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FDMA (Frequency Division Multiple Access), TDMA
(Time Division Multiple Access) and CDMA (Code Division Multiple Access)
are the three basic multiple access schemes. FDMA divides radio channels
into a range of radio frequencies and is used in the traditional analog
cellular system. With FDMA, only one subscriber is assigned to a channel
at a time. Other conversations can access this channel only after the
subscriber's call has terminated or after the original call is handed off
to a different channel by the system. FDMA cellular standards include AMPS
(Advanced Mobile Phone Service) and TACS (Total Access Communications
System). TDMA is a common multiple access technique employed in digital
cellular systems. It divides conventional radio channels into time slots
to obtain higher capacity. Its standards include North American Digital
Cellular, Global System for GSM (Mobile Communications), and PDC (Personal
Digital Cellular). As with FDMA, no other conversations can access an
occupied TDMA channel until the channel is vacated. CDMA uses a radically
deferent approach. It assigns each subscriber a unique "code" to
put multiple users on the same wideband channel at the same time. The
codes, called "pseudo-random code sequences", are used by both
the mobile station and the base station to distinguish between
conversations. The IS-95 CDMA standard was adopted by the TIA
(Telecommunications Industry Association) and became a digital cellular
standard in 1992. The J-STD-008 standard for personal communications
services was also accepted by ANSI. CDMA is the first digital technology
which meets the exacting standards of the CTIA (Cellular
Telecommunications Industry Association). Depending on the level of
mobility of the system, it provides 10 to 20 times the capacity of AMPS,
and 4 to 7 times the capacity of TDMA. CDMA is the only one of the three
technologies that can efficiently utilize spectrum allocation and offer
service to many subscribers without requiring extensive frequency
planning. All CDMA users can share the same frequency channel because
their conversations are distinguished only by digital code, while TDMA
operators have to coordinate the allocation of channels in each cell in
order to avoid interfering with adjacent channels. The average transmitted
power required by CDMA is much lower than what is required by analog, FDMA
and TDMA technologies. |
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