15.2 ANALOG COMMUNICATION SYSTEMS 709Cell
with base station
Telephone
central office MTSOFigure 15.2.31Cellular telephone concept in mobile radio system.
band trunk lines, which carry speech signals from many users. When the two parties hang up
upon completion of the telephone call, the radio channel then becomes available for another user.
During the telephone conversation, if the signal strength drops below a preset threshold, the
MTSO monitors and finds a neighboring cell that receives a stronger signal and automatically
switches (in a fraction of a second) the mobile user to the base station of the adjacent cell. If a
mobile user is outside of the assigned service area, the mobile telephone may be placed in a roam
mode, which allows the user to initiate and receive calls.
In analog transmission between the base station and the mobile user, the 3-kHz wide audio
signal is transmitted via FM using a channel bandwidth of 30 kHz. Such a large bandwidth
expansion (by a factor of 10) is needed to obtain a sufficiently large SNR at the output of
the FM demodulator. Since the use of FM is indeed wasteful of the radio frequency spectrum,
cellular telephone systems based on digital transmission of digitized compressed speech are later
developed. With the same available channel bandwidth, the system then accommodates a four-
to tenfold increase in the number of simultaneous users.
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).
Withanalog cellular,orAMPS(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,orD-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 eitherCDMA(Code Division Multiple Access) orTDMA(Time
Division Multiple Access) technology; but CDMA phones won’t work in TDMA areas, and vice
versa. Adual-modeunit 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, adual-banddigital
phone (which switches to the lower digital frequency) and atrimodephone (which works in