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.
|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|
|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.
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.
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.