1. ECE 3125
Communication Engineering II
ECE 3242
Digital Communication
Semester I, 2012-2013
1
Semester I, 2012-2013
Lecturer
Prof. Dr. Mohammad Umar Siddiqi
Office: E2-3-10.12
umarsiddiqi@iium.edu.my
03-61964458; 012-6549103
6. What is Electrical / Electronic /
Optical Communication?
It is the transfer of
information from one place toinformation from one place to
another through
electrical/electronic/optical
signals
6
7. Basic Digital Communication System Model
Encode
Transmit
Pulse
modulateSample Quantize
Format
Digital info.
Textual
info.
Analog
info.
source
7
Demodulate/
Detect
Channel
Receive
Low-pass
filter
Decode
Pulse
waveformsBit stream
Format
Textual
info.
Analog
info.
Digital info.
sink
11. Electrical / Electronic / Optical
Communication
* Transfer of information from one place to
another through electrical/electronic/optical signals
11
another through electrical/electronic/optical signals
* Key feature is the transfer of random signals
over transmission channels with random behaviour
12. This (communication process) should be accomplished
* as efficiently as possible
(bandwidth and power)
bits/sec/Hz for Digital Communication
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bits/sec/Hz for Digital Communication
* with as much fidelity as possible (for analogue com) - SNR
* with as much reliability as possible (for digital com) – BER
* as securely as possible (using cryptographic schemes)
* as economically as possible
13. Analog vs. Digital Communication
• Analog Communication Systems
– Transmit one waveform out of infinite
possibilities
– Rx “guesses” one out of infinite possibilities– Rx “guesses” one out of infinite possibilities
• Digital Communication Systems
– Transmit one waveform out of finite (limited)
possibilities
– Rx “guesses” one out of finite possibilities
14. Why Digital?
• DSP allows us to condition signal more
flexibly than analog circuits (channel
coding, encryption, equalization))
• Robustness against noise and interference
15. Basic Digital Communication System Model
Encode
Transmit
Pulse
modulateSample Quantize
Format
Digital info.
Textual
info.
Analog
info.
source
15
Demodulate/
Detect
Channel
Receive
Low-pass
filter
Decode
Pulse
waveformsBit stream
Format
info.
Textual
info.
Analog
info.
Digital info.
sink
16. DCS – Essential Elements
• Formatting for compatibility
• Modulation for adaptation to the channel
– Base-band– Base-band
– Band-pass
• Demodulation for extracting the information
• Synchronization for aligning the
timing/carrier frequency between XMT and
RCV
17. Formatting
• To provide proper “interface” between the
information source/sink and the DCS
• to maintain compatibility• to maintain compatibility
• Sampling, quantization, source coding
• Producing bit stream
18. Modulation
• Convert the bit stream into waveform
suitable for transmission
• Base-band: signal with spectrum extending
from zero up to finite bandwidth
– E.g. PCM, M-ary PAM, RZ, NRZ
• Pass-band: signal with spectrum extending
from a carrier frequency (>>signal
bandwidth) up to finite bandwidth
– E.g. M-ary PSK, M-QAM, GMSK
• Include pulse shaping (filtering)
19. Demodulation
• The inverse of modulation
• To recover the message from received
waveformwaveform
• Detection is the process of deciding the
digital meaning of the waveform
20. Synchronization
• XMT and RCV are in general separated
(disjointed) by some distance
• Each has local time and frequency reference• Each has local time and frequency reference
• Imperfect match will result in power loss
21. Mathematical Representation
• r(t): received signal
• si(t): transmitted message symbol
• hc(t): channel impulse response
• n(t): additive noise• n(t): additive noise
• M: size of message symbol
22. Performance Metrics
• ACS (Analog)
– Waveform is member of infinite set
– Signal fidelity (resemblance to the transmitted
waveform)
– Signal to Noise Ratio (SNR), Mean Squared– Signal to Noise Ratio (SNR), Mean Squared
Error (MSE) between transmitted waveform and
received one
• DCS (Digital)
– Waveform is member of finite (alphabet) set
– Probability of error
