The digital cellular mobile communication system divides the communication range into several cells with a certain distance apart. Mobile users can move from one cell to another, relying on terminal tracking of the base station to ensure uninterrupted communication. Mobile users can also roam from one city to another, and even make calls to their original registered user terminals in another country. The composition of a digital cellular mobile communication system mainly consists of three parts: a control switching center, several base stations, and many mobile terminals. By controlling the switching center to enter the public wired telephone network, communication between mobile phones and fixed phones, as well as between mobile phones and mobile phones, is achieved. The transmission direction from the base station to the mobile station is called the downlink (or forward link), and the transmission direction from the mobile station to the base station is called the uplink (or reverse link). At present, the second generation mobile communication system is widely used, which adopts narrowband time division multiple access (TDMA) and narrowband code division multiple access (CDMA) digital access technology. The national and regional standards that have been formed include the GSM system in Europe, the IS-54 and IS-95 systems in the United States, and the PDC system in Japan. China mainly adopts the European GSM system
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