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WHAT IS CDMA?
(Code Division Multiple Access) A method for transmitting real-time signals over a shared
portion of the spectrum. The foremost application of CDMA is the digital cellular phone technology
that operates in the 800MHz band and 1.9GHz . Unlike GSM and TDMA, which divides the spectrum
into different time slots CDMA uses a spread spectrum technique to assign a code to each
conversation. After the speech codec converts voice to digital, CDMA spreads the voice stream over
the full 1.25MHz bandwidth of the CDMA channel coding each stream separately so it can be
decoded at the receiving end. The rate of the spreading signal is known as the “chip rate,” as each
bit in the spreading signal is called a “chip” voice conversations use the full bandwidth at the same
time. One bit from each conversation is multiplied into 128 coded bits by the spreading techniques,
giving the receiving side an enormous amount of data it can average just to determine the value of
one bit.
CDMA phones are noted for their excellent call quality and low current draw CDMA is less
costly to implement, requiring fewer cell sites than the GSM and TDMA digital cell phone systems
and providing three to five times the calling capacity. It provides more than 10 times the capacity of
the analog cell phone system (AMPS). CDMA is also expected to become the third-generation (3G)
technology for GSM
CDMA transmission has been used by the military for secure phone calls. Unlike FDMA and
TDMA methods, CDMA’s wide spreading signal makes it difficult to detect and jam.
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 temporary 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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MULTIPLE ACCESS COMPARISON
It is easier to understand CDMA if it is compared with other multiple access technologies.
The following sections describe the fundamental differences between a Frequency Division
Multiple Access Analog technology (FDMA), a Time Division Multiple Access Digital technology
(TDMA) and a Code Division Multiple Access Digital technology (CDMA).
FDMA - Frequency Division Multiple Access
FDMA is used for standard analog cellular. Each user is assigned a discrete slice of the RF
spectrum. FDMA permits only one user per channel since it allows the user to use the channel
100% of the time. Therefore, only the frequency “dimension” is used to define channels.
TDMA - Time Division Multiole Access
The key point to make about TDMA is that users are still assigned a discrete slice of RF
spectrum, but multiple users now share that RF carrier on a time slot basis. Each of the users
alternate their use of the RF channel. Frequency division is still employed, but these carriers are
now further sub-divided into some number of time slots per carrier.
A user is assigned a particular time slot in a carrier and can only send or receive information
at those times. This is true whether or not the other time slots are being used. Information flow is
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not continuous for any user, but rather is sent and received in “bursts.” The bursts are reassembled at the receiving end, and appear to provide continuous sound because the process is
very fast.
CDMA - Code Division Multiple Access
IS-95 uses a multiple access spectrum spreading technique called Direct Sequence
(DS) CDMA.
Each user is assigned a binary, Direct Sequence code during a call. The DS code is a signal
generated by linear modulation with wideband Pseudorandorn Noise (PN) sequences. As a result,
DS CDMA uses much wider signals than those used in other technologies. Wideband signals reduce
interference and allow one-cell frequency reuse.
There is no time division, and all users use the entire carrier, all of the time.
Figure 3: DS-CDMA
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The International Cocktail Party
To illustrate the conceptual differences among the multiple access technologies, the
“International Cocktail Party” analogy will be applied. Picture a large room and a number of people,
in pairs, who would like to hold conversations. The people in each pair only want to talk and listen
to each other, and have no interest in what is being said by the other pairs. In order for these
conversations to take place, however, it is necessary to define the environment for each
conversation.
First, let us apply this analogy to an FDMA system. An FDMA environment would be
simulated by building walls in the single large room, creating a larger number of small rooms. A
single pair of people would enter each small room and hold their conversation. When that
conversation is complete, the pair of people would leave and another pair would be able to enter
that small room.
In a TDMA environment, each of these small rooms would be able to accommodate multiple
conversations “simultaneously.” For example, with a 3 slot TDMA system such as IS-54, each
“room” would contain up to 3 pairs of people, with each pair taking turns talking. Think of each pair
having the right to speak for 20 seconds during each minute, With pair A able to use 0:01 second
through 0:20 second, pair B using 0:21 second through 0:40 second, and pair C using 0:41 second
through 0:60 second. Even if there are fewer than three pairs in the small room, each pair is still
limited to its 20 seconds per minute.
Now, for CDMA, get rid of all of the little rooms. Pairs of people will enter the single large
room. However, if every pair uses a different language, they can all use the air in the room as a
carrier for their voices and experience little interference from the other pairs. The analogy here is
that the air in the room is a wideband “carrier” and the languages are represented by the “codes”
assigned by the CDMA system. In addition, language “filters” are incorporated, people speaking
German will hear virtually nothing from those speaking Spanish, etc.
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We can continue to add pairs, each speaking a unique language (as defined by the unique
code) until the overall “background noise” (interference from other users) makes it too difficult for
some of the people to understand the other in their pair (frame erasure rates get too high). By
controlling the voice volume (signal strength) of all users to no more than necessary, we maximize
the number of conversations which can take place in the room (maximize the number of users per
carrier).
Therefore, the maximum number of users, or effective traffic channels, per carrier depends
on the amount of activity that is going on in each channel, and is therefore not precise. It is a “soft
overload” concept where an additional user (or conversation, in our analogy) can usually be
accommodated if necessary, at the “cost” of a bit more interference to the other users.
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 Time Division
Multiple Access (TDMA). TDMA digital standards include North American Digital Cellular (known by
its standard number IS-54), Global System for Mobile Communications (GSM), and Personal Digital
Cellular (PDC).
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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.
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”.
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CDMA TECHNOLOGY
Though CDMA 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.
The standard data rate of a CDMA call is 9600 bits per second (9.6 kilobits per second). This
initial data is “spread,” including the application of digital codes to the data bits, up to the
transmitted rate of about 1.23 megabits per second. The data bits of each call are then transmitted
in combination with the data bits of all of the calls in the cell. At the receiving end, the digital codes
are separated out, leaving only the original information which was to be communicated. At that
point, each call is once again a unique data stream with a rate of 9600 bits per second. 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 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 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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Introduction to Spread Spectrum Communications
CDMA is a form of Direct Sequence Spread Spectrum communications. In general, Spread
Spectrum communications is distinguished by three key elements:
1.
The signal occupies a bandwidth much greater than that which is necessary to send the
information. This results in many benefits, such as immunity to interference and jamming
and multi-user access
2.
The bandwidth is spread by means of a code which is independent of the data. The
independence of the code distinguishes this from standard modulation schemes in which
the data modulation will always spread the spectrum somewhat.
3.
The receiver synchronizes to the code to recover the data. The use of an independent code
and synchronous reception allows multiple users to access the same frequency band at the
same time.
In order to protect the signal, the code used is pseudo-random. It appears random, but is
actually deterministic, so that the receivefcan reconstruct the code for synchronous detection. This
pseudo-random code is also called pseudo-noise (PN).
Three Types of Spread Spectrum Communications
There are three ways to spread the bandwidth of the signal:

Frequency hopping. The signal is rapidly switched between different frequencies within
the hopping bandwidth pseudo-randomly, and the receiver knows before hand where to
find the signal at any given time.

Time hopping. The signal is transmitted in short bursts pseudo-randomly, and the receiver
knows beforehand when to expect the burst.

Direct sequence. The digital data is directly coded at a much higher frequency. The code is
generated pseudo-randomly, the receiver knows how to generate the same code, and
correlates the received signal with that code to extract the data.
Direct Sequence Spread Spectrum
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CDMA is a Direct Sequence Spread Spectrum system. The CDMA system works directly on
64 kbit/sec digital signals. These signals can be digitized voice, ISDN channels, modem data, etc.
Signal transmission consists of the following steps:
1.
A pseudo-random code is generated, different for each channel and each successive
connection.
2.
The Information data modulates the pseudo-random code (the Information data is
“spread”).
3.
The resulting signal modulates a carrier.
4.
The modulated carrier is amplified and broadcast.
Signal reception consists of the following steps:
1. The carrier is received and amplified.
2. The received signal is mixed with a local carrier to recover the spread digital signal.
3. A pseudo-random code is generated, matching the anticipated signal.
4. The receiver acquires the received code and phase locks its own code to it.
5. The received signal is correlated with the generated code, extracting the Information
data.
The main Problem with Direct Sequence is the Near-Far effect. If there are more then one
users active, the transmitted power of non-reference users is suppressed by a factor dependent on
the (partial) cross correlation between the code of the reference user and the code of a nonreference user. However when a non- reference user is closer to the receiver then the referenceuser, it is possible that the interference caused by this non-reference user (however suppressed)
has more power the reference user. Now only the non-reference user will be received, this nasty
property is called the near-far effect
One way to beat the near-far effect can be exploited in cellular systems. In such systems the
base station takes care that all users have such a power that the received power at the base station
is equal for all users.
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In non-cellular systems the influence of the near-far effect can be reduced by using the
frequency-hopping spread spectrum technique.
CDMA uses a form of direct sequence. Direct sequence is, in essence, multiplication of a
more conventional communication waveform by a pseudonoise (PN) ±1 binary sequence in the
transmitter.
Spreading takes place prior to any modulation, entirely in the binary domain, and the
transmitted signals are carefully bandlimited.
A second multiplication by a replica of the same +1 sequence in the receiver recovers the
original signal.
The noise and interference, being uncorrelated with the PN sequence, become noise-like
and increase in bandwidth when they reach the detector. The signal-to- noise ratio can be
enhanced by narrowband filtering that rejects most of the interference power. The SNR is
enhanced by the so-called processing gain W/R, where W is the spread bandwidth and R is the data
rate.
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Frequency Hopping
When using Frequency Hopping, the carrier frequency is ‘hopping’ according to a known
sequence (of length). In this way the bandwidth is also increased. If the channels are nonoverlapping the factor of spreading is , this factor is equal to the Processing Gain. The process of
frequency hop is shown below:
There are two kinds of Frequency Hopping Techniques.
• Slow Frequency Hopping (SFH)
In this case one or more data bits are transmitted within one Frequency Hop.
An advantage is that coherent data detection is possible. A disadvantage is that if one
frequency hop channel is jammed, one or more data bits are lost.
So we are forced to use error correcting codes.
• Fast Frequency Hopping (FFH)
In this technique one data bit is divided over more Frequency Hops. Now error correcting
codes are not needed. An other advantage is that diversity can be applied. Every
frequency hop a decision is made whether a -1 or a 1 is transmitted, at the end of each
data bit a majority decision is made. A disadvantage is that coherent data detection is not
possible because of phase discontinuities. The applied modulation technique should be
FSK or MFSK.
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As nearby non-reference users are not constantly in the same frequency slot a the reference
user, the near-far effect has less influence.
Hybrid System: DS/(F)FH
The DS/FFH Spread Spectrum technique is a combination of direct-sequence and frequencyhopping. One data bit is divided over frequency-hop channels (carrier frequencies). In each
frequency-hop channel one complete PN-code of length is added to the data signal (see figure,
where is taken to be 5). Using the FFH scheme in stead of the SFH scheme causes the bandwidth to
increase, this increase however is neglectable with regard to the enormous bandwidth already in
use.
CODING
CDMA uses unique spreading codes to spread the baseband data before transmission. The
signal is transmitted in a channel, which is below noise level. The receiver then uses a correlator to
despread the wanted signal, which is passed through a narrow bandpass filter. Unwanted signals
will not be despread and will not pass through the filter. Codes take the form of a carefully
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designed one/zero sequence produced at a much higher rate than that of the baseband data. The
rate of a spreading code is referred to as chip rate rather than bit rate.
Generating Pseudo-Random Codes
For each channel the base station generates a unique code that changes for every
connection. The base station adds together all the coded transmissions for every subscriber. The
subscriber unit correctly generates its own matching code and uses it to extract the appropriate
signals. Note that each subscriber uses several independent channels.
In order for all this to occur, the pseudo-random code must have the following properties:
1. It must be deterministic. The subscriber station must be able to independently generate the
code that matches the base station code.
2. It must appear random to a listener without prior knowledge of the code (i.e. it has the
statistical properties of sampled white noise).
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3. The cross-correlation between any two codes must be small (see below for more information
on code correlation).
4. The code must have a long period (i.e. a long time before the code repeats itself).
Code Correlation
In this context, correlation has a specific mathematical meaning. In general the correlation
function has these properties:

It equals 1 if the two codes are identical

It equals 0 of the two codes have nothing in common

Intermediate values indicate how much the codes have in common. The more they have in
common, the harder it is for the receiver to extract the appropriate signal. There are two
correlation functions:

Cross-Correlation: The correlation of two different codes. As we’ve said, this should be as
small as possible.

Auto-Correlation: The correlation of a code with a time-delayed version of itself. In order to
reject multi-path interference, this function should equal 0 for any time delay other than
zero.
The receiver uses cross-correlation to separate the appropriate signal from signals meant
for other receivers, and auto-correlation to reject multi-path interference.
Pseudo-Noise Spreading
The FEC coded Information data modulates the pseudo-random code,, : - Some terminology related to the pseudo-random code:

Chipping Frequency (fe): the bit rate of the PN code.

Information rate (f): the bit rate of the digital data. -

Chip: One bit of the PN code.
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Epoch: The length of time before the code starts repeating itself (the period of the code).
The epoch must be longer than the round trip propagation delay (The epoch is on the order
of several seconds).
The bandwidth of a digital signal is twice its bit rate. The bandwidth of the combination of
the two, information data (f) and the PN code, for fc >fi, can be approximated by the bandwidth of
the PN code.
System Capacity
The capacity of a system is approximated by
C max 
Gp
1
Eb
1 
No
Where
Cmax Is the maximum number of simultaneous calls
Gp Is the processing gain
Eb
No
Is the total signal to noise ratio per bit,and
 Is the cell interference factor
The capacity is directly proportional to the processing gain. Capacity is also inversely
proportional to the signal to noise ratio of the received signal. So, the smaller the transmitted
signal, the larger the system capacity. Both the RCS and FSU control the power transmitted by the
other so that the received signal is as small as possible while maintaining a minimum signal to noise
ratio. This maximizes system capacity.
THE SPREADING PROCESS
WCDMA uses Direct Sequence spreading, where spreading process is done by directly
combining the baseband information to high chip rate binary code. The Spreading Factor is the
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ratio of the chips (UMTS = 3. 84Mchips/s) to baseband information rate. Spreading factors vary
from 4 to 512 in FDD UMTS. Spreading process gain can in expressed in dBs (Spreading factor 128 =
21dB gain).
CDMA spreading
HANDOVER
Handover occurs when a call has to be passed from one cell to another as the user moves
between cells. In a traditional “hard” handover, the connection to the current cell is broken, and
then the connection to the new cell is made. This is known as a “break-before-make” handover.
Since all cells in CDMA use the same frequency, it is possible to make the connection to the new
cell before leaving the current cell. This is known as a “make-before-break” or “soft” handover. Soft
handovers require less power, which reduces interference and increases capacity. Mobile can be
connected to more that two BTS the handover. “Softer” handover is a special case of soft handover
where the radio links that are added and removed belong to the same Node B.
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CDMA soft handover
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MULTIPATH AND RAKE RECEIVERS
One of the main advantages of CDMA systems is the capability of using signals that arrive in
the receivers with different time delays. This phenomenon is called multipath. FDMA and TDMA,
which are narrow band systems, cannot discriminate between the multipath arrivals, and resort to
equalization to mitigate the negative effects of multipath. Due to its wide bandwidth and rake
receivers, CDMA uses the multipath signals and combines them to make an even stronger signal at
the receivers. CDMA subscriber units use rake receivers. This is essentially a set of several receivers.
One of the receivers (fingers) constantly searches for different multipaths and feeds the
information to the other three, fingers. Each finger then demodulates the signal corresponding to a
strong multipath. The results are then combined together to make the signal stronger.
INTERFERENCE REJECTION
CDMA technology is inherently resistant to interference and jamming. A common problem
with urban communications is multi-path interference. Multi-path interference is caused by the
broadcast signal traveling over different paths to reach the receiver. The receiver then has to
recover the signal combined with echoes of varying amplitude and phase. This results in two types
of interference:

Inter-chip interference: The reflected signals are delayed long enough that successive bits
(or chips, in this case) in the demodulated signals overlap, creating uncertainty in the data.

Selective fading: The reflected signals are delayed long enough that they are randomly out
of phase, and add destructively to the desired signal, causing it to fade.
Combating Interference
Two methods are commonly used to combat multi-path interference:
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 Rake filter: Correlators are set up at appropriate time intervals to extract all the echoes. The
relative amplitude and phase of each echo is measured, and each echo signal is phase corrected
and added to the signal.

Adaptive Matched Filter. This filter is “matched” to the transfer function (i.e. the propagation
characteristics) of the signal path. It phase shifts the echo signals and adds them to maximize
the received signal.
POWER CONTROL
CDMA is interference limited multiple access system. Because all users transmit on the
same frequency, internal interference generated by the system is the most significant factor in
determining system capacity and call quality. The transmit power for each user must be reduced to
limit interference, however, the power should be enough to maintain the required Eb/No (signal to
noise ratio) for , satisfactory call quality. Maximum capacity is achieved when Eb/No of every user
is at the minimum level needed for the acceptable channel performance. As the MS moves around,
the RF environment continuously changes due to fast and slow fading, external interference,
shadowing, and other factors. The aim of the dynamic power control is to limit transmitted power
on both the links while maintaining link quality under all conditions.
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CDMA BENEFITS
BENEFIT 1: CDMA CAPACITY INCREASES
CDMA anti Cell Reuse
Eb/No and Interference Threshold
Examples of Capacity Improvements
Other influence on Capacity
BENEFIT 2: IMPROVED CALL QUALITY
Advanced Error Detection and! Error Correction Sophisticated Vocoders
Multiple Levels of Diversity
Soft Handoff
Precise Power Control
BENEFIT 3: SIMPLIFIED SYSTEM PLANNING
BENEFIT 4: ENHANCED PRIVACY
BENEFIT 5: IMPROVED COVERAGE
BENEFIT 6: INCREASED PORTABLE TALKTIME
BENEFIT 7:BANDWIDTH ON DEMAND
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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. Each benefit will be described in detail in the following subsections.
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
Benefit 1: CDMA Capacity Increases
Capacity gains in cellular systems can be attained in one of two ways:
1. By getting more channels per MHz of spectrum.
2. By getting more channel reuse per unit of geographic area.
NAMPS is an example of a system technology which achieves greater capacity through
method #1 (more channels per MHz of spectrum). Instead of one channel in 30 kHz as in AMPS,
NAMPS gets three channels in 30 kHz, thereby providing three times the capacity of AMPS.
GSM is an example of a system which uses method #2 (more channel reuse per unit of
geographic area). GSM allows for a 9dB C/I (carrier to interference ratio) instead of the traditional
17dB C/I used in TACS (the analog FDMA technology in the 900 MHz band). This allows GSM to
place cell sites closer together and translates to about two times the capacity of TACS.
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FREQUENCY REUSE
Central to the cellular concept is the concept of frequency reuse. Although there are
hundreds of channels available, if each frequency were assigned to only one cell, total system
capacity would equal to the total number of channels, only a few thousand subscribers per system.
By reusing channels in multiple cells the system can grow without geographical limits.
Typical cellular reuseis easily rationalized by considering an idealized system. If we assume
that propagation is uniformly R and that cell boundaries are at the equisignal points, then a planar
service area is optimally covered by the classical hexagonal array of cells
Seven sets of channels are used, one set in each colored cell. This seven-cell unit is then
replicated over the service area.
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No similarly colored cells are adjacent, and therefore there are no adjacent cells using the
same channel. While real systems do not ever look like these idealized hexagonal tilings of a plane,
the seven-way reuse is typical of that achieved in practice. The capacity of a K-way reuse pattern is
simply the total number of available channels divided by K. With K=7 and 416 channels, there are
approximately 57 channels available per cell.
Eb/No and Interference Threshold
Eb/No provides a measure of the performance of a CDMA link between the mobile and the
cell. It represents the signal to noise ratio for a single bit on the reverse link. It is the ratio in dB
between the energy of each information bit and the noise spectral density. The noise is a
combination of background interference and the interference created by other users on the
system.
A decrease in the Eb/No ratio indicates that the relative level of interference, as compared to
the level of the voice information, is increasing. This will lower the voice quality of the
conversation. While all digital cellular systems use error correction coding, systems based on
narrowband digital modulation generally use less sophisticated schemes which use up less
bandwidth. In order to keep voice quality high, therefore, the operators of narrowband systems
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require a higher Eb/No. This leads to a need to limit the number of users on the system, lowering
capacity.
CDMA, on the other hand, uses advanced forward error correction coding as well as a digital
demodulator, lowering CDMA’s required Eb/No ratio. Using a lower Eb/No to reach voice quality
standards, CDMA achieves more capacity and uses less transmitter power than narrowband
systems.
CDMA describes Eb/No noise interference in terms of the Frame Erasure Rate (FER). Using an
interference threshold, the CDMA system erases frames of information that contain too many
errors. The FER, then, describes the number of frames that were erased due to poor quality.
Therefore, as the Eb/No level increases, the FER decreases, and system voice quality is improved.
Conversely, the higher the acceptable FER, the higher the overall cell site capacity. These two
parameters, frame erasure rate and voice quality, must be balanced against each other.
Benefit 2: Improved Call Quality
Cellular telephone systems using CDMA are able to provide higher quality sound and fewer
dropped calls than systems based on other technologies. A number of features inherent in the
system produce this high quality.

Advanced error detection and error correction schemes greatly increase the likelihood that
frames are interpreted correctly.

Sophisticated vocoders offer high speed coding and reduce background noise.

CDMA takes advantage of various types of diversity to improve speech quality:

frequency diversity (protection against frequency selective fading)

spatial diversity (two receive antennas)

path diversity (rake receiver improves reception of a signal experiencing multipath
“interference,” and actually enhances sound quality)


time diversity (interleaving and coding)
Soft Handoffs contribute to high voice quality by providing a “make before break”
connection. “Softer” Handoffs between sectors of the same cell provide similar benefits.
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Precise power control assures that all mobiles are very close to the optimum power level to
provide the highest voice quality possible.

The voice quality for CDMA has been rated very high in mean opinion score
Advanced Error Detection and Error Correction
The IS-95 CDMA air interface standard specifies powerflul error detection and correction
algorithms. Corrupted voice data can be detected and either corrected or manipulated to minimize
the impact of data errors on speech quality.
Sophisticated Vocoders
PCM is the vocoding standard used in landline systems. It is simple, which was necessary in
the 1 960s, but not very efficient. It has the sound quality wireless would like to match. Wired
communications still uses PCM, since bandwidth has become rather inexpensive via fiber optic
cable and/or microwave links. Wireless vocoders, on the other hand, are constrained by
bandwidth. Several types of vocoding standards currently exist, offering operators the choice
between higher capacity and better voice quality. Initial CDMA systems use an 8 kilobit per second
(kbps) variable rate speech vocoder, revision IS-96A. The vocoder transmits 8 kbps of voice
information at 9.6 kbps, when overhead and error correction bits are added.
The CDMA vocoder also increases call quality by suppressing background noise. Any noise
that is constant in nature, such as road noise, is eliminated. Constant background sound is viewed
by the vocoder as noise which does not convey any intelligent information, and is removed as much
as possible. This greatly enhances voice clarity in noisy environments, such as the inside of cars, or
in noisy public places.
Multiple Levels of Diversity
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CDMA takes advantage of a number of types of diversity, all of which lead to improved
speech quality. The four types are frequency diversity, spatial diversity, path diversity and time
diversity.
Frequency Diversity
With radio, fades or “holes” in frequency will occur. Fades occur in a multi-path
environment when two or more signals combine and cancel each other out. Narrow band
transmissions are especially prone to this phenomenon. For wide band signals such as CDMA, this is
much less of a problem. The wide band signal is, of course, also subjected to frequency selective
fading, but the majority of the signal is unaffected and the overall effect is minimal.
Figure 5: CDMA Quality Benefits from Frequency Diversity
As an example, consider what happens when there is a 12 dB deep, 400 kHz wide, frequency
selective fade. For a wide band CDMA signal which spans 1.25 MHz, this fade affects only about 1/3
of the entire signal’s bandwidth. Since the energy of a phone call is spread across the entire signal,
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the effect of the fade is looked at as an average, and represents an overall drop in signal of
approximately 2 dB.
If this same 400 kHz, 12 dB fade falls on top of a narrow band 30 kHz signal, as in AMPS or
IS-54 TDMA systems, the results are quite different. The entire 30 kHz signal is then affected by this
fade. The result will be an overall drop in signal of the full 12 dB. This is a much more serious hit to
the signal, and could lead to severe degradation in voice quality, or even a dropped call.
Similarly, CDMA is more resistant to interference or ‘ In a typical narrow band technology
such as AMPS or TDMA, if this narrow band jammer was at the same frequency as the signal of
interest, and was of sufficient magnitude, it would totally disrupt the information signal.
However, a narrow band jammer has little effect on a CDMA signal. In the CDMA
despreading process, when the received signal is combined with the original spreading code, the
signal of interest correlates with the spreading code and the desired signal “jumps” out of the
noise. A narrow band jammer is a random signal, so it will not correlate with any spreading code.
Therefore, in the CDMA despreading process the energy of the narrow band jammer is spread
across the spectrum and does not interfere with the desired signal of interest. This fundamental
immunity to interference is one of the most attractive benefits of CDMA.
Spatial Diversity
Spatial Diversity refers to the use of two receive antennas separated by some physical
distance. The principle of spatial diversity recognizes that when a mobile is moving about, it creates
a pattern of signal peaks and nulls. When one of these nulls falls on one antenna it will cause the
received signal strength to drop. However, if a second antenna is placed some physical distance
away, it will be outside of the signal null area and thus receive the signal at an acceptable signal
level.
Path Diversity
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With radio communications, there is usually more than one RF path from the transmitter to
the receiver. Therefore, multiple versions of the same signal are usually present at the receiver.
However, these signals, which have arrived along different paths, are all time shifted with respect
to each other because of the differences in the distance each signal has traveled. This “multipath”
effect is created when a transmitted signal is reflected off of objects in the environment (buildings,
mountains, planes, trucks, etc.). These reflections, combined with the transmitted signal, create a
moving pattern of signal peaks and nulls. When a narrow band receiver moves through these nulls
there is a sudden drop in signal strength. This fading will cause either lower, more noisy speech
quality or if the fading is severe enough, the loss of signal and a dropped call
Figure6: CDMA Quality Benefits from Path Diversity
Although multipath is usually detrimental to an analog or TDMA signal, it is actually an
advantage to CDMA ,since the rake receiver can use multipath to improve a signal. The CDMA
receiver has a number of receive ‘fingers’ which are capable of receiving the various multipath
signals. The receiver locks onto the three strongest received multipath signals, time shifts them,
and then sums them together to produce a signal that is better than any of the individual signal
components. Adding the multipath signals together enhances the signal rather than degrading it.
Time Diversity
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CDMA systems use a number of forward error correcting codes, followed by interleaving.
Error correction schemes are most effective when bit errors in the data stream are spread more
evenly over time. By separating the pieces of data over time, a sudden disruption in the CDMA data
will not cause a corresponding disruption in the voice signal. When the frames are pieced back
together by the decoder, any disrupted voice data will have been in small pieces over a relatively
longer stretch of the actual speech, reducing or eliminating the impact on the voice quality of the
call. Interleaving, which is common to most digital communication systems, ensures that
contiguous pieces of data are not transmitted consecutively. Even if you lose one small piece of a
word, chances are great that the rest of the word will get through clearly.
Soft Handoff
With traditional hard handoffs, which are used in all other types of cellular systems, the
mobile drops a channel before picking up the next channel. When a call is in a soft handoff
condition, a mobile user is monitored by two or more cell sites and the transcoder circuitry
compares the quality of the frames from the two receive cell sites on a frame-by-frame basis. The
system can take advantage of the moment-by-moment changes in signal strength at each of the
two cells to pick out the best signal.
This ensures that the best possible frame is used in the CDMA decoding process. The
transcoder can literally toggle back and forth between the cell sites involved in a soft handoff on a
frame-by-frame basis, if that is what is required to select the best frame possible.
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Figure 7: CDMA Soft Handoff Improves Frame Quality
Soft handoffs also contribute to high call quality by providing a “make before break”
connection. This eliminates the short disruption of speech one hears with non-CDMA technologies
when the RF connection breaks from one cell to establish the call at the destination cell during a
handoff. Narrow band technologies “compete” for the signal, and when Cell B “wins” over Cell A,
the user is dropped by cell A (hard handoff). In CDMA the cells “team up” to obtain the best
possible combined information stream. Eventually, Cell A will no longer receive a strong enough
signal from the mobile, and the transcoder will only be obtaining frames from Cell B. The handoff
will have been completed, undetected by the user. CDMA handoffs do not create the “hole” in
speech that is heard in other technologies.
Figure 8: CDMA Soft Handoff Utilizes Two or More Cells
Some cellular systems suffer from the “ping pong effect” of a call getting repetitively
switched back and forth between two cells when the subscriber unit is near a cell border. At worst,
such a situation increases the chance of a call getting dropped during one of the handoffs, and at a
minimum, causes noisier handoffs. CDMA soft handoff avoids this problem entirely. And finally,
because a CDMA call can be in a soft handoff condition with up to three cells at the same time, the
chances of a loss of RF connection (a dropped call) is greatly reduced.
CDMA also provides for “softer” handoffs. A “softer” handoff occurs when a subscriber is
simultaneously communicating with more than one sector of the same cell.
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Precise Power Control
CDMA power control not only increases capacity (as described earlier) but also increases
speech quality by minimizing and overcoming interference. CDMA’s power control algorithms are
all designed to reduce the overall signal strength level to the bare minimum required to maintain a
quality call.
Benefit 3: Simplified System Planning
All users on a CDMA carrier share the same RF spectrum. This N1/S reuse of frequencies
(where S = number of sectors per cell) is one factor which gives CDMA its greater capacity over
AMPS and other technologies, but it also makes certain aspects of system planning more
straightforward. Engineers will no longer have to perform the detailed frequency planning which is
necessary in analog and TDMA systems.
Benefit 4: Enhanced Privacy
Increased privacy over other cellular systems, both analog and digital, is inherent in CDMA
technology. It is extremely difficult for someone to jam the CDMA signal. In addition, since the
digitized frames of information are spread across a wide slice of spectrum, it is unlilely that a casual
eavesdropper will be able to listen in on a conversation.
Benefit 5: Improved Coverage
A CDMA cell site has a greater range than a typical analog or digital cell site. Therefore
fewer CDMA cell sites are required to cover the same area. CDMA’s greater range is due to the fact
that CDMA uses a more sensitive receiver than other technologies.
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Benefit 6: Increased Portable Talk Time
Because of precise power control and other system characteristics, CDMA subscriber units
normally transmit at only a fraction of the power of analog and TDMA phones. This will enable
portables to have longer talk and standby time.
Benefit 7: Bandwidth on Demand
A wideband CDMA channel provides a common resource that all mobiles in a system utilize
based on their own specific needs, whether they are transmitting voice, data, facsimile, or other
applications. At any given time, the portion of this “bandwidth pooi” that is not used by a given
mobile is available for use by any other mobile. This provides a tremendous amount of flexibility - a
flexibility that can be exploited to provide powerful features, such as higher data rate services. In
addition, because mobiles utilize the “bandwidth pool” independently, these features can easily
coexist on the same CDMA channel.
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CDMA IMPLEMENTATION
CDMA Channels
CDMA traffic channels are different: they are dependent on the equipment platform. Channels
are designated in three ways-effective traffic channels, actual traffic channels and physical traffic
channels.

The number of “Effective” traffic channels includes the traffic carrying channels less the soft
handoff channels. The capacity of an effective traffic channel is equivalent to the traffic
carrying capacity of an analog traffic channel.

The number of “Actual” traffic channels includes the effective traffic channels, plus channels
allocated for soft handoff.

The number of “Physical” traffic channels includes the Pilot channels, the Sync channels, the
Paging channels, the Soft Handoff Overhead channels and the Effective (voice and data)
traffic channels.
CDMA Forward Channels
Pilot Channel
The pilot channel is used by the mobile unit to obtain initial system synchronization and to
provide time, frequency, and phase tracking of signals from the cell site.
Sync Channel
This channel provides cell site identification, pilot transmit power, and the cell site pilot
pseudo-random (PN) phase offset information. With this information the mobile units can establish
the System Time as well as the proper transmit power level to use to initiate a call.
Paging Channel
The mobile unit will begin monitoring the paging channel after it has set its timing to the
System Time provided by the sync channel. Once a mobile unit has been paged and acknowledges
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that page, call setup and traffic channel assignment information is then passed on this channel to
the mobile unit.
Forward Traffic Channel
This channel carries the actual phone call and carries the voice and mobile power control
information from the base station to the mobile unit.
CDMA Reverse Channels
Access Channel
When the mobile unit is not active on a traffic channel, it will communicate to the base
station over the access channel. This communication includes registration requests, responses to
pages, and call originations. The access channels are paired with a corresponding paging channel.
Reverse Traffic Channel
This channel carries the other half of the actual phone call and carries the voice and mobile
power control information from the mobile unit to the base station.
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CDMA Modulation
Both the Forward and Reverse Traffic Channels use a similar control structure consisting of
20 millisecond frames. For the system, frames can be sent at either 14400, 9600, 7200, 4800, 3600,
2400, 1800, or 1200 bps. For example, with a Traffic Channel operating at 9600 bps, the rate can
vary from frame to frame, and can be 9600, 4800, 2400, or 1200 bps. The receiver detects the rate
of the frame and processes it at the correct rate. This technique allows the channel rate to
dynamically adapt to the speech or data activity. For speech, when a talker pauses, the
transmission rate is reduced to a low rate. When the talker speaks, the system instantaneously
shifts to using a higher transmission rate. This technique decreases the interference to other CDMA
signals and thus allows an increase in system capacity. CDMA starts with a basic data rate of 9600
bits per second. This is then spread to a transmitted bit rate, or chip rate (the transmitted bits are
called chips), of 1.2288 MHz. The spreading process applies digital codes to the data bits, which
increases the data rate while adding redundancy to the system.
The chips are transmitted using a form of QPSK (quadrature phase shift keying) modulation
which has been filtered to limit the bandwidth of the signal. This is added to the signal of all the
other users in that cell. When the signal is received, the coding is removed from the desired signal,
returning it to a rate of 9600 bps. When the decoding is applied to the other users’ codes, there is
no despreading; the signals maintain the 1.2288 MHz bandwidth. The ratio of transmitted bits or
chips to data bits is the coding gain. The coding gain for the IS-95 CDMA system is 128, or 21 dB.
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Input data
CDMA works on Information data from several possible sources, such as digitized voice or
ISDN channels. Dat rates can vary, here are some examples:
Data Source
Voice
Data Rate
Pulse Code Modulation (PCM)
64KBits/Sec
Adaptive Differential Pulse code Modulation
32KBits/Sec
(ADPCM)
Low Delay Code Excited Linear Prediction (LD-
16KBits/Sec
CELP)
ISDN
Bearer Channel (B-Channel)
64KBits/Sec
Data Channel (D-Channel)
16KBits/Sec
The system works with 64 kBits/sec data, but can accept input rates of 8, 16, 32, or 64
kBits/sec. Inputs of less than 64 kBits/sec are padded with extra bits to bring them up to 64
kBits/sec. For inputs of 8, 16, 32, or 64 kBits/sec, the system applies Forward Error Correction (FEC)
coding, which doubles the bit rate, up to 128 kbits/sec. The Complex Modulation scheme (which
we’ll discuss in more detail later), transmits two bits at a time, in two bit symbols. For inputs of less
than 64 kbits/sec, each symbol is repeated to bring the transmission rate up to 64 kilosymbols/sec.
Each component of the complex signal carries one bit of the two bit symbol, at 64 kBits/sec, as
shown below
Transmitting Data
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The resultant coded signal next modulates an RF carrier for transmission using Quadrature
Phase Shift Keying (QPSK). QPSK uses four different states to encode each symbol. The four states
are phase shifts of the carrier spaced 90_ apart. By convention, the phase shifts are 45, 135, 225,
and 315 degrees. Since there are four possible states used to encode binary information, each state
represents two bits. This two bit “word” is called a symbol.
Complex Modulation
Algebraically, a carrier wave with an applied phase shift, (t), can be expressed as a sum of
two components, a Cosine wave and a Sine wave, as:
A(t)Cos(0o t -I (1)) l(t)Cos(o 0 -‘ Q(L)S t)
1(t) is called the real, or In-phase, component of the data, and Q(t) is called the imaginary,
or Quadrature-phase, component of the data. We end up with two Binary PSK waves
superimposed. These are easier to modulate and later demodulate.
This is not only an algebraic identity, but also forms the basis for the actual
modulation/demodulation scheme. The transmitter generates two carrier waves of the same
frequency, a sine and cosine. 1(t) and Q(t) are binary, modulating each component by phase
shifting it either 0 or 180 degrees. Both components are then summed together. Since 1(t) and Q(t)
are binary, we’ll refer to them as simply I and Q.
The receiver generates the two reference waves, and demodulates each component. It is
easier to detect 1 80_ phase shifts than 90_ phase shifts. The following table summarizes this
modulation scheme. Note that I and Q are normalized to 1.
Symbol
I
Q
Phase Shift
00
+1
+1
45 o
01
+1
-1
315 o
10
-1
+1
135 o
11
-1
-1
225o
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For Digital Signal Processing, the two-bit symbols are considered to be complex numbers, I
+jQ.
Working with Complex Data
In order to make full use of the efficiency of Digital Signal Processing, the conversion of the
Information data into complex symbols occurs before the ni The system generates complex PN
codes made up of 2 independent components, PNi +jPNq. To spread the Information data the
system performs complex multiplication between the complex PN codes and the complex data.
Summing Many Channels Together
Many channels are added together and transmitted simultaneously. This addition happens
digitally at the chip rate. Remember, there are millions of chips in each symbol. For clarity, let’s say
each chip is represented by an 8 bit word (it’s slightly more complicated than that, but those details
are beyond the scope of this discussion).
At the Chip Rate

Information data is converted to two bit symbols.

The first bit of the symbol is placed in the I data stream, the second bit is placed in the Q
data stream.

The complex PN code is generated. The complex PN code has two independently generated
components, an I component and a Q component.

The complex Information data and complex PN code are multiplied together. For each
component (I or Q):
At the Symbol Rate
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Since the PN-code has the statistical properties of random noise, it averages to zero over
long periods of time (such as the symbol period). Therefore, fluctuations in I and Q, and hence the
phase modulation of the carrier, that occur at the chip frequency, average to zero. Over the symbol
period the modulation averages to one of the four states of QPSK, which determine what the
symbol is.
The symbol only sees the QPSK, and obeys all the statistical properties of QPSK transmission,
including Bit Error Rate.
Receiving Data
The receiver performs the following steps to extract the Information:
• Demodulation
• Code acquisition and lock
• Correlation of code with signal
• Decoding of Information data
Demodulation
The receiver generates two reference waves, a Cosine wave and a Sine wave. Separately
mixing each with the received carrier, the receiver extracts 1(t) and Q(t). Analog to Digital
converters restore the 8-bit words representing the I and Q chips.
Code Acquisition and Lock
The receiver, as described earlier, generates its own complex PN code that matches the code
generated by the transmitter. However, the local code must be phase- locked to the encoded data.
Correlation and Data Despreading
Once the PN code is phase-locked to the pilot, the received signal is sent to a correlator that
multiplies it with the complex PN code, extracting the I and Q data meant for that receiver. The
receiver reconstructs the Information data from the I and Q data.
Automatic Power Control
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The RCS gets bombarded by signals from many FSUs. Some of these FSUs are close and their
signals are much stronger than FSUs farther away. This results in the Near/Far problem inherent in
CDMA communications. System .Capacity is also dependant on signal power. For these reasons,
both the RCS and FSU measure the received power and send signals to control the other’s transmit
power.
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CONCLUSION
The world is demanding more from wireless communication technologies than ever before.
More people around the world are subscribing to wireless services and consumers are using their
phones more frequently. Add in exciting Third-Generation (3G) wireless data services and
applications - such as wireless email, web, digital picture taking/sending and assisted-GPS position
location applications - and wireless networks are asked to do much more than just a few years ago.
And these networks will be asked to do more tomorrow.
This is where CDMA technology fits in. CDMA consistently provides better capacity for voice
and data communications than other commercial mobile technologies, allowing more subscribers
to connect at any given time, and it is the common platform on which 3G technologies are built.
In a world of finite spectrum resources, CDMA enables many more people to share the
airwaves at the same time than do alternative technologies. The CDMA air interface is used in both
2G and 3G networks. 2G CDMA standards are branded cdmaOne and include IS-95A and IS-95B.
CDMA is the foundation for 3G services: the two dominant IMT-2000 standards, CDMA2000 and
WCDMA, are based on CDMA.
cdmaOne: The Family of IS-95 CDMA Technologies
cdmaOne describes a complete wireless system based on the TIA/EIA IS-95 CDMA standard,
including IS-95A and IS-95B revisions. It represents the end-to-end wireless system and all the
necessary specifications that govern its operation. cdmaOne provides a family of related services
including cellular, PCS and fixed wireless (wireless local loop).
CDMA2000: Leads the 3G revolution
CDMA2000 represents a family of ITU-approved, IMT-2000 (3G) standards and includes
CDMA2000 l and CDMA2000 1xEV technologies. They deliver increased network capacity to meet
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growing demand for wireless services and high-speed data services. CDMA2000 lx was the world’s
first 3G technology commercially deployed (October 2000).
CDMA is the fastest growing wireless technology and it will continue to grow at a faster
pace than any other technology. It is the platform on which 2G and 3G advanced services are built.
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