Modulation (6): Digital modulation€¦ · 04/02/2012 · FSK = Frequency Shift Keying ! PSK =...
Transcript of Modulation (6): Digital modulation€¦ · 04/02/2012 · FSK = Frequency Shift Keying ! PSK =...
Modulation (6): Digital modulation
Luiz DaSilva Professor of Telecommunications [email protected] +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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