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.