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SCINET SPREAD SPECTRUM RADIO COMMUNICATIONS
Radio Communications between Associates and Operations Center

SCiNetEDR® uses the spread spectrum (digital transmission segments at low power and different frequencies) through an interface between the computer terminal and the radio card, at T1 transmission speeds (1.5 MB per second).

SCiNetEDR® Worldwide Communications Network Through Spread Spectrum Radio Frequencies. By splitting information into digital segments and then transmitting them at low power and at different frequencies, millions of people can send and receive messages at the same time. It’s been said over and over again that radio-electric space is a valuable and scarce resource, which should be rationed in the same way as water in the desert. This deeply rooted idea is comes from the concept of traditional transmitters and receivers, whose functioning is restricted to narrow specialized bands of the electromagnetic spectrum in order to minimize interference. For this reason, governments manage radio channels by issuing licenses as though they were drops of precious water. In some countries they even have auctions and bids for the allocation of the different types of bands: commercial radio and television broadcasts, military and police transmissions, taxi servers, CB communications, and cellular phone users. But advances in digital communications have opened the way for a new model. »
	» Transmitters can now make use of spread spectrum techniques in order to share channels without creating conflict with one other. Information can be split into segments in bundles of ones and zeros that are then radio-transmitted, each bundle being sent at low power through different channels or frequencies. In principle, millions of transmitters could work on the same frequency band at the same time within the same metropolitan area, moving hundreds of megabits per second.
This shared use of the spectrum presents a challenge to traditional practices. In the past, when allocating the narrow bands of commercial frequencies, governments issued licenses to companies, such as cellular telephone and personal communications service companies. These would charge users for services offered the same way that a telephone company would charge a subscriber. In the new economic model, there is no need for intermediaries. Users can communicate directly with each other free of charge, even if they are miles away and other people are using the same radio channels. This essential change has led to a review of the regulatory practices of governments, which have already designated certain frequency bands to be used freely by spread spectrum radio devices.

What is this technical revolution based on?

Traditional radio broadcasting worked by transmitting information at high power through a narrow frequency band. By operating within a tiny portion of the electromagnetic spectrum, each transmitter left room for others to work in without creating interference in neighboring frequencies. However, it turns out to be more cost-effective to broadcast information through a diametrically opposed method, which is to spread the information at low power using a smaller fraction of the spectrum.

At first it is not intuitively easy to understand the advantages: to see that the technique helps to split the spectrum "pie" more equally by allowing an almost unlimited number of individuals to have an almost invisible “nibble” at it. Although spread spectrum radio devices use wider bands than they actually require, they avoid interference because they broadcast their transmissions at minimum power and with only a fragment of information on each frequency used. Signals emitted are so weak that they are almost imperceptible above the background noise, which means that spread spectrum has an added advantage: other receivers have great difficulty in intercepting transmissions. In practice the intended receiver could be the only one to know what is being transmitted.

Beginnings...

At first, the most attractive feature of spread spectrum was stealth. During the Second World War, the Allies were interested in an intriguing device.

The idea was extremely simple: instead of transmitting information on a single channel, which the enemy might detect by chance during transmission, the device moved the channel continuously, transmitting a snippet of information here and another one there, according to a secret code known only by the transmitter and the intended receiver.

This unceasing frequency hopping prevented the enemy from picking up information out of the surrounding noise.

However, subsequent progress in circuit electronics made spread spectrum feasible. The semiconductor chips, crammed with thousands of transistors, can put forth packages of digital data according to an apparently random pattern through a great many channels.

The receiver, designed to capture signals according to the exact and exclusive sequence of the transmitting radio, rearranges the fragments of information received from the various frequencies.

Fig. 1 RADIO SIGNALS (in red) have traditionally been transmitted at high power and in a continuous fashion along a single narrow frequency band (a). Today, engineers know how to get better performance out of the radio spectrum by distributing each signal among various channels, as shown by the frequency hopping technique (b).

Furthermore, when the receiver misses a data package or comes across one that is corrupt, it can inform the transmitter so that it will be sent again. In addition, it can correct errors in advance, a technique that increases the likelihood that data will be received properly at the first go.

Fig. 2 DIRECT SEQUENCING is another technique for distributing a low power signal throughout the radio spectrum. A "10110" digital message (a) is mixed with a coded sequence (b). Next, the resulting signal is transmitted in such a way that each bit of the original is sent several times on a different frequency. This redundancy increases the likelihood of the message passing through even in densely populated urban areas, where interference is a problem. Next a receiver uses the same coded sequence in order to (d) decode the transmission and thereby decipher the original digital message (e)

Electronic techniques have brought us another method of spread spectrum. In the Direct Sequencing method, the information transmitted is mixed with a coded signal that sounds like noise to the outside listener.

In this alternative to the frequency hopping method, each bit of data is sent simultaneously over several frequencies, with, of course, the transmitter and the receiver both synchronized to the same coded sequence.

More recently, subsequent breakthroughs in chip technology have enabled signal processors to crush data at breakneck speeds; low consumption chips which are, to boot, fairly cheap.

These technological enhancements open the way to more refined spread spectrum techniques, including some hybrids that combine the best characteristics of frequency hopping and direct sequencing, as well as other data coding procedures.

The new procedures put up a fierce resistance to interference, to parasites and to the echo or ghost effect; this effect, which is frequency-dependent, may confuse the receiver by introducing delays as the signal bounces off buildings, the earth's surface and various atmospheric layers.

Where is this technique leading to?

Transmitters and receivers are almost completely digital. This trend, combined with the rapid development of wireless cellular systems, will open up a wide range of services related to spread spectrum. We can already make use of these services thanks to intelligent networks using "intelligent" transceivers and switches, such as the SCiNetEDR® System.

Such devices know, for example, which of the various spread spectrum techniques they must use in each situation in order to ensure accurate transmission of all of the information. Today, the Internet represents the best example of a self-regulating mechanism needed for the radio diffusion environment. The creation of a new but similar decentralized structure to optimize the shared distribution of the radio-electric spectrum will require a significant effort.

We think the deployment and growth of this system are objectives that are attainable by having increasingly more "intelligent" electronic circuits, and we can imagine a set of autonomous protocols incorporated into these intelligent devices. With the multiplication of advanced transmitters, society must accept the incorporation of incentives, both positive and negative, into the bosom of that same network infrastructure in order to make the best possible use of a common shared resource: the radio spectrum.

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