Multiplexing and Multiple Acess

8/12/2019 Multiplexing and Multiple Acess

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Unit 4 : MULTIPLEXING AND

MULTIPLE - ACCESS

TECHNIQUES

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MULTIPLE - ACCESS TECHNIQUES

Syllabus Content (16HRS)

4.1 Explain multiplexing necessity & Types and multiple

access -1 Hr

4.2 Explain FDM, and TDM With diagram -1 Hr 4.3 FDM AT&T ANALOG Hierarchy & Applications of

FDM -1 Hr

4.4 & 4.5 Calculate the number of signals that can be

transmitted in a Given channel using FDM and TDMtechniques. - 2 Hr

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4.6 TDM Digital Hierarchy Types of TDM - 1 Hr

4.7 Explain Statistical Time Division Multiplexing- 1

Hr

4.8 Explain the types of Multiple Access - FDMA,TDMA -1 Hr

4.9 Describe the principle of spread-spectrum

communication -1 Hr 4.10 4.11 And distinguish between frequency

hopping & Direct-sequence system- 2Hr

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4.12 Explain CDMA -1 Hr

4.13 Compare the three major types of

Multiplexing -1 Hr

4.14 Compare the three major types of Multipleaccess- 1 Hr

4.15 Explain synchronous Transmission and

Framing -1 Hr 4.16 Discuss the advantages of synchronous

communication -1 Hr

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4.1 Explain multiplexing necessity &Types and multiple access

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Necessity of Multiplexing

It has been observed that most of the individual

data-communicating devices typically require

modest data rate. But, communication media

usually have much higher bandwidth. Forexample, we can use coaxial cable, optical fiber,

microwave which has several mega bytes of

bandwidth and as a consequence the

communication media provide you much higherbandwidth. On the other hand individual users

have lesser data to send.

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And as a consequence, two communicating

stations do not utilize the full capacity of a data

link.

Another observation is that higher the data ratethe most cost effective is the transmission

facility. For example if the data rate is small then

the cost per byte or per kilo byte is more and if

the capacity is large say several Gigabytes then

the cost per byte or cost per kilo byte is much

less.

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So, based on this observation it has been found

that we can use a technique known as

multiplexing.

Multiplexing is used in situations where thetransmitting media is having higher bandwidth,

but the signals have lower bandwidth. Hence

there is a possibility of sending number of

signals simultaneously over the sametransmitting media.

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Essentially we are trying to share the channel

capacity or bandwidth of a particular media by

several signals by several users. That means we have

to utilize the channel capacity fully. Multiplexing can be used to achieve following goals

To send a large number of signals simultaneously.

To reduce the cost of transmission To make effective use of the available bandwidth

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Types of Multiplexing

Multiplexing may be defined as a technique whichallows many users to share a commoncommunication channel simultaneously.

TYPES OF MULTIPLEXING

Frequency Division Multiplexing

Wave length Division Multiplexing

Time Division Multiplexing

i) Synchronous Time Division Multiplexingii) Asynchronous Time Division Multiplexing or

Statistical Time Division Multiplexing

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Multiple access

Multiple Access: - Multiple Access is the method

in which a transmission medium is access by

different users by sharing the resources of the

medium. A transmission channel could bewireless or wired channel. User can shares the

bandwidth, time or power capacity of these

channels transmitting the signals trough them.

Types of Multiple Access: - FDMA,TDMA,CDMA

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4.2 Explain FDM, and TDM With diagram

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In frequency division multiplexing, the available bandwidth

of a single physical medium is subdivided into several

independent frequency channels. Independent message

signals are translated into different frequency bands using

modulation techniques, which are combined by a linear

summing circuit in the multiplexer, to a composite signal.

The resulting signal is then transmitted along the single

channel by electromagnetic means as shown in figure.


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The carriers used to modulate the individual message signals are

called sub-carriers, shown as f1, f2, , fn in above Fig.


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At the receiving end the signal is applied to a

bank of band-pass filters, which separates

individual frequency channels. The band pass

filter outputs are then demodulated anddistributed to different output channels as

shown in above Fig.(FDM Demultiplexing

process)

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If the channels are very close to one other, it leads to

inter-channel cross talk. Channels must be separated

by strips of unused bandwidth to prevent inter-channelcross talk. These unused channels between each

successive channel are known as guard bands as

shown in Fig.


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FDM can be used with analog signals. A number of signalscarried simultaneously on the same communication channel

by allocating to each signal a different frequency band.

FDM are commonly used in radio broadcasts and TV

networks.

Wavelength-division multiplexing (WDM) is conceptually same

as the FDM, except that the multiplexing and demultiplexing

involves light signals transmitted through fiber-optic channels.

WDM is a process in which different sources of information

(channels) are propagated down an optical fiber on differentwavelengths where the different wavelengths do not interfere

with each other.

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In frequency division multiplexing, all signals

operate at the same time with different

frequencies, but in Time-division multiplexing all

signals operate with same frequency at differenttimes.

This is a base band transmission system, where

an electronic commutator sequentially samples

all data source and combines them to form acomposite base band signal.

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Now this composite base band signal travels

through the media and is being demultiplexed

into appropriate independent message signals

by the corresponding commutator at thereceiving end.

The incoming data from each source are briefly

buffered. Each buffer is typically one bit or one

character in length.

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The buffers are scanned sequentially to form a

composite data stream.

The scan operation is sufficiently rapid so that

each buffer is emptied before more data canarrive.

The composite signal can be transmitted directly

or through a modem

The multiplexing operation is shown in Fig.

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As shown in the Figure the composite signal has some

dead space between the successive sampled pulses,

which is essential to prevent inter-channel cross talks.


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Along with the sampled pulses, onesynchronizing pulse is sent in each cycle. These

data pulses along with the control information

form a frame.

Each of these frames contain a cycle of time

slots and in each frame, one or more slots are

dedicated to each data source.

The maximum bandwidth (data rate) of a TDMsystem should be at least equal to the same

data rate of the sources.

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TDM is called synchronous mainly because

each time slot is preassigned to a fixed source.

The time slots are transmitted irrespective of

whether the sources have any data to send ornot. Hence, for the sake of simplicity of

implementation, channel capacity is wasted.

Both multiplexing and demultiplexingoperation for synchronous TDM are shown in

figure below.

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TDM (also known as synchronous time

division multiplexing)-TDM can be used with

digital signals or analog signals carrying digital

data. In TDM, data from various sources arecarried in respective frames. Each frame

consists of a set of time slots, and each source

is assigned a time slot per frame.

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4.3.FDM AT&T ANALOG Hierarchy &

Applications of FDM

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FDM AT&T ANALOG Hierarchy

For better utilization of the infrastructure,

analog signals are multiplexed to provide lines

of higher bandwidth.

FDM is used to combine many lines into fewer

lines in a hierarchical manner. The hierarchical

system used by AT&T comprises of groups,

super groups, master groups and jumbogroups.

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In FDM AT&T analog hierarchy,12 voice

channels (each of 4KHz ) are multiplexed onto

a higher bandwidth line to form a group. A group

has 48KHz(12X4KHz) of bandwidth andsupports 12 voice channels.

At the next level, five such groups can be

multiplexed to form a Supergroup. A super group

has bandwidth of 240 KHz (5X48KHz) andsupports 60 voice channels.

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At the next level, 10 supergroups are

multiplexed to create a master group. A master

group must have 240MHz of bandwidth, but

need of guard bands between the supergroupsincreases the necessary bandwidth to 2.52MHz.

A master group supports up to 600 voice

channels.

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Finally, six master groups can be combined into

a jumbo group. A jumbo group must have

15.12MHz of bandwidth, but it is increased to

16.984MHz to allow for guard bands betweenthe master. A jumbo group supports up to 3600

voice channels.

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Applications of FDM

FDM are commonly used in radio broadcasts CableTV and TV networks.

The cable TV providers give one cable, one coaxial

cable to the premise connected to the TV. That one

coaxial cable apparently is carrying a number of

channels, may be hundreds of channels these days.

What is done once again is that, all the frequencies,

which can travel down this cable, are broken into anumber of logical channels and each channel is

dedicated to one particular station .That is how we

carry multiple video channels on a single cable.33


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4.4. & 4.5. Calculate the number of

signals that can be transmitted in agiven channel using FDM and TDMtechniques.

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Number of signals that can be transmitted

in a given channel using TDM techniques-T1 system

Let us consider TDM-PCM T1 system developed

by Bell Telephone laboratory for transmitting

telephone signals by high speed digital

transmission link. The T1 system multiplexes 24 voice channels

onto a single line using TDM techniques. Each

voice signal is sampled at 8KHz that is sampled

every 125 S (i.e.; sampling interval=1/8KHZ= 125S )

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Number of signals that can be

transmitted in a given channel usingTDM techniques-T1 system

These samples are then converted to serial

digital words by Analog to Digital converter and

then transmitted one after another sequentially.

Each sample is an 8 bit word where 7 bits ofmagnitude and 1 bit of supervisory purposes.

As there are 8 bits/Channel(sample) and since

24 channels are multiplexed total number of bits

can be given as,

Total number of bits= 24X8=192 bits

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transmitted in a given channel usingTDM techniques-T1 system

One additional bit is then added to this train of

pulses for maintaining synchronization between

transmitter and receiver. Hence this pulse is

known as Synchronizing pulse and it is alsoknown as Framing bit. Framing bit is added at

the beginning of each frame. Thus one frame

has a total of 193 bits.

Total number of bits/ frame = 193 ,

Sampling rate=8KHz ,

then the actual data rate = 1.544Mbps(193X8)

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transmitted in a given channel usingTDM techniques-T1 telephone system

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transmitted in a given channel usingTDM techniques

Q. Calculate the maximum number of messages, eachof bandwidth 4KHz which can be transmitted using

binary PCM-TDM system with 256 quantum levels andpulse allocation width of 0.625S. ?

Ans:- With 256 quantization levels used, the number of

pulses used in one group ,P, is given by

P=log2(No. of levels)=log2(256) =8

each pulse has a pulse duration of 0.625 S.

Time for each pulse group = (0.625S)8 =5 S.

Message bandwidth =4KHz, then sampling frequency (using

Nyquist criteria) = 2X4KHz=8KHz.

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transmitted in a given channel usingTDM techniques

Sampling interval(time period between two samples)

= 1/sampling frequency = 1/8KHz = 1/(8x103) =125 S.

Hence total number of message can be transmitted

= 125 S/5 S= 25.

Here each code group is using 8 pulse, actually 7

pulses are used for coding and the 8th one is reserved

for synchronization. Similarly out of 25 message which

could be transmitted, one is used signalling and frame

synchronization.

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transmitted in a given channel usingFDM techniques

Q. Five channels, each with a 100-kHz bandwidth, areto be multiplexed together. What is the minimum

bandwidth of the link if there is a need for a guard band

of 10 kHz between the channels to prevent interference?

Ans:- For five channels, we need at least four guardbands. This means that the required bandwidth is at

least

=(5 100) + (4 10) = 540 kHz,as shown in Figure below

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transmitted in a given channel usingFDM techniques

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4.6 TDM Digital Hierarchy Types of TDM

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TDM Digital Hierarchy

Telephone companies implemented TDM

through a hierarchy of digital signal called DS

service. The figure shows TDM digital hierarchy.

A DS-0 service is a single digital channel of64Kbps.

DS-1 is a 1.5444Mbps service. It can be used to

multiplexed 24DS-0 channels.1.544Mbps is 24

times 64 Kbps plus 8 Kbps of overhead.

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DS-2 is a 6.312 Mbpsservice. It can be used to

multiplexed 4 DS-1 channels(or 96 DS-0

channels). 6.312Mbps is 96times 64 Kbps plus 168

Kbps of overhead. DS-3 is a 44.376 Mbpsservice. It can be used to

multiplexed 7 DS-2 channels(or 28 DS-1channels

or 672 DS-0channel ). 6.312Mbps is 672times 64

Kbps plus 1.368 Mbps of overhead.

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DS-4 is a 274.176 Mbpsservice. It can be used to

multiplexed 6 DS-3 channels(or 42 DS-2channels

or168 DS-1channel or 4032 DS-0channel ).

274.176 Mbpsis 4032times 64 Kbps plus 16.128Mbps of overhead.

DS-0 , DS-1 etc are the names of service. To

implement these services telephone companies

use T-lines(T1 to T4). The capacities of T-linesprecisely match to data rates of DS lines.(see

table below)

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Types of TDM

Time Division Multiplexing can be classified as

i) Synchronous Time Division Multiplexing

ii) Asynchronous Time Division Multiplexing or

Statistical Time Division Multiplexing

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Types of TDM

Synchronous TDM is called synchronous mainly

because each time slot is preassigned to a fixed

source. The time slots are transmitted

irrespective of whether the sources have anydata to send or not. Hence, for the sake of

simplicity of implementation, channel capacity is

wasted.

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4.7. Statistical TDM

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Statistical TDM

Statistical Time Division Multiplexing :- One

drawback of the synchronous TDM approach, as

discussed earlier, is that many of the time slot in

the frame are wasted. It is because, if aparticular terminal has no data to transmit at

particular instant of time, an empty time slot will

be transmitted. An efficient alternative to this

synchronous TDM is statistical TDM, This is alsocalled asynchronous TDM or Intelligent TDM.

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Statistical TDM

It dynamically allocates the time slots on

demand to separate input channels, thus saving

the channel capacity.

In statistical multiplexing, the number of slots ineach frame is less than the number of input

lines. The multiplexer check each input lines in

round robin fashion and it allocates a slot for

input lines , if the line has data to send;otherwise it skips the line and check the next

line.

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Statistical TDM

In case of statistical TDM, the data in each slot

must have an address part, which identifies the

source of data. Since data arrive from and are

distributed to I/O lines unpredictably, addressinformation is required to assure proper delivery.

This leads to more overhead per slot. Relative

addressing can be used to reduce overheads.

In case of statistical TDM there is no need ofsynchronization bit at frame level.

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Statistical TDM

In case of synchronous TDM synchronization

and preassigned relationship between input and

output serve as an address. In this case there is

a need of synchronization bit or framing bit. The figure shows comparison between

synchronous TDM and statistical TDM

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TDM slot comparison

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4.8.Multiple AccessFDMA, TDMA

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Multiple AccessFDMA, TDMA

Multiple Access: - Multiple Access is the method

in which a transmission medium is acess by

different users by sharing the resources of the

medium. A transmission channel could bewireless or wired channel. User can shares the

bandwidth, time or power capacity of these

channels transmitting the signals trough them.

FDMA:- Frequency division multiple accessassigns an unique frequency band to each

users.

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Frequency division multiple access

These channels are assigned on demand tousers who request service. During the period of

communication or call, no other user can share

the same channel

The Bandwidth of the FDM channels are

relatively narrow. That is FDMA uses a narrow

band channels and its inter-symbol interference

is low. FDMA is a continuous transmission scheme,

such that synchronous bit or framing bits are not

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Frequency division multiple access

FDMA systems have higher cell site system cost

as compared to TDMA system. This is because

of FDMA system uses different carrier signaling

for different channel, use of more band passfilters and also use of duplexers.

Advantages:- Simplest method that could be

implemented with conventional telephony.

(AMPS) Disadvantages:-Inter modulation Distortion. Loss

of bandwidth due to guard band.

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Time division multiple access

TDMA:- Time division multiple access technique

allows different users to share a transmission

medium using different time slots for

transmission that is in TDMA bandwidth is timeshare. This method is suitable for digital signal.

Here time is divided into frames of equal

duration.

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TDMA frame is the period during which each

user gets a chance to transmit data at least

once. Each user occupies a cyclic repeating time

slot, so a channel may be thought of a particulartime slot that reoccurs in every frame.TDM

system transmit data in a buffer and burst

method. The transmission of any users is non

continuous.

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TDMA shares a single carrier frequency for

different users where each user makes use of

non overlapping time slots.

The no. of time slots per frame depends uponavailable bandwidth and modulation technique

used.

Data transmission for users of TDMA system is

non continuous but occurs in a buffer and burstmethod. This result in low battery consumption

and also hands off process is much simpler.

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TDMA uses different time slots for transmission

and reception, thus duplexer are not required.

Even if FDD is used, a switch is required to

switch between transmitter and receiver usingTDMA.

TDMA has an advantage in that it is possible to

allocate different number of time slots per frame

to different users.

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4.9.Principle of spread-spectrum

communication

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Spread-spectrum

Spread spectrum is a modulation and

multiplexing technique that distributes a signals

and its side bands over a very wide band width.

This is also known as wideband modulation. Spread spectrum multiple access uses signals

which have a transmission bandwidth much

greater than the minimum required bandwidth.

A pseudo-noise(PN) sequence convertsnarrowband signal to wideband noise like

signals before transmission.

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Spread-spectrum

Spread spectrum multiple access is not verybandwidth efficient when used by single user.

However, since many user can share the same

spread spectrum bandwidth without interfering

with one another, spread specrum systems

become bandwidth efficient in a multiple user

environment.

Spread spectrum multiple access Providesimmunity to all multiple interference and robust

multiple access capability

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Spread-spectrum

Spread spectrum is a means of transmission in

which the signal occupies a bandwidth in excess

of the minimum necessary to send the

information; the band spread is accomplished bymeans of a PN code which is independent of the

data, and a synchronized reception with the

code at the receiver is used for de-spreading

and subsequent data recovery.

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4.10 & 4.11. Distinguish between frequencyhopping & Direct-sequence system-

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Frequency hopping & Direct


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Frequency hopping & Direct-sequence system-

There are two main types of SSMA :

Frequency hopped multiple access(FHMA)

Direct sequence multiple access(DSMA)

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Frequency Hopping

Frequency hopping multiple access is a digitalmultiple access system in which carrier

frequency of individual users are varied in a

pseudorandom fashions (predetermined

sequence) within in a wide band channel.

It causes the carrier to hop from frequency to

frequency over a wide band according to a

sequence defined by the PN code. Frequency hopped signal changes channel at

rapid intervals.

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Frequency Hopping

Provides a level of security when a large numberof channels are used.

FHMA allows multiple user to simultaneously

occupy the same spectrum at the same time isshown in figure below

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Frequency Hopping

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Frequency Hopping

In a FH transmitter:

The digital data is broken into uniform sized

bursts which are transmitted on different carrier

frequencies.

The instantaneous bandwidth of any one

transmission burst is much smaller than the

total spread bandwidth.

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Frequency Hopping

The pseudorandom change of the carrier

frequencies of the user randomizes the

occupancy of a specific channel at any

given time.

In FH receiver:

A locally generated PN code is used tosynchronize the receivers instantaneous

frequency with that of the transmitter.

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Frequency Hopping

At any given point in time, a frequencyhopped signal only occupies a single, relatively

narrow channel.

FHMA systems often employ energy efficient

constant envelope modulation.

Linearity is not an issue, and the power ofmultiple users at the receiver does not

degrade FHMA performance.

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Direct sequence Spread Spectrum

In Direct sequence spread spectrum the serialbinary data is mixed with a higher frequency

pseudorandom binary code at a faster rate

and result is used to phase modulated carrier. Direct sequence multiple access is also called

CDMA.

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Distinguish between frequency


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hopping & Direct-sequencesystem

Parameters DS FH

Bandwidth PN sequence clock rate

or chip rate

The tuning range of

frequencies

Synchronization Very critical Less critical

Spectrum Very wide narrow

Near-far problem More likely to occur Less likely to occur

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Distinguish between frequency hopping


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Distinguish between frequency hopping& Direct-sequence system

FHSS:In FHSS systems, the two modulationprocesses are as follows:

The original message modulates the carrier,

thus generating a narrow band signal.

The frequency of the carrier is periodically

modified (hopped) following a specific

spreading code.(In FHSS systems, thespreading code is a list of frequencies to be

used for the carrier signal).

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The amount of time spent on each hop isknown as dwell time.

Redundancy is achieved in FHSS systems by

the possibility to execute re-transmissions onfrequencies (hops) not affected by noise.

DSSS:- In DSSS systems, the two modulation

processes are as follows

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The original message is modulated by thespreading code. In DSSS systems, the

spreading code is a sequence of bits (known

as chips), and the first modulation step is aXOR operation executed between the message

and the spreading code (process known as

"chipping"). The result of the first modulationstep is that a "0" bit of message is converted

into a chip sequence representing the "0" bit.

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"1" bit of message is converted into anotherchip sequence, representing the "1" bit.

Instead of transmitting the original message

bit, a chip sequence representing the bit willbe transmitted.

The sequences representing the message bits

modulate the carrier signal.

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Redundancy is achieved in DSSS systems bythe presence of the message bit on each chip

of the spreading code. Even if some of the

chips of the spreading code are affected bynoise, the receiver may recognize the

sequence and take a correct decision

regarding the received message bit.

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Noise and Interference Immunity

The issue: Capacity of the system to operate

without errors when other radio signals are

present in the same band. FHSS systems operate with SNR (Signal to

Noise Ratio) of about 18 dB.

DSSS systems, because of the more effective

modulation technique used (PSK), can operate

with SNR as low as 12 dB.

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Near / Far problem

The issue: The problems generated in DSSS

systems by transmitters located close to

receivers of other systems are known as Near /Far problems. The interfering signals described

above may be generated for example by the

transmitter of one system (System A) located

close to the receiver of a different system(system B).

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4.12 Explain CDMA -1 Hr

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CDMA


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CDMA

CDMA:-Code division multiple access is atechnique in which user can use entire

bandwidth all the time. This is achieved by

combining the data with a PN code (PseudoNoise) at higher rate than the data rate. Thus

using available spectrum fully since PN

modulation spreads the signal spectrum. This is

also known as Spread Spectrum multipleaccess.

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CDMA


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CDMA

The spreading signal is a pseudo-noise codesequence that has a chip rate which is orders ofmagnitudes greater than the data rate of themessage.

All users in CDMA system, use the same carrierfrequency and may transmit simultaneously.

In CDMA, the narrowband message signal ismultiplied by a very large bandwidth signal

called the spreading signal.

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CDMA


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CDMA

Each user has its own pseudorandom codewordwhich is approximately orthogonal to all othercode words.

The receiver performs a time correlation

operation to detect only the specific desiredcodeword.

The near-far problem

when many mobile users share the samechannel, the strongest received mobile signal

will capture the demodulator at a base station

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CDMA


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CDMA

In CDMA, stronger received signal levels raise the noisefloors at the base station demodulators for the weaker

signals, thereby decreasing the probability that weaker

signals will be received.

Solution: power control

Power control is provided by each base station in a

cellular system and assures that each mobile within the

base station coverage area provides the same signal

level to the base station receiver. This solves the

problem of a near by subscriber over powering the base

station

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CDMA


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CDMA

CDMA has a soft capacity. Increasing the number of

users in a CDMA system raises the noise floor in a

linear manner.

Multipath fading may be substantially reduced, becausethe signal is spread over a large bandwidth

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Multiplexing

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TDM Vs FDM


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TDM Vs FDM


1.It is a technique for transmitting several massages on

one channel by dividing time domain slots. One slot for

each massage.

2. It requires commutator at the transmitting end and adistributor, working in perfect synchronization with

commutator at the receiving end.

3. perfect synchronization b/w transmitter and receiver is

required.

4. crosstalk problem is not severe in TDM .

5. It is usually preferred for digital signal transmission.

6. It does not require very complex circuitry

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TDM Vs FDM


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TDM Vs FDM

In FDM technique to transmit several messages on onechannel, message signals are distributed in frequency

spectrum such that they do not overlap.

FDM requires modulator, filters and demodulators.

Synchronization b/w transmitter and receiver is notrequired.

FDM suffers from crosstalk problem due to imperfect

bandpass filter.

It is usually pre ferred for analog signal transmission.

It requires complex circuitry at transmitter and reciver.

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FDM Vs WDM


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FDM Vs WDM

Wave-division multiplexing (WDM) isconceptually the same as FDM, except that

the multiplexing and demultiplexing involves

light signals transmitted through fiber-opticschannels. The idea is same that is combining

different frequency signals. However, the

difference is that the frequencies are veryhigh.

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Multiple access

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Comparison of FDMA, TDMA,


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CDMA

Feature FDMA TDMA CDMAHigh carrier

frequency stability

Required Not necessary Not necessary

Timing/synchroni

zation

Not required Required Required

Near-far problem No No Yes,power controltech.

Variable

transmission rate

Difficult Easy Easy

Fading mitigation Equalizer not

needed

Equalizer may be

needed

RAKE receiver

possiblePower monitoring Difficult Easy Easy

Zone size Any size Any size Large size difficult

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4.15 Explain synchronous Transmissionand Framing

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Synchronous Transmission


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yand Framing

Synchronous Transmission:- In SynchronousTransmission start and stop bits are not used.

Characters are sent in groups called blocks and special

synchronization characters (pre-determined group of

bits) are also can be send along with the blocks toachieved the synchronization between transmitter and

receiver.

Synchronous Transmission is used in high speed data

transmission e.g.; Ethernet ,token Ring.

Synchronous Transmission is more efficient and but

expensive to implement.

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Synchronous Transmission


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yand Framing

To ensure that the receiver correctly reads the incomingbits, i.e., knows the incoming bit boundaries to interpret a

1 and a 0, a known bit pattern is used between the

frames.

The receiver looks for the anticipated bit and startscounting bits till the end of the frame.

Then it starts over again with the reception of another

known bit.

These bits (or bit patterns) are called synchronizationbit(s) or Farming bits (end of a frame)

They are part of the overhead of transmission.

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Framing bits


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Framing bits

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4.16 Discuss the advantages ofsynchronous communication -

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Advantages of synchronous


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communication

Synchronous Transmission is used in high speed datatransmission e.g.; Ethernet ,token Ring.

Synchronous Transmission is more efficient, that is

overhead is less in compare to Asynchronous

Transmission. It can also perform error detection as transmitter and

receiver are synchronized with each other.

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QUIZ


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QUIZ

In FDM hierarchy total no of channels inMaster group are

a) 12

b) 600 c) 60

d) 100

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QUIZ


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QUIZ

In synchronous TDM time slot are ---------

a) plenty

b) wasted

c) utilized fully d) none

105

QUIZ


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QUIZ

Asynchronous TDM time slots are ------

a) plenty

b) wasted

c) utilized fully

d) none

106

QUIZ


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QUIZ

10 In North american hierarchy no ofchannels the basic group contains ---

a) 12

b) 24

c) 30

d) 50

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QUIZ


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QUIZ

A multiplexing technique where time slotsare allocated on demand basis is ----

a ) TDM

b) Svnchronous TDM

c ) Asynchronous TDM

d ) SDM

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QUIZ


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QUIZ

14 A multiplexing system where signals areseparated on frequency slots but jumbled

together in time domain is ---------

a ) FDM b) TDM

c ) WDM

d ) SDM

109

QUIZ


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QUIZ

16 An example for FDM is

a) Telephone system

b) Cable TV System

c) RADAR

d ) none

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QUIZ


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QUIZ

15 A multiplexing system where signals areseparated on Time slots but jumbled

together in frequency domain is ---------

a ) FDM b) TDM

c ) WDM

d ) SDM

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QUIZ


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QUIZ

17 An example for TDM is

a) Telephone system

b) Cable TV System

c) RADAR

d ) none

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QUIZ


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QUIZ

19 Frequency assignment multiple access isrepresented as

a) TDMA

b) FDMA

c) CDMA

d) FAMA

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QUIZ


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QUIZ

Spread spectrum technology is used in ---------

a) TDMA

b) FDMA

c) CDMA

d) FAMA

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QUIZ


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QUIZ

Frequency Division multiple access isrepresented as

a) TDMA

b) FDMA

c) CDMA

d) FAMA

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QUIZ


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QUIZ

Time Division multiple access isrepresented as

a) TDMA

b) FDMA

c) CDMA

d) FAMA

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Review Question


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Q

In what situation multiplexing is used? Ans:-Multiplexing is used in situations where the

transmitting media is having higher bandwidth, but the

signals have lower bandwidth. Hence there is a

possibility of sending no of signals simultaneously.

Multiplexing can be used to achieve following goals

To send a large number of signals simultaneously.

To reduce the cost of transmission

To make effective use of the available bandwidth

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Review Question


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Review Question

Q. Why sync pulse is required in TDM?

Ans: In TDM, in each frame time slots are pre-assigned

and are fixed for each input sources. In order to

identify the beginning of each frame, a sync pulse isadded at the beginning of every frame.

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Review Question


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Review Question

Q. How is the wastage of bandwidth in TDM isovercome by Statistical TDM?

Ans: It dynamically allocates the time slots on demand to

separate input channels, thus saving the channel

capacity. As with Synchronous TDM, statistical

multiplexers also have many I/O lines with buffer

associated to each of them. During the input, the

multiplexer scan the input buffers, collecting data untill

the frame is filled and send the frame.At the receiving

end, the demultiplexer receive the frame and distributesthe data to the appropriate buffers.

In case of statistical TDM, the data in each slot must

have an address part, which identify the sorce of data.119

Review Question


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Review Question

Q. Why guard bands are used in FDM?

Ans: In FDM, a number of signals are sent

simultaneously on the same medium by allocating

separate frequency band or channel to each signal.

Guard bands are used to avoid interference between two

successive channels.

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Review questions


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q

Compare between FDMA and FDM?

Compare between FDM,TDM?

Compare FDMA,TDMA and CDMA ? What do you mean by multiple access? Explain

FDMA,TDMA, CDMA ?

Explain the use TDM in telephony and its digital

hierarchy?

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References


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Data Communications and Networking ByBehrouz A.Forouzan

Wireless Communications Principles and

Practice Theodore S.Rappaport,

Data communication nptel lectures and Notes.

Date post:	03-Jun-2018
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Multiplexing and Multiple Acess

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