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Digital Modulation
Saravanan [email protected]
Department of Electrical EngineeringIndian Institute of Technology Bombay
August 19, 2013
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Digital Modulation
DefinitionThe process of mapping a bit sequence to signals for transmission over achannel.
InformationSource
SourceEncoder
ChannelEncoder Modulator
Channel
DemodulatorChannelDecoder
SourceDecoder
InformationDestination
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Digital Modulation
Example (Binary Baseband PAM)1→ p(t) and 0→ −p(t)
t
A
p(t)
t
−A
−p(t)
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Classification of Modulation Schemes• Memoryless
• Divide bit sequence into k -bit blocks• Map each block to a signal sm(t), 1 ≤ m ≤ 2k
• Mapping depends only on current k -bit block
• Having Memory
• Mapping depends on current k -bit block and L− 1 previous blocks• L is called the constraint length
• Linear
• Complex baseband representation of transmitted signal has theform
u(t) =∑
n
bng(t − nT )
where bn’s are the transmitted symbols and g is a fixed basebandwaveform
• Nonlinear
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Signal Space Representation
Signal Space Representation of Waveforms• Given M finite energy waveforms, construct an orthonormal basis
s1(t), . . . , sM(t)→ φ1(t), . . . , φN(t)︸ ︷︷ ︸Orthonormal basis
〈φi , φj〉 =∫ ∞−∞
φi(t)φ∗j (t) dt ={
1 if i = j0 otherwise
• Each si(t) is a linear combination of the basis vectors
si(t) =N∑
n=1
si,nφn(t), i = 1, . . . ,M
• si(t) is represented by the vector si =[si,1 · · · si,N
]T• The set {si : 1 ≤ i ≤ M} is called the signal space representation or
constellation
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Constellation Point to Waveform
si,N
si,N−1
...
si,2
si,1
si(t)
×
×
...
×
×
φ1(t)
φ2(t)
φN−1(t)
φN(t)
+
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Waveform to Constellation Point
si(t)
si,N
si,N−1
...
si,2
si,1×
×
...
×
×
φ∗1(t)
φ∗2(t)
φ∗N−1(t)
φ∗N(t)
∫
∫
...
∫
∫
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Gram-Schmidt Orthogonalization Procedure• Algorithm for calculating orthonormal basis for s1(t), . . . , sM(t)
• Consider M = 1φ1(t) =
s1(t)‖s1‖
where ‖s1‖2 = 〈s1, s1〉• Consider M = 2
φ1(t) =s1(t)‖s1‖
, φ2(t) =γ(t)‖γ‖
where γ(t) = s2(t)− 〈s2, φ1〉φ1(t)
• Consider M = 3
φ1(t) =s1(t)‖s1‖
, φ2(t) =γ1(t)‖γ1‖
, φ3(t) =γ2(t)‖γ2‖
where
γ1(t) = s2(t)− 〈s2, φ1〉φ1(t)
γ2(t) = s3(t)− 〈s3, φ1〉φ1(t)− 〈s3, φ2〉φ2(t)
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Gram-Schmidt Orthogonalization Procedure• In general, given s1(t), . . . , sM(t) the k th basis function is
φk (t) =γk (t)‖γk‖
where
γk (t) = sk (t)−k−1∑i=1
〈sk , φi〉φi(t)
is not the zero function
• If γk (t) is zero, sk (t) is a linear combination of φ1(t), . . . , φk−1(t). It doesnot contribute to the basis.
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Gram-Schmidt Procedure Example
2 t
1
s1(t)
2 t
1
-1
s2(t)
3 t
-1
1
s3(t)
3 t
-1
1
s4(t)
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Gram-Schmidt Procedure Example
2 t
1√2
φ1(t)
2 t
1√2
− 1√2
φ2(t)
2 3 t
-1
1
φ3(t)
s1 =[√
2 0 0]T
s2 =[0√
2 0]T
s3 =[√
2 0 1]T
s4 =[−√
2 0 1]T
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Properties of Signal Space Representation• Energy
Em =
∫ ∞−∞|sm(t)|2 dt =
N∑n=1
|sm,n|2 = ‖sm‖2
• Inner product〈si(t), sj(t)〉 = 〈si , sj〉
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Modulation Schemes
Pulse Amplitude Modulation• Signal Waveforms
sm(t) = Amp(t), 1 ≤ m ≤ M
where p(t) is a pulse of duration T and Am ’s denote the M possibleamplitudes.
• Example M = 2, p(t) is a real pulseA1 = −A,A2 = A for a real number A
A-A
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Pulse Amplitude Modulation• Example M = 4, p(t) is a real pulse
A1 = −3A,A2 = −A,A3 = A,A4 = 3A
-3A -A A 3A
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Phase Modulation• Complex envelope of phase modulated signals
sm(t) = p(t)e j π(2m−1)M , 1 ≤ m ≤ M
where p(t) is a real baseband pulse of duration T
• Corresponding passband signals
spm(t) = Re
[√2sm(t)e j2πfc t
]=√
2p(t) cos(π(2m − 1)
M
)cos 2πfc t
−√
2p(t) sin(π(2m − 1)
M
)sin 2πfc t
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Constellation for PSK
11
10
01
00
QPSK, M = 4
000
001011
010
110
111 101
100
Octal PSK, M = 8
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Quadrature Amplitude Modulation• Complex envelope of QAM signals
sm(t) = (Am,i + jAm,q)p(t), 1 ≤ m ≤ M
where p(t) is a real baseband pulse of duration T
• Corresponding passband signals
spm(t) = Re
[√2sm(t)e j2πfc t
]=√
2Am,ip(t) cos 2πfc t −√
2Am,qp(t) sin 2πfc t
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Constellation for QAM
16-QAM
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Thanks for your attention
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