Spread Spectrum Development Tools

Over the last eight or nine years a new commercial marketplace has been emerging. Called spread spectrum, this field covers the art of secure digital communications that is now being exploited for commercial and industrial purposes. In the next several years hardly anyone will escape being involved, in some way, with spread spectrum communications. 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. Direct sequence spread spectrum .....

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A number of advantages are:

- Low power spectral density. As the signal is spread over a large frequency-band, the Power-Spectral Density is getting very small, so other communications systems do not suffer from this kind of communications. However the Gaussian Noise level is increasing. Interference limited operation. In all situations the whole frequency-spectrum is used.

- Privacy due to unknown random codes. The applied codes are - in principle - unknown to a hostile user. This means that it is hardly possible to detect the message of an other user.

- Applying spread spectrum implies the reduction of multi-path effects. Random access possibilities. Users can start their transmission at any arbitrary time.

- Good anti-jam performance.
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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.
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How "The Bad Boy Of Music" And "The Most Beautiful Girl In The World" Catalyzed A Wireless Revolution--In 1941 By Anna Couey

Today, spread spectrum holds the potential to revolutionize wireless communications, because it renders radio spectrum--a resource currently deemed so precious that only the largest of corporations can afford to buy it--plentiful enough for all of us.

Spread spectrum is a highly efficient way of using radio waves to communicate, because it enables multiple users to share radio frequencies at the same time, without interfering with each other. Up until now, each radio station, for example, has been allocated a specific frequency over which to broadcast their programs. Since there are only so many bands of frequency within the entire radio spectrum, frequency bands are carefully allocated by the FCC for particular uses--such as commercial broadcasts, military communications, police radios, ham radios, etc.
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Direct Sequence is one of the most popular forms of spread spectrum. This is probably a result of the simplicity with which direct sequencing can be implemented. In this form of modulation, a pseudorandom noise generator creates a high-speed pseudonoise code sequence. This sequence is transmitted at a maximum bit rate called the chip rate. The pseudo-random code sequence is used to directly modulate the narrow-band carrier signal; thus, it directly sets the transmitted radio frequency (RF) bandwidth. The chip rate has a direct correlation to the spread of the information. The information is demodulated at the receiving end by multiplying the signal by a locally generated version of the pseudo-random code sequence. While direct sequence is a very popular form of spread spectrum transmission, it is not by any means the only method available. Another popular from of implementing spread spectrum takes an entirely different approach to spreading then that of direct sequencing.

Frequency Hopping is a from of spread spectrum in which spreading takes place by hopping from frequency to frequency over a wide band. The specific order in which the hopping occurs is determined by a hopping table generated with the help of a pseudo-random code sequence. The rate of hopping is a function of the information rate. The order of frequencies that is selected by the receiver is dictated by the pseudorandom noise sequence. While the transmitted spectrum of a frequency-hopping signal is quite different from that of a direct sequence signal, it is sufficient to note that the data is spread out over a signal band larger than is necessary to carry it. In both cases, the resultant signal appears noise-like and the receiver utilizes a similar technique to the one employed in transmitting in order to recover the original signal.
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The objective of this project is to introduce the reader to the concept of spread spectrum in 'general' terms and then proceed with a detailed, technical discussion of various signaling techniques, including mathematical derivations. The project ends with a description of one of the civilian applications such as cellular code division multiple access (CDMA) system as described by the IS-95: air-interference specification standard which was released in July 1993. It is hoped that this project will introduce the reader to spread spectrum techniques through theoretical discussion and multiple examples. It will also provide information beyond the introductory materials through a description of specific application of spread spectrum technique
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The advantages of using spread spectrum in data and voice communications are summarized by Pahlavan [7] as follows:

1.spread-spectrum signals can be overlaid onto bands where other systems are already operating, with minimal performance impact to or from the other systems.

2.the anti-multipath characteristics of spread-spectrum signaling and reception techniques are attractive in applications where multipath is likely to be prevalent.

3.the anti-interference characteristics of spread-spectrum are important in some applications, such as networks operating on manufacturing floors, where the signal interference environment can be harsh.

4.cellular systems designed with CDMA spread-spectrum technology offer greater operational flexibility and possibly a greater overall system capacity than do systems built on frequency division multiplexing (FDMA) or time division multiplexing (TDMA) access methods.

5.the convenience of unlicensed spread-spectrum operation in ISM bands in the United States is attractive to manufacturers and users alike.
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