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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.
These Low Probability of Intercept (LPI) and anti-jam (AJ) features are why the military has used Spread Spectrum for so many years.
Spread signals are intentionally made to be much wider band than the information they are carrying to make them more noise-like.
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.
In both places, a new spread spectrum personal communications network (PCN) will share the 1.85-1.9-gigahertz band with local electric and gas utilities. " Spread Spectrum uses wide band, noise-like signals.
The FCC licensee, Millicom Inc., a New York City-based cellular telephone company, expects to enlist 45000 subscribers. Because Spread Spectrum signals are noise-like, they are hard to detect.Over the last eight or nine years a new commercial marketplace has been emerging.Called spread spectrum, this field covers the art of secure digital communications that is now being exploited for commercial and industrial purposes.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.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.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.