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CDMA
CDMA
WHAT IS CDMA
MULTIPLE A CCESS COMPARISON
CDMA TECHNOLOGY
CDMA BENEFITS
CDMA IMPLEMENTA HON
CONCLUSION
CDMA
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.
CDMA
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.
CDMA
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
CDMA
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 not
continuous for any user, but rather is sent and received in “bursts.”
The bursts are re-assembled 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
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
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
CDMA
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.
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).
CDMA
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).
CDMA
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”.
CDMA
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
CDMA
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.
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).
CDMA
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
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.
CDMA
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 non-reference user. However when a non- reference user is
closer to the receiver then the reference-user, 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.
In non-cellular systems the influence of the near-far effect can
be reduced by using the frequency-hopping spread spectrum
technique.
CDMA
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.
CDMA
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 non-overlapping 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
CDMA
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.
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 frequency-hopping. 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.
CDMA
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 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
CDMA
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).
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.
CDMA
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.
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
β+=
11
max
o
b
p
NEG
C
Where
maxC Is the maximum number of simultaneous calls
Gp Is the processing gain
CDMA
o
bN
E Is the total signal to noise ratio per bit,and
β 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 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
CDMA
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.
CDMA soft handover
CDMA
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.
CDMA
Combating Interference
Two methods are commonly used to combat multi-path
interference:
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.
CDMA
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
CDMA
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.
CDMA
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.
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
CDMA
Seven sets of channels are used, one set in each colored cell.
This seven-cell unit is then replicated over the service area.
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.
CDMA
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 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
CDMA
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.
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.
CDMA
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
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.
CDMA
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, the effect of the fade is looked at as an
average, and represents an overall drop in signal of approximately 2
dB.
CDMA
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
CDMA
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
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
CDMA
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
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
CDMA
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.
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
CDMA
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.
Precise Power Control
CDMA power control not only increases capacity (as described
earlier) but also increases speech quality by minimizing and
CDMA
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.
CDMA
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.
CDMA
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.
CDMA
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 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.
CDMA
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.
CDMA
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 Data Rate
Voice Pulse Code Modulation (PCM) 64KBits/Sec
Adaptive Differential Pulse code Modulation
(ADPCM)
32KBits/Sec
Low Delay Code Excited Linear Prediction
(LD-CELP)
16KBits/Sec
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
CDMA
Transmitting Data
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, ψ(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
CDMA
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
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).
CDMA
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
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
CDMA
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
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.
CDMA
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
CDMA
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
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.
REFERENCE
Wireless Networked Communication -Jay Ranade
Principles of Communication - Taub & Schilling
Principle of Wireless Network - Kaveh Pahlavan
Prashant Krishnamurthi
www.rf.rfglobalnet.com
www.bee.net
www.cas.et.tudelft.nl
www.unstsworld.com
CDMA
ABSTRACT
Code-Division Multiple Access, a digital cellular technology that
uses spread-spectrum techniques. Unlike competing systems, such
as GSM, that use TDMA, CDMA does not assign a specific frequency
to each user. Instead, every channel uses the full available spectrum.
Individual conversations are encoded with a pseudo-random digital
sequence.
As the term implies, CDMA is a form of multiplexing which allows
numerous signals to occupy a single transmission channel, optimizing
the use of available bandwidth. The technology is used in ultra-high-
frequency (UHF) cellular telephone systems in the 800-M1-Iz and 1.9-
GHz bands.
CDMA employs analog-to-digital conversion (ADC) in combination
with spread spectrum technology. Audio input is first digitized into
binary elements. The frequency of the transmitted signal is then made
to vary according to a defined pattern (code), so it can be intercepted
only by a receiver whose frequency response is programmed with the
same code, so it follows exactly along with the transmitter frequency.
There are trillions of possible frequency-sequencing codes; this
enhances privacy and makes cloning difficult.
The CDMA channel is nominally 1.23 MHz wide. CDMA networks
use a scheme called soft handoff, which minimizes signal breakup as
a handset passes from one cell to another. The combination of digital
and spread spectrum modes supports several times as many signals
per unit bandwidth as analog modes. CDMA is compatible with other
cellular technologies; this allows for nationwide Roaming.
CDMA
ACKNOWLEDGEMENT
I thank God Almighty for the successful completion of
my seminar.
I express my sincere gratitude to Dr. M N Agnisharman
Namboothiri, Head of the Department, Information Technology.
I am deeply indebted to Staff-in-charge, Miss. Sangeetha Jose
and Mr. Biju, for their valuable advice and guidance. I am also
grateful to all other members of the faculty of Information
Technology department for their co-operation.
Finally, I wish to thank all my dear friends, for their
whole-hearted co-operation, support and encouragement.