What is Digital Modulation

7/27/2019 What is Digital Modulation

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What is digital modulation ?The techniques used to modulate digital information so that it can be transmitted via microwave, satellite or

down a cable pair are different to that of Analog transmission.

Advantages of Amplitude-shift keying (ASK): The main advantage of ASK modulation is generation of ASKis very much easy. Both ASK modulation and demodulation processes are relatively inexpensive. The ASK

technique is also commonly used to transmit digital data over optical fiber. There are many other advantages of

ASK, Such as Amplitude-shift keying transmitters are very simple and transmitter current is lower than FSKOne important advantage of ASK is it need lees bandwidth than FSK.

Disadvantages of Amplitude-shift keying (ASK): Unfortunately, ASK is linear and sensitive to atmosphericnoise, distortion, propagation condition on different routes in PSTN. It requires excessive bandwidth and is

therefore a waste of power.

Frequency-shift keying (FSK):-In "frequency shift keying (FSK)", the frequency of a sinusoidal carrier is shifted between two discrete values.

One of these frequencies (f 1) represents a binary "1" and the other value (f 0) represents a binary "0". The

representation of digital data using FSK is as shown in Fig. Note that there is no change in the amplitude of the

carrier.

Advantages of FSK:

• FSK is relatively easy to implement.

• It has better noise immunity than ASK. Therefore the probability of error free reception of data is high.

Disadvantages of FSK:



1. The major disadvantage is its high b

2. Therefore FSK is extensively used i

3. The FSK is not preferred for the

increases.

4. This increases the channel bandwidt

5. As the telephone lines have a v

requirement of FSK at higher speed. T

Bi

Cons

BPSK (also sometimes called PRK, pha(PSK). It uses two phases which are sep

matter exactly where the constellation po0° and 180°. This modulation is the mdistortion to make the demodulator rea

bit/symbol (as seen in the figure) and so i

In the presence of an arbitrary phase-shi

to tell which constellation point is which.BPSK is functionally equivalent to 2-QA

The gene

andwidth requirement as' discussed earlier.

low speed modems having bit rates below 1

high speed modems because with increase

h required to transmit the FSK signal.

ry low bandwidth, it is not possible to

herefore FSK is preferred only for the low s

ary phase-shift keying (BPSK)

ellation diagram example for BPSK.

e reversal keying, or 2PSK) is the simplest forated by 180° and so can also be termed 2-PS

ints are positioned, and in this figure they are sst robust of all the PSKs since it takes the hch an incorrect decision. It is, however, onl

s unsuitable for high data-rate applications.

t introduced by the communications channel, t

As a result, the data is often differentially encoM modulation.

ral form for BPSK follows the equation:

2

00 bits/sec.

in speed, the bit rate

satisfy the bandwidth

eed modems.

m of phase shift keying . It does not particularly

hown on the real axis, at ighest level of noise or

able to modulate at 1

e demodulator is unable

ded prior to modulation



The bit error rate (

or

Since there is only one bit per symbol, th

Qua

Constellat

Each adjacent symbol only differs by on

PSK, or 4-QAM. (Although the root con

waves are exactly the same.) QPSK useWith four phases, QPSK can encode tw

the bit error rate (BER) — sometimes mi

The mathematical analysis shows that Q

system while maintaining the same bandbandwidth needed. In this latter case, the

differently is a common confusion when

The implementation of QPSK is more

higher-order PSK. Writing the symbols i

to transmit them:

The probability of symbol error may be a

QAM

BER) of BPSK in AWGN can be calculated as:

is is also the symbol error rate.

rature phase-shift keying (QPSK)

ion diagram for QPSK with Gray coding.

bit.Sometimes this is known as quaternary P

cepts of QPSK and 4-QAM are different, the r

s four points on the constellation diagram, equbits per symbol, shown in the diagram with

sperceived as twice the BER of BPSK.

PSK can be used either to double the data rate

idth of the signal, or to maintain the data-rateBER of QPSK is exactly the same as the BER

onsidering or describing QPSK.

general than that of BPSK and also indicate

n the constellation diagram in terms of the sin

pproximated:

Quadrature amplitude modulation)

3

[5]

K, quadriphase PSK, 4-

sulting modulated radio

ispaced around a circle ray coding to minimize

compared with a BPSK

of BPSK but halving the of BPSK - and deciding

the implementation of

and cosine waves used



QAM (quadrature amplitude modulationa single channel, thereby doubling the

(PAM) in digital systems, especially in w

In a QAM signal, there are two carriers,(one quarter of a cycle, from which the t

called the Q signal. Mathematically, on

cosine wave. The two modulated carrie

carriers are separated, the data is extracteinformation.

QAAlthough QAM appears to increase the e

both amplitude and phase variations, it

noise because the states are closer togedifferent decision point. Receivers for

amplifiers that are able to remove any a

case with QAM.The second limitation is also associated

modulated signal is amplified in a radio t

QAM that contains an amplitude compoless efficient and consume more power, a

Domestic satellite c

A potential domestic satellite communi

earth stations. The system is compose

terrestrial telephone network through a santenna) earth terminals located near th

Two types of demand assignment mul

utilization efficiency and reduce interfervariable destination multiple access for

multiple access (SSMA) in the small ea

code synchronizing system and an AFCsatellite have demonstrated the simultan

SSMA code synchronization and AFC ch

The communications subsystem is the m

• It is usually composed by:

is a method of combining two amplitude-modeffective bandwidth. QAM is used with puls

ireless applications.

each having the same frequency but differingrm quadrature arises). One signal is called the

of the signals can be represented by a sine

s are combined at the source for transmission

d from each, and then the data is combined int

advantages and disadvantages fficiency of transmission for radio communicat

has a number of drawbacks. The first is that i

ther so that a lower level of noise is neededse with phase or frequency modulation are b

mplitude noise and thereby improve the noise

ith the amplitude component of the signal. W

ransmitter, there is no need to use linear amplifi

nent, linearity must be maintained. Unfortunatnd this makes them less attractive for mobile a

ommunication system using small earth stati

ations system is presented which can accom

of a master earth station with a 12.8-m a

atellite telephone switching center, and up to 1subscribers which can transmit up to three t

iple access techniques are utilized in the sy

nce to other communication systems, namelyhe master earth station to small earth station li

rth station to master earth station link, for wh

system without pilot signal are employed. Eous operation of 50 channels, with satisfactor

aracteristics.

ASIC TRANSMISSION THEORY

jor component of a communication satellite pa

4

ulated (AM) signals into amplitude modulation

in phase by 90 degrees I signal, and the other is

ave, and the other by a

. At the destination, the

the original modulating

ons systems by utilising

t is more susceptible to

to move the signal to a oth able to use limiting

reliance. This is not the

en a phase or frequency

ers, whereas when using

ely linear amplifiers are plications.

ons

modate numerous small

tenna connected to the

00 small (2-m diameter lephone channels each

tem to improve power

time division multiplex- nk and spread spectrum

ich a high-speed SSMA

periments using the CS error rate performance

load.



– one or more antennas, which receive an

– a set of receivers and transmitters that

• The receiver-transmitter units are know

carriers. “Multiple accesses” is the techniseveral earth stations which contend for

• The sharing technique may be achieved

• by sharing the transponder’s band

• by sharing the transponder’s avail

• by allowing coded signals to over

station then separates the signals

In electronics, noise temperature is one

component or source. The power spectral

kelvins) that would produce that level of

where:

• is the power (in watts)

• is the total bandwidth (Hz) ove

• is the Boltzmann constant (1.3

• is the noise temperature (K)

d transmit over wide bandwidths microwave si mplify and retransmit the incoming signals.

as transponders. Signals transmitted to/from a

que to share the available capacity of a satelliteccessing it.

:

width in separate frequency slots (FDMA)

ability in discrete time slots (TDMA) lap in time and frequency (CDMA or spread sp

y recognizing which of the codes is destined fo

System Noise Temperature

ay of expressing the level of available noise po

density of the noise is expressed in terms of th

Johnson – Nyquist noise, thus:

r which that noise power is measured

81×10−23

J/K, joules per kelvin)

5

nals

satellite are known as

transponder among

ctrum). Each earth

r it.

er introduced by a

temperature (in



Thus the noise temperature is proportionthat would be absorbed from the compon

function of frequency, unlike that of an i

resistor at all frequencies.

The G/T of the satellite is the "figure of

noise that is added to the signal. The bigG/T is bigger, the resulting C/N in the up

The noise temperature of an amplifier isamplifiers in cascade, the noise temperat

where

• = resulting noise temperature

• = noise temperature of the firs

• = noise temperature of the sec

• = noise temperature of the thir

• = power gain of the first comp

• = power gain of the second co

D

The term uplink chain is used to refer to

frequency signal for sending out data. Thvary widely. The downlink chain is built

UPLINK CHAIN

How it works:

1. Digital data is sent to the modulat

Intermediate Frequency range (70

Broadcast to organize communic2. The Intermediate Frequency is pi

mixes the intermediate frequency

modulated data.3. Noise is removed from the signal

Klystron, Travelling Wave Tube

4. The final cleaned signal is transm5. The feed horn at the focal point o

focuses into a directional transmi

l to the power spectral density of the noise,ent or source by a matched load. Noise tempera

eal resistor which is simply equal to the actual

G/T ratio

erit" and gives you an idea of the ratio betwee

er the G/T the more gain and the less noise is alink that can be achieved will be higher with th

ommonly measured using the Y-factor methodre of the cascade can be calculated using the F

referred to the input

component in the cascade

nd component in the cascade

component in the cascade

onent in the cascade

mponent in the cascade

sign of Uplink and Downlink:-

he series of pieces of equipment that are used t

e description provided here is imprecise as theusing nearly the same equipment in reverse ord

or which takes the data and converts it into a m

-140 Mhz). The modulators use standards such

tion over the microwave link. ped to an "up converter" (usually via shielded c

with a higher frequency to produce a final freq

via either a band pass filter or other means and

r Solid State amplifier.

itted down the wave guide to the dish. the dish emits the high frequency radio transm

sion at the satellite.

6

. That is the power ure is generally a

emperature of the

the input gain and the

ded to the signal. If the same uplink power.

If there are multiple iis equation:[4]

produce a radio

xact configuration can r.

dulated signal in the

as Digital Video

axial cable) which

ency which carries the

hen it is amplified in a

ission, which the dish



7

DOWNLINK CHAIN

How it works:

1. The satellite transmits a signal containing data

2. The signal is received at the sattellite dish3. The signal is amplified and fed to the Down Converter

4. The Down Converter downmixes the signal to create an intermediate frequency5. The intermediate frequency is fed to the demodulator and converted into a data signal6. The datastream is forwarded into the network via a router.

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What is Digital Modulation