Modulation (6): Digital modulation

Luiz DaSilva Professor of Telecommunications dasilval@tcd.ie +353-1-8963660 Adapted from material by Dr Nicola Marchetti

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digital    modula,on  

discrete data (text, video, images)

continuous data (sound)

sample  &    

quan,za,on  

[1001100111000]

e.g.,  phase  (PSK),    amplitude/phase  (QAM)  

Digital modulation q The idea of modulation is still the same as in the

analog case – the amplitude, frequency, phase of an RF carrier or a combination of them, is varied according to the information to be transmitted

q Now, the input are discrete signals q Time sequence of pulses or symbols

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Advantages q Robustness to channel impairments

q Easier multiplexing of various sources of information: voice, text, video …

q Can accommodate digital error-control codes

q Enables encryption of the transferred signals (uses discrete math è cryptography)

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Line coding q Baseband signals represented as line codes

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Unipolar NRZ

Bipolar RZ

Manchester NRZ

Tb

Tb

Tb

V 0

V

-V V

-V

1 0 1 0 1 0 1

(N)RZ = (Non-) Return to Zero

Symbols and bits q  The modulating signal is represented as a time-sequence of symbols or pulses q  Each symbol has m finite states: That means each symbol carries n bits of information where n = log2m bits/symbol

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... 0 1 2 3 T

One symbol

Modulator  

Basic modulation schemes

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BASIC  MODULATION  SCHEMES  

Modulation schemes q  ASK = Amplitude Shift Keying q  FSK = Frequency Shift Keying q  PSK = Phase Shift Keying q  DPSK = Differential Phase Shift Keying q  MSK = Minimum Shift Keying (a form of FSK) q  CPM = Continuous Phase Modulation (a form of FSK) q  Hybrids

e.g., ASK+PSK = QAM = Quadrature Amplitude Modulation (phase and amplitude together)

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Applications

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Symbol rate q  In the general M-ary signaling case, the processor accepts k input bits at a time, and instructs the modulator to produce one of an available set of M = 2k waveform types q  The waveform types are called symbols q  Any one symbol will have a given time period T and contain a certain amount of energy E

q  Symbol rate is the ratio of the bit rate to the number of bits represented per symbol

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PSK

q E is the symbol energy and T is the symbol

duration

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TtMi

ttTEts ii

≤≤

=

+=

0,...,2,1

))(cos(2)( 0 φω

MiMiti

,...,2,1,/2)(

=

= πφ

Binary phase shift keying (BPSK) q  Use two sine wave phases to encode bits

q Phases are separated by 180 degrees q Simple to implement, inefficient use of

bandwidth (only 1 bit/symbol) q Very robust, used extensively in satellite

communication

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0binary )2cos()(1binary )2cos()(

2

1

πθπ

θπ

++=

+=

ccc

ccc

tfAtstfAts

Q

0 State

1 State

Example

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Data

Carrier

Carrier+ π

BPSK waveform

1 1 0 1 0 1

“ones”

“zeros”

Quadrature phase shift keying (QPSK) q  Multilevel modulation technique: 2 bits per symbol q  Two times more spectrally efficient, more complex receiver than BPSK

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Q

11 State

00 State 10 State

01 State

Phase of Carrier: π/4, 3π/4, 5π/4, 7π/4

Example

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Frequency shift keying (FSK) q  The frequency of the carrier is changed according to the message state (in Binary FSK (BFSK), high (1) or low (0))

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0)(bit Tt0 ))22cos(()(1)(bit Tt0 ))22cos(()(

b2

b1

=≤≤Δ−=

=≤≤Δ+=

tffAtstffAts

c

c

ππ

ππ

1 1 0 1

Data

FSK Signal

higher frequency lower frequency higher frequency

Example

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ASK

q Ei(t) is the symbol energy and T is the symbol duration

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TtMi

tTtEts i

i

≤≤

=

+=

0,...,2,1

)cos()(2)( 0 φω

QAM

q Ei(t) is the symbol energy and T is the symbol duration

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TtMi

ttTtEts i

ii

≤≤

=

+=

0,...,2,1

))(cos()(2)( 0 φω

Example

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