dept. of ee, ndhu 1 chapter four bandpass modulation and demodulation
TRANSCRIPT
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1Dept. of EE, NDHU
Chapter Four
Bandpass Modulation and Demodulation
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2Dept. of EE, NDHU
Bandpass Signaling
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3Dept. of EE, NDHU
Why Modulate?
• The transmission of EM fields through space is accomplished with the antenna
• The size of the antenna depends on the wavelength l
– Telephone industry benchmark of l/4 as the antenna dimension
– Example : 3kHz baseband signal needs about 15 miles for the antenna diameter
– Example: 900MHz signal needs about 8cm for the antenna diameter
• Bandpass modulation is an essential step for all systems involving radio transmis
sion
• Modulation can separate the different signals (Ex. FDMA)
• Modulation can also be used to place a signal in a frequency band where design r
equirement can be easily met
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4Dept. of EE, NDHU
Digital Bandpass Modulation Techniques
• Bandpass modulation is the process by which an information signal is converted
to a sinusoidal waveform (carrier waveform)
• Three features can be used to distinguish the sinusoidal waveform
– Amplitude, frequency, phase
• Coherent detection
– The receiver exploits knowledge of the carrier’s phase to detect the signa
– PSK, FSK, ASK, CPM, and Hybrid forms
• Non-coherent detection
– The receiver does not utilize the carrier’s phase reference information
– DPSK, FSK, ASK, CPM, and Hybrid forms
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5Dept. of EE, NDHU
Digital Modulations
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6Dept. of EE, NDHU
Detection of Signals in Gaussian Noise
• Bandpass model of the detection process is virtually identical to the baseband mo
del
• Decision regions
– Minimum error decision rule is to choose the signal class that the distance d(r,si) is mi
nimized, where r is the received signal
• Correlation receiver
– Transform the received waveform into a point in the decision space
– Determine in which decision region the point is located
Choose the si(t) whose index corresponds to max zi(T)
MidttstrTzT
ii ,,1 , )()()(
0
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7Dept. of EE, NDHU
Decision Regions
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8Dept. of EE, NDHU
Correlator Receiver with Reference Signals
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9Dept. of EE, NDHU
Binary Correlator Receiver
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10Dept. of EE, NDHU
Coherent Detection of PSK
• BPSK signal
• Decision stage chooses the signal with largest output value of matched
filter
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11Dept. of EE, NDHU
Sampled Matched Filter
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12Dept. of EE, NDHU
Coherent Detection of MPSK
• MPSK signal
• Signal space and decision regions for a QPSK (M=4) system
– As shown in Fig.4.11
– Make a decision by the phase information
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13Dept. of EE, NDHU
Demodulator for MPSK Signals
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14Dept. of EE, NDHU
Coherent Detection of FSK
• FSK signal
• The distance between any two signal vectors is
• Choose the largest output of matched filter
otherwise 0
for Therefore
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cos2
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15Dept. of EE, NDHU
Signal Space for a 3-ary FSK Signal
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16Dept. of EE, NDHU
Signal Space for DPSK
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17Dept. of EE, NDHU
Detection of Differential PSK
• Differential encoding for the PSK signal
• Signaling characteristics
• Non-coherent detection
• Compare with PSK and DPSK
– PSK detection is with only one noise signal
– DPSK detection is with two noise signal (differentially decoding)
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18Dept. of EE, NDHU
Binary Differential PSK Example
Suboptimum detection
Optimum detection
)()1()( kmkckc
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19Dept. of EE, NDHU
Non-coherent Detection of FSK
Quadrature Receiver
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20Dept. of EE, NDHU
Non-coherent Detection of FSK
Non-coherent detection of FSK with envelop detector
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21Dept. of EE, NDHU
Tone Spacing for Non-coherent Orthogonal FSK Signaling
• Two tones f1 and f2 are orthogonal
– For a transmitted tone f1, the sampled envelop of the receiver output
filter tuned to f2 is zero
• Minimum tone spacing for orthogonal FSK signaling
– Non-coherently detected FSK
– Coherent FSK signaling is 2/T
Tff
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1 is spacing toneminimum
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21
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22Dept. of EE, NDHU
Minimum Tone Spacing for Non-coherent Orthogonal FSK
• For binary FSK, bandwidth is two times the tone spacing
• For M-ary FSK, bandwidth is M/T
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23Dept. of EE, NDHU
D8PSK Modulator
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24Dept. of EE, NDHU
D8PSK Demodulator
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25Dept. of EE, NDHU
Error Performance for Binary Systems
• Bit error probability for BPSK signaling
• Probability of bit error for coherent detected, differential encoded binary PSK
• Probability of bit error for coherently detected binary orthogonal FSK
• Probability of bit error for non-coherently detected binary orthogonal FSK
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26Dept. of EE, NDHU
Binary DPSK
• DPSK signaling
• Pairs of DPSK signals, S1(t) and S2(t) are orthogonal
• DPSK detection can be implemented by matching signal envelopes
• Bit error probability is similar to the one for non-coherently detected binary FSK
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27Dept. of EE, NDHU
DPSK Detection
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28Dept. of EE, NDHU
Bit Error Probability of Binary Systems
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29Dept. of EE, NDHU
M-ary Signals and Performance
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30Dept. of EE, NDHU
Ideal Probability of Bit Error Performance
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31Dept. of EE, NDHU
Bit Error Performance for M-ary Orthogonal Signaling
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32Dept. of EE, NDHU
Bit Error Performance for Multiple Phase Signaling
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33Dept. of EE, NDHU
M-ary Signaling
• M-ary signaling instructs the modulator to produce one of M=2k waveforms
• M-ary multiple phase signaling
– The BER curve moves in the direction of degraded error performance as k increa
ses
– A larger bit rate can be transmitted within the same bandwidth as k increases
• M-ary orthogonal signaling
– The BER curve moves in the direction of improved error performance as k i
ncreases
– The required system bandwidth increases as k increases
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34Dept. of EE, NDHU
Vectorial View of MPSK Signaling
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35Dept. of EE, NDHU
Relation Between Eb/N0 and S/N
• General relationship between Eb/N0 and S/N
• For the QPSK signaling– QPSK bit stream is usually partitioned into an even and odd stream; each ne
w stream is at half the bit rate of the original stream
– Each of the quadrature BPSK signals has half of the average power of the ori
ginal QPSK signal (as shown in Fig. 4.31)
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36Dept. of EE, NDHU
Vectorial View of MFSK Signaling
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37Dept. of EE, NDHU
Symbol Error Performance for Coherent FSK Signaling
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38Dept. of EE, NDHU
Eb/N0 and SNR in the MFSK
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39Dept. of EE, NDHU
Symbol Error Versus Bit Error for FSK Signaling
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40Dept. of EE, NDHU
Symbol Error Performance for M-ary Systems
• Symbol error performance for coherently detected M-ary PSK
• Symbol error performance for differentially coherent detection of MPSK signal
• Probability of symbol error for coherently detected MFSK signal
• Probability of symbol error for non-coherently detected MFSK signal
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41Dept. of EE, NDHU
Symbol Error Performance for Coherently Detected MPSK
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42Dept. of EE, NDHU
Symbol Error Performance for Coherently Detected MFSK
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43Dept. of EE, NDHU
Symbol Error Performance for Non-coherently Detected MFSK
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44Dept. of EE, NDHU
Bit Error Versus Symbol Error Probability
• Orthogonal signal
2
1limget weincreases, as
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45Dept. of EE, NDHU
Bit Error Versus Symbol Error Probability
• Multiple Phase signals with Gray coded
• For BPSK and QPSK signaling
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EE
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