Q. Advanced Mobile Phone System?

Cellular systems employed cells with a radius in the range of 5-18 km. The base station usually transmitted at a power level of 35 W or less, and the mobile users transmitted at a power level of about 3 W, so that signals did not propagate beyond immediately adjacent cells. By making the cells smaller and reducing the radiated power, frequency reuse, bandwidth efficiency, and the number of mobile users have been increased. With the advent of small and powerful integrated circuits (which consume very little power and are relatively inexpensive), the cellular radio concept has been extended to various types of personal communication services using low-power hand-held sets (radio transmitter and receivers).

With analog cellular,or AMPS (Advanced Mobile Phone System), calls are transmitted in sound waves at 800 MHz to 900 MHz. This was the first mobile phone technology available in early 1980s. Digital cellular,or D-AMPS (Digital AMPS), transmits calls in bits at the same frequency as analog cellular, with improved sound quality and security. To send numerous calls at once, D-AMPS phones use either CDMA (Code Division Multiple Access) or TDMA (Time Division Multiple Access) technology; but CDMA phones won't work in TDMA areas, and vice versa. A dual-mode unit can switch to analog transmission outside of the more limited digital network.

PCS (Personal Communications Service) phones transmit at 1800 MHz to 1900 MHz and are smaller and more energy efficient. To get around the limited coverage, a dual-band digital phone (which switches to the lower digital frequency) and a trimode phone (which works in AMPS, D-AMPS, or PCS areas) has been developed. The GSM (Global System for Messaging communications) is the most widely accepted transmission method for PCS phones.