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The ABCs of Spread Spectrum -- A Tutorial

Tutorial on Spread Spectrum

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Introduction to Spread Spectrum

by Randy Roberts, Director of RF/Spread Spectrum Consulting

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.

The IEEE Spectrum of August, 1990 contained an article entitled 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. This article 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.

"A case in point is a two-year demonstration project the Federal Communications Commission (FCC) authorized in May (1990) for Houston, Texas, and Orlando, Fla. In both places, a new spread spectrum personal communications network (PCN) will share the 1.85-1.9-gigahertz band with local electric and gas utilities. The FCC licensee, Millicom Inc., a New York City-based cellular telephone company, expects to enlist 45000 subscribers.

"The demonstration is intended to show that spread-spectrum users can share a frequency band with conventional microwave radio users--without one group interfering with the other -- thereby increasing the efficiency with which that band is used. . . . "

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 narrowband 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 signals are intentionally made to be much wider band than the information they are carrying to make them more noise-like.

Spread Spectrum signals use fast codes that run many times the information bandwidth or data rate. These special "Spreading" codes are called "Pseudo Random" or "Pseudo Noise" codes. They are called "Pseudo" because they are not real gaussian noise.

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 Watts per Hertz, than narrowband transmitters. This lower transmitted power density characteristic gives spread signals a big plus. Spread 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.

More Details on Spread Spectrum

Over the last 50 years, a class of modulation techniques usually called "Spread Spectrum," has been developed. This group of modulation techniques is characterized by its wide frequency spectra. The modulated output signals occupy a much greater bandwidth than the signal's baseband information bandwidth. To qualify as a spread spectrum signal, two criteria should be met:

  1. The transmitted signal bandwidth is much greater than the information bandwidth.

  2. Some function other than the information being transmitted is employed to determine the resultant transmitted bandwidth.

A Spectrum Analyzer Photo of a Direct Sequence (DS) Spread Spectrum signal.

Most commercial part 15.247 spread spectrum systems transmit an RF signal bandwidth as wide as 20 to 254 times the bandwidth of the information being sent. Some spread spectrum systems have employed RF bandwidths 1000 times their information bandwidth. Common spread spectrum systems are of the "direct sequence" or "frequency hopping" type, or else some combination of these two types (called a "hybrid").

A Spectrum Analyzer Photo of a Frequency Hop (FH) Spread Spectrum signal.

There are also "Time Hopped" and "Chirp" systems in existence. Time hopped spread spectrum systems have found no commercial application to date. However, the arrival of cheap random access memory (RAM) and fast micro-controller chips make time hopping a viable alternative spread spectrum technique for the future. "Chirp" signals are often employed in radar systems and only rarely used in commercial spread spectrum systems.

Direct sequence systems -- Direct sequence spread spectrum systems are so called because they employ a high speed code sequence, along with the basic information being sent, to modulate their RF carrier. The high speed code sequence is used directly to modulate the carrier, thereby directly setting the transmitted RF bandwidth. Binary code sequences as short as 11 bits or as long as [2^(89) - 1] have been employed for this purpose, at code rates from under a bit per second to several hundred megabits per second.

The result of modulating an RF carrier with such a code sequence is to produce a signal centered at the carrier frequency, direct sequence modulated spread spectrum with a (sin x/x)2 frequency spectrum. The main lobe of this spectrum has a bandwidth twice the clock rate of the modulating code, from null to null. The sidelobes have a null to null bandwidth equal to the code's clock rate. Figure 1 illustrates the most common type of direct sequence modulated spread spectrum signal. Direct sequence spectra vary somewhat in spectral shape depending upon the actual carrier and data modulation used. The signal illustrated is that for a binary phase shift keyed (BPSK) signal, which is the most common modulation signal type used in direct sequence systems.

Frequency hopping systems -- The wideband frequency spectrum desired is generated in a different manner in a frequency hopping system. It does just what its name implies. That is, it "hops" from frequency to frequency over a wide band. The specific order in which frequencies are occupied is a function of a code sequence, and the rate of hopping from one frequency to another is a function of the information rate. The transmitted spectrum of a frequency hopping signal is quite different from that of a direct sequence system. Instead of a [(sin x)/x]^2-shaped envelope, the frequency hopper's output is flat over the band of frequencies used. Figure 2 shows an output spectrum of a frequency hopping system. The bandwidth of a frequency hopping signal is simply w times the number of frequency slots available, where w is the bandwidth of each hop channel.

"Inside" Spread Spectrum

This section is intended to gently introduce the reader to the more intricate aspects of the rapidly growing world of spread spectrum, wireless local and wide area networks, as well as introduce the evolution (some may call it explosion) in new communications technologies such as PCN/PCS. We will also try to thoroughly define new terms and concepts the first time we use them.

As an introduction, a little history lesson and a few definitions seem to be in order. Spread Spectrum (SS) dates back to World War II. A German lady scientist was granted a patent on a simple frequency hopping CW system. The allies also experimented with spread spectrum in World War II. These early research and development efforts tried to provide countermeasures for radar, navigation beacons and communications. The U. S. Military has used SS signals over satellites for at least 25 years. An old, but faithful, highly capable design like the Magnavox USC-28 modem is an example of this kind of equipment. Housed in two or three six foot racks, it had selectable data rates from a few hundred bits per second to about 64 kBits per second. It transmitted a spread bandwidth of 60 MHZ. Many newer commercial satellite systems are now converting to SS to increase channel capacity and reduce costs.

Over the last twenty years, many spread spectrum signals have appeared on the air. The easiest way to characterize these modulations is by their frequency spectra. These SS signals occupy a much greater bandwidth than needed by the information bandwidth of the transmitted data. To rate being called an SS signal, two technicalities must be met:

  • The signal bandwidth must be much wider than the information bandwidth.

  • Some code or pattern, other than the data to be transmitted, determines the actual on-the-air transmit bandwidth.

In today's commercial spread spectrum systems, bandwidths of 10 to 100 times the information rates are used. Military systems have used spectrum widths from 1000 to 1 million times the information bandwidth. There are two very common spread spectrum modulations: frequency hopping and direct sequence. At least two other types of spreading modulations have been used: time hopping and chirp.

What Exactly is Spread Spectrum?

One way to look at spread spectrum is that it trades a wider signal bandwidth for better signal to noise ratio. Frequency hop and direct sequence are well-known techniques today. The following paragraphs will describe each of these common techniques in a little more detail and show that pseudo noise code techniques provide the common thread through all spread spectrum types.

Frequency hopping is the easiest spread spectrum modulation to use. Any radio with a digitally controlled frequency synthesizer can, theoretically, be converted to a frequency hopping radio. This conversion requires the addition of a pseudo noise (PN) code generator to select the frequencies for transmission or reception. Most hopping systems use uniform frequency hopping over a band of frequencies. This is not absolutely necessary, if both the transmitter and receiver of the system know in advance what frequencies are to be skipped. Thus a frequency hopper in two meters, could be made that skipped over commonly used repeater frequency pairs. A frequency hopped system can use analog or digital carrier modulation and can be designed using conventional narrow band radio techniques. De-hopping in the receiver is done by a synchronized pseudo noise code generator that drives the receiver's local oscillator frequency synthesizer.

The most practical, all digital version of SS is direct sequence. A direct sequence system uses a locally generated pseudo noise code to encode digital data to be transmitted. The local code runs at much higher rate than the data rate. Data for transmission is simply logically modulo-2 added (an EXOR operation) with the faster pseudo noise code. The composite pseudo noise and data can be passed through a data scrambler to randomize the output spectrum (and thereby remove discrete spectral lines). A direct sequence modulator is then used to double sideband suppressed carrier modulate the carrier frequency to be transmitted. The resultant DSB suppressed carrier AM modulation can also be thought of as binary phase shift keying (BPSK). Carrier modulation other than BPSK is possible with direct sequence. However, binary phase shift keying is the simplest and most often used SS modulation technique.

An SS receiver uses a locally generated replica pseudo noise code and a receiver correlator to separate only the desired coded information from all possible signals. A SS correlator can be thought of as a very special matched filter -- it responds only to signals that are encoded with a pseudo noise code that matches its own code. Thus, an SS correlator can be "tuned" to different codes simply by changing its local code. This correlator does not respond to man made, natural or artificial noise or interference. It responds only to SS signals with identical matched signal characteristics and encoded with the identical pseudo noise code.

What Spread Spectrum Does

The use of these 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 narrowband 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.

A typical commercial direct sequence radio, might have a processing gain of from 11 to 16 dB, depending on data rate. It can tolerate total jammer power levels of from 0 to 5 dB stronger than the desired signal. Yes, the system can work at negative SNR in the RF bandwidth. Because of the processing gain of the receiver's correlator, the system functions at positive SNR on the baseband data.

Besides being hard to intercept and jam, spread spectrum signals are hard to exploit or spoof. Signal exploitation is the ability of an enemy (or a non-network member) to listen in to a network and use information from the network without being a valid network member or participant. Spoofing is the act of falsely or maliciously introducing misleading or false traffic or messages to a network. SS signals also are naturally more secure than narrowband radio communications. Thus SS signals can be made to have any degree of message privacy that is desired. Messages can also, be cryptographically encoded to any level of secrecy desired. The very nature of SS allows military or intelligence levels of privacy and security to be had with minimal complexity. While these characteristics may not be very important to everyday business and LAN (local area network) needs, these features are important to understand.

Some Spread Spectrum Terms Defined

Spread spectrum technology seems to present an alphabet soup to most newcomers. We define some of the more commonly used terms in this field in the following text box. For a complete glossary, see our complete Glossary.

A Brief Spread Spectrum Glossary

  • AJ: Anti-Jam, designed to resist interference or jamming.
  • BPSK: Binary Phase Shift Keying -- Digital DSB suppressed carrier modulation.
  • CDMA: Code Division Multiple Access -- a way to increase channel capacity.
  • CHIP: The time it takes to transmit a bit or single symbol of a PN code.
  • CODE: A digital bit stream with noise-like characteristics.
  • CORRELATOR: The SS receiver compponent that demodulates a Spread Spectrum signal.
  • DE-SPREADING: The process used by a correlator to recover narrowband information from a spread spectrum signal.
  • WIRELESS LAN: Wireless Local Area Network - a 1,000-foot or less range computer-to-computer data communications network.
  • PCN: Personal Communication Network. PCNs are usually short range (hundreds of feet to 1 mile or so) and involve cellular radio type architecture. Services include digital voice, FAX, mobile data and national/international data communications.
  • PCS: Personal Communication System. PCSs are usually associated with cordless telephone type devices. Service is typically digital voice only.
  • PN: Pseudo Noise - a digital signal with noise-like properties.
  • RF: Radio Frequency - generally a frequency from around 50 kHz to around 3 GHz. RF is usually referred to whenever a signal is radiated through the air.
  • SS: Spread Spectrum, a wideband modulation which imparts noise-like characteristics to an RF signal.
  • WIRELESS UAN: Wireless Universe Area Network - a collection of wireless MANs or WANs that link together an entire nation or the world. UANs use very small aperture (VSAT) earth station gateway technology.


Our world is rapidly changing -- computers have gone from mainframes to palmtops. Radio communications has gone from lunchbox sized (or trunk mounted/remote handset car phone) to cigarette-pack-sized micro-cellular telephone technology. The technical challenges of this progress are significant. The new opportunities created by this new technology are also significant. We've talked here about some of the very basic principles in spread spectrum and talked about evolving career opportunities -- isn't it time somebody did something about moving forward in the new millennium?

About the Author:

Randy Roberts has over 30 years experience in communications, electronics and spread spectrum system design. He graduated with a BSEE in 1970 from UC Irvine. For many years prior to his retirement he operated RF/Spread Spectrum Consulting, an independent product development, publishing, strategic planning and training company. He is the founder and former publisher of Spread Spectrum Scene Online.

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Quick Reference Guide

A - C D - F G - I J - L
M - O P - R S - U V - Z

Acquisition: The initial process of aligning a spread spectrum receiver's local PN sequence with the corresponding sequence received from the transmitter. After acquisition, synchronization must be maintained in order to despread the RF signal, and is accomplished through one of several code tracking techniques.

AJ: See Antijam.

Antijam (AJ): The inherent ability of a spread spectrum radio receiver to attenuate and overcome narrowband electromagnetic interference or intentional jamming transmissions. Commonly spelled with a hyphen: "anti-jam."

Appended Code: A PN sequence that is intentionally truncated and restarted after N chips, where N is longer than the natural length of the sequence. Compare this to a truncated code, where the sequence is truncated short of the natural sequence length.

Balanced QPSK Modulation: A QPSK modulation scheme where the I (in-phase) channel of an RF signal is modulated by one PN code, the Q (quadrature-phase) channel of the signal is modulated by a second PN code, and both channels are modulated by the same data source. Compare this to dual-channel QPSK modulation, where the I and Q channels are modulated by two distinct data sources.

Barker Code: Barker codes, originally developed for radar, are short (13 bits or less) sequences that are normally used in one-shot schemes, as compared to most other spreading codes which run continually. For example, one might be used as a preamble to a long PN sequence for the sole purpose of simplifying synchronization. The most notable property of Barker codes is that the minor peaks of their autocorrelation functions always consist of -1,0, and +1. Barker sequences are not the natural product of linear feedback shift registers, but rather are hard-coded. Following is the complete list of Barker codes:

R2: 10 (or 11)
R3: 110
R4: 1011 or (1001)
R5: 11101
R7: 1110010
R11: 11100010010
R13: 1111100110101

BER: See Bit Error Rate.

Bit Error Rate (BER): Numerically equal to the number of erroneous bits divided by the total number of bits received through an RF communication channel. The bit error rate always increases with lower channel signal-to-noise ratio.

Bit Error Rate Tester: Often abbreviated as either BER tester or BERT, a laboratory instrument used to measure the bit error rate of a digital signal transmitted over an RF communication channel. A bit error rate tester typically consists of a pseudorandom sequence generator at the radio transmitter to simulate a data bit stream, and an error-detector at the radio receiver to count the number of received errors.

Bit Inversion Modulation: Same as code inversion modulation.

BPSK Modulation: Biphase shift keying. Modulation of an RF carrier via phase shifting, usually at 0 and 180 degrees.

CDMA: The term CDMA.refers either to the generic form of code division multiple access, or to one of the practical forms of CDMA in use today, particularly cdma2000 and CDMA One.

cdma2000: Also known as IMT-CDMA Multi-Carrier or IS-136, cdma2000 is a code-division multiple access (CDMA) version of the IMT-2000 standard developed by the International Telecommunication Union (ITU). The cdma2000 standard was created for third-generation (3G) mobile wireless technology. cdma2000 can support mobile data communications at speeds ranging from 144 kbps to 2 Mbps. Versions have been developed by Ericsson and Qualcomm. cdma2000 is often misspelled as CDMA 2000 (two words), or as CDMA2000 (all caps).

CDMA One: Also written as cdmaOne, CDMA One refers to the original IS-95 code-division multiple access (CDMA) wireless interface protocol that was standardized in 1993 by the International Telecommunication Union (ITU). It is considered a second-generation (2G) mobile wireless technology. Today, there are two versions of IS-95, called IS-95A and IS-95B. The IS-95A protocol employs a 1.25-MHz carrier, operated in radio-frequency bands of either 800 MHz or 1.9 GHz, and supports data speeds of up to 14.4 Kbps. IS-95B can support data speeds of up to 115 kbps by bundling up to eight channels.

CDMA Repeater: A stand-alone device that receives CDMA signals and retransmits them at a higher power level for the purpose of improving coverage in focused areas like tunnels, indoor settings, dense urban sites, and sports stadiums.

Chip: A single bit of a pseudonoise sequence.

Chip Rate: The rate at which bits of a pseudonoise sequence are shifted, expressed in Hz. Also known as spread rate.

Chirping: A less common form of spread spectrum employing a swept-frequency pulse, called a chirp, to spread the signal spectrum. Chirping is more commonly used in radar and ranging applications than in data communications.

Code: A binary bit stream. In spread spectrum, code refers to the pseudorandom sequence used to spread an information signal across a frequency band. It is more specifically referred to as a pseudonoise code.

Code Division Multiple Access (CDMA): CDMA technology exploits the orthogonality property of certain families of PN codes in order to increase channel capacity. Typically, each user is given a unique spreading code. To communicate with a particular user, the sender must use the same code assigned to that user. This technique permits many users to operate simultaneously over the same frequency band. Gold codes and Walsh codes are often used in CDMA systems.

Code Inversion Modulation: Also known as phase inversion modulation and bit inversion modulation, a popular means by which a binary data stream is modulated into a spread spectrum signal. In a direct sequence system, the data is modulo-2 added with the PN sequence prior to modulation of the carrier. In theory, this is equivalent to multiplying a PN-modulated PSK signal with the data. This is an important point to recognize, as it can be used in demonstrating the fact that multiplication of the received signal by the same PN sequence at the receiver will result in a data-modulated PSK signal, and the data can be recovered through standard PSK demodulation techniques.

Code Orthogonality: See Orthogonality.

Correlation: The process of synchronizing the phase of a local PN sequence within an SS radio receiver with the received PN sequence in order to despread and recover the narrowband data signal from a spread signal. Sometimes referred to as a despreading in direct sequence systems, or dehopping in frequency hopping systems.

Also, the process of determining the degree of cross-correlation, or similarity, between the two sequences.

Correlator: The SS radio receiver component that synchronizes the phase of a local PN sequence with the received PN sequence in order to despread and recover the narrowband data signal from a spread signal.. Sometimes referred to as a despreader in direct sequence systems, or dehopper in frequency hopping systems. A sliding correlator is a common type of correlator.

Also, a device or circuit that determines the degree of cross-correlation, or similarity, between the two sequences.

Cross-Correlation: The mathematically derived measure of similarity between two functions or signals. Cross-correlation also refers to the process of determining this similarity, and is accomplished by multiplying the two signals together and integrating the result over time. If the result is zero, the two signals are said to be uncorrelated, or orthogonal.

Dehopper: See Correlator. Often spelled with a hyphen: "de-hopper."

Dehopping: See Correlation. Often spelled with a hyphen: "de-hopping."

Delay-Locked Loop Tracker: A type of PN tracker where synchronization between the local PN sequence and the received PN sequence is maintained by measuring the cross-correlation levels between the received sequence and both an early and late version of the punctual (non-shifted) local sequence, and adjusting the phase of the local sequence such that the two cross-correlation levels are equal. The early sequence is always 1/2 chip early relative to the punctual sequence, and the late sequence is 1/2 chip late. Thus, maintaining equal cross-correlation levels ensures maximum correlation with the punctual sequence, since it is precisely in the middle. The delay-locked loop, or delay-lock loop, is sometimes called an early-late detection loop or early late gate synchronizer.

Despreader: See Correlator. Often spelled with a hyphen: "de-spreader."

Despreading: See Correlation. Often spelled with a hyphen: "de-spreading."

Direct Sequence CDMA (DS-CDMA): The most prevalent form of code division multiple access, employing direct sequence spectrum spreading..

Direct Sequence Spread Spectrum (DSSS or DS): A modulation technique where a pseudorandom sequence directly phase modulates a (data-modulated) carrier, thereby increasing the bandwidth of the transmission and lowering the spectral power density (i.e. the power level at any given frequency). The resulting RF signal has a noise-like spectrum, and in fact can intentionally be made to look like noise to all but the intended radio receiver. The received signal is despread by correlating it with a local pseudorandom sequence identical to and in synchronization with the sequence used to spread the carrier at the radio transmitter.

Direct Spread Modulation: Same as direct sequence spread spectrum.

DS or DSSS: See Direct Sequence Spread Spectrum.

DS-CDMA: See Direct Sequence CDMA.

Dual-Channel QPSK Modulation: A QPSK modulation scheme where the I (in-phase) channel of an RF signal is modulated by one PN code, the Q (quadrature-phase) channel of the signal is modulated by a second PN code, and where the I and Q channels are modulated by two distinct data sources. Compare this to balance QPSK modulation, where both channels are modulated by the same data source.

Eb: The energy of an information bit. Eb is expressed in Joules, or equivalently in Watts per Hertz.

Epoch: A strobe signal which indicates when a pseudonoise sequence repeats.

FCC Part 15 Rules: See Part 15 Rules.

Feedback Pattern: See Feedback Taps.

Feedback Taps: The taps of a linear feedback shift register that are fed back to the input of the register. Also, a specification of which taps are fed back. The latter sense of the term is also known as feedback tap set or feedback pattern.

FH or FHSS: See Frequency Hopping Spread Spectrum.

Fibonacci Form LFSR: A form of linear feedback shift register where multiple taps from the register are modulo-2 summed and the result fed back to the shift register's input. Also known as a simple shift register generator (SSRG). Compare this to the Galois form LFSR, where the shift register's output is fed back at multiple points along the shift register.

Frequency Hopping Spread Spectrum (FHSS or FH): A spread spectrum modulation technique whereby the radio transmitter frequency-hops from channel to channel in a predetermined but pseudorandom manner. The RF signal is dehopped at the radio receiver using a frequency synthesizer controlled by a pseudorandom sequence generator synchronized to the transmitter's pseudorandom sequence generator. A frequency hopper may be fast-hopped, where there are multiple hops per data bit, or slow-hopped, where there are multiple data bits per hop.

Galois Form LFSR: A form of linear feedback shift register where the shift register's output is fed back to multiple inputs along the shift register. At each of these inputs, the sequence being fed back is modulo-2 summed with the output of the prior register. Also known as a multiple-return shift register generator (MRSRG) or modular shift register generator (MSRG). Compare this to the Fibonacci form LFSR, where multiple taps of the shift register are modulo-2 summed and fed back to the input of the shift register.

GMSK Modulation: Gaussian minimum shift keying. A form of MSK where the shaping function is bell shaped ("normal" curve).

Gold Code: One of a family of pseudonoise codes that exhibits minimal, well defined, cross-correlation levels with all other members of the family. This property is often exploited in CDMA spread spectrum systems. A Gold code is generated through modulo-2 addition of two PN codes of equal length. Distinct members of a Gold code family are determined by the chip (bit) offset of one code relative to the other. Selection of preferred pairs of PN codes, which results in optimal Gold code performance, has been thoroughly studied and documented. A balanced Gold code is one in which the number of ones exceeds the number of zeros by one, a trait shared by all m-sequences. An orthogonal Gold code is one in which an extra zero is appended to the end of the naturally-generated sequence in order to make the number of ones and zeros the same. Without this extra zero, the code would not be perfectly orthogonal with other members of the family.

GPS: Global Positioning System. Known also as NAVSTAR, a satellite-based radio positioning systems that provide 24-hour three-dimensional position, velocity and time information to suitably equipped users anywhere on or near the surface of the Earth, and sometimes off the earth. The system employs spread spectrum technology in a 24-satellite constellation, 20,000 Km above the earth in six orbital planes. NAVSTAR is operated by the U.S. Department of Defense, and was the first global positioning system widely available to civilian users.

Initial Fill: The initial content of a linear feedback shift register, or other PN sequence generating device. Also known as the preset code.

ISM Band: Industry, Scientific and Medical frequency band, as designated by the FCC. Unlicensed 902 - 928 MHz, 2.4 - 2.4835 GHz and 5.725 - 5.850 GHz bands, with RF power up to 1 watt at the lower band. Frequency hopping, direct sequence, and other spread spectrum transmissions are allowed. The ISM band frequencies are often abbreviated as 902 MHz or 915 MHz, 2.4 GHz, and 5.7 GHz or 508 GHz, respectively.

Jam: To intentionally or maliciously interference with another radio signal.

Jammer: A device that transmits an energetic RF signal with the intention of interfering with another radio signal.

Jammer-to-Signal Ratio (JSR or J/S Ratio): The dimensionless ratio of the jammer signal received to the signal-of-interest (SOI) received, over the SOI bandwidth. Usually expressed in dB.

Jamming: The typically intentional or malicious interference with another radio signal. Spread spectrum transmissions inherently attenuate jamming signals. See Antijam.

JPL Code: Named after Jet Propulsion Laboratories, where it was invented, a pseudonoise code generated through the modulo-2 addition of two PN codes of differing lengths. (Compare this to Gold codes, where the two summed codes are of identical length.) Certain properties of JPL codes can be exploited to attain fast acquisition at the radio receiver.

JSR or J/S Ratio: See Jammer-to-Signal Ratio.

Kasami Code: Kasami codes are similar to Gold codes in that they are produced by exclusive-ORing two distinct sequences. The twist in the case of Kasami codes is that both these sequences are produced by a single linear feedback shift register. One sequence is the output of the LFSR, whereas the other is derived from the first by decimating it by a factor of N, and then repeating it N times. For example, if the original sequence is 15 chips long, and it is decimated it by a factor of 5, this results in 3 chips (every fifth chip). Then these three chip are repeated 5 times to produce the second sequence of 15 chips. Finally, this sequence is exclusive-ORed with the original to obtain the Kasami sequence.

LAN: See Local Area Network.

LFSR: See Linear Feedback Shift Register.

Linear Feedback Shift Register (LFSR): A logic shift register using feedback and XOR (exclusive-or, or modulo-2 addition) elements that produces linear recursive sequences. Two practical implementations of LFSR are the Fibonacci form and Galois form.

Linear Recursive Sequence (LRS): A periodic sequence of bits generated through the use of a logic shift register with linear feedback, known as a linear feedback shift register. The most common type of sequence used in spread spectrum systems. Given a proper set of feedback taps, the sequence produced can be of maximal length and have certain desirable properties. Such a sequence is referred to as an m-sequence.

Local Area Network (LAN): Relatively small (building-wide) network of computers connected together via transmission cable and using one of various RF communication protocols.

Low Probability of Intercept (LPI): The property of a transmitter which, because of its low power, high directivity, frequency variability, or other design features, is difficult to detect or identify. In the case of spread spectrum, LPI is achieved either through the lowering of the power spectrum at any given frequency by means of spectrum spreading, or through the frequency agility provided by frequency hopping.

Low Probability of Intercept Radar(LPIR): A radar system which, because of its low peak power output, the way in which it is operated, or other design features, is difficult to detect or identify. In the case of spread spectrum, LPI is achieved either through the lowering of the power spectrum at any given frequency by means of spectrum spreading, or through the frequency agility provided by frequency hopping.

LPI: See Low Probability of Intercept.

LPIR: See Low Probability of Intercept Radar.

LRS: See Linear Recursive Sequence.

M-Sequence: See Maximal Length Sequence.

Maximal Length Sequence (M-Sequence, MLS): A linear recursive sequence of period 2n-1 chips (bits), where n is the number of stages in the linear feedback shift register generating the sequence. Since this constitutes every possible state of the register, it is the longest sequence that can be generated. Only certain combinations of feedback taps will produce an m-sequence, also referred to as a maximal sequence. M-sequences, also known as pseudonoise (PN) sequences and pseudorandom bit sequences (PRBS), have favorable noise-like properties that make them particularly useful in spread spectrum applications.

Maximal Sequence: Same as maximal length sequence.

MLS: See Maximal Length Sequence.

Modular Shift Register Generator (MSRG): Same as Galois form LFSR.

MRSRG: See Multiple-Return Shift Register Generator.

MSK Modulation: Minimum shift keying. A modulation technique which uses waveform shaping to significantly lower the main sidelobes. MSK can be considered a form of OQPSK.

MSRG: See Modular Shift Register Generator.

Multipath: The presence of multiple copies of a single RF signal arriving at a radio receiver's antenna simultaneously. Signals that are in phase will add to one another, and signals that are out of phase will cancel.

Multipath Fading: Multipath fading, a.k.a. Rayleigh fading, occurs when a direct-path transmitted wave destructively interferes with reflections of itself at the receiving end. The destructive interference is a result of the reflected waves arriving at the receiving end out of phase with the direct-path transmitted wave. Multipath interference can vary in intensity depending on the amount of destructive interference that takes place.

Multiple-Return Shift Register Generator (MRSRG): Same as Galois form LFSR.

No: The amount of noise energy accumulated over one period of an information bit. No is expressed in Joules, or equivalently in Watts per Hertz.

OQPSK Modulation: Offset Quadriphase Shift Keying. Similar to QPSK, but with an initial phase offset, of usually 45 degrees, in one of its two binary channels. As a result, the phase never jumps by more than 90 degrees at any given data transition. OQPSK, also known as staggered QPSK (SQPSK), has a lower envelope modulation than does QPSK.

Orthogonal Code: A PN code is said to be orthogonal with another if their cross-correlation, a mathematical measure of similarity, is zero. Orthogonality ensures that the two codes will not interfere with one another when present on the same communication channel.

Orthogonality: A property exhibited between two PN codes whose cross-correlation, a mathematical measure of similarity, is zero. Orthogonality ensures that the two codes will not interfere with one another when present on the same communication channel.

Part 15 Rules: That part of the Federal Communication Commission's (FCC) regulations which regulates unlicensed use of the ISM bands for wireless networking and other uses, and that includes spread spectrum in certain bands.

PCN: Personal Communication Network. PCNs are usually short range (100s of feet to 1 mile or so) and involve cellular radio type architecture, sometimes utilizing spread spectrum. Services include digital voice, FAX, mobile data and national/international data communications.

PCS: Personal Communication System (or Services). Usually associated with cordless telephone-like devices, and personal data assistant devices. These services are typically digital and often employ spread spectrum technologies. Within the U.S., the 1.9 GHz band has been allocated for PCS systems; the allocated spectrum is 120 MHz wide and is licensed as two 30 MHz segments for the 51 major trading areas, and three 10 MHz segments for the 493 basic trading areas.

Phase Inversion Modulation: Same as code inversion modulation.

PN Acquisition: See Acquisition.

PN Code: See Pseudonoise Code.

PN Sequence: See Pseudonoise Sequence.

PN Correlation: See Correlation.

PN Correlator: See Correlator.

PN Synchronization: See Synchronization.

PN Synchronizer: See Synchronizer.

PN Tracker: See Tracker.

PN Tracking: See Tracking.

PNG: Pseudonoise generator. Same as pseudonoise code generator.

PRBS: See Pseudorandom Bit Sequence.

PRG: Pseudorandom generator. Same as pseudonoise code generator.

PRN: Pseudorandom noise. Same as pseudonoise code.

Preset Code: See Initial Fill.

Processing Gain: Also known as process gain, the ratio of the bandwidth of a spread spectrum signal to the data rate of the information signal being spread. As a rule-of-thumb, this ratio determines the level of interference rejection exhibited by the system, and thus the anti-jam performance.

Pseudonoise Code (PN Code): Also called pseudonoise (PN) sequence, any of a group of binary sequences that exhibit random noise-like properties. PN sequences are distinguishable from truly random sequences in that they inherently or deliberately exhibit periodicity (i.e. they repeat). An integral part of all spread spectrum systems, PN sequences are usually generated using a liner feedback shift register. Often spelled with a hyphen: "pseudo-noise code."

In the strict sense, pseudonoise sequence and pseudorandom sequence are synonymous with maximal sequence. However, the terms are often used informally to include both maximal and nonmaximal sequences.

Pseudonoise (PN) Code Generator: Also called a pseudonoise sequence generator or pseudorandom sequence generator, a hardware or software device that generates a pseudonoise code. Often implemented in the form of a linear feedback shift register.

Pseudonoise Sequence (PN Sequence): Same as pseudonoise code. Often spelled with a hyphen: "pseudo-noise sequence."

Pseudonoise Sequence Generator: Same as pseudonoise code generator.. Commonly spelled with a hyphen: "pseudo-noise sequence generator."

Pseudorandom Bit Sequence (PRBS): Same as pseudonoise code. Often spelled with a hyphen: "pseudo-random bit sequence." Also known as pseudorandom bit stream and pseudorandom binary sequence.

Pseudorandom Sequence: Same as pseudorandom bit sequence. Often spelled with a hyphen: "pseudo-random sequence."

Pseudorandom Sequence Generator: Same as pseudonoise code generator.. Commonly spelled with a hyphen: "pseudo-random sequence generator." Also known as pseudorandom bit sequence generator and pseudorandom binary sequence generator.

PSK Modulation: Phase shift keying. Modulation of an RF carrier via phase shifting. Binary PSK, quaternary PSK, and offset-quaternary PSK are three common forms of PSK.

QPSK Modulation: Quadriphase shift keying. Modulation of an RF carrier via phase shifting, usually at 0, 90, 180, and 270 degrees. Also known as quaternary phase shift keying.

Rake Receiver: A receiver technique which exploits multipath phenomenon to improve system performance. Multiple baseband correlators are used to individually process multiple multipath components. The correlator outputs are then added to increase total signal strength.

Simple Shift Register Generator (SSRG): Same as Fibonacci form LFSR.

Sinc Function: Defined as sin(x)/x, the sinc function is mathematically equivalent to the Fourier transform of a rectangular function. Consequently, a rectangular pulse in the time domain appears as a sinc function in the frequency domain. Accordingly, in digital radio communications where rectangular waveforms dominate, sinc-like power spectra are observed. However, in an effort to prevent the sinc function's sidelobes from interfering with neighboring frequency bands, pulse shaping is usually performed in an effort to attenuate all but the central, or main, lobe of the function. In the time domain, this appears as a smoothing or rounding of the discontinuous edges of the pulse.

Signal-to-Noise Ratio (SNR or S/N Ratio): The dimensionless ratio Eb/(No+Io), or bit energy divided by the noise-plus-interference energy accumulated over one bit period. Usually expressed in dB.

Sliding Correlator: A simple type of PN correlator where the local PN sequence in an SS radio receiver is slid relative to the received PN sequence until the phase of the two sequences match. Sliding is usually done in discrete steps so the cross-correlation, or similarity, between the two sequences can be measured before the local sequence is again slid.

SNR or S/N Ratio: See Signal-to-Noise Ratio.

Spectrum Spreading: The act of spreading the bandwidth of an information signal to be transmitted to a remote radio receiver. The receiver despreads the transmission to recover the original information signal.

Spread Spectrum (SS): A wideband modulation technique which imparts noise-like characteristics to an RF signal. This communication technique spreads a signal over a wide range of frequencies for transmission and then despreads it to the original data bandwidth at the radio receiver. Spread spectrum's advantages and properties include low probability of intercept, antijam capabilities, CDMA multiplexing, and FCC Part 15 license free operation.

Spread Spectrum Generator: A laboratory instrument used to generate or simulate spread spectrum signals. Spread spectrum generators are used in both the development and testing of spread spectrum systems, such as CDMA, PCS, cellular, and wireless LAN systems.

Spreading: See Spectrum Spreading.

Spreading Code: Any pseudonoise code used to spread the data signal's frequency spectrum within a direct sequence spread spectrum system. Spreading codes are usually generated with a linear feedback shift register.

SQPSK Modulation: Staggered quadriphase shift keying. Same as OQPSK.

SS: See Spread Spectrum.

SSRG: See Simple Shift Register Generator.

Synchronization: The process within a spread spectrum radio of maintaining alignment between the local PN sequence and the received PN sequence. Synchronization is broken down into two steps: initial acquisition followed by tracking.

Synchronizer: The component within a spread spectrum radio that maintains alignment between the local PN sequence and the received PN sequence. Synchronization is broken down into two steps: initial acquisition followed by tracking.

Tau-Dither Tracker: A type of PN tracker where synchronization between the local PN sequence and the received PN sequence is maintained by intentionally shifting, or dithering, the local sequence back and forth relative to the received sequence by a small amount, and measuring the change in their cross-correlation level, for the purpose of maximizing the level.

TDMA: See Time Division Multiple Access.

Time Division Multiple Access (TDMA): A method of digital multiplexing whereby each signal is sent and received at predesignated time slots, in a series of time slots shared by multiple signals. The radio transmitter and receiver must be time-synchronized. Public telephone networks typically use TDMA.

Tracker: The component of an SS radio receiver which tracks the received PN sequence. (See Tracking.) Tau-dither trackers and delay-locked loop trackers are two common types of tracker.

Tracking: After initial acquisition, the process of maintaining alignment of the local PN sequence of an SS radio receiver relative to the corresponding sequence received from the radio transmitter, in order to despread the spread signal.

Truncated Code: A PN sequence that is intentionally truncated and restarted after N chips, where N is shorter than the natural length of the sequence. Compare this to an appended code, where the sequence is truncated long of the natural sequence length.

Walsh Code: One of a group of specialized PN codes having good autocorrelation properties but poor cross-correlation properties. Walsh codes are the backbone of the CDMA One and cdma2000 cellular systems, and are used to support the individual channels used simultaneously within a cell. Walsh codes are generated in firmware by applying the Hadamard transform on 0 repeatedly.

WAN: See Wide Area Network.

WCDMA or W-CDMA: See Wideband CDMA.

Wide Area Network (WAN): Large network formed by bridging smaller LANs or using dial-up lines. WANs can span the globe.

Wideband CDMA (WCDMA or W-CDMA): A form of CDMA technology where the bandwidth is appreciably greater than that provided by the digital cellular systems introduced in the 1990s. The bandwidth of WCDMA, which will be the standard for third-generation (3G) cellular systems of the early 2000s, is expected to be around 5 MHz.

Wireless: An all-encompassing buzzword which describes what traditionally has been called "radio", but which typically also implies inclusion of some of the newer cellular or digital radio technologies, including spread spectrum.

Wireless Local Area Network (WLAN or W-LAN): A short range computer-to-computer wireless data communications network. In the United States, operation is in the 2.4 GHz and 5.8 GHz unlicensed ISM bands using spread spectrum technology.

WLAN or W-LAN: See Wireless Local Area Network.