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Digital Communications( 디지털 통신 )
중앙대학교 전자전기공학부이정우
Email: [email protected]: http://cau.ac.kr/~jwlee2
office: 신공학관 409 호Phone: 820-5734
2
• Main text: class notes
• References
– “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 4 번 ( 각 5%), 참여도 (10%)
Introduction toDigital Communications
4
Open Systems Interconnection (OSI)Data Communication Model
Covered in Digital Communications
5
Communication System
Channelencoder
Sourceencoder
Modulator
Demodulator
Channel
Channeldecoder
Sourcedecoder
Error controlcoding
Compression
Transmitter
Receiver
waveform
received signal
channelcodeword
estimatedchannel
codeword
sourcecodeword
estimatedsource
codeword
messagesignal
estimatedmessage
signal
Data
User
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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)– 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.
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Channel Capacity
• AWGN channel
C = W log2 (1 + SNR) bit/sec,
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 e
ntropy with the parameter ρ.
• BEC
C = 1 – ε ,where ε is the erasure probability.
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Channel Capacity
• Channel capacity of AWGN channel
12
Modulation
• Modulation: – 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– 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.
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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
• 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
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Error Control Coding
18
Digital Communication Problem
• Elements of digital communication system
messagesignal m(t)0 → − 11 → + 1for duration T
transmittedsignal s(t)
carrier waveAccos(2πfc t), where fc=1/T
noise w(t)
channel output(received signal) x(t)
+
+
T
dt0
decisionmakingdevice
say 1 if yT > 0
say 0, otherwise
threshold=0local carriercos(2πfc t)
receivedsignal x(t)
transmitter(phase shift keying modulation)
channel
receiver
yT
correlator
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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.
Topics inDigital Communications
21
Topics
1. Fourier Transform, Random Processes
2. Baseband Communications
• signaling, matched filter, equalization, etc.
3. Passband Communications
• signaling, coherent/noncoherent demodulation, Rayleigh and Rician distribution, etc.
4. Optimality
• Maximum-likelihood (ML), maximum a posteriori (MAP)
5. Error Control Coding
6. Multiple Access Communications
7. Spread Spectrum Communications