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                               Spread Spectrum Technology

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. This transmission technique 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.

Introduction

FCC rule changes in 1985, combined with the continuing evolution of digital technology has catalyzed the development of spread spectrum data communication radios. These radios offer significant performance and operation benefits to end-users.

In this section, the operation of spread spectrum radio technology is presented, performance differences between spread spectrum and conventional radios, regulatory implications, and applications are discussed.

Conventional Radio Operation - Narrow Band Radio Signal

The conventional radio signal is referred to as narrow-band, which means that it contains all of its power in a very narrow portion of the radio frequency bandwidth. Due to the relatively small portion of the radio band that an individual radio transmission occupies, the FCC has traditionally favored these conventional radios. However, as a result of the very narrow frequency, these radios are prone to interference (a single interfering signal at or near their frequency can easily render the radio inoperable).

Spread Spectrum Radio Operation

Spread spectrum is a technique that takes a narrow band signal and spreads it over a broader portion of the radio frequency band. This has the operational advantage of being resistant it interference, however, due to unfounded concerns over the increased frequency space it occupies, the FCC until recently, did not permit commercial use of the technology.

Spread Spectrum Radio Signal

In performing Spread Spectrum, the transmitter takes the input data and spreads it in a predefined method. Each receiver must understand this predefined method and despread the signal before the data can be interpreted.

There are two basic methods to performing the spreading: (1) Frequency Hopping, and (2) Direct Sequencing. Frequency hopping spreads its signals by "hopping" the narrow band signal as a function of time. Direct sequencing spreads its signal by expanding the signal over a broad portion of the radio band.

The FCC allows the use of Spread Spectrum technology in three radio bands, 902-928 MHz, 2400-2483.5 MHz and 5752.5-5850 MHz for transmission under 1 Watt of power. This power limit prevents interference within the band over long distances.

Advantages to Spread Spectrum

No FCC Site License - The FCC will grant a one time license on the radio product. After that license is granted, the product can be sold anywhere in the U.S.

Interference Immunity - Spread Spectrum radios are inherently more noise immune than conventional radios. Thus they will operate with higher efficiency than conventional technology.

Multi-Channel - Conventional radios operate on a specific frequency controlled by a matched crystal oscillator. The specific frequency is allocated as a part of the FCC site license, and the equipment must remain on that frequency (except for very low power devices such as cordless phones).

Spread Spectrum data radios offer the opportunity to have multiple channels which can be dynamically changed through software. This allows for many applications such as repeaters, redundant base station and overlapping antenna cells.
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