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Spread Spectrum Primer

Spread Spectrum is the art of secure digital communications that is now being exploited for commercial and industrial purposes. Hardly anyone can escape being involved, in some way, with spread spectrum communications these days. 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 orcountry wide networks for passing faxes, computer data, email, or multimedia data.

On this page and on our Tutorial Page, we endeavor to provide you with some basic knowledge about this fascinating and useful technology. Please explore some of our introductory topics below:

Contents of This Page

Virginia Polytechnic Institute's SS Introduction
- Direct Sequence Spread Spectrum
- Frequency Hopped Spread Spectrum
Which is Better, Direct Sequence or Frequency Hopping?
Other Spread Spectrum Resources on SSS Online
Reference Books on Spread Spectrum

Virginia Polytechnic Institute's Spread Spectrum Introduction

The term spread spectrum describes a modulation technique which makes the sacrifice of bandwidth in order to gain signal-to-noise performance. Basically, the SS system is a system in which the transmitted signal is spread over a frequency much wider than the minimum bandwidth required to send the signal. The fundamental premise is that, in channels with narrowband noise, increasing the transmitted signal bandwidth results in an increased probability that the received information will be correct. If total signal power is interpreted as the area under the spectral density curve, then signals with equivalent total power may have either a large signal power concentrated in a small bandwidth or a small signal power spread over a large bandwidth.

From a system viewpoint, the performance increase for very wideband systems is referred to as "process gain". This term is used to describe the received signal fidelity gained at the cost of bandwidth. The numerical advantage is obtained from Claude Shannon's equation describing channel capacity:

C=W log2 (1+ S/N)

where C = Channel capacity in bits, W = Bandwidth in Hertz, S = Signal Power, and N = Noise Power

From this equation, the result of increasing the bandwidth becomes apparent. By increasing W in the equation, the S/N may be decreased without decreased BER performance. The process gain (GP) is what actually provides increased system performance without requiring a high S/N. This is described mathematically as:

GP = BWRF/RINFO

where BWRF = RF Bandwidth in Herz and RINFO = Information rate in bits/second.

The baseband signal is spread out to BWRF over the channel (see Fig. 1). Then at the receiving end, the signal is de-spread by the same amount by a correlation with a desired signal generated by the spreading technique (more on the different spreading techniques later). When the received signal is matched to the desired signal the baseband/information signal is retrieved.

Fig. 1 Bandwidth Spreading

Signal Spreading works quite well in situations with strong narrowband interference signals, since the SS signal has a unique form of frequency diversity. The actual signal spreading may be achieved with one of three basic techniques. These include: direct sequence, frequency hopped and pulsed FM or hybrid forms.

Direct Sequence Spread Spectrum (DSSS)

This is probably the most widely recognized form of spread spectrum. The DSSS process is performed by effectively multiplying an RF carrier and a pseudo-noise (PN) digital signal. First, the PN code is modulated onto the information signal using one of several modulation techniques (e.g. BPSK, QPSK, etc ). Then, a doubly balanced mixer is used to multiply the RF carrier and PN modulated information signal. This process causes the RF signal to be replaced with a very wide bandwidth signal with the spectral equivalent of a noise signal. The demodulation process (for the BPSK case) is then simply the mixing/multiplying of the same PN modulated carrier with the incoming RF signal. The output is a signal that is a maximum when the two signals exactly equal one another or are "correlated." The correlated signal is then filtered and sent to a BPSK demodulator.

The signals generated with this technique appear as noise in the frequency domain. The wide bandwidth provided by the PN code allows the signal power to drop below the noise threshold without loss of information. The spectral content of an SS signal is shown in Fig. 2. Note that this is just the spectrum of a BPSK signal with a (sin x / x)2 form.

Fig. 2 BPSK DSSS Spectrum

The bandwidth in DSSS systems is often taken as the null-to-null bandwidth of the main lobe of the power spectral density plot (indicated as 2Rc in Fig. 2). The half power bandwidth of this lobe is .88 Rc, where Rc is the chip rate. Therefore, the bandwidth of a DSSS system is a direct function of the chip rate; specifically 2Rc/RINFO. This is just an extension of the previous equation for process gain. It should be noted that the power contained in the main lobe comprises 90 percent of the total power. This allows a narrower RF bandwidth to accommodate the received signal with the effect of rounding the received pulses in the time domain.

One feature of DSSS is that QPSK may be used to increase the data rate. This increase of a factor of two bits per symbol of transmitted information over BPSK causes an equivalent reduction in the available process gain. The process gain is reduced because for a given chip rate, the bandwidth (which sets the process gain) is halved due to the two-fold increase in information transfer. The result is that systems in a spectrally quiet environment benefit from the possible increase in data transfer rate.

Frequency Hopped Spread Spectrum (FHSS)

Frequency hopping relies on frequency diversity to combat interference. This is accomplished by multiple frequency, code-selected FSK. Basically, the incoming digital stream is shifted in frequency by an amount determined by a code that spreads the signal power over a wide bandwidth. In comparison to binary FSK, which has only two possible frequencies, FHSS may have 2*10^20 or more.

The FHSS transmitter is a pseudo-noise PN code controlled frequency synthesizer. The instantaneous frequency output of the transmitter jumps from one value to another based on the pseudo-random input from the code generator (see Fig. 3). Varying the instantaneous frequency results in an output spectrum that is effectively spread over the range of frequencies generated.

Fig.3 FHSS Spectrum

In this system, the number of discrete frequencies determines the bandwidth of the system. Hence, the process gain is directly dependent on the number of available frequency choices for a given information rate.

Another important factor in FHSS systems is the rate at which the hops occur. The minimum time required to change frequencies is dependent on the information bit rate, the amount of redundancy used, and the distance to the nearest interference source.

Visit the Virginia Tech Site for More Great SS & Wireless Information
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Which is Better, DS or FH Spread Spectrum?

This article is re-printed from "Spread Spectrum Scene" magazine (paper version), Volume 3, Number 3, page 8, and was written by Randy Roberts, RF/SS Consulting (Retired)

This frequently asked question is really rather hard to answer. There is no real unbiased way to compare today's crop of commercial direct sequence radios to the frequency hoppers that are available. Sure, claims and counter claims abound, but the truth is hard to find.

Why? A little history helps explain what has evolved in the commercial SS world. First of all, the FCC's first Part 15 rules (published in 1989), did not require any SS radio to have processing gain - nor did these initial rules differentiate between fast and slow hopping. Thus the earliest SS radios produced, could use almost anything as long as they met the then defined Part 15 rules.

Some of these early radios used post detection correlation and thus, were not "TRUE DIRECT SEQUENCE" radios, at all. Only when correlation is done before detection, can all of the anti-jam and anti-interference benefits of direct sequence be seen. Some of the early hoppers changed frequency so slowly that they transmitted tens of thousands of bits on a single frequency dwell (and made no provision for error detection - let alone correction).

It's no wonder then, that some of these early radios (of either variety) were very short of the long hyped interference immunity that they were supposed to have. In fact, in Europe and the United Kingdom, Direct Sequence has gotten such a bad name from early trials with overly simple Direct Sequence radios, that frequency hoppers have almost become a standard.

The FCC tried to rectify this situation in 1992, with new Part 15 technical rules that require a minimum processing gain and better definitions of hopping speed and numbers of hopping channels required. But, out of intense lobbying efforts, came "grandfather provisions" that allowed existing approved designs to be sold for 5 years beyond 1989. The most recent actions of the FCC, however, have granted "dispensations" to those "grandfathered" manufacturers who yelled the loudest. The "deal" that was struck allows slow hoppers and post detection correlation (Non-TRUE DIRECT SEQUENCE) radios to continue to be sold if they keep their power output below 100 milliwatts.

So if a manufacturer cannot furnish a radio with significantly more power than 100 mW, they are probably peddling an old, inferior design - Caveat Emptor!

So the answer to the which is better is still unclear -- neither is any good, if it's an old design! Fast hoppers (no more than a few bits per frequency dwell) can have almost identical performance to Direct Sequence.

Real (or TRUE) DS and FH radios can each be vulnerable to certain kinds of interference. No one modulation is best against any and ail interferers! However, the best that can be done with SS is to use a hybrid, or combination of DS and FH, that adapts to channel conditions in real time. The BEST SS modulation is thus seen to be not either DS or FH -- but both, when used optimally against adverse interference, multipath and channel conditions.

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Other Spread Spectrum Resources on SSS Online

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Reference Books on Spread Spectrum

Click on a Title Below for a Direct Link to Purchase

Buy This Book	*Spread Spectrum Cdma Systems for Wireless Communications (Artech House Mobile Communications Series), by Savo Glisic, Branka Vucetic (Contributor). Hardcover - 383 pages (April 1997). Our Price: Click for Price*	Buy This Book	*Spread Spectrum Systems : With Commercial Applications, by Robert C. Dixon. Hardcover - 592 pages 3rd edition (April 1994). Our Price: Click for Price*
Cover not Available	*Spread Spectrum Communications Handbook, by Marvin K. Simon, Jim K. Omura (Contributor), Robert A. Scholtz (Contributor), Barry K. Levitt. Hardcover - 1228 pages Revised edition (May 1994). Our Price: Click for Price*	Buy This Book	*Spread Spectrum Communications Handbook, Electronic Edition, by Marvin K. Simon, Jim K. Omura, Robert A. Scholtz. Hardcover: 1248 pages, first edition (September 26, 2001). Our Price: Click for Price*
Buy This Book	*Synchronization in Digital Communications: Phase-, Frequency-Locked Loops, and Amplitude Control (Wiley Series in Telecommunications), by Heinrich Meyr, Gerd Ascheid. Hardcover: 528 pages (March 1990). Our Price: Click for Price*	Buy This Book	*Cdma : Principles of Spread Spectrum Communication (Addison-Wesley Wireless Communications), by Andrew J. Viterbi. Hardcover - 245 pages (June 1995). Our Price: Click for Price*
Cover not Available	*Introduction to Spread Spectrum Communications, by Roger L. Peterson, Rodger E. Ziemer, & David E. Borth. Hardcover - 700 pages 1 edition (March 24, 1995). Our Price: Click for Price*	Buy This Book	*Digital Communications, by John G. Proakis. Hardcover - 1024 pages 4th edition (August 15, 2000). Our Price: Click for Price*

Also see our "SS Library" for more recommended books on Spread Spectrum.

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