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Code Division Multiple Access (CDMA) is a spread-spectrum
technology that is used for implementing cellular telephone
service. Spread spectrum is a family of digital communication
techniques originally used in military communications and
control applications. Spread spectrum uses carrier waves that
consume a much wider bandwidth than that required for simple
point-to-point communication at the same data rate. This
results in the carrier wave looking more like random noise than
real communication between a sender and receiver. Originally,
there were two motivations for implementing spread spectrum:
to resist enemy efforts to jam vital communications and to hide
the fact that communication was even taking place.
For cellular telephony, spread-spectrum technology underlies
CDMA, which is a digital multiple access technique specified
by the Telecommunications Industry Association (TIA)
as IS-95. Commercial applications of CDMAbecame possible
because of two key developments. One was the availability of
low-cost, high-density digital integrated circuits, which
reduce the size, weight, and cost of the mobile phones. The
other was the realization that optimal multiple access communication
depends on the ability of all mobile phones to regulate
their transmitter power to the lowest level that will
achieve adequate signal quality.
CDMAchanges the nature of the mobile phone from a predominately
analog device to a predominately digital device.
CDMAreceivers do not eliminate analog processing entirely,
but they separate communication channels by means of a
pseudorandom modulation that is applied and removed in
the digital domain, not on the basis of frequency. This allows
multiple users to occupy the same frequency band; this frequency
reuse results in high spectral efficiency.
TDMAsystems commonly start with a slice of spectrum,
referred to as a “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, TDMAsystems, designed to coexist with
AMPS systems, divide 30 kHz of spectrum into three channels.
By comparison, GSM systems create eight timedivision
channels in 200-kHz-wide carriers.
Wideband Usage
With CDMAsystems, multiple conversations simultaneously
share the available spectrum in both the time and frequency
dimensions. The available spectrum is not
“channelized” in frequency or time as in Frequency Division
Multiple Access (FDMA) or TDMA systems, respectively.
Instead, the individual conversations are distinguished
through coding; that is, at the transmitter, each conversation
is processed with a unique spreading code that is used
to distribute the signal over the available bandwidth. The
receiver uses the unique code to accept the energy associated
with a particular code. The other signals present are each
identified by a different code and simply produce background
noise. In this way, many conversations can be carried
simultaneously within the same block of spectrum.
The following analogy is used commonly to explain how
CDMAtechnology works. Four speakers are simultaneously
giving a presentation, and they each speak a different native
language: Spanish, Korean, English, and Chinese. If English is your native language, you only understand
the words of the English speaker and tune out the Spanish,
Korean, and Chinese speakers. You hear only what you know
and recognize. The rest sounds like background noise. The
same is true for CDMA. Each conversation is specially
encoded and decoded for a particular user. Multiple users
share the same frequency band at the same time, yet each
user hears only the conversation he or she can interpret.
CDMAassigns each subscriber a unique code to put multiple
users on the same wideband channel at the same time.
These codes are used to distinguish between the various conversations.
The result of this access method is increased callhandling
capacity.
One of the unique aspects of CDMAis that while there are
ultimate limits to the number of phone calls that a system
can handle, this is not a fixed number. Rather, the capacity
of the system depends on how coverage, quality, and capacity
are balanced to arrive at the desired level of system performance.
Since these parameters are tightly intertwined,
operators cannot have the best of all worlds: 3 times wider
coverage, 40 times capacity, and high-quality sound. For
example, the 13-kbps vocoder provides better sound quality
but reduces system capacity compared with an 8-kbps
vocoder. Higher capacity might be achieved through some
degree of degradation in coverage and/or quality.
System Features
CDMAhas been adapted for use in cellular communications
with the addition of several system features that enhance
efficiency and lower costs.
Mobile Station Sign-on On power-on, the mobile station
already knows the assigned frequency for CDMAservice in
the local area and will tune to that frequency and search for
pilot signals. Multiple pilot signals typically will be found,
each with a different time offset. This time offset distinguishes
one base station from another. The mobile station
will pick the strongest pilot and establish a frequency reference
and a time reference from that signal. Once the mobile
station becomes synchronized with the base station’s system
time, it can then register. Registration is the process by
which the mobile station tells the system that it is available
for calls and notifies the system of its location.
Call Processing The user makes a call by entering the digits
on the mobile station keypad and hitting the “Send” button. If
multiple mobile stations attempt a link on the access channel
at precisely the same moment, a collision occurs. If the base
station does not acknowledge the access attempt, the mobile
station will wait a random time and try again. On making contact,
the base station assigns a traffic channel, whereupon
basic information is exchanged, including the mobile station’s
serial number. At this point, the conversation mode is started.
As a mobile station moves from one cell to the next, another
cell’s pilot signal will be detected that is strong enough for it
to use. The mobile station will then request a “soft handoff,”
during which it is actually receiving both signals via different
correlative elements in the receiver circuitry. Eventually, the
signal from the first cell will diminish, and the mobile station
will request from the second cell that the soft handoff be terminated.
Abase station does not hand off the call to another
base station until it detects acceptable signal strength.
This soft handoff technique is a significant improvement
over the handoff procedure used in analog FM cellular systems,
where the communication link with the old cell site is
momentarily disconnected before the link to the new site is
established. For a short time, the mobile station is not connected
to either cell site, during which the subscriber hears
background noise or nothing at all. Sometimes the mobile
stations Ping-Pong between two cell sites as the links are
handed back and forth between the approaching and
retreating cell sites. Other times, the calls are simply
dropped. Because a mobile station in the CDMAsystem has
more than one modulator, it can communicate with multiple
cells simultaneously to implement the soft handoff.
At the end of a call placed over the CDMA system, the
channel will be freed and may be reused. When the mobile
station is turned off, it will generate a power-down registration
signal that tells the system that it is no longer available
for incoming calls.
Voice Detection and Encoding With voice activity detection,
the transmitter is activated only when the user is speaking.
This reduces interference levels—and, consequently, the
amount of bandwidth consumed—when the user is not
speaking. Through interference averaging, the capacity of
the system is increased. This allows systems to be designed
for the average rather than the worst interference case.
However, the IS-95 CDMAstandard requires that no interfering
signal be received that is significantly stronger than
the desired signal, since it would then jam the weaker signal.
This has been called the “near-far problem” and means
that high cell capacity does not necessarily translate into
high overall system capacity.
The speech coder used in CDMAoperates at a variable
rate. When the subscriber is talking, the speech coder operates
at the full rate; when the subscriber is not talking, the
speech coder operates at only one-eighth the full rate. Two
intermediate rates are also defined to capture the transitions
and eliminate the effect of sudden rate changes. Since the
variable-rate operation of the speech coder reduces the average
bit rate of the conversations, system capacity is increased.
Privacy Increased privacy is inherent in CDMAtechnology .
CDMAphone calls will be secure from the casual eavesdropper
because, unlike a conversation carried over an analog system,
a simple radio receiver will not be able to pick out individual
digital conversations from the overall RF radiation in a frequency
band.
ACDMAcall starts with a standard rate of 9.6 kbps. This
is then spread to a transmitted rate of about 1.25 Mbps.
“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 the original rate of 9.6 kbps.
Because of the wide bandwidth of a spread-spectrum signal,
it is very difficult to identify individual conversations for
eavesdropping. Since a wideband 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 analog
technologies, the power of the signal is concentrated in a
narrower band, which makes it easier to detect with a radio
receiver tuned to that set of frequencies.
The use of wideband spread-spectrum signals also offers
more protection against cloning, an illegal practice whereby
a mobile phone’s electronic serial number is taken over the
air and programmed into another phone. All calls made from
a cloned phone are “free” because they are billed to the original
subscriber.
Power Control CDMA systems rely on strict control of power at
the mobile station to overcome the so-called near-far problem.
If the signal from a near mobile station is received at the cell
site receiver with too much power, the cell site receiver will
become overloaded and prevent it from picking up the signals
from mobile stations located farther away. The goal of CDMA
is to have the signals of all mobile stations arrive at the base
station with exactly the same power level. The closer the
mobile station is to the cell site receiver, the lower is the power
necessary for transmission; the farther away the mobile station,
the greater is the power necessary for transmission.
Two forms of adaptive power control are employed in
CDMAsystems: open loop and closed loop. Open-loop power
control is based on the similarity of loss in the forward and
reverse paths. The received power at the mobile station is
used as a reference. If it is low, the mobile station is assumed
to be far from the base station and transmits with high
power. If it is high, the mobile station is assumed to be near
the base station and transmits with low power. The sum of
the two power levels is a constant.
Closed-loop power control is used to force the power from
the mobile station to deviate from the open-loop setting. This
is achieved by an active feedback system from the base station
to the mobile station. Power control bits are sent every
1.25 millisecond (ms) to direct the mobile station to increase
or decrease its transmitted power by 1 decibel (dB). Lack of
power control to at least this accuracy greatly reduces the
capacity of CDMAsystems.
With these adaptive power-control techniques, the mobile
station transmits only enough power to maintain a link. This
results in an average power requirement that is much lower
than that for analog systems, which do not usually employ
such techniques. CDMA’s lower power requirement translates
into smaller, lightweight, longer-life batteries—approximately
5 hours of talk time and over 2 days of standby
time—and makes possible smaller, lower-cost hand-held
computers and hybrid computer-communications devices.
CDMAphones can easily weigh in at less than 8 ounces.
Spatial Diversity Among the various forms of diversity is
that of spatial diversity, which is employed in CDMA, as well
as in other multiple access techniques, including FDMAand
TDMA. Spatial diversity helps to maintain the signal during
the call handoff process when a user moves from one cell to
the next. This process entails antennas in two different cell
sites maintaining links with one mobile station. The mobile
station has multiple correlative receiver elements that are
assigned to each incoming signal and can add these.
CDMAuses at least four of these correlators: three that
can be assigned to the link and one that searches for alternate
paths. The cell sites send the received data, along with
a quality index, to the MTSO, where a choice is made regarding
the better of the two signals.
Not all these features are unique to CDMA; some can be
exploited by TDMA-based systems as well, such as spatial
diversity and power control. These already exist in all TDMA
standards today, while soft handoff is implemented in the
European Digital Enhanced Cordless Telecommunications
(DECT) standard, which is based on TDMA.
Summary
There are still conflicting performance claims for TDMAand
CDMA. Since both TDMAand CDMAhave become TIAstandards—
IS-54 and IS-95, respectively—vendors are now aiming
their full marketing efforts toward the cellular carriers.
Proponents of each technology have the research to back up
their claims of superior performance. Of the two, CDMAsuffered
a credibility problem early on because its advocates
made grandiose performance claims for CDMAthat could not
be verified in the real-world operating environment. In some
circles, this credibility problem lingers today. Of note, however,
is that both technologies have been successful in the
marketplace, each having been selected by many cellular carriers
around the world. Both are capable of supporting emerging
PCS networks and providing such services as wireless
Internet access, Short messaging Service, voice mail, facsimile,
paging, and video. Although TDMA-based Global System
for Mobile (GSM) telecommunications is the dominant standard
in the global wireless market, the use of CDMAis growing
rapidly. GSM’s head start in the market gives it a much
larger presence and practically guarantees that GSM will continue
to lead the digital cellular market for the next 5 years.
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