digital modulation techniques and ofdm

7/30/2019 digital modulation techniques and OFDM

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TOPIC: DIGITALMODULATIONTECHNIQUE-

ASK,PSK,FSK,QPSKAND OFDM

Presented by:

Sima Medhi(4086)

Smriti Rekha Goswami(4082)

Jyotirmoy Deka(4081)

Pankaj Changkakati(4079)


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Introduction

Modulation is the process by which some characteristics of carriersignal varies in accordance with a modulating signal.

In digital communication, the modulating signal consist of binarydata. This data is used to modulate a carrier wave with fixed

frequency.

There are 3 basic modulation technique:

> ASK,FSK,PSK


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ASK Amplitude shift keying is the simplest form of digital modulation technique.

In ASK the amplitude of the carrier is switched depending on the input digital signal keeping

phase and frequency constant.

Only one energy carrier there and it is switched ON and OFF for a specific duration

depending upon the input binary sequence. Hence it is also known as ON-OFF Keying (OOK).

ASK demonstrates poor performance, as it is heavily affected by noise.

The binary ASK signal s(t) is given by:

s(t)= for symbol 1

= 0 for symbol 0

Fig. Amplitude Shift keying waveforms. (a) Unmodulated

carrier.

(b) Unipolar bit sequence. (c) ASK Waveform.


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Fig. Generation of binary ASK. Fig. Coherent detection of binary ASK signals.

GENERATION AND DETECTION OF ASK SIGNAL:


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FSK

In FSK two sinusoidal carrier waves of the same amplitude but different frequencies are used torepresent binary symbols 1 and 0 of the message respectively.

The binary FSK wave S(t) may be represented as

S(t)= for symbol 1

= for symbol 0

Noise immunity is higher than ASK.

Bandwidth requirement is higher than ASK.

Baseband

Data

BFSK

modulatedsignal

1 10 0

fc2 fc2fc1 fc1

Fig. FSKwaveforms


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Fig. Block diagrams for (a) binary FSK transmitter

(b) Coherent binary FSK receiver


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In PSK the binary symbols 1 & 0 of the message modulate the phase of the carrier with

a phase difference of 180 degree keeping amplitude and frequency constant .

PSK signal S(t) is given by

S(t)= for symbol 1

= for symbol 0

It has the best noise performance among these three digital modulation technique.

Fig. Phase-shift keying waveforms. (a) Unmodulated carrier (b) Phase shift keying waveform.

PSK


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Fig. Detection of PSKFig. Generation of binary PSK


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Three constellation diagrams

c. FSK


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QPSKQuadrature Phase Shift Keying (QPSK) can be interpreted as two independent BPSK systems , one on

the I-channel and one on Q-channel

Also a type of linear modulation scheme

Quadrature Phase Shift Keying (QPSK) has twice the bandwidth efficiency of BPSK, since 2 bits aretransmitted in a single modulation symbol.

The phase of the carrier takes on 1 of 4 equally spaced values, such as whereeach value of phase corresponds to a unique pair of message bits.

The QPSK signal for this set of symbol states may be defined as:

/4,3/4,5/4,7/4

QPSK signal s(t) is given by:

s(t)= dibit 10

= dibit 00

= dibit 01

= dibit 11


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QPSK


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Fig. Signal Space/ constellation diagram for QPSK


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Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation

scheme

First break the data into small portions

Then use a number of parallel orthogonal sub-carriers to transmit the data

Multiple sub-channels (sub-carriers) carry samples sent at a lower rate

OFDM will provide much higher bandwidth efficiency.

OFDM is one of the main techniques proposed to be employed in Wireless Systems

OFDM


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14

Serial

to

Parallel

X0

XN-1

x0

xN-1

IFFT

Parallel

to

Serial

and

add CP

Add

CP

DACRF Section

Input

Symbols

OFDM Transmitter


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15

ADCand

Remove

CP

Serial to

ParallelFFT

Parallel

to Serial

and

Decoder

X0

XN-1

x0

xN-1

Output

Symbols

OFDM Receiver


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OFDM TRANSMISSION AND RECEPTIION


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OFDM

OFDM is a special case ofFrequency DivisionMultiplexing (FDM)

For FDM

No special relationship between the carrierfrequencies

Guard bands have to be inserted to avoidAdjacent Channel Interference (ACI)

For OFDM

Bandwidth efficient . This is due to thefact that in OFDM the spectra ofindividual subcarriers are allowed tooverlap. In fact, the carriers are carefullychosen to be orthogonal one another.

As it is well known, the orthogonalsignals do not interfere, and they can beseparated at the receiver by correlationtechniques.


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MULTI PATH EFECT AND FADING :

An OFDM signal offers an advantage in channel having frequency selective fading response.

Only very few subcarriers are affected all the others are perfectly ok that can be recovered.


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DELAYED SPREAD AND CYCLIC PREFIX TO MITIGATE IT IN OFDM

In composite, the splash of the first symbolbecome noise and affect at the beginning of the

next symbol.

To mitigate this noise at the front of the symbol,

the symbol is moved further to catch the delay

spread.

Cyclic prefix is done by circular convolution.

Orthogonality is lost when multipath channel isinvolved.

Cyclic prefix can restore the orthogonality.


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Amongst the above described modulation

techniques OFDM is the most efficient

technique .

OFDM is now used in 4G wireless

technology and still research is going on to

make it more efficient.

CONCLUSION


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digital modulation techniques and ofdm

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