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How can you continue in your call, while the person you are calling driven his or her car? Dr. Majidah H. Majeed Communication Department |
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The answer to this question is included three sides, if it is assumed that you are living in a city of a certain area First, the city is divided into cells, each cell is served by at least fixed-location transceiver known as site or base station. These cells cover different land area to provide radio coverage over wider area than that of one cell so the person’s mobile go from one cell to another that means the coverage area extend while he is driving his car and the base station will provide enough coverage area. The coverage area of the city and a single cell are demonstrated as follows:-
Second, By the aid of what is known the frequency reuse which is utilized in mobile communication, which means that the same band of radio frequencies will be presented and do not loose from the radio transmitter sites within a geographic area that separated by sufficient distance to permit minimal interference with each other. That means one can continuo his calling by providing the band of frequencies he used while his friend is driving from one cell to another. Since the cell phones and base stations use low-power transmitters, the same frequencies can be reused in non-adjacent cells Third, when a mobile unit moves from one cell to another it moves into a region with a completely new set of frequencies. Initially such activity resulted in the loss of the call. In order to avoid losing calls a procedure termed handover was introduced. The base station control unit generates a handover to another cell when it is required. In GSM, the mobile station continuously monitors the sensing power levels of serving cell and the neighboring cells. If the power levels of an adjacent cell is more than that of the current cell a handover is performed and allowing the mobile unit to communicate with the base station of the new cell with new set of frequencies. The handover can be between two sectors with the same base station, between two base station control units, between two mobile station control units belonging to the same operator or even between two networks. This is clarified is illustrated in Figure.
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