What is CDMA (Code Division Multiple Access)?
One of the most important concepts to any cellular telephone system is that of "multiple access", meaning that multiple,
simultaneous users can be supported. In other words, a large number of users share a common pool of radio channels and any
user can gain access to any channel (each user is not always assigned to the same channel). A channel can be thought of as
merely a portion of the limited radio resource which is temporarily allocated for a specific purpose, such as someone's phone call.
A multiple access method is a definition of how the radio spectrum is divided into channels and how channels are allocated to the
many users of the system.
Current Cellular Standards
Different types of cellular systems employ various methods of multiple access. The traditional analog cellular systems, such as
those based on the Advanced Mobile Phone Service (AMPS) and Total Access Communications System (TACS) standards,
use Frequency Division Multiple Access (FDMA). FDMA channels are defined by a range of radio frequencies, usually
expressed in a number of kilohertz (kHz), out of the radio spectrum. For example, AMPS systems use 30 kHz "slices" of
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.
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).
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