Think Silicon: Spread Spectrum Modulation

Living in a world of Digital Communication,we would need to know how secure the comunication is and what are the ways handled to have it secure.This article describes about Spread Spectrum Technology,the art of secure digital communications which was once restricted only for Military communicaton and that is now being exploited for commercial and industrial purposes too.

Applications for commercial spread spectrum range from "wireless" LAN's (computer to computer local area networks), to integrated bar code scanner/palmtop computer/radio modem devices for warehousing, to digital dispatch, to digital cellular telephone communications, to "information society" city/area/state or country wide networks for passing faxes, computer data, email, or multimedia data.
What is Spread Spectrum?
The process of using a second modulating signal which is independent of the data and has the effect of increasing the bandwidth of the transmitted signal to well beyond the bandwidth of the data signal.

How Spread Spectrum Works?
Spread Spectrum uses wide band, noise-like signals. Because Spread Spectrum signals are noise-like, they are hard to detect. Spread Spectrum signals are also hard to Intercept or demodulate. Further, Spread Spectrum signals are harder to jam (interfere with) than narrowband signals. 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.

Spread Spectrum transmitters use similar transmit power levels to narrow band transmitters. 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. 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 Modulation is distinguished from wideband modulation schemes such as wideband Frequency Modulation (FM) by noting that in spread spectrum the waveform causing the spreading is independent of the data being transmitted. This permits the spreading waveform to be selected based on improving system performance in some way. PN sequences are selected as the spreading signals since they uniformly spread the signal power over the available bandwidth and provide other critical advantages such as permitting universal frequency reuse.

Three Types of Spread Spectrum Communications:

Frequency hopping. The signal is rapidly switched between different frequencies within the hopping bandwidth pseudo-randomly, and the receiver knows before hand where to find the signal at any given time.

Time hopping. The signal is transmitted in short bursts pseudo-randomly, and the receiver knows beforehand when to expect the burst.

Direct sequence. The digital data is directly coded at a much higher frequency. The code is generated pseudo-randomly, the receiver knows how to generate the same code, and correlates the received signal with that code to extract the data.