MODULATION TECHNIQUES

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mwo1974

After graduating from the University of Michigan in 1936 with bachelor's degrees in mathematics
and electrical engineering, Shannon obtained a research assistant's position at the
Massachusetts Institute of Technology (MIT). There, among other duties, he worked with the
noted researcher Vannevar Bush, helping to set up differential equations on Bush's differential
analyzer . A summer internship at American Telephone and Telegraph's Bell Laboratories in New
York City in 1937 inspired much of Shannon's subsequent research interests. In 1940 he earned
both a master's degree in electrical engineering and a Ph.D. in mathematics from MIT. He joined
the mathematics department at Bell Labs in 1941, where he first contributed to work on
antiaircraft missile control systems. He remained affiliated with Bell Labs until 1972. Shannon
became a visiting professor at MIT in 1956, a permanent member of the faculty in 1958, and
professor emeritus in 1978.
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Shannon's master's thesis, "A Symbolic Analysis of Relay and Switching Circuits" (1940), used
Boolean algebra to establish the theoretical underpinnings of digital circuits. Because digital
circuits are fundamental to the operation of modern computers and telecommunications
equipment, this dissertation was called one of the most significant master's theses of the 20th
century. In contrast, his doctoral thesis, "An Algebra for Theoretical Genetics" (1940), was not
as influential.
In 1948 Shannon published "A Mathematical Theory of Communication," which built on the
foundations of other researchers at Bell Labs such as Harry Nyquist and R.V.L. Hartley.
Shannon's paper, however, went far beyond the earlier work. It established the basic results of
information theory in such a complete form that his framework and terminology are still used.
(The paper appears to contain the first published use of the term bit to designate a single binary
digit.)
An important step taken by Shannon was to separate the technical problem of delivering a
message from the problem of understanding what a message means. This step permitted
engineers to focus on the message delivery system. Shannon concentrated on two key questions
in his 1948 paper: determining the most efficient encoding of a message using a given alphabet
in a noiseless environment, and understanding what additional steps need to be taken in the
presence of noise.
Shannon solved these problems successfully for a very abstract (hence widely applicable) model
of a communications system that includes both discrete (digital) and continuous (analog)
systems. In particular, he developed a measure of the efficiency of a communications system,
called the entropy (analogous to the thermodynamic concept of entropy, which measures the
amount of disorder in physical systems), that is computed on the basis of the statistical
properties of the message source.
Shannon's formulation of information theory was an immediate success with communications
engineers and continues to prove useful. It also inspired many attempts to apply information
theory in other areas, such as cognition, biology, linguistics, psychology, economics, and physics.
In fact, there was so much enthusiasm in this direction that in 1956 Shannon wrote a paper,
"The Bandwagon," to moderate some overenthusiastic proponents.
Renowned for his eclectic interests and capabilities--including such activities as juggling while
riding a unicycle down the halls of Bell Labs--Shannon produced many provocative and
influential articles on information theory, cryptography, and chess-playing computers, as well as
designing various mechanical devices.
Four years after he published his ground-breaking theory, Shannon invented an electrical mouse
with a telephone relay switch brain. Its ability to find its way through a maze.................

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