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Introduction to Communication Systems

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A characteristic of electrical communication systems is the presence of uncertainty. This uncertainty is due in part to the inevitable presence in any system of unwanted signal perturbation, broadly referred to as noise, and in part to the unpredictable nature of information itself. Systems analysis in the presence of such uncertainty requires the use of probabilistic techniques.

Noise has been an ever-present problem since the early days of electrical communication, but it was not until the 1940s that probabilistic systems analysis procedures were used to analyze and optimize communication systems operating in its presence. It is also somewhat surprising that the unpredictable nature of information was not widely recognized until the publication of Claude Shannon's mathematical theory of communications in the late 1940s. This work was the beginning of the science of information theory.

Major historical facts related to the development of electrical communications are given in the table below. It provides an appreciation for the accelerating development of communications-related inventions and events down through the years.

 Year Event
1791 Alessandro Volta invents the galvanic cell, or battery
1826 Georg Simon Ohm establishes a law on the voltage-current relationship in resistors
1838 Samuel F. B. Morse demonstrates the telegraph
1864 James C. Maxwell predicts electromagnetic radiation
1876 Alexander Graham Bell patents the telephone
1887 Heinrich Hertz verifies Maxwell's theory
1897 Guglielmo Marconi patents a complete wireless telegraph system
1904 John Fleming patents the thermionic diode
1905 Reginald Fessenden transmits speech signals via radio
1906 Lee De Forest invents the triode amplifier
1915 The Bell System completes a U.S. transcontinental telephone line
1918 B. H. Armstrong perfects the superheterodyne radio receiver
1920 J. R. Carson applies sampling to communications
1925-27 First television broadcasts in England and the United States
1931 Teletypewriter service is initialized
1933 Edwin Armstrong invents frequency modulation
1936 Regular television broadcasting began by the BBC
1937 Alec Reeves conceives pulse-code modulation (PCM)
WWII Radar and microwave systems are developed; Statistical methods are applied to signal extraction problems
1944 Computers put into public service (government-owned)
1948 The transistor is invented by W. Brattain, J. Bardeen, & W. Shockley
1948 Claude Shannon's "A Mathematical Theory of Communications" is published
1950 Time-division multiplexing is applied to telephony
1956 First successful transoceanic telephone cable
1959 Jack Kilby patens the "solid circuit" - the precursor to the integrated circuit
1960 First working laser demonstrated by T. H. Mainman of Hughes Research Labs (patent awarded to G. Gould after 20-year dispute with Bell Labs)
1962 First communications satellite, Telstar I, launched
1966 First successful FAX (facsimile) machine
1967 U.S. Supreme Court Carterfone decision opens the door for modem development
1968 Live television coverage of the moon exploration
1969 First Internet started - APPANET
1970 Low-loss optical fiber developed
1971 Microprocessor invented
1975 Ethernet patent filed
1976 Apple I home computer invented
1977 Live telephone traffic carried by fiber-optic cable system
1977 Interplanetary grand tour launched; Jupiter, Saturn, Uranus, and Neptune
1979 First cellular telephone network started in Japan
1981 IBM personal computer developed and sold to public
1981 Hayes Smartmodem marketed (automatic dial-up allowing computer control)
1982 Compact disk (CD) audio based on 16-bit PCM developed
1983 First 16-bit programmable digital signal processors sold
1984 Divestiture of AT&T's local operations into seven Regional Bell Operating Companies
1985 Desktop polishing programs first sold, Ethernet developed
1988 First commercially available flash memory (later applied in cellular phones, etc.)
1988 ADSL (asymmetric digital subscribe lines) developed
1990s Very small aperture satellite (VSATs) become popular
1991 Application of echo cancellation results in low-cost 14,400 bits/s modems
1993 Invention of turbo coding allows approach to Shannon limit
mid-1990s Second-generation (2G) cellular systems fielded
1995 Global Positioning System reaches full operational capability
1996 All-digital phone systems result in modems with 56 kbps download speeds

Widespread personal and commercial applications of the Internet

High-definition TV becomes mainstream

2001 Fielding of 3G cellular telephone systems begins; WiFi and WiMAX allow wireless access to the Internet and electronic devices wherever mobility is desired
2000s Wireless sensor networks, originally conceived for military applications, find civilian applications such as environmental monitoring, healthcare applications, home automation, and traffic control as well
2010s Introduction of fourth-generation cellular radio. Technological convergence of communication-related devices - e.g., cell phones, television, personal digital assistants, etc.


It is an interesting fact that the first electrical communication system, the telegraph, was digital - that is, it conveyed information from point to point by means of a digital code consisting of words composed of dots and dashes. The subsequent invention of the telephone 38 years after the telegraph, wherein voice waves are conveyed by an analog current, swung the pendulum in favor of this more convenient means of word communication for about 75 years.

One may rightly ask, in view of this history, what the almost complete domination by digital formatting in today’s world? There are several reasons, among which are: (1) Media integrity - a digital format suffers much less deterioration in reproduction than does an analog record; (2) Media integration - whether a sound, picture, or naturally digital data such as a word file, all are treated the same when in digital format; (3) Flexible interaction - the digital domain is much more convenient for supporting anything from one-on-one to many-to-many interactions; (4) Editing - whether text, sound, images, or video, all are conveniently and easily edited when in digital format.

With this brief introduction and history, we now look in more detail at the various components that make up a typical communication system.

1. The Block Diagram of a Communication System

The following figure shows a commonly used model for a single-link communication system. Although it suggests a system for communication between two remotely located points, this block diagram is also applicable to remote sensing systems, such as radar or sonar, in which the system input and output may be located at the same site. Regardless of the particular application and configuration, all information transmission systems invariably involve three major subsystems - a transmitter, the channel, and a receiver. We usually think in terms of systems for transfer of information b twenty remotely located points. It is emphasized, however, that the techniques of systems analysis are not limited to such systems. 

Input Transducer

The wide variety of possible sources of information results in many different forms for message. Regardless of their exact form, however, messages may be categorized as analog or digital. The former may be modeled as functions of a continuous-time variable (for example, pressure, temperature, speech, music), whereas the latter consist of discrete symbols (for example, written text or a sampled/quantized analog signal such as speech). Almost invariably, the message produced by a source must be converted by a transducer to a form suitable for the particular type of communication system employed. For example, in electrical communications, speech waves are converted by a microphone to voltage variations. Such a converted message is referred to as the message signal. Therefore, a signal can be interpreted as the variation of a quantity, often a voltage or current, with time.


The purpose of the transmitter is to couple the message to the channel. Although it is not uncommon to find the input transducer directly coupled to the transmission medium, as for example in some intercom systems, it is often necessary to modulate a carrier wave with the signal from the input transducer. Modulation is the systematic variation of some attribute of the carrier, such as amplitude, phase, or frequency, in accordance with a function of the message signal. There are several reasons for using a carrier and modulating it. Important ones are (1) for ease of radiation, (2) to reduce noise and interference , (3) for channel assignment, (4) for multiplexing or transmission of several messages over a single channel, and (5) to overcome equipment limitations. Several of these reasons are self-explanatory; others, such as the second, will become more meaningful later.

In addition to modulation, other primary functions performed by the transmitter are filtering, amplification, and coupling the modulated signal to the channel (for example, through an antenna or other appropriate device).


The channel can have many different forms; the most familiar, perhaps, is the channel that exists between the transmitting antenna of a commercial radio station and the receiving antenna of a radio. In this channel, the transmitted signal propagates through the atmosphere, or free space, to the receiving antenna. However, it is not uncommon to find the transmitter hard-wired to the receiver, as in most local telephone systems. This channel is vastly different from the radio example. However, all channels have one thing in common: the signal undergoes degradation from transmitter to receiver. Although this degradation may occur at any point of the communication system block diagram, it is customarily associated with the channel alone. This degradation often results from noise and other undesired signals or interference but also may include other distortion effects as well, such as fading signal levels, multiple transmission paths, and filtering. More about these unwanted perturbations will be presents shortly.


The receiver’s function. Is to extract the desired message from the received signal at the channel output and to convert it to a form suitable for the output transducer. Although amplification may be one of the first operations performed by the receiver, especially in radio communications, where the received signal may be extremely weak, the main function of the receiver is to demodulate the received signal. Often it is desired that the receiver output be a scaled, possibly delayed, version of the message signal at the modulator input, although in some cases a more general function of the input message is desired. However, as a result of the presence of noise and distortion, this operation is less than ideal. Ways of approaching the ideal case of perfect recovery will be discussed as we proceed.

Output Transducer

The output transducer completes the communication system. This device converts the electrical signal at its input into the form desired by the system user. Perhaps the most common output transducer is a loudspeaker or ear phone.


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