Spread Spectrum

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Introduction

In today’s wireless communications the technology has come a long way since its beginnings back in the early nineteen hundreds and has become so advance that most countries have their own regulator. In the UK the regulator for wireless standards is called Ofcom which has been established since 2003. In the US the regulator is Federal Communications Commission and has been established since 1934. These regulators govern what frequency band companies can purchase to provide a wireless mobile service to customers.
The term spread spectrum is used in data communications and is a technique in which the transmitted signal is distributed over a wide frequency band, much wider than the minimum bandwidth required to transfer the information. The signal distributing is accomplished by deploying a pseudo-noise code which is not directly connected to the data. The pseudo-noise code is used as a modulation waveform to expand the baseband signal over a bandwidth much greater than the signal information bandwidth. At the receiver a reception synchronized to the code is used to demodulate the signal and collect the data.

In the diagram above you can see what the information signal looks like before it’s been modulated and after.
Spread spectrum first became well known way back in 1942 when it received U.S patent which was granted to Hollywood actress Hedy Lamarr and pianist George Antheil. It was described as a safe radio link to control torpedoes but the technology never became commercial available until the 1980s and is now used for applications that involve radio links in hostile environments.
The spread spectrum technology became popular with the military because it provides a low probability of intercept and anti-jam featur...

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...he carrier to hop from frequency to frequency at random but predictable sequence over a wide band according to the pseudo-random noise (PN) code. The higher the data rate of the information the faster the hops occur.
The next modulation technique is Direct-Sequence Spread Spectrum (DSSS) which applies the (PN) code directly to the data entering the carrier modulator. The baseband signal is spread out into a much larger bandwidth greater than the minimum bandwidth required to transmit the information.
Another modulation technique is time-hopping spread spectrum (THSS) which is used to achieve anti-jamming (AJ) or low probability of intercept (LPI).
The final modulation technique is chirp spread spectrum (CSS) that uses wideband linear frequency modulated chirp pulses to encode information. (A chirp is a signal in which a frequency increases or decreases in time).

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