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Direct sequence systems -- Direct sequence spread spectrum systems are so called because they employ a high speed code sequence, along with the basic information being sent, to modulate their RF carrier.The high speed code sequence is used directly to modulate the carrier, thereby directly setting the transmitted RF bandwidth.
There are also "Time Hopped" and "Chirp" systems in existence.
Time hopped spread spectrum systems have found no commercial application to date.
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 main lobe of this spectrum has a bandwidth twice the clock rate of the modulating code, from null to null.
The sidelobes have a null to null bandwidth equal to the code's clock rate.However, the arrival of cheap random access memory (RAM) and fast micro-controller chips make time hopping a viable alternative spread spectrum technique for the future."Chirp" signals are often employed in radar systems and only rarely used in commercial spread spectrum systems.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.Spread and narrow band signals can occupy the same band, with little or no interference.This capability is the main reason for all the interest in Spread Spectrum today.Spread Spectrum signals use fast codes that run many times the information bandwidth or data rate.These special "Spreading" codes are called "Pseudo Random" or "Pseudo Noise" codes.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.