About Spread Spectrum Technology

The article "Spread Spectrum goes commercial," by Donald L. Schilling of City College of New York, Raymond L. Pickholtz of George Washington University, and Laurence B. Milstein of UC San Diego, that appeared in the IEEE Spectrum, August, 1990 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. Such an advantage opens up crowded frequency spectra to vastly expanded use."

Introduction

In 1985, the FCC (Federal Communications Commission) allocated three frequency bands for a radio transmission technique know as Spread Spectrum communications, originally developed by the military. The three bands are 902-928 MHz, 2400-2483.5 MHz and 5752.5-5850 MHz. Spread Spectrum has much greater immunity to interference and noise compared to conventional radio transmission techniques. In addition, an increasing number of users can use the same frequency (similar to cellular). These rules are designed to drive usage towards local data communications. Under the regulations, users of FCC certified Spread Spectrum products do not require a license from the FCC. The only requirement is that the manufacturers of Spread Spectrum products must meet FCC Spread Spectrum regulations.

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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 narrow band 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 Spectrum signals are intentionally made to be a 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 Wats per Hertz, than narrow band transmitters. This lower transmitted power density characteristic gives Spread Spectrum signals a big plus. Spread Spectrum 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.

What Spread Spectrum Does

The use of special pseudo noise codes in Spread Spectrum (SS) communications makes signals appear wide band and noise-like. It is this very characteristic that makes SS signals possess the quality of Low Probability of Intercept. SS signals are hard to detect on narrow band equipment because the signal's energy is spread over a bandwidth of maybe 100 times the information bandwidth.

The spread of energy over a wide band, or lower spectral power density, makes SS signals less likely to interfere with narrow band communications. Narrow band communications, conversely, cause little to no interference to SS systems because the correlation receiver effectively integrates over a very wide bandwidth to recover an SS signal. The correlator then "spreads" out a narrow band interferer over the receiver's total detection bandwidth. Since the total integrated signal density or SNR at the correlator's input determines whether there will be interference or not. All SS systems have a threshold or tolerance level of interference beyond which useful communication ceases. This tolerance or threshold is related to the SS processing gain. Processing gain is essentially the ratio of the RF bandwidth to the information bandwidth.