1. Digital Communications (디지털 통신)
중앙대학교 전자전기공학부
이정우
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office: 신공학관 409호
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2. Main text: class notes References Test
“Digital transmission of information” by Richard E. Blahut, Addison-Wesley, 1990.
“Communication systems” by Simon Haykin, John Wiley & Sons, 2001.
Test
중간고사 (35%), 기말고사 (35%), 과제물 4번 (각 5%), 참여도 (10%)

3. Introduction to Digital Communications

4. Open Systems Interconnection (OSI) Data Communication Model
Covered in Digital Communications

5. Communication System Transmitter Source encoder Channel encoder
message
signal
Source
encoder
source
codeword
Channel
encoder
channel
codeword
Modulator
Data
waveform
Error control
coding
Channel
Compression
received signal
estimated
message
signal
estimated
source
codeword
estimated
channel
codeword
Source
decoder
Channel
decoder
Demodulator
User
Receiver

6. Communication System Two basic modes of communications Broadcasting
Single powerful transmitter and many receivers
TV, Radio, etc.
Point-to-point communication
Link between a single transmitter and a receiver
Telephone

7. Communication Resources
Two primary resources
Transmitted power
Average power of transmitted signal
Channel bandwidth
Band of frequencies allocated for transmission
System design objective
Use two resources as efficiently as possible.
Power limited vs. Band limited

8. Communication Channels
Guided propagation vs. free propagation
telephone channels, coaxial cables, optical fibers, etc.
broadcast channels, mobile radio channels, satellite channels, etc.
Random error vs. burst error
Deep-space channels, satellite channels
Radio channels, wire and cable, magnetic recording channel, etc.
Discrete vs. continuous
Binary symmetric channel (BSC), binary erasure channel (BEC)
Additive white Gaussian noise (AWGN) channel, Fading channels (Rayleigh, Rician), etc.

9. Channel Capacity Shannon (1948) Definition of channel capacity C
If you transmit information at a rate R < C, then the error-free transmission is possible.
Definition of channel capacity C
Maximum rate at which information can be transmitted across the channel without error.
Goal of communication system design in power-limited environment:
achieve a target error rate of data transmission with as low signal power as possible.
Similar to achieving the capacity bound as close as possible with less power.

10. Channel Capacity AWGN channel C = W log2 (1 + SNR) bit/sec, BSC
where W denotes the channel bandwidth and SNR denotes the signal to noise ratio.
BSC
C = 1 – H(ρ) ,
where ρ is the error probability of the channel and H(ρ ) is the entropy with the parameter ρ.
BEC
C = 1 – ε ,
where ε is the erasure probability.

11. Channel Capacity
Channel capacity of AWGN channel

12. Modulation Modulation: Demodulation:
Modifies the message signal into a form suitable for transmission over the channel.
Demodulation:
Recreates the original message signal from a degraded version of the transmitted signal after propagation through the channel.
Due to the presence of noise, the original message signal cannot be recreated exactly.
The degradation is influenced by the type of modulation scheme.

13. Modulation Continuous wave (CW) modulation Pulse modulation
Carrier is a sinusoidal wave.
Amplitude modulation (AM), frequency modulation (FM), phase modulation (PM)
Pulse modulation
Carrier is a periodic sequence of rectangular pulses.
Pulse-amplitude modulation (PAM) or amplitude-shift keying (ASK), pulse-duration modulation (PDM), pulse-position modulation (PPM)
Other names: frequency shift keying (FSK), amplitude-shift keying (ASK), on-off keying (OOK), phase-shift keying (PSK), M-ary orthogonal keying, etc.

14. Modulation Another benefit: Multiplexing
Combines several message signals for their simultaneous transmission over the same channel.
Frequency-division multiplexing (FDM)
CW modulation is used.
Assigns message signal distinct carrier frequency.
Time-division multiplexing (TDM)
Pulse modulation is used.
Different time slots
Code-division multiplexing (CDM)
Each message is identified by a distinctive code.
Message signals are permitted to overlap in both time and frequency.

15. Error Control Coding
Channel encoder produces a new sequence of symbols called the channel codeword.
Controlled redundancy exists in the construction of channel codeword.
Channel codeword is longer than source codeword.
Benefits
In principle:
If you transmit information at a rate R < C, then the error-free transmission is possible.
In practice:
Reduce the error rates
Reduce the transmitted power requirements

16. Error Control Coding Classification Block codes Tree codes
Hamming, BCH, RS, Golay, Goppa, Algebraic geometric codes (AGC), LDPC codes
Tree codes
Convolutional codes, turbo codes
Linear codes
Hamming, BCH, RS, Golay, Goppa, AGC, LDPC, turbo, etc.
Nonlinear codes
Nordstrom-Robinson, Kerdock, Preparata, etc.
Systematic codes vs. Nonsystematic codes

17. Error Control Coding

18. Digital Communication Problem
Elements of digital communication system
transmitter
(phase shift keying modulation)
message
signal m(t)
0 → − 1
1 → + 1
for duration T
channel output
(received signal) x(t)
transmitted
signal s(t)  + 
channel
carrier wave
Accos(2πfc t),
where fc=1/T
noise w(t) yT
decision
making
device
say 1 if yT > 0
received
signal x(t) 
say 0, otherwise
correlator
local carrier
cos(2πfc t)
receiver
threshold=0

19. Digital Communication Problem
Theoretical issues
Justification of the receiver structure.
Finding a random variable describing the noise.
Determining the probability of decision of errors.
Practical issues
Choice of modulation scheme conserving bandwidth in a cost-effective manner.
Design of channel encoder/decoder to be close to the channel capacity.
Synchronization of the carrier frequencies in modulator and demodulator.

20. Topics in Digital Communications

21. Topics Fourier Transform, Random Processes Baseband Communications
signaling, matched filter, equalization, etc.
Passband Communications
signaling, coherent/noncoherent demodulation, Rayleigh and Rician distribution, etc.
Optimality
Maximum-likelihood (ML), maximum a posteriori (MAP)
Error Control Coding
Multiple Access Communications
Spread Spectrum Communications