Digital Communication: Signals, Modulation, and Demodulation, Slides of Digital Communication Systems

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Department of Electrical and Computer Engineering

Chapter 3

Introduction to Digital Communication

Overview

 Introduction  Digital representation of signals  Baseband digital transmission  Digital modulation  Coherent demodulation

Introduction

 The figure illustrates a functional block diagram of a typical digital communication system showing the key components of the system.  We will briefly review and discuss the functions of these key elements of the communication system.  Source Output: The output from an information source could be an analog signal such as voice or video signal or digital signal that is discrete in time and having a finite number of characters.  Messages from the source are converted into a sequence of binary digits. Ideally, the source message should be represented by as few as possible binary digits.

Introduction

 Source Encoding: The process of efficiently converting source outputs into a sequence of binary digits, called information sequence. The representation of the source output in binary form should have as little or no redundancy (data compression)  Channel Encoding: Introduce, in a controlled manner, some redundancy in the binary information sequence. The redundancy can be used at the receiver to overcome the effects of noise and other interferences on the transmission channel.  Trivial example: Repeat each binary digit n times

Introduction

 This is an example of binary modulation in which each bit from the encoder is transmitted separately, called Binary Phase-Shift Keying – BPSK.  Alternatively, modulator may transmit b coded information bits at a time using distinct waveforms si(t), i= 0,1,2…..M-1,called M-ary modulation.

Introduction

 Communication Channel: Physical medium that is used to send the signal from the transmitter to the receiver. Examples include:  Wireless transmission- the atmosphere or free space  Wireline, optical fiber, coaxial cables  Storage channels: Information storage and retrieval devices - magnetic tapes, compact discs, etc  Transmitted signals are corrupted, in a random manner, by a variety of additive noise such as thermal noise, atmospheric noise, man made noise, etc and also attenuated in amplitude.  Channels can be modeled in a variety of ways that take into account the particular properties of the channel

Introduction

 The probability of error is in general a function of the code characteristics, the type of waveforms used, the transmitter power, the channel characteristics and the method of demodulation and decoding.

Introduction

Major Factors that help the growth of Digital Communication

1. Impact of the Computer: Computers are processors and sources of data as well as tools for communication.
2. Digital communication offers flexibility and compatibility : The adoption of a common digital format makes it possible for a transmission system to handle many different sources of information in a flexible manner.
3. Improved reliability due to improved theory, microelectronics and system design.

Introduction

Different goals between analog and digital communication systems:  Analog communication systems: to reproduce the transmitted waveform accurately.  Use signal to noise ratio to assess the quality of the system  Digital communication systems: the transmitted symbol to be identified correctly by the receiver  Use the probability of error of the receiver to assess the quality of the system

Overview

 Introduction  Digital representation of signals  Baseband digital transmission  Digital modulation  Coherent demodulation

Pulse-Coded Modulation (PCM)

 Sample the message signal above the Nyquist frequency  Quantize the amplitude of each sample  Encode the discrete amplitudes into a binary codeword  Note: PCM isn’t modulation in the usual sense; it’s a type of Analog-to-Digital Conversion.

The PCM Process

Line Coding

 The bits of PCM, DPCM etc need to be converted into some electrical signals.  Line coding encodes the bit stream for transmission through a line, or a cable.  Line coding was used former to the wide spread application of channel coding and modulation techniques.  Nowadays, it is used for communications between the CPU and peripherals, and for short-distance baseband communications, such as the Ethernet.

Line Coding