In first generation (1G) analog cellular systems, signal strength measurements are made by the base stations and supervised by the MSC. Each base station constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the base station tower. In addition to measuring the RSSI of calls in progress within the cell, a spare receiver in each base station, called the locator receiver, is used to scan and determine signal strengths of mobile users which are in neighboring cells. The locator receiver is controlled by the MSC and is used to monitor the signal strength of users in neighboring cells which appear to be in need of handoff and reports all RSSI values to the MSC. Based on the locator receiver signal strength information from each base station, the MSC decides if a handoff is necessary or not.
In second generation (2G) systems, handoff decisions are mobile assisted. In mobile assisted handoff (MAHO), every mobile station measures the received power from surrounding base stations and continually reports the results of these measurements to the serving base station. A handoff is initiated when the power received from the base station of a neighboring cell begins to exceed the power received from the current base station by a certain level or for a certain period of time.
In Third generation (3G) systems, the majority of handoff are intra-frequency soft handoffs.A soft handoff perform between two sectors belonging to different BS but not necessarily to the same BSC is class 2-way soft handoff.When a mobile is moving away from BS it continuously measures the pilot signal coming from all nearby BS and select the best BS for handoff.Thus the signal coming from pilot channel is used to make handoff decision.If the strength of pilot signal coming from old BS decreases then handoff decision is taken.
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