Digital Modulation - Engineeringmodulation for many wireless systems. But M-PSK for M>8 is not used...
Transcript of Digital Modulation - Engineeringmodulation for many wireless systems. But M-PSK for M>8 is not used...
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Digital Modulation
Lecture 17
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s t E tT
t
i Mt T
io1
2
1 20
( ) ( ) cos( )
, ,...,
= +
=
≤ ≤
ω Φ
Amplitude Shift Keying (ASK)
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Baseband filtered ASK
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FSK Waveforms
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s t ET
t
i Mt T
i i( ) cos( )
, ,...,
= +
=
≤ ≤
2
1 20
ω Φ
Frequency Shift Keying (FSK)
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s t ET
t i M
i Mt T
i o( ) cos( / )
, ,...,
= +
=
≤ ≤
2 2
1 20
ω π
Phase Shift Keying (PSK)
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Baseband filtered PSK
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s t E tT
t t
i Mt T
ii
o i( ) ( ) cos[ ( )]
, ,...,
= +
=
≤ ≤
2
1 20
ω Φ
Amplitude & Phase Shift Keying (APK)
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2,4 and 8-PSK constellations
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SER of coherent M-PSK
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Why M-PSK ? (M>4)
• The last figure clearly demonstrates that as M becomes larger than 4, there is a power efficiency penalty. The question is why do we pay this penalty. The answer is in the next figure.
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Bandwidth of M-PSK
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Is FSK used in cellular systems?
• We know that FSK is a basic digital modulation format.
• Is it frequently used in cellular systems? • If not, why not?
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Signal separation in FSK
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SER performance of M-FSK, M=2k
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What about QAM?
• We now know that PSK is the most popular modulation for many wireless systems. But M-PSK for M>8 is not used in practice. Clearly as M becomes large, putting the points on a single circuit reduces the distance for a given average power (or energy) as shown next.
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a
b
c d
4 different 8-QAM constellations
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Average Peak Peak to average
a 6 10 1.66
b 6.8 11.8 1.7
c 6 8 1.33
d 4.7 7.46 1.58
Comparison of the 8-QAM constellations
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AWGN
Loss of Synch
Inter_ ference
Phase Noise
16QAM with different impairments
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Bandwidth requirements
For PSK or QAM for FSK
B r rM
B Mr M rM
Brr
M
sb
sb
s
b
= + =+
= =
− < ≤
( ) ( )log log
::: ( ):
1 1
0 1
2 2
αα
α α
: bandwidth in Hzsymbol rate in spsbit rate in bpsroll off factor number of points in the constellation
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Detection Types
• Coherent detection • Differential detection (also called
differentially coherent detection) • Noncoherent detection (e.g. envelope
detection)
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Non-coherent detection of ASK
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Coherent ASK detection
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Carrier recovery for ASK
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Noncoherent FSK Detection
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PLL based detection of FSK
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Coherent FSK Detection
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Envelope detection of M-FSK
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Coherent detection of BPSK
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Coherent detection of PSK
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State diagram of QPSK
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Phase changes in QPSK
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Envelope variations in QPSK