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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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).
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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.
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A CDMA call starts with a standard rate of 9600 bits per second (9.6 kilobits per second).
This is then spread to a transmitted rate of about 1.23 Megabits per second. Spreading
means that digital codes are applied to the data bits associated with users in a cell.
These data bits are transmitted along with the signals of all the other users in that
cell. When the signal is received, the codes are removed from the desired signal,
separating the users and returning the call to a rate of 9600 bps.
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Traditional uses of spread spectrum are in military operations. Because of the wide
bandwidth of a spread spectrum signal, it is very difficult to jam, difficult to interfere
with, and difficult to identify. This is in contrast to technologies using a narrower
bandwidth of frequencies. Since a wide and spread spectrum signal is very hard to detect,
it appears as nothing more than a slight rise in the "noise floor" or
interference level. With other technologies, the power of the signal is concentrated in a
narrower band, which makes it easier to detect. Increased privacy is inherent in CDMA
technology. CDMA phone calls will be secure from the casual eavesdropper since, unlike an
analog conversation, a simple radio receiver will not be able to pick individual digital
conversations out of the overall RF radiation in a frequency band.
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Synchronization
In the final stages of the encoding of the radio link from the base station to the mobile,
CDMA adds a special "pseudo-random code" to the signal that repeats itself after
a finite amount of time. Base stations in the system distinguish themselves from each
other by transmitting different portions of the code at a given time. In other words, the
base stations transmit time offset versions of the same pseudo-random code. In order to
assure that the time offsets used remain unique from each other, CDMA stations must remain
synchronized to a common time reference.
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The Global Positioning System (GPS) provides this precise common time reference. GPS is a
satellite based, radio navigation system capable of providing a practical and affordable
means of determining continuous position, velocity, and time to an unlimited number of
users.
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"The
Balancing Act"
CDMA cell coverage is dependent upon the way the system is designed. In fact, three
primary system caracteristics-Coverage, Quality, and Capacity-must be balanced off of each
other to arrive at the desired level of system performance.
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In a CDMA system these three characteristics are tightly inter-related. Even higher
capacity might be achieved through some degree of degradation in coverage and/or quality.
Since these parameters are all intertwined, operators cannot have the best of all worlds:
three times wider coverage, 40 times capacity, and "CD" quality sound. For
example, the 13 kbps vocoder provides better sound quality, but reduces system capacity as
compared to an 8 kbps vocoder.
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CDMA Benefits
When implemented in a cellular telephone system, CDMA technology offers numerous benefits
to the cellular operators and their subscribers. The following is an overview of the
benefits of CDMA.
1. Capacity increases of 8 to 10 times that of an AMPS analog system and 4 to 5
times that of a GSM system
2. Improved call quality, with better and more consistent sound as compared to
AMPS systems
3. Simplified system planning through the use of the same frequency in every
sector of every cell
4. Enhanced privacy
5. Improved coverage characteristics, allowing for the possibility of fewer cell sites
6. Increased talk time for portables
7. Bandwidth on demand
In a cellular system, each cell base station consists of a
powerful transmitter/receiver, controller, and ideally, an omni-directional antenna
resulting in a more or less circular radiation pattern. You've probably seen this
represented graphically as a regular hexagon. However, creating an area of reliable
coverage is much more than laying out a hexagon pattern on a map - it is an art. Sometimes
a base station uses three 120° directional antennas at each of the cell's corners or at
its center to provide higher radiating power in directions where signal power is likely to
be lost, such as a building or down a street. In special cases, a single directional beam
may be focused on an area of particular difficulty.
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At the other end, each cellular phone, or "mobile", consists of a low power
transmitter/receiver, antenna, and a microprocessor control unit, which samples the
available communication channels until it finds one with the best reception. If the base
station's strength diminishes during a call, the microprocessor switches (tunes) the
mobile to another channel and the transmission is picked up from there. This automatic
channel selection is called "handoff," and is normally inaudible to the user.
(Though it can be deadly to data transmissions). Each cell phone is personalized with an
Electronic Serial Number (ESN) specific to the unit, the Mobile Identification Number
(MIN) otherwise known as the cellular phone number, and two other numbers, all
reprogrammable except the ESN. Each call is validated at the base station cell by matching
the ESN and the MIN in its database. .
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