum and by measuring power levels. When the power level of the user begins to fade, the cell tower determines which cell is the closest cell. Upon finding this information, the current cell tower sends an over-the-air message to the new cell tower and to the cell phone. At this point, the new cell tower picks up the call and the old one drops the call as the cell phone switches frequencies. This type of handoff is called a "hard handoff" since the audio feed is lost for between 10 milliseconds and 100 milliseconds while the new tower picks up the signal. Often these "hard" handoffs fail when the new tower tries to pick the call up, leading to frequent dropped calls. 

  In most systems, each cell tower typically receives a 1.8 MHz frequency spectrum.

          Different systems have different polices and methods for managing handoff requests. Some systems handle handoff requests in the same way they handle originating calls. In such systems, the probability that a handoff request will not be served by a new base station is equal to the blocking probability of incoming calls. However, from the user’s point of view, having a call abruptly terminated while in the middle of a conversation is more annoying than being blocked occasionally on a new call attempt. To improve the quality of service as perceived by the users, various methods have been devised to prioritize handoff requests over call initiation requests when allocating voice channels.