Spread-spectrum techniques are methods by which energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference and jamming, and to prevent detection. Spread Spectrum techniques have some powerful properties which make them an excellent candidate for networking applications. There are a number of good practical reasons why spread spectrum modulation is technically superior to the intuitively more obvious techniques such as AM and FM, and all of the hybrids which lie in between.
• The Ability to Selectively Address. Spread the signal, and use the proper encoding method, then the signal can only be decoded by a receiver which knows the transmitter's code. Therefore by setting the transmitter's code, target a specific receiver in a group, or vice versa. This is termed Code Division Multiple Access.
• Bandwidth Sharing. It is entirely feasible to have multiple pairs of receivers and transmitters occupying the same bandwidth. This would be equivalent to having say ten TV channels all operating at the same frequency. In a world where the radio spectrum is being busily carved up for commercial broadcast users, the ability to share bandwidth is a valuable capability.
• Security from Eavesdropping. If an eavesdropper does not know the modulation code of a spread spectrum transmission, all the eavesdropper will see is random electrical noise rather than something to eavesdrop. If done properly, this can provide almost perfect immunity to interception.
• Immunity to Interference. If an external radio signal interferes with a spread spectrum transmission, it will be rejected by the demodulation mechanism in a fashion similar to noise. Therefore we return to the starting point of this discussion, which is that spread spectrum methods can provide excellent error rates even with very faint signals.
• Difficulty in Detection. Because a spread spectrum link puts out much less power per bandwidth than a conventional radio link, having spread it over a wider bandwidth, and a knowledge of the link's code is required to demodulate it, spread spectrum signals are extremely difficult to detect. This means that they can coexist with other more conventional signals without causing catastrophic interference to narrowband links.
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