Bandpass Bandpass Modulation & Modulation & Demodulation Demodulation Detection Detection CHAPTER 4 CHAPTER 4 School of Computer and Communication School of Computer and Communication Engineering, Amir Razif Arief b. Jamil Engineering, Amir Razif Arief b. Jamil Abdullah Abdullah EKT 431: Digital EKT 431: Digital Communications Communications
Last time we talked about: Another source of error due to filtering effect of the system: Inter-symbol interference (ISI) The techniques to reduce ISI Pulse shaping to achieve zero ISI at the sampling time Equalization to combat the filtering effect of the channel
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Bandpass Bandpass Modulation Modulation
& & DemodulatioDemodulation Detectionn Detection
CHAPTER CHAPTER 44
School of Computer and Communication School of Computer and Communication Engineering, Amir Razif Arief b. Jamil Engineering, Amir Razif Arief b. Jamil
AbdullahAbdullahEKT 431: Digital EKT 431: Digital Communications Communications
Last time we talked Last time we talked about:about:
Another source of error due to filtering effect Another source of error due to filtering effect of the system: of the system:
Inter-symbol interference (ISI)Inter-symbol interference (ISI)The techniques to reduce ISIThe techniques to reduce ISI
Pulse shapingPulse shaping to achieve zero ISI at the to achieve zero ISI at the sampling timesampling timeEqualizationEqualization to combat the filtering effect to combat the filtering effect of the channelof the channel
Test 1
0
20
40
60
80
100
0 10 20 30 40 50 60 70Student ID
Scor
e
Series1
Average 65.9
Max 97.0
Min 35.0
Stdev 13.9
ModulationModulationDigital Bandpass Modulation Digital Bandpass Modulation TechniquesTechniquesDetection of Signals in Gaussian Detection of Signals in Gaussian NoiseNoiseCoherent DetectionCoherent DetectionNoncoherent DetectionNoncoherent DetectionM-ary Signaling and PerformanceM-ary Signaling and Performance
Baseband Modulation and Baseband Modulation and Demodulation DetectionDemodulation Detection
Today, we are going to Today, we are going to talk about:talk about:
Some bandpass modulation schemes used in Some bandpass modulation schemes used in DCS for transmitting information over channelDCS for transmitting information over channel
M-PAM, M-PSK, M-FSK, M-QAMM-PAM, M-PSK, M-FSK, M-QAMHow to detect the transmitted information at How to detect the transmitted information at the receiverthe receiver
is the process of digital symbols are is the process of digital symbols are transform into waveforms that are transform into waveforms that are compatible with the characteristics of the compatible with the characteristics of the channel.channel.
Bandpass Modulation~Bandpass Modulation~the shape modulate the carrier wave. For the shape modulate the carrier wave. For radio transmission the carrier is converted to radio transmission the carrier is converted to an electromagnetic (EM) field for an electromagnetic (EM) field for propagation to the desired destination.propagation to the desired destination.
Through carrier wave modulation the size of Through carrier wave modulation the size of antenna can be antenna can be reduced reduced for high frequency for high frequency transformation.transformation.FDMA Uses to FDMA Uses to minimizeminimize the effect of the effect of interference, to separate different signal interference, to separate different signal transmitting using single channel. transmitting using single channel.
Block Diagram of a DCSBlock Diagram of a DCSDigital ModulationDigital Modulation
~ ~ digital symbols transform into waveforms that are digital symbols transform into waveforms that are compatible with channel characteristics.compatible with channel characteristics.
Baseband modulationBaseband modulation~ shaped pulses modulate a sinusoid carrier wave ~ shaped pulses modulate a sinusoid carrier wave (carrier) for radio transmission the carrier is converted (carrier) for radio transmission the carrier is converted to EM.to EM.
FormatSourceencode
FormatSourcedecode
Channelencode
Pulsemodulate
Bandpassmodulate
Channeldecode
Demod. SampleDetect
Channel
Digital modulation
Digital demodulation
Digital Bandpass Digital Bandpass Modulation TechniquesModulation Techniques
Bandpass modulationBandpass modulation~ ~ The process of converting a data signal to a The process of converting a data signal to a sinusoidal waveform where its sinusoidal waveform where its amplitudeamplitude, , phasephase or or frequencyfrequency, or a combination of them, are varied in , or a combination of them, are varied in accordance with the transmitting data. accordance with the transmitting data.
Carrier wave;Carrier wave;wherewhere
oo~ radian frequency~ radian frequency(t)~ phase(t)~ phasef & f & ~ frequency ~ frequency denotes average symbol energy given by denotes average symbol energy given by
Coherent detection~Coherent detection~The receiver exploits the knowledge of the carrier’s The receiver exploits the knowledge of the carrier’s phase to detect the signals.phase to detect the signals.
noncoherent detection~noncoherent detection~The receiver does The receiver does not utilizenot utilize the the phasephase reference reference information.information.
)](cos[)()( 0 tttAts
Noncoherent demodulation~Noncoherent demodulation~Refers to systems employing demodulator Refers to systems employing demodulator that are designed to operate without that are designed to operate without knowledge of the absolute value of the knowledge of the absolute value of the incomming signal’s phaseincomming signal’s phaseIt It reduce complexityreduce complexity compare to coherent compare to coherent demodulation and increase probability error demodulation and increase probability error (P(PEE))
Phasor representation of sinusoidPhasor representation of sinusoid
eejwotjwot consists of consists of (1) inphase (real) component and (1) inphase (real) component and (2) quadrature (imaginary) component (2) quadrature (imaginary) component that are orthogonal to each other. that are orthogonal to each other.
where is the baseband pulse shape where is the baseband pulse shape with energy .with energy .
We assume here (otherwise will be stated):We assume here (otherwise will be stated): is a rectangular pulse shape with is a rectangular pulse shape with unit energy.unit energy.Gray coding is used for mapping bits to Gray coding is used for mapping bits to symbols.symbols. denotes average symbol energy given denotes average symbol energy given by by
where is the baseband pulse shape where is the baseband pulse shape with energy.with energy.
We assume here (otherwise will be stated):We assume here (otherwise will be stated): is a rectangular pulse shape with is a rectangular pulse shape with unit energy.unit energy.Gray coding is used for mapping bits to Gray coding is used for mapping bits to symbols.symbols. denotes average symbol energy given denotes average symbol energy given by by
)(tgT
)(tgT
TtttitTEtgts ici
Ti 0 )()1(cos2)()(
gE
sE
M
i is EM
E1
1
Frequency Shift Keying Frequency Shift Keying (FSK)(FSK) General analytic expression for General analytic expression for FSKFSK;;
where where wwii have M discrete values, s have M discrete values, s(t)(t) is an is an arbitrary constantarbitrary constant
symbol transition depicts a symbol transition depicts a gentle shiftgentle shift from from one frequency tone to another. True for one frequency tone to another. True for CPFSK only.CPFSK only.
In MFSK the change to a different tone is In MFSK the change to a different tone is quite abrupt.quite abrupt.
OrthogonalOrthogonal signal, signal with mutually signal, signal with mutually perpendicular vector.perpendicular vector.
MiTt
tTEts ii ,...,1
0]cos[2)(
Contd.. Frequency Shift Keying (FSK)Contd.. Frequency Shift Keying (FSK)FSK Bandwidth;FSK Bandwidth;f ~ (fm-fs)/2f ~ (fm-fs)/2f ~ frequency space and mark between f ~ frequency space and mark between different absolute (Hz) deviation frequency.different absolute (Hz) deviation frequency.fm-fs ~ absolute different between mark and fm-fs ~ absolute different between mark and space frequency.space frequency.
Contd.. Frequency Shift Keying (FSK)Contd.. Frequency Shift Keying (FSK)FSKFSK
Amplitude Shift Keying Amplitude Shift Keying (ASK)(ASK) General analytic expression for General analytic expression for ASKASK;;
where swhere sii(t) will have M discrete values(t) will have M discrete values where where wwii have M discrete values, s have M discrete values, s(t)(t) is an is an
arbitrary constantarbitrary constant
MiTt
tTEtsi ,...,1
0]cos[2)( 0
Phase Shift Keying Phase Shift Keying (PSK)(PSK)
General analytic expression for General analytic expression for PSKPSK;;
where swhere sii(t) will have M discrete values(t) will have M discrete valuesE ~ energyE ~ energyT~ durationT~ durationFor BPSK, M=2For BPSK, M=2Shift sShift sii(t) phase state zero and(t) phase state zero and
BPSK ReceiverBPSK Receiver Bit value 0 –sine wave Bit value 1 –inverted sine wave Very simple PSK Low spectral efficiency Robust , used in satellite system
Two Dimensional Two Dimensional Modulation, demodulation Modulation, demodulation
No need for a reference in phase with the No need for a reference in phase with the received carrierreceived carrier
Less complexity compared to coherent Less complexity compared to coherent detection at the price of higher error rate.detection at the price of higher error rate.
Differential encoding of the messageDifferential encoding of the messageThe symbol phase changes if the The symbol phase changes if the
current bit is different from the current bit is different from the previous bit. previous bit.
,...,MiTtttTEts ii 1 ,0 ,)(cos2)( 0
)())1(()( nTTnnT ikk
)(1 t
i
1s2s 0
0 1 2 3 4 5 6 7 1 1 0 1 0 1 11 1 1 0 0 1 1 1
0 0
Symbol index: Data bits:Diff. encoded bitsSymbol phase:
k
kmk
Non-Coherent Detection …Non-Coherent Detection … Coherent detection for diff encoded Coherent detection for diff encoded
mod.mod.assumes slow variation in carrier-phase assumes slow variation in carrier-phase
mismatch during two symbol intervals.mismatch during two symbol intervals.correlates the received signal with basis correlates the received signal with basis
functionsfunctionsuses the phase difference between the uses the phase difference between the
current received vector and previously current received vector and previously estimated symbolestimated symbol