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USE OF MODULATION TECHNIQUES IN

TELECOMMUNICATION

Varun Kumar Sen, RB6703B50, 3460070010Dept. of ECE, Lovely Professional University, Phagwara, Punjab. 144402

e-mail id: varunsen@ymail.com

ABSTRACT:

Modulation techniques are methods used to encode

digital information data in analog information. This

term paper describes in detail various digital

modulation techniques for telecommunication.Among others, these include quardrature phase shift

keying(QPSK), used in second generation digital

cellular mobile systems in North America and Japan,

Gaussian minimum-shift keying (GMSK), employed

in the GSM system in Europe. In this paper I explain

all these techniques and application in

telecommunication system. In telecommunications,

modulation is the process of varying a periodicwaveform, i.e. a tone, in order to use that signal toconvey a message, in a similar fashion as a musician

may modulate the tone from a musical instrumentby

varying its volume , timing andpitch.Normally a

high-frequencysinusoidwaveformis used as carrier

signal. The three keyparameters of a sine wave are

its amplitude ("volume"), itsphase ("timing") and its

frequency ("pitch"), all of which can be modified in

accordance with a low frequency informationsignal

to obtain the modulated signal.

1. INTRODUCTION:

The techniques used to modulate digital information

so that it can be transmitted via microwave, satellite

or down a cable pair is different to that of analogue

transmission. The data transmitted via satellite ormicrowave is transmitted as an analogue signal. The

techniques used to transmit analogue signals are used

to transmit digital signals. The problem is to convert

the digital signals to a form that can be treated as an

analogue signal that is then in the appropriate form to

either be transmitted down a twisted cable pair or

applied to the RF stage where is modulated to a

frequency that can be transmitted via microwave or

satellite. The equipment that is used to convert digitalsignals into analogue format is a modem. The word

modem is made up of the words modulator and

demodulator. A modem accepts a serial data stream

and converts it into an analogue format that matches

the transmission medium. In the selecting a suitable

modulation technique for telecommunication system,

consideration must be given to achieving the

following:

1. high bandwidth efficiency

2. high power efficiency

3. low carrier-to-co channel interference power

ratio

4. low out-of-band radiation

5. low sensitivity to multipath fading

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6. low cost and ease implementation

To optimize all these features at the same time is not

possible as each has its practical limitation and also is

related to the others. For example, to achieve high

bandwidth efficiency one may choose to use high-

level modulation. However, if this is done two

consequent disadvantages are introduced. Firstly, the

power efficiency of the system is reduced. Secondly,

the bandlimited high level modulated signal has a

large envelope variation which, when the signal is

passed through a power efficiency nonlinear

amplifier, generates large out of band radiation, this

in turn, introduces interference to adjacent channels.

The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel, for

example over thepublicswitched telephone network

(where a filter limits the frequency range to between

300 and 3400 Hz) or a limited radio frequency band.

On other, telecommunication is the assisted

transmission over distance for the purpose of

communication.

2. THEORY:

Modulation is the process of varying one waveform

in relation to another waveform and the device that

performs modulation is known as a modulation and

device that performs the inverse operation of

modulation is known as a demodulator. And the

device that can perform both operations is a modem.

There are basically three types of modulation:

1). Digital modulation:

It is also known as digital to analog conversion. In

this type of modulation analog carrier signal is

modulated by a digital bit stream.

2). Analog modulation:

In this type of modulation, modulation is applied

continuously in response to the analog information

signal.

3). Pulse modulation:

This method is used for transfer a narrowband analog

signal.

2.1 MODULATION TECHNIQUES:

We have three basic modulation techniques:

a. AM (amplitude modulation)

b. FM (frequency modulation)

c. PM (phase modulation)

In all mentioned techniques a carrier signal is

signal frequency that is used to carry data.

a. Amplitude modulation:

In amplitude modulation technique, it modified

the amplitude of the carrier to represent 1 or 0.

Fig. 1: Amplitude Modulation

This modulation technique is simple to design

but noise spikes on transmission medium

interfere with carrier signal.

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b. Frequency modulation:

This technique modified the frequency of the carrier

signal to represent the 1s or 0s.

Fig. 2: Frequency Modulation

Half Duplex FSK

Fig. 3: Simplex/Half Duplex FSK

In this diagram, 0 is represented by original carrier

frequency and 1 by higher frequency.

This technique has great immunity to noise on

transmission medium and loss of signal easily

detected.

Frequency Shift Keying or FSK is the frequency

modulation of a carrier to represent digital

intelligence. For Simplex or Half Duplex operation, a

single carrier (1170 Hz) is used - communication can

only be transmitted in one direction at a time. A Mark

or 1 is represented by 1270 Hz, and a Space or 0 is

represented by 1070 Hz. The following diagram

shows the Voice Channel with Simplex/Half Duplex

FSK.

Full Duplex FSK

For Full Duplex, (data communication in both

directions simultaneously) the upper bandwidth of

the Voice Channel is utilized. Another carrier is

added at 2125 Hz. A Mark or 1 is represented by

2225 Hz, and a Space or 0 is represented by 2025 Hz.

The phone number and starts the connection) uses the

lower carrier (1170 Hz) and the answer modem (the

one which answers the ringing phone line) uses the

upper carrier (2125 Hz). This allocation of carriers is

done automatically by the modem's hardware. The

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following diagram shows the Voice Channel with

Full Duplex FSK.

Fig.4: Full Duplex FSK

Example of Originates Frequency Modulated Carrier:

Fig. 5: Frequency Modulation Carrier

The originate modem transmits on the 1170 Hz

carrier and receives on the 2125 Hz carrier. The

answer modem receives on the 1170 Hz carrier and

transmits on the 2125 Hz carrier. This way both

modems can be transmitting and receiving

simultaneously.

Fig. 6: FSK Modem

The FSK modem described above is used for 300

baud modems only. We can not use this for higher

modem because higher data rates require more

bandwidth: this would require that the Mark and

Space frequencies for each band be moved farther

apart (the originate and answer bands become wider).

The two carriers would have to move farther apart

from each other to prevent crosstalk (interference

with each other). The limit for present phone lines is

1200 Baud Half Duplex (one way) used by Bell 202

compatible modems.

c. Phase modulation:

This technique modified the phase of the carrier

to represent 0s and 1s.

Fig.7: Phase Modulation

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In this technique, carrier phase shift at every

occurrence of the 1 bit but remains unaffected for 0

bit.

Any digital modulation scheme uses a finite number

of distinct signals to represent digital data. PSK uses

a finite number of phases; each assigned a unique

pattern of binary bits. Usually, each phase encodes an

equal number of bits. Each pattern of bits forms the

symbol that is represented by the particular phase.

The demodulator, which is designed specifically for

the symbol-set used by the modulator, determines the

phase of the received signal and maps it back to the

symbol it represents, thus recovering the original

data. Aim of pulse modulation method to transfer a

narrowband analog signal. For example, a cell phone

over a wideband low pass channel or, in some of

scheme, as a bit stream over another digital

transmission system.

Phase modulation further categories into following

methods:

a. BSK- Binary shift keying

b. QPSK - Quardrature Phase Shifted Keying

1. BSK- Binary Shift Keying:

BPSK (also sometimes called PRK, Phase Reversal

Keying, or 2PSK) is the simplest form of phase shift

keying (PSK). It uses two phases which are separated

by 180 and so can also be termed 2-PSK. It does not

particularly matter exactly where the constellation

points are positioned, and in this figure they are

shown on the real axis, at 0 and 180. Thismodulation is the most robust of all the PSKs since it

takes the highest level of noise or distortion to make

the demodulator reach an incorrect decision. It is,

however, only able to modulate at 1 bit/symbol and

so is unsuitable for high data rate applications when

bandwidth is limited.

Fig. 8: Constellation diagram for BPSK

Implementation

Binary data is often conveyed with the following

signals:

for binary "0",for binary "1"

where fcis the frequency of the carrier-wave. Hence,

the signal-space can be represented by the single

basis function where 1 is represented by. This

assignment is, of course, arbitrary. The use of this

basis function is shown at the end of the next section

in a signal timing diagram. The topmost signal is a

BPSK-modulated cosine wave that the BPSK

modulator would produce.

2. QPSK - Quardrature Phase Shift Keying

Quardrature Phase Shift Keying employs shifting the

phase of the carrier at a 600 baud rate plus an

encoding technique. QPSK is used in Bell 212A

compatible modems and V.22 - both are 1200 bps

Full Duplex standards. The originate modem

transmits at 1200 Hz and receives on 2400 Hz. Theanswer modem receives on 1200 Hz and transmits on

2400 Hz.

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Fig. 9: QPSK Modem

The digital information is encoded using 4 (Quad)

level differential PSK at 600 baud.

Remember that baud indicates how fast the analog

signal is changing in the Voice Channel. The data is

encoded as follows:

DIBIT Phase Shift

00 +90

01 0

10 180

11 270

For every change in the baud rate (phase shift), we

can decode 2 bits. This leads to:

2 bits x 600 baud = 1200 bps

Example of Carrier Phase Modulation:

Fig. 10: Carrier Phase Modulation

3. Differential phase-shift keying (DPSK)

Differential phase shift keying (DPSK) is a common

form of phase modulation that conveys data by

changing the phase of the carrier wave. As mentioned

for BPSK and QPSK there is an ambiguity of phase if

the constellation is rotated by some effect in the

communications channel through which the signal

passes. This problem can be overcome by using the

data to change rather than set the phase. For example,

in differentially-encoded BPSK a binary '1' may be

transmitted by adding 180 to the current phase and a binary '0' by adding 0 to the current phase. In

differentially-encoded QPSK, the phase-shifts are 0,

90, 180, -90 corresponding to data '00', '01', '11',

'10'. This kind of encoding may be demodulated in

the same way as for non-differential PSK but the

phase ambiguities can be ignored. Thus, each

received symbol is demodulated to one of the M

points in the constellation and a comparator then

computes the difference in phase between this

received signal and the preceding one. The difference

encodes the data as described above. The modulated

signal is shown below for both DBPSK and DQPSK

as described above. It is assumed that the signal starts

with zero phase, and so there is a phase shift in both

signals at t = 0.

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QAM - Quardrature Amplitude Modulation

QAM Sometimes known as quaternary or

quadriphase PSK, 4-PSK, or 4- QAM [6], QPSK uses

four points on the constellation diagram, equispaced

around a circle. With four phases, QPSK can encode

two bits persymbol. Analysis shows that this may be

used either to double the data rate compared to a

BPSK system while maintaining the bandwidth of the

signal or to maintain the data-rate of BPSK but halve

the bandwidth needed.

Quadrature amplitude modulation (QAM) is both an

analog and a digital modulation scheme. It conveys

two analog message signals, or two digital bit

streams, by changing or modulating the amplitudes of

two carrier waves, using the amplitude-shift keying

(ASK) digital modulation scheme or amplitude

modulation (AM) analog modulation scheme. These

two waves, usually sinusoids, are out of phase with

each other by 90 and are thus called Quadrature

carriers or Quadrature components hence the name of

the scheme. The modulated waves are summed, and

the resulting waveform is a combination of both

phase-shift keying (PSK) and amplitude-shift keying,

or in the analog case of phase modulation (PM) and

amplitude modulation. In the digital QAM case, a

finite number of at least two phases and at least two

amplitudes are used. PSK modulators are often

designed using the QAM principle, but are not

considered as QAM since the amplitude of the

modulated carrier signal is constant. Quardrature

Amplitude Modulation refers to QPSK with

Amplitude Modulation. Basically, it is a mix of phase

modulation and amplitude modulation. QAM phase

modulates the carrier and also modulates the

amplitude of the carrier.

Fig. 11: Phase Modulated and Amplitude Modulated

Carrier

There are two types: 8-QAM and 16-QAM. 8-QAM

encodes 3 bits of data (23=8) for every baud and 16-

QAM encodes 4 bits of data (24=16) for every baud.

Both are used in the V.32 standard for 9600 bps

modem (milestone for communications). 8-QAMtransfers 4800 bps and 16-QAM transfers 9600 bps.

The baud rate used with QAM is 2400 baud half-

duplex. 16-QAM has 12 phase angles, 4 of which

have 2 amplitude values. 16-QAM changes phase

with every baud change.

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Fig. 12: 16-QAM Phasor Diagram

Higher transfer rates use much more complex QAM

methods. For example, V.32bis (14.4 kbps) uses a 64 point constellation to transfer 6 bits per baud.

Compare that to the above 16 point constellation.

Digital QAM

Like all modulation schemes, QAM conveys data by

changing some aspect of a carrier signal, or the

carrier wave, (usually a sinusoid) in response to a

data signal. In the case of QAM, the amplitude of two

waves, 90 degrees out-of-phase with each other (in

Quadrature) are changed or modulated to represent

the data signal. represent the data signal. Amplitude

modulating two carriers in quardrature can be

equivalently viewed as both amplitude modulating

and phase modulating a single carrier. Phase

modulation (analog PM) and phase-shift keying

(digital PSK) can be regarded as a special case of

QAM, where the magnitude of the modulating signal

is a constant, with only the phase varying. This can

also be extended to frequency modulation and

frequency-shift keying (FSK), for these can be

regarded as a special case of phase modulation.

Fig 13: An 8QAM

3 CONCLUSION:

The result shows that in telecommunications,

modulation is the process of varying a periodic

waveform, i.e. a tone, in order to use that signal to

convey a message, in a similar fashion as a musician

may modulate the tone from a musical instrument by

varying its volume, timing and pitch. Normally a

high-frequency sinusoid waveform is used as carrier

signal. This term paper introduces the concepts of

digital modulation used in many communications

systems today and modulation techniques like ASK,FSK, BPSK, QPSK, and QAM.

REFERENCES:

1. Watkins-Johnson Company Tech-notes,

Vol. 8, Page no. 5, 1981

2. Taub, Herbert and Donald L. Schilling,

Principles of Communication System,

Tata McGraw Hill Book Company, 2003

3. Federal Communication System

4. Electronics and Communication

Engineering Journal, Page no 125, June

1993

5. www.docstoc.com

6. www.slideshare .com

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