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A seminar on

Spresd Spectrum Technology

Submitted for partial fulfillment of award of 

Batchler Of Technology

degree

In

Electronics & Communication Engineering

By

Arjun Singh PalAshish Kumar Yadav

 Name of Guide

Sri Rajesh Singh

Sri Rakesh BajpaiSri Brijesh Tripathi

Sri Harish Mishra

Feroze Gandhi Inst. Of Engg. & Tech. , Raebareli

SEPTEMBER, 2010

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ACKNOWLEDGEMENT

I owe my deepest gratitude to the head of the department, Mr.Rakesh Bajpai (E & C.) for giving me an ample opportunity to carry out this seminar.

My heartful thanks to my seminar guide Mr. Rajesh Singh who has helped us in preparation of this seminar. We also thank Mr. Harish Mishra and all the faculty members of 

E.C. department , F.G.I.E.T.

It is indeed with deep sense of gratitude that we express our regards to our guide

Mr. Brijesh Tripathi for his tireless supervision encouragement throughout the seminar.

I am also very thankful to all the members and supporting staff. of FGIET for their corporation and support throughout the knowledge of seminar.

I want to express my thanks to all my friend who helped me in out of my doubts and gavetheir suggestion during the preparation.

With gratitude

Arjun Singh Pal

Ashish Kumar Yadav

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  Certificate

Certified that Arjun Singh Pal & Ashish Kumar Yadav have carried out work 

 presented in this seminar entitled “Spread Spectrum Technology” for the award of 

Bachelor Of Technology from Uttar Pradesh Technical University, Lucknow under my

supervision. The seminar embodies result of original work and studies carried out by

these Students themselves and the contents of the topic do not form the basis for the

award of any other degree to the candidate or to anybody else.

( Mr. Rajesh Singh)

Designation: Lecturer 

Date:06-10-2010

Abstract

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This application note is a tutorial overview of spread-spectrum principles. The primary goal of 

this paper is to provide a concise introduction to the use of spread spectrum in wireless

communications. The symbiotic relationship between wireless communications and digital

microelectronics is also developing rapidly. Indeed, mobile and personal communications is now

at parity with the computer industry as a major driver in the development of high-speed, low- power integrated circuits. Underlying this is the deep relationship between the theories of signal

 processing and communication. This is the motivation behind the second objective of this article:

to present the concepts and capabilities of spread spectrum so that engineers versed in DSP can

quickly grasp its utility and exploit opportunities for further advancing the performance of 

spread-spectrum systems. To these ends, we first present a simple digital communication system

that operates in discrete time. Then we build upon this model to show the basic concepts and

resulting benefits of spread spectrum. Then we introduce the two basic spread-spectrum

techniques of direct-sequence (DS) and frequency hopping (FH), followed by two methods that

have been recently proposed. Next we present simple models for the multipath fading channel,

and introduce solutions employing both DS and FH techniques. We then briefly discuss

descriptive links to other nonspread modulation techniques and conclude with a brief 

introduction to the literature for further investigation by the reader The discussion covers both

direct-sequence and fast-hopping methods. Theoretical equations are given to allow performance

estimates. Relation to CDMA and TDMA is provided. A schematic of a code sequence generator 

is shown. Spectral plots illustrate direct-sequence spread-spectrum (DSSS) and frequency-

hopping spread-spectrum (FHSS) methods.

"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 -- therebyincreasing the efficiency with which that band is used. . . . "

  CONTENTS

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1. Introduction………………………………………………………………..…………...…...7

1.1 A short history…………………………………………………………………………...7

1.2 Inside Spectrum Spectrum………………………………………………………….…....8

2. Definitions……………………………………………………………………...…………....8

figure 1………………………………………………………………..……………………...9

2.1What Exactly is Spread Spectrum? …………...…………………………………….....…9

2.2 What Spread Spectrum Does?..........................................................................................10

3. Theoretical Justification for Spread Spectrum ……………………………………........11

3.1 Bandwidth Effects of the Spreading Operation ………………………………………...12

figure 2………………………………………………………………………………….…...12

3.2Bandwidth Effects of the Despreading Operation…………………………………….....13

figure 3……………………………………………………………………………………....13

4.Waste of Bandwidth Due to Spreading Is Offset by Multiple Users……………………..13

figure 4……………………………………………………………………………………….13

5.Benefits of Spread Spectrum………………………………………………………………..13

5.1 Resistance to Interference and Antijamming Effects………………………………........13

figure 5……………………………………………………………………………….............14

5.2 Resistance to Interception..................................................................................................14

figure 6.....................................................................................................................................14

5.3 Resistance to Fading (Multipath Effects)............................................. ............................15

figure 7....................................................................................................................................15

6. Spread Spectrum Allows CDMA.........................................................................................15

6.1 FDMA—Frequency Division Multiple Access.................................................................15

figure 8.....................................................................................................................................16

6.2 TDMA—Time Division Multiple Access.........................................................................16

figure 9.....................................................................................................................................16

6.3 CDMA—Code Division Multiple Access.........................................................................16

figure 10...................................................................................................................................16

7. Spread Spectrum and (De)coding "Keys”..........................................................................17

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figure 11.................................................................................................................................17

8. Different Modulation Spreading Techniques for Spread Spectrum................................18

figure 12................................................................................................................................18

8.1 Direct-Sequence Spread Spectrum (DSSS).....................................................................19

figure 13.................................................................................................................................19

8.2 Frequency-Hopping Spread Spectrum (FHSS)................................................................20

figure 14.................................................................................................................................20

8.3 Time-Hopping Spread Spectrum (THSS)........................................................................21

figure 15.................................................................................................................................21

9. Some Spread Spectrum Terms Defined..............................................................................21

10. Applications Of Spread Spectrum.....................................................................................22

11. Conclusion............................................................................................................................26

12. References............................................................................................................................27

1. Introduction

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As spread-spectrum techniques become increasingly popular, electrical engineers outside the

field are eager for understandable explanations of the technology. There are books and websites

on the subject, but many are hard to understand or describe some aspects while ignoring others

(e.g., the DSSS technique with extensive focus on PRN-code generation). 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 andindustrial 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 articlesummarized 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.

1.1 A Short History

Spread-spectrum communications technology was first described on paper by an actress and a

musician! In 1941 Hollywood actress Hedy Lamarr and pianist George Antheil described a

secure radio link to control torpedos. They received U.S. Patent #2.292.387. The technology was

not taken seriously at that time by the U.S. Army and was forgotten until the 1980s, when it

 became active. Since then the technology has become increasingly popular for applications that

involve radio links in hostile environments.

Typical applications for the resulting short-range data transceivers include satellite-positioning

systems (GPS), 3G mobile telecommunications, W-LAN (IEEE® 802.11a, IEEE 802.11b, IEEE

802.11g), and Bluetooth. Spread-spectrum techniques also aid in the endless race between

communication needs and radio-frequency availability—situations where the radio spectrum is

limited and is, therefore, an expensive resource.

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1.2 "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 usethem.

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. Ittransmitted 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.

2. Definitions

Different spread-spectrum techniques are available, but all have one idea in common: the key

(also called the code or sequence) attached to the communication channel. The manner of 

inserting this code defines precisely the spread-spectrum technique. The term "spread spectrum"

refers to the expansion of signal bandwidth, by several orders of magnitude in some cases, which

occurs when a key is attached to the communication channel.

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The formal definition of spread spectrum is more precise: an RF communications system in

which the baseband signal bandwidth is intentionally spread over a larger bandwidth by injecting

a higher frequency signal (Figure 1). As a direct consequence, energy used in transmitting the

signal is spread over a wider bandwidth, and appears as noise. The ratio (in dB) between the

spread baseband and the original signal is called processing gain. Typical spread-spectrum

 processing gains run from 10dB to 60dB.

To apply a spread-spectrum technique, simply inject the corresponding spread-spectrum code

somewhere in the transmitting chain before the antenna (receiver). (That injection is called the

spreading operation.) The effect is to diffuse the information in a larger bandwidth. Conversely,

you can remove the spread-spectrum code (called a despreading operation) at a point in the

receive chain before data retrieval. A despreading operation reconstitutes the information into its

original bandwidth. Obviously, the same code must be known in advance at both ends of the

transmission channel. (In some circumstances, the code should be known only by those two

 parties.)

 Figure 1. Spread-spectrum communication system.

2.1 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.

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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 suppressedcarrier 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, naturalor artificial noise or interference. It responds only to SS signals with identical matched signal

characteristics and encoded with the identical pseudo noise code.

2.2 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.

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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 isdesired. 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.

3. Theoretical Justification for Spread Spectrum

Spread-spectrum is apparent in the Shannon and Hartley channel-capacity theorem:

C = B × log2 (1 + S/N) (Eq. 1)

In this equation, C is the channel capacity in bits per second (bps), which is the maximum data

rate for a theoretical bit-error rate (BER ). B is the required channel bandwidth in Hz, and S/N is

the signal-to-noise power ratio. To be more explicit, one assumes that C, which represents the

amount of information allowed by the communication channel, also represents the desired

 performance. Bandwidth (B) is the price to be paid, because frequency is a limited resource. The

S/N ratio expresses the environmental conditions or the physical characteristics (i.e., obstacles,

 presence of jammers, interferences, etc.).

There is an elegant interpretation of this equation, applicable for difficult environments, for 

example, when a low S/N ratio is caused by noise and interference. This approach says that one

can maintain or even increase communication performance (high C) by allowing or injecting

more bandwidth (high B), even when signal power is below the noise floor. (The equation does

not forbid that condition!)

Modify Equation 1 by changing the log base from 2 to e (the Napierian number) and by noting

that ln = loge. Therefore:

C/B = (1/ln2) × ln(1 + S/N) = 1.443 × ln(1 + S/N) (Eq. 2)

Applying the MacLaurin series development for 

ln(1 + x) = x - x²/2 + x³/3 - x 4/4 + ... + (-1)k+1xk /k + ...:

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C/B = 1.443 × (S/N - 1/2 × (S/N)² + 1/3 × (S/N)³ - ...) (Eq. 3)

S/N is usually low for spread-spectrum applications. (As just mentioned, the signal power 

density can even be below the noise level.) Assuming a noise level such that S/N << 1,

Shannon's expression becomes simply:

C/B ≈ 1.433 × S/N (Eq. 4)

Very roughly:

C/B ≈ S/N (Eq. 5)

Or:

  N/S ≈ B/C (Eq. 6)

To send error-free information for a given noise-to-signal ratio in the channel, therefore, one

need only perform the fundamental spread-spectrum signal-spreading operation: increase the

transmitted bandwidth. That principle seems simple and evident.

3.1 Bandwidth Effects of the Spreading Operation

Figure 2 illustrates the evaluation of signal bandwidths in a communication link.

 Figure 2. Spreading operation spreads the signal energy over a wider frequency bandwidth.

Spread-spectrum modulation is applied on top of a conventional modulation such as BPSK or 

direct conversion. One can demonstrate that all other signals not receiving the spread-spectrum

code will remain as they are, that is, unspread.

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3.2 Bandwidth Effects of the Despreading Operation

Similarly, despreading can be seen in Figure 3.

 Figure 3. The despreading operation recovers the original signal.

Here a spread-spectrum demodulation has been made on top of the normal demodulationoperations. One can also demonstrate that signals such as an interferer or jammer added during

the transmission will be spread during the despreading operation!

4. Waste of Bandwidth Due to Spreading Is Offset by Multiple Users

Spreading results directly in the use of a wider frequency band by a factor that corresponds

exactly to the "processing gain" mentioned earlier. Therefore spreading does not spare the

limited frequency resource. That overuse is well compensated, however, by the possibility thatmany users will share the enlarged frequency band (Figure 4).

 Figure 4. The same frequency band can be shared by multiple users with spread-spectrum

techniques.

5. Benefits of Spread Spectrum

5.1 Resistance to Interference and Antijamming Effects

There are many benefits to spread-spectrum technology. Resistance to interference is the most

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important advantage. Intentional or unintentional interference and jamming signals are rejected

 because they do not contain the spread-spectrum key. Only the desired signal, which has the key,

will be seen at the receiver when the despreading operation is exercised. See Figure 5.

 Figure 5. A spread-spectrum communication system. Note that the interferer's energy is spread while the data signal is despread in the receive chain.

You can practically ignore the interference, narrowband or wideband, if it does not include the

key used in the despreading operation. That rejection also applies to other spread-spectrum

signals that do not have the right key. Thus different spread-spectrum communications can be

active simultaneously in the same band, such as CDMA. Note that spread spectrum is a

wideband technology, but the reverse is not true: wideband techniques need not involve spread-

spectrum technology.

5.2 Resistance to Interception

Resistance to interception is the second advantage provided by spread-spectrum techniques.

Because nonauthorized listeners do not have the key used to spread the original signal, those

listeners cannot decode it. Without the right key, the spread-spectrum signal appears as noise or 

as an interferer. (Scanning methods can break the code, however, if the key is short.) Even better,

signal levels can be below the noise floor, because the spreading operation reduces the spectral

density. See Figure 6. (Total energy is the same, but it is widely spread in frequency.) The

message is thus made invisible, an effect that is particularly strong with the direct-sequence

spread-spectrum (DSSS) technique. (DSSS is discussed in greater detail below.) Other receivers

cannot "see" the transmission; they only register a slight increase in the overall noise level!

 Figure 6. Spread-spectrum signal is buried under the noise level. The receiver cannot "see" the

transmission without the right spread-spectrum keys.

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5.3 Resistance to Fading (Multipath Effects)

Wireless channels often include multiple-path propagation in which the signal has more than one

 path from the transmitter to the receiver (Figure 7). Such multipaths can be caused by

atmospheric reflection or refraction, and by reflection from the ground or from objects such as

 buildings.

 Figure 7. Illustration of how the signal can reach the receiver over multiple paths.

The reflected path (R) can interfere with the direct path (D) in a phenomenon called fading.Because the despreading process synchronizes to signal D, signal R is rejected even though it

contains the same key. Methods are available to use the reflected-path signals by despreading

them and adding the extracted results to the main one.

6. Spread Spectrum Allows CDMA

 Note that spread spectrum is not a modulation scheme, and should not be confused with other 

types of modulation. One can, for example, use spread-spectrum techniques to transmit a signalmodulated by FSK or BPSK. Thanks to the coding basis, spread spectrum can also be used as

another method for implementing multiple access (i.e., the real or apparent coexistence of 

multiple and simultaneous communication links on the same physical media). So far, three main

methods are available.

6.1 FDMA—Frequency Division Multiple Access

FDMA allocates a specific carrier frequency to a communication channel. The number of 

different users is limited to the number of "slices" in the frequency spectrum (Figure 8). Of the

three methods for enabling multiple access, FDMA is the least efficient in term of frequency- band usage. Methods of FDMA access include radio broadcasting, TV, AMPS, and

TETRAPOLE.

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 Figure 8. Carrier-frequency allocations among different users in a FDMA system.

6.2 TDMA—Time Division Multiple Access

With TDMA the different users speak and listen to each other according to a defined allocation

of time slots (Figure 9). Different communication channels can then be established for a unique

carrier frequency. Examples of TDMA are GSM, DECT, TETRA, and IS-136.

 Figure 9. Time-slot allocations among different users in a TDMA system.

6.3 CDMA—Code Division Multiple Access

CDMA access to the air is determined by a key or code (Figure 10). In that sense, spread

spectrum is a CDMA access. The key must be defined and known in advance at the transmitter 

and receiver ends. Growing examples are IS-95 (DS), IS-98, Bluetooth, and WLAN.

 Figure 10. CDMA systems access the same frequency band with unique keys or codes.

One can, of course, combine the above access methods. GSM, for instance, combines TDMA

and FDMA. GSM defines the topological areas (cells) with different carrier frequencies, and sets

time slots within each cell.

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7. Spread Spectrum and (De)coding "Keys"

At this point, it is worth restating that the main characteristic of spread spectrum is the presence

of a code or key, which must be known in advance by the transmitter and receiver(s). In moderncommunications the codes are digital sequences that must be as long and as random as possible

to appear as "noise-like" as possible. But in any case, the codes must remain reproducible, or the

receiver cannot extract the message that has been sent. Thus, the sequence is "nearly random."

Such a code is called a pseudo-random number (PRN) or sequence. The method most frequently

used to generate pseudo-random codes is based on a feedback shift register . 

One example of a PRN is shown in Figure 11. The shift register contains eight data flip-flops

(FF). At the rising edge of the clock, the contents of the shift register are shifted one bit to the

left. The data clocked in by FF1 depends on the contents fed back from FF8 and FF7. The PRNis read out from FF8. The contents of the FFs are reset at the beginning of each sequence length.

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 Figure 11. Block diagram of a sample PRN generator.

Many books are available on the generation of PRNs and their characteristics, but that

development is outside the scope of this basic tutorial. Simply note that the construction or 

selection of proper sequences, or sets of sequences, is not trivial. To guarantee efficient spread-

spectrum communications, the PRN sequences must respect certain rules, such as length,

autocorrelation, cross-correlation, orthogonality, and bits balancing. The more popular PRNsequences have names: Barker, M-Sequence, Gold, Hadamard-Walsh, etc. Keep in mind that a

more complex sequence set provides a more robust spread-spectrum link. But there is a cost to

this: more complex electronics both in speed and behavior, mainly for the spread-spectrum

despreading operations. Purely digital spread-spectrum despreading chips can contain more than

several million equivalent 2-input NAND gates, switching at several tens of megahertz.

8. Different Modulation Spreading Techniques for Spread Spectrum

Different spread-spectrum techniques are distinguished according to the point in the system at

which a PRN is inserted in the communication channel. This is very basically illustrated in the

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RF front-end schematic in Figure 12.

 Figure 12. Several spreading techniques are applied at different stages of the transmit chain.

If the PRN is inserted at the data level, this is the direct-sequence form of spread spectrum

(DSSS). (In practice, the pseudo-random sequence is mixed or multiplied with the information

signal, giving an impression that the original data flow was "hashed" by the PRN.) If the PRN

acts at the carrier-frequency level, this is the frequency-hopping form of spread spectrum

(FHSS). Applied at the LO stage, FHSS PRN codes force the carrier to change or "hop"

according to the pseudo-random sequence. If the PRN acts as an on/off gate to the transmitted

signal, this is a time-hopping spread-spectrum technique (THSS). There is also the "chirp"technique, which linearly sweeps the carrier frequency in time.

One can mix all the above techniques to form a hybrid spread-spectrum technique, such as DSSS

+ FHSS. DSSS and FHSS are the two techniques most in use today.

8.1 Direct-Sequence Spread Spectrum (DSSS)

With the DSSS technique, the PRN is applied directly to data entering the carrier modulator. The

modulator, therefore, sees a much larger bit rate, which corresponds to the chip rate of the PRN

sequence. Modulating an RF carrier with such a code sequence produces a direct-sequence-

modulated spread spectrum with ((sin x)/x)² frequency spectrum, centered at the carrier 

frequency.

The main lobe of this spectrum (null to null) has a bandwidth twice the clock rate of the

modulating code, and the side lobes have null-to-null bandwidths equal to the code's clock rate.

Illustrated in Figure 13 is the most common type of direct-sequence-modulated spread-spectrum

signal. Direct-sequence spectra vary somewhat in spectral shape, depending on the actual carrier 

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and data modulation used. Below is a binary phase shift keyed (BPSK) signal, which is the most

common modulation type used in direct-sequence systems.

 Figure 13. Spectrum-analyzer photo of a DSSS signal. Note the original signal (nonspread)

would only occupy half of the central lobe. 

8.2 Frequency-Hopping Spread Spectrum (FHSS)

The FHSS method does exactly what its name implies—it causes the carrier to hop from

frequency to frequency over a wide band according to a sequence defined by the PRN. The speed

at which the hops are executed depends on the data rate of the original information. One can,

however, distinguish between fast frequency hopping (FFHSS) and low frequency hopping

(LFHSS).

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

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over the band of frequencies used (see Figure 14). The bandwidth of a frequency-hopping signal

is simply N times the number of frequency slots available, where N is the bandwidth of each hop

channel.

 Figure 14. Spectrum-analyzer photo of a FHSS signal.

8.3 Time-Hopping Spread Spectrum (THSS)

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 Figure 15. THSS block diagram.

Figure 15 illustrates THSS, a method not well developed today. Here the on and off sequences

applied to the PA are dictated according to the PRN sequence.

9. 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

For more definitions of spread spectrum terms, please visit our Technical 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.

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• 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 toan 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.

10. APPLICATIONS OF SPREAD SPECTRUM

• Space Systems• Test Systems and Equipment

• Personal Communications

• Wireless Local Area Network (WLAN)

Designers of communications systems are often concerned with the efficiency with which the

systems utilize the signal energy and bandwidth. In the majority of communications systems

these are the most important issues. In some cases, however, there exist situations in which it is

necessary for for the system to resist external interference, to operate low spectral energy, to

 provide multiple access capability without external control, and to provide a secure channel,

inaccessible to the outside listeners. Thus, it is sometimes necessary to sacrifice some of the

efficiency of the system in order to to enhance these features. SS technique allows us to

accomplish such objectives.

Space Systems

In space stations, which are continuously accessible to interference, SS methods have proved

effective. This is especially true for communication satellites. In general the satellites do not

employ processing on-board as it adds to the complexity and would limit the number of users of 

the satellite. A simple repeating satellite is used, so all SS modulation and demodulation must be

done on the ground.

With no on-board processing the satellite is forced to transmit an uplink interference signal

which reduces the spacecraft transmitter power to send the desired signal. another disadvantage

of no on board processing is that every receiver would have to acquire a SS demodulator.

When on-board processing is used, effective transmitter power requirements can be reduced

 because de modulation of the uplink signal suppresses any interference, by an amount

approximately equal to the process gain.

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Global Positioning System (GPS) is a satellite based navigation system developed and operated

 by the US Department of Defense. The idea behind GPS is to transmit SS signals that allow a

range Measurement, from an unknown satellite location. With knowledge of the transmitter 

location and the distance to the satellite, then the receiver can locate itself on a sphere whose

radius is the distance measured. After receiving signals and making range measurement on other satellites, the receiver can calculate its position based on the intersection of several spheres.

GPS permits users to determine their 3-D position, velocity and time. This service is available

for military and commercial users around the clock, in all weather, anywhere in the world.

GPS uses NAVSTAR (NAVigation Satellite Timing And R anging) satellites. The constellation

consists of 21 operational satellites and 3 active spares. This provides a GPS receiver with 4 to

12 usable satellites 'in view' at any time. A minimum of 4 satellites allows the GPS card to

compute Latitude, longitude, altitude and GPS system time. The NAVSTAR satellites orbit the

earth at an altitude of 10898 Nautical miles in six 55 degree orbital planes, with 4 satellites ineach plane. The orbital period of each satellite is approximately 12 hours.

The GPS satellite signal contains information to identify the satellite, as well as provide

 positioning, timing, ranging data and satellite status. The satellites are identified by space vehicle

number or the pseudo random code number. The satellites transmit on two L-band frequencies:

1.57542 GHz (L1) and 1.22760 (L2). The L1 signal has a sequence encoded on the carrier 

frequency by a modulation technique which contains two codes, a precision (P) code and a

course/acquisition (C/A) code. The L2 code contains only P code which is encrypted for military

and authorized commercial users.

The control segment of the system consists of the worldwide system of tracking and monitoring

stations. The monitor stations measure signals from the GPS satellites and relay the information

they collect to the Master Control Station located at Colorado Springs. This station uses this data

to compute precise orbital models for the entire GPS constellation. This information is then

formatted into updated navigational messages for each satellite.

The user segment consists of the GPS receivers, processors and antennas utilized for the

 positioning and timing by the military and community. Users figure their position on Earth by

measuring their distance to a group of satellites. Each GPS satellite transmits an accurate

 position and time signal. The user's receiver measures the time delay for the signal to reach the

receiver. By knowing the distance to 4 points in space, the receiver is able to triangulate a three

dimensional position.

The specifications for the GPS are shown below:

Precise positioning System (PPS)- P code

• 17.8m horizontal accuracy

• 22.7m vertical accuracy

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• 100ns time accuracy

• Standard Positioning System (SPS)- C/A code

• 100m horizontal accuracy

• 156m vertical accuracy

Test Systems And Equipment

SS techniques can be used in test systems. Transmission test sets characterize data transmission

systems. A pseudo noise code sequence modulates the system under test. The code signal passes

through the transmission path to the test set, which locks on the received code sequence and

compares it to its local reference for any differences. Any difference is recorded as an error.

Another type of transmission test set allows the transmission system to be tested while in

operation. This application makes use of the low power density of direct sequence modulation by

transmitting the DS signal simultaneously with the desired system information signal. Mutual

interference is low because the test signal is transmitted well below the desired signal.

Personal Communications

The advantages in using SS in data and voice communications are as follows:

1. SS 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-mutipath characteristics of SS signaling and reception techniques are attractive

in applications where multipath is likely to be prevalent.

3. The anti-interference characteristics of SS 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 Code Division Multiple Access (CDMA) spread spectrum

technology offer greater operational flexibility and possibly a greater overall system

capacity then do systems built on frequency division multiplexing (FDMA) or time

division multiplexing (TDMA) access methods.

5. The convenience of unlicensed SS operation is attractive to manufacturers.

For these reasons many companies began development of SS systems. the voice-orientated

digital cellular and personal communication services manufactures are using CDMA. CDMA

implemented with direct sequence SS (DS-SS) signaling is among the most promising

multiplexing technologies for cellular telecommunications services.

THe advantages of DS-SS for these services include superior operation in multipath

environments, flexibility in the allocation of channels, privacy and the ability to operate

asynchronously. Also among the attractive features of SS CDMA is the ability to share

 bandwidth with narrow band communication without undue degradation of either systems

 performance. In CDMA SS transmission, user channels are created by assigning different codes

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to different users. This type of system provides a degree of privacy, by controlling the

distribution of user-unique code sequences.

Wireless Local Area Network (WLAN)

A WLAN is a flexible data communication system implemented as an extension to, or an

alternative for, a wired local area network. WLANs transmit and receive data over the air,

minimizing the need for wired connections. Thus, WLANs combine data connectivity with user 

mobility and enable movable LANs.

Most WLAN systems use SS technology. Both frequency hopping and direct sequence are used.

To operate a WLAN system using SS the receiver must know the parameters of the SS signal

 being broadcast.

WLANs are being used in such areas as health care, retail, manufacturing, Warehousing and

academic arenas. These industries have profited from the productivity gains of using hand held

terminals and note book computers to transmit real time information to centralize hosts for 

 processing. WLANs offer the following productivity, convenience, and cost advantages over 

wired networks:

• Mobility;- This supports productivity and service opportunities not possible with wired

networks.

• Installation speed and simplicity;- Installing a WLAN system can be fast and easy, and

can eliminate the need to install cable through the walls and ceiling of a building.• Installation flexibility;- Wireless technology allows the network to go where wire cannot.

• Reduce cost;- While the initial investment required for WLAN hardware can be higher 

then the cost of a wired LAN hardware, overall installation costs and life cycle expenses

can be significantly lower. Long term cost benefits are greatest in dynamic environments

requiring frequent moves, extensions and changes.

11. Conclusions

A complete spread-spectrum communication link requires various advanced and up-to-date

technologies and disciplines: an RF antenna, a powerful and efficient PA, a low-noise and highly

linear  LNA, compact transceivers, high-resolution ADCs and DACs, rapid low-power digital

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signal processing (DSP), etc. Though designers and manufacturers compete, they are also joining

in their effort to implement spread-spectrum systems.

The most difficult area is the receiver path, especially at the despreading level for DSSS, because

the receiver must be able to recognize the message and synchronize with it in real time. The

operation of code recognition is also called correlation. Because correlation is performed at thedigital-format level, the tasks are mainly complex arithmetic calculations including fast, highly

 parallel, binary additions and multiplications.

The most difficult aspect of today's receiver design is synchronization. More time, effort,

research, and money have gone toward developing and improving synchronization techniques

than toward any other aspect of spread-spectrum communications. Several methods can solve the

synchronization problem, and many of them require a large number of discrete components to

implement. Perhaps the biggest breakthroughs have occurred in DSP and in application-specific

integrated circuits (ASICs). DSP provides high-speed mathematical functions that analyze,

synchronize, and decorrelate a spread-spectrum signal after slicing it in many small parts.

12. References

1. Sklar B., ‘A structured Overview of Digital communications - A Tutorial Review - Part I’,

IEEE Communications Magazine, August 1983

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2. Sklar B., ‘A structured Overview of Digital communications - A Tutorial Review - Part II’,

IEEE Communications Magazine, October 1983

3. B. Sklar, ’Digital Communications’, Prentice-Hall , 1988 , chap 10

4. J.G. Proakis,’Communication Systems Engineering’, Prentice-Hall, 1994, chap 11

5. Glover and Grant , ‘Digital Communications’, Prentice Hall, 1997, chap 15

6. Viterbi, ‘CDMA Principles of Spread Spectrum Communication’, Addison-Wesley, 1995

7. Raymond L. Pickholtz, Donald L. Schilling and Laurence B. Milstein, ‘Theory of 

spreadspectrum

communications- A Tutorial’, IEEE Trans. on Communications, vol. com-30, no. 5,

May 1982, pp. 855-884

8. R.C. Dixon, ‘Spread Spectrum Systems with commercial applications’, John Wiley & Sons,

Inc., 1994

9. J.K. Holmes, Coherent Spread Spectrum Systems, John Wiley & Sons, 1982

10. M.K. Simon, ‘Spread Spectrum Communications Handbook’, Mc Graw-Hill, Inc., 1994

11. Shimon Moshavi, Bellcore, ‘Multi-user Detection for DS-CDMA Communications’, IEEE

communications magazine, October 1996, pp. 124-136

12. E. Dinan and B. Jabbari, “Spreading Codes for Direct Sequence CDMA and Wideband

CDMA cellular Networks”, IEEE communications magazine, October 1996, pp. 124-136

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