Spread Spectrum Research Papers

"The demonstration is intended to show that spread-spectrum users can share a frequency band with conventional microwave radio users--without one group interfering with the other -- thereby increasing the efficiency with which that band is used. Spread Spectrum signals are also hard to Intercept or demodulate.Further, Spread Spectrum signals are harder to jam (interfere with) than narrowband signals.

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Over the last 50 years, a class of modulation techniques usually called "Spread Spectrum," has been developed.

This group of modulation techniques is characterized by its wide frequency spectra.

There are also "Time Hopped" and "Chirp" systems in existence.

Time hopped spread spectrum systems have found no commercial application to date.

Because Spread Spectrum signals are so wide, they transmit at a much lower spectral power density, measured in Watts per Hertz, than narrowband transmitters.

This lower transmitted power density characteristic gives spread signals a big plus.Figure 1 illustrates the most common type of direct sequence modulated spread spectrum signal.Direct sequence spectra vary somewhat in spectral shape depending upon the actual carrier and data modulation used.The IEEE Spectrum of August, 1990 contained an article entitled Spread Spectrum Goes Commercial, by Donald L. Pickholtz of George Washington University, and Laurence B. This article summarized the coming of commercial spread spectrum: "Spread-spectrum radio communications, long a favorite technology of the military because it resists jamming and is hard for an enemy to intercept, is now on the verge of potentially explosive commercial development.The reason: spread-spectrum signals, which are distributed over a wide range of frequencies and then collected onto their original frequency at the receiver, are so inconspicuous as to be 'transparent.' Just as they are unlikely to be intercepted by a military opponent, so are they unlikely to interfere with other signals intended for business and consumer users -- even ones transmitted on the same frequencies.The modulated output signals occupy a much greater bandwidth than the signal's baseband information bandwidth.To qualify as a spread spectrum signal, two criteria should be met: Most commercial part 15.247 spread spectrum systems transmit an RF signal bandwidth as wide as 20 to 254 times the bandwidth of the information being sent.Such an advantage opens up crowded frequency spectra to vastly expanded use."A case in point is a two-year demonstration project the Federal Communications Commission (FCC) authorized in May (1990) for Houston, Texas, and Orlando, Fla.Binary code sequences as short as 11 bits or as long as [2^(89) - 1] have been employed for this purpose, at code rates from under a bit per second to several hundred megabits per second.The result of modulating an RF carrier with such a code sequence is to produce a signal centered at the carrier frequency, direct sequence modulated spread spectrum with a (sin x/x)2 frequency spectrum.


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