1. Bandpass Modulation & Demodulation Detection
CHAPTER 4
Bandpass Modulation & Demodulation Detection
School of Computer and Communication Engineering, Amir Razif Arief b. Jamil Abdullah
EKT 431: Digital Communications

2. 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

3. Average
65.9
Max
97.0
Min
35.0
Stdev
13.9

4. Baseband Modulation and Demodulation Detection
Digital Bandpass Modulation Techniques
Detection of Signals in Gaussian Noise
Coherent Detection
Noncoherent Detection
M-ary Signaling and Performance

5. Today, we are going to talk about:
Some bandpass modulation schemes used in DCS for transmitting information over channel
M-PAM, M-PSK, M-FSK, M-QAM
How to detect the transmitted information at the receiver
Coherent detection
Non-coherent detection

6. Modulation Modulation~
is the process of digital symbols are transform into waveforms that are compatible with the characteristics of the channel.
Bandpass Modulation~
the shape modulate the carrier wave. For radio transmission the carrier is converted to an electromagnetic (EM) field for propagation to the desired destination.
Through carrier wave modulation the size of antenna can be reduced for high frequency transformation.
FDMA Uses to minimize the effect of interference, to separate different signal transmitting using single channel.

7. Block Diagram of a DCS Digital Modulation
~ digital symbols transform into waveforms that are compatible with channel characteristics.
Baseband modulation
~ shaped pulses modulate a sinusoid carrier wave (carrier) for radio transmission the carrier is converted to EM.
Format
Source
encode
decode
Channel
Pulse
modulate
Bandpass
Demod.
Sample
Detect
Digital modulation
Digital demodulation

8. Digital Bandpass Modulation Techniques
~ The process of converting a data signal to a sinusoidal waveform where its amplitude, phase or frequency, or a combination of them, are varied in accordance with the transmitting data.
Carrier wave;
where
wo~ radian frequency
f(t)~ phase
f & w~ frequency
denotes average symbol energy given by
Coherent detection~
The receiver exploits the knowledge of the carrier’s phase to detect the signals.
noncoherent detection~
The receiver does not utilize the phase reference information.

9. Noncoherent demodulation~
Refers to systems employing demodulator that are designed to operate without knowledge of the absolute value of the incomming signal’s phase
It reduce complexity compare to coherent demodulation and increase probability error (PE)
Phasor representation of sinusoid
ejwot consists of
(1) inphase (real) component and
(2) quadrature (imaginary) component that are orthogonal to each other.

10. Bandpass Modulation Bandpass signal:
where is the baseband pulse shape with energy
We assume here (otherwise will be stated):
is a rectangular pulse shape with unit energy.
Gray coding is used for mapping bits to symbols.
denotes average symbol energy given by

11. Contd… Bandpass Modulation

12. Bandpass Modulation Bandpass signal:
where is the baseband pulse shape with energy.
We assume here (otherwise will be stated):
is a rectangular pulse shape with unit energy.
Gray coding is used for mapping bits to symbols.
denotes average symbol energy given by

13. Frequency Shift Keying (FSK)
General analytic expression for FSK;
where wi have M discrete values, s(t) is an arbitrary constant
symbol transition depicts a gentle shift from one frequency tone to another. True for CPFSK only.
In MFSK the change to a different tone is quite abrupt.
Orthogonal signal, signal with mutually perpendicular vector.

14. Contd.. Frequency Shift Keying (FSK)
FSK Bandwidth;
Df ~ (fm-fs)/2
Df ~ frequency space and mark between different absolute (Hz) deviation frequency.
fm-fs ~ absolute different between mark and space frequency.
B= |(fs-fb) - (fm-fb)|
= |fs-fm| + 2fb
= 2Df + fb

15. Contd.. Frequency Shift Keying (FSK)

16. Amplitude Shift Keying (ASK)
General analytic expression for ASK;
where si(t) will have M discrete values
where wi have M discrete values, s(t) is an arbitrary constant

17. Phase Shift Keying (PSK)
General analytic expression for PSK;
where si(t) will have M discrete values
E ~ energy
T~ duration
For BPSK, M=2
Shift si(t) phase state zero and
(p) 1800.

18. Contd… Phase Shift Keying (PSK)
BPSK transmitter
BPSK Receiver
Bit value 0 –sine wave
Bit value 1 –inverted sine wave
Very simple PSK
Low spectral efficiency
Robust , used in satellite system

19. Two Dimensional Modulation, demodulation and detection (M-PSK)‏
M-ary Phase Shift Keying (M-PSK)‏

21. Two Dimensional Mod.,… (MPSK)‏
BPSK (M=2)‏
“0”
“1”
8PSK (M=8)‏
“110”
“000”
“001”
“011”
“010”
“101”
“111”
“100”
QPSK (M=4)‏
“00”
“11”
“10”
“01”

22. Two Dimensional Mod.,…(MPSK)‏
Coherent detection of MPSK
Compute
Choose
smallest

23. Two Dimensional Mod., (M-QAM)‏
M-ary Quadrature Amplitude Mod. (M-QAM)‏

24. Two dimensional mod (M-QAM)‏
“0000” “0001” “0011” “0010” 1 3 -1 -3
“1000” “1001” “1011” “1010”
“1100” “1101” “1111” “1110”
“0100” “0101” “0111” “0110”
16-QAM

25. Two dimensional mod., (M-QAM)‏
Coherent detection of M-QAM
ML detector
Parallel-to-serial
converter
ML detector

26. Multi-Dimensional Modulation, Demodulation & Detection
M-ary Frequency Shift keying (M-FSK)‏

27. Multi-dimensional mod.,…(M-FSK)‏
ML detector:
Choose
the largest element
in the observed vector

28. Non-Coherent Detection
No need for a reference in phase with the received carrier
Less complexity compared to coherent detection at the price of higher error rate.

29. Non-Coherent Detection …
Differential coherent detection
Differential encoding of the message
The symbol phase changes if the current bit is different from the previous bit.
Symbol index:
Data bits:
Diff. encoded bits
Symbol phase:
Lecture 7

30. Non-Coherent Detection …
Coherent detection for diff encoded mod.
assumes slow variation in carrier-phase mismatch during two symbol intervals.
correlates the received signal with basis functions
uses the phase difference between the current received vector and previously estimated symbol

31. Non-Coherent Detect…
Optimum differentially coherent detector
Sub-optimum differentially coherent detector
Performance degradation about 3 dB by using sub-optimal detector
Decision
Delay T
Decision
Delay T

32. Non-coherent detection …
Non-coherent detection of BFSK
Decision stage: + -