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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