Post on 17-Oct-2020
Lecture 1 Introduction
I-Hsiang WangNational Taiwan University
ihwang@ntu.edu.tw
2018.09.13
Principle of Communications, Fall 2018
Outline• The basic architecture of communication systems
• Source-channel separation
• Overview of this course
!2
A Communication System
• The simplest point-to-point abstraction
• By nature/applications: source, channel, destination
• Engineers’ design: the encoder + the decoder
!3
Encoder Channel DecoderSource Destination
Noise
Source and Channel
• The (information) source‣ generates the message to be delivered‣ message: text, audio, image, video, etc.. → abstracted into signals‣ continuous-time and continuous-valued signals → waveforms!
• Channel‣ abstraction of the physical medium: light, sound, wire, fiber, EM
radiation, etc.‣ distorts the transmitted signal → noisy channel (stochastic models)
!4
Encoder Channel DecoderSource Destination
Noise
Encoder and Decoder
• The Encoder‣ converts the message into physical signals ready for transmission‣ functions include sampling, quantization, compression,
modulation, error-correction coding, etc.
• The Decoder‣ reconstructs the message from the received signals‣ depending on the goal of the destination, can also carry out other
tasks such as computing a function of the messages.
!5
Encoder Channel DecoderSource Destination
Noise
A Digital Communication System
• A digital communication system uses digital sequence (bits) as an interface between source coding and channel coding
• Why separation?‣ Digital hardware is cheap, reliable, and scalable. ‣ Source coding and channel coding can be independently designed. ‣ Separation attains optimal transmission efficiency (Shannon).‣ Bits as the universal currency of the digital world → a channel can be extended to a network
!6
SourceEncoderSource
NoisyChannel
ChannelEncoder
Destination SourceDecoder
ChannelDecoder
Binary Interface
Bits
Bits
message waveform
signal waveform
Source Coding
• Sampling: ‣ (continuous-time) waveform → (discrete-time) sequence‣ (Review: Sampling Theorem, Nyquist Rate)
• Quantization: ‣ continuous-valued seq. → discrete-valued seq.
• Compression: ‣ discrete-valued seq. → bit seq., remove redundancy
!7
input (message) waveform
Sampler Quantizer Discrete Encoder
Discrete Decoder
Table LookupFilter
output (message) waveform
analog sequence
discrete sequence
Binary Interface
Source Coding
s(t)
s(t){s[m]}
{s[m]}
{dm}
{dm}
{bi}
{bi}
Channel Coding
• Error correcting code (ECC) encoder: bit seq. → Longer bit seq.‣ protect information bits by adding redundancy into the information bit seq.
• Symbol Mapper: bit seq. → discrete-valued seq.‣ map the coded bits to constellation points, a step in digital modulation
• Pulse Shaper: sequence → baseband waveform‣ bridge the digital world and the analog world, a key step in digital modulation
• Up Converter: baseband waveform → passband waveform‣ move the signal to the “right” frequency band
!8
ECC Encoder
Symbol Mapper
Pulse Shaper
Filter + Sampler
Symbol Demapper
ECC Decoder
codedbits
discrete sequence
Binary Interface
Channel Coding
Information bits
Up Converter
Down Converter
baseband waveform Noisy
Channel
passband waveform
{bi}
{bi}
{ci}
{ci}
{um}
{um}
xb(t)
yb(t)
x(t)
y(t)
This Course: All about Channel Coding• Build a bridge between the cyber and the physical world‣ Digital modulation and demodulation without noise
• Overcoming noise in narrow-band channels‣ Optimal detection under noise‣ Reliable communication with error correction codes
• Overcoming inter-symbol interference in wide-band ch.‣ Equalization techniques‣ Orthogonal frequency division multiplex (OFDM)
• Overcoming fading in wireless channels‣ Diversity techniques
!9