Advanced Mobile Phone System

AMPS (Advanced Mobile Phone System) was invented by Bell Labs and first installed in the United States in 1982. In all mobile phone systems, a geographic region divides up into cells, which is why the devices are sometimes called cell phones. 

In AMPS, cells are smaller and typically 10 to 20 km across digital systems. Each cell uses some set of frequencies, which are not used by any neighbors. The key idea that gives cellular systems far more capacity than previous systems is the use of relatively small cells and the reuse of transmission frequencies in nearby cells. Thus, the cellular design increases the system capacity by at least some order of magnitude. Smaller cells mean that less power is needed, which leads to smaller and cheaper transmitters and handsets. 

Working of AMPS :

• In an area where the number of users has grown to a point, where the system is overloaded, the power can be reduced and the overloaded cells split into small micro-cells to permit more frequency reuse. At the center of each cell, there is a base station to which all telephones in the cell transmit. 
• The base station consists of a computer and transmitter/receiver connected to the antenna. In a small system, all the base stations are connected to a single device called MSC (Mobile Switching Center) or MTSO(Mobile Telephone Switching Office). 
• The MSCs are essential for ending offices, as in telephonic systems, and are in fact connected to at least one telephone system's end office.
• At any instant mobile phone is logically in one specified cell and under the control of that cell's base station. The mobile phone is then informed of its new boss (when the user leaves the current cell) and if a call is in progress, it is asked to switch to a new channel.
• This process is called handoff, which takes about 300 msec. Channel assignment is done by the MSC, the nerve center of the system.  

Features :

• It is an analog system and is based upon initial electromagnetic spectrum allocation for cellular service by the federal communication commission.
• It uses FDMA (frequency division multiple) access for multiple simultaneous conversations.
• When the number of conversations is very high, it requires high bandwidth.
• It was the first to use hexagonal cells.
• Cells in AMPS are 10 km to 20 km across.
• Since it was analog technology, it suffers from noise and eavesdropping.