Subtyping is a notion in programming language theory where a subtype, which is a data type, is related to a supertype based on the notion of substitutability, where program elements such as functions and subroutines that are written for the supertype will still operate if given the subtype instead. Subtypes are an essential concept in...
Spatial division multiple access (SDMA) is a channel access method used in mobile communication systems which reuses the same set of cell phone frequencies in a given service area. Two cells or two small regions can make use of the same set of frequencies if they are separated by an allowable distance (called the reuse distance).
SDMA increases the capacity of the system and transmission quality by focusing the signal into narrow transmission beams. Through the use of smart antennas with beams pointed at the direction of the mobile station, SDMA serves different users within the same region. Mobile stations operating outside the bounds of these directed beams experience a near zero interference from other mobile stations operating under the same base station with the same radio frequency. Since the beams are focused, the radio energy frequency can have increased base station range. This attribute of SDMA allows base stations to have larger radio coverage with less radiated energy. This narrow beam width also allows greater gain and clarity. Under traditional mobile phone network systems, the base station radiates radio signals in all directions within the cell without knowledge of the location of the mobile station. SDMA technology channels radio signals based on the location of the mobile station. Through this method, the SDMA architecture saves on valuable network resources and prevents redundant signal transmission in areas where mobile devices are currently inactive. The main advantage of SDMA is frequency reuse. Provided the reuse distance is preserved in the network architecture, interference can be near zero, even if mobile stations use the same allocated frequencies.
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