Реферат: Что является CDMA (Разделение Кодекса Многократный Доступ) (?)

spectrum for each channel. Narrowband AMPS (NAMPS) requires only 10 kHz per channel. TACS channels are 25 kHz wide.

With FDMA, only one subscriber at a time is assigned to a channel. No other conversations can access this channel until the

subscriber's call is finished, or until that original call is handed off to a different channel by the system. A common multiple access

method employed in new digital cellular systems is the Time Division Multiple Access (TDMA). TDMA digital standards include

North American Digital Cellular (know by its standard number IS-54), Global System for Mobile Communications (GSM), and

Personal Digital Cellular (PDC). TDMA systems commonly start with a slice of spectrum, referred to as one "carrier". Each

carrier is then divided into time slots. Only one subscriber at a time is assigned to each time slot, or channel. No other

conversations can access this channel until the subscriber's call is finished, or until that original call is handed off to a different

channel by the system. For example, IS-54 systems, designed to coexist with AMPS systems, divide 30 kHz of spectrum into

three channels. PDC divides 25 kHz slices of spectrum into three channels. GSM systems create 8 time-division channels in 200

kHz wide carriers.

The CDMA Cellular Standard

With CDMA, unique digital codes, rather than separate RF frequencies or channels, are used to differentiate subscribers. The

codes are shared by both the mobile station (cellular phone) and the base station, and are called "pseudo-Random Code

Sequences." All users share the same range of radio spectrum. For cellular telephony, CDMA is a digital multiple access

technique specified by the Telecommunications Industry Association (TIA) as "IS-95." In March 1992, the TIA established the

TR-45.5 subcommittee with the charter of developing a spread-spectrum digital cellular standard. In July of 1993, the TIA gave

its approval of the CDMA IS-95 standard. IS-95 systems divide the radio spectrum into carriers which are 1,250 kHz (1.25

MHz) wide. One of the unique aspects of CDMA is that while there are certainly limits to the number of phone calls that can be

handled by a carrier, this is not a fixed number. Rather, the capacity of the system will be dependent on a number of different

factors. This will be discussed in later sections.

CDMA Technology

Though CDMA's application in cellular telephony is relatively new, it is not a new technology. CDMA has been used in many

military applications, such as anti-jamming (because of the spread signal, it is difficult to jam or interfere with a CDMA signal),

ranging (measuring the distance of the transmission to know when it will be received), and secure communications (the spread

spectrum signal is very hard to detect).

Spread Spectrum

CDMA is a "spread spectrum" technology, which means that it spreads the information contained in a particular signal of interest

over a much greater bandwidth than the original signal. A CDMA call starts with a standard rate of 9600 bits per second (9.6