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Spread Spectrum Communications Communications

Dr. Fahim Aziz UmraniDepartment of Telecommunication, Room # 213

Institute of Information & Communication Technologies (IICT), Mehran UET, Jamshoro

[email protected]

Spread Spectrum Communications Title of Subject : Spread Spectrum CommunicationsDisciplines : B.E. Telecommunication EngineeringTerm : (7th Term)Effective : 08TL Batch to OnwardsPre-requisites : ADC Co-requisite: NILAssessment : 20% Sessional Work, 80% Written ExaminationMarks : Theory : 100 Practical : 50Credit Hours : 4 + 2Minimum Contact Hours: 52 + 26

AimsTo introduce the concepts and applications of spread spectrum communications systems.ObjectivesUpon completion of this course work the students should be able to:Understand the basic concept and terminology of spread communications.Have awareness and familiarity of practical application of spread spectrum to various communication system such as CDMA systemsReview of noise and fading channels modelsIntroduction to Spread Spectrum (SS) Communications:Concept and definitions of SS communications as compared with conventional types of communications, Origin of SS communications, Implementations of SS

Department of Telecommunication, Mehran UET 2

Concept and definitions of SS communications as compared with conventional types of communications, Origin of SS communications, Implementations of SScommunications, Spreading Sequences, Orthogonal and Semi-orthogonal sequences.PN Sequences: Pseudo-noise (PN) sequences generation and its properties (balance, run and correlation properties).M-sequences: Generation of m-sequence using shift registers (Fibonacci and Galois feedback generators), characteristics of m-sequence (near balance, correlation andsecurity etc.), polynomial representation of m-sequences,Gold Sequences: Why Gold codes, generation of Gold codes, finding preferred pair of m-sequences, correlation properties of Gold sequences.Orthogonal Sequences: Walsh Codes, Motivation, Generation and its properties, application of Walsh Codes, Variable length orthogonal codes (motivation, generationand properties).Spread Spectrum Communication Systems:Fundamental of Spread Spectrum:: Concept of Spectrum and Bandwidth, Definition of SS Signals, Types of SS signals, Benefits of SS techniques.Analysis of SS Systems: Direct Sequence SS systems, Frequency-Hopped SS systems, Synchronization of SS communication system (Acquisition and Tracking)Application of Spread Spectrum: Anti-jamming, Ranging, multipath suppression, code-division multiple access, recent commercial applications.CDMA Systems: Introduction to 2G/3G standards, CDMA 2000 Systems & Architecture, WCDMA System and Architecture.

Recommended Books:R. Petersons, R. Ziemer “Introduction to Spread Spectrum Communications”, Prentice-Hall 1995.R. C. Dixon, “Spread Spectrum System with Commercial Applications”, 4th ed. Wiley 2000.Andrew J. Viterbi, “CDMA Principles of Spread spectrum Communication”, Adison-Wiley, 2004.

Spread Spectrum Communications Tentative Teaching Plan

Name of Teacher: Dr. Fahim Aziz Umrani Subject: Spread Spectrum Communications Batch: 08TL Year: 4th Term: 7th

Term Starting Date: 03 – 01 – 2011 Term Suspension date: 23 – 04 –2011

Sr. # Topics No. of Lectures 01 Introduction to the subject, Review of noise and fading channels models 0202 Concept and definitions of SS communications 0203 Origin of SS communications 0204 Implementations of SS communications 0205 Spreading Sequences, Orthogonal and Semi-orthogonal sequences 0206 PN Sequences: Pseudo-noise (PN) sequences generation and its properties (balance, run & correlation prop.). 0207 M-sequences: Generation of m-sequence using shift registers (Fibonacci and Galois feedback generators) 0208 Characteristics of m-sequence (near balance, correlation and security etc.) 0209 Polynomial representation of m-sequences 0210 Gold Sequences: Why Gold codes, generation of Gold codes, 0211 Finding preferred pair of m-sequences, correlation properties of Gold sequences. 0212 Orthogonal Sequences: Walsh Codes, Motivation, Generation and its properties 02


12 Orthogonal Sequences: Walsh Codes, Motivation, Generation and its properties 0213 Application of Walsh Codes, Variable length orthogonal codes (motivation, generation and properties). 0214 Fundamental of Spread Spectrum: Concept of Spectrum and Bandwidth, 0215 Definition of SS Signals, Types of SS signals, 0216 Benefits of SS techniques 0217 Analysis of SS Systems: Direct Sequence SS systems, 0218 Frequency-Hopped SS systems 0219 Synchronization of SS communication system (Acquisition and Tracking) 0220 Application of Spread Spectrum: Anti-jamming, Ranging 0221 Multipath suppression 0222 Code-Division Multiple Access 0223 Recent Commercial Applications 0224 CDMA Systems: Introduction to 2G/3G standards 0225 CDMA 2000 Systems & Architecture 0226 WCDMA System and Architecture 02

Total Lectures 52

Spread Spectrum Communications

The beginning – Mobile Communication



Wireless Evolution


Analog voice

2009: Apple iphone 3G

Digital voice + 7.2 Mbps data + GPS+ Full internet browsing+ multimedia messaging+ multimedia entertainment


Impact of Wireless Communication



Wireless Trends



Cellular Wireless Evolution

4G

3G IMT 2000 Global standard | Wideband CDMA (‘00s)

System beyond IMT-2000 (IMT-Advanced)LTE/LTE-Advanced, WiMax (802.16m)


1G

3G

2G

Analog speech | FDMA (‘80s)AMPS

Digital modulation & roaming | TDMA & CDMA (‘90s)GSM, IS-95, PDC

UMTS/WCDMA/HSPA, CDMA2000, TD-CDMA


Towards 4G§ ITU’s System beyond IMT-2000 (IMT-Advanced) is

set to introduce 4G.

§ 3GPP is currently developing evolutionary/revolutionary systems toward 4G:

Department of Telecommunication, Mehran UET

§ 3GPP is currently developing evolutionary/revolutionary systems toward 4G: Long Term Evolution (LTE) and LTE-advanced.

§ IEEE 802.16-based WiMax is also evolving towards 4G through 802.16m.

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Wireless Background§ Introduction

§ Fundamental Limits

§ Multiplexing & Multiple Access Schemes


§ Multiplexing & Multiple Access Schemes

§ Broadband wireless channel basics

§ Cellular Systems

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Digital Communication System



Digital versus analog§ Advantages of digital communications:

ØRegenerator receiver

Originalpulse

Regeneratedpulse


ØDifferent kinds of digital signal are treated identically.

Data

Voice

Media

Propagation distance

A bit is a bit!


Digital Communication System§ In figure, the input information source is converted to

binary digits (bits), the bits are then grouped to form digital messages or message symbols. ØEach such symbol (mi, where i =1,..., M) can be regarded as

a member of a finite alphabet set containing M members.


a member of a finite alphabet set containing M members.ØM –ary is usually given to those cases where M >2

§ For systems that use channel coding (error correcting codes), a sequence of message symbols becomes transformed to a sequence of channel symbols (code symbols), where each channel symbol is denoted ui.

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Basic Terms & their meaning§ Formatting transforms the source information into bits. § Modulation is the process by which message symbols or channel symbols are converted to

waveforms (line codes) that are compatible with the requirements of transmission channel. § The term baseband refers to a signal whose spectrum extends from (or near) dc up to some

finite value, usually less than few megahertz. § The term bandpass is used to indicate that the baseband waveform gi(t) is frequency

translated by a carrier wave to a frequency that is much larger than the spectral content of


translated by a carrier wave to a frequency that is much larger than the spectral content of gi(t). Ø Bandpass modulation is required whenever the transmission medium will not support propagation

of pulse-like waveform such as RF transmission. For such cases, the medium requires a bandpasswaveforms si(t).

§ Equalization can be described as a filtering option that is used in or after the demodulator to reverse any degrading effects on the signal that were caused by the channel. Ø An equalizer is implemented to compensate for (i.e., remove or diminish) any signal distortion

caused by a non-ideal channel impulse response hc(t).

§ Demodulation is defined as recovery of a waveform (baseband pulse), and detection is defined as decision-making regarding the digital meaning of that waveform.

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Basic terms & their meanng§ Source coding produces Analog-t-digital (A/D) conversion (for Analog

sources) and removes redundant (unneeded) information. Note that a typical DCS would either use the source coding option (for both digitizing and compressing the source information), or it would use the simpler formatting transformation (for digitizing alone). A system would not use both source coding and formatting, because the former already includes the essential step of digitizing the information.


§ Encryption prevents unauthorized users from understanding messages and from injecting false messages into the system.

§ Channel coding, for a given data rate, can reduce the probability of error, PE, or reduce the required signal-to-noise ratio to achieve a desired PE at the expense of transmission bandwidth or decoder complexity.

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Basic Terms & their meaning§ Demodulation is typically accomplished with signal attributes

(particularly phase), the process is termed coherent; when phase information is not used, the process is termed non-coherent.

§ Waveform coding involves the use of new waveforms, yielding improved detection performance over that of the original waveforms.


waveforms.

§ Structured sequences involve the use of redundant bits to determine whether or not an error has occurred due to noise on the channel. One of these techniques, known as automatic repeat request (ARQ), simply recognizes the occurrence of an error and requests that the sender retransmit the message; other techniques, known as forward error correction (FEC), are capable of automatically correcting the errors (within specified limits).

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Basic Terms & their meaning§ Symbol (digital message). A symbol is a group of k bits considered

as a unit. We refer to this unit as a message symbol mi.

§ Data rate. This quantity in bits per second is given by R=k/T=(1/ T)log2M bits/s, where k bits identify a symbol from an M=2k –symbol alphabet, and T is the k-bit symbol duration.


§ Signal to Noise Ratio (SNR): The figure of merit for analog communication system is a fidelity criterion, such as SNR, percent distortion, or expected mean-square error between the transmitted and received waveforms. A figure for digital communication system is the probability of incorrectly detecting a digit, or the probability of error, PEs.

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Fundamental Constraint

§ Shannon’s capacity upper bound ØAchievable data rate is fundamentally limited by

bandwidth and signal-to-noise ratio



Fundamental Constraints§ Fundamental constraints for high data rate communications



Challenges for Wireless Communications

§ Multipath radio propagation

§ Spectrum limitations

§ Limited energy


§ Limited energy

§ User mobility

§ Resource management

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Duplexing§ Two ways to duplex downlink (base station to mobile) and

uplink (mobile to base station)Ø Frequency division duplex (FDD)Ø Time division duplex (TDD)



Wireless Channel

§ Wireless channel experiences multipath-radio propagation



Multipath Radio Propagation



Multipath Channel§ Multipath channel causes:

Ø Inter-symbol interference and fading in the time domainØ Frequency-selectivity in the frequency domain



Multipath Channel§ For broadband wireless channel, ISI and frequency-

selectivity becomes severe.

§ To resolve the ISI and frequency-selectivity in the channel, various measures are used. Ø Channel equalization in the time domain or frequency domain


Ø Channel equalization in the time domain or frequency domainØ Multi-carrier multiplexing

• Orthogonal frequency division multiplexing (OFDM)Ø Frequency hoppingØ Channel-adaptive schedulingØ Channel coding Ø Automatic repeat request (ARQ) and hybrid ARQ (H-ARQ)

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Mobile User§ When the user is mobile, the channel becomes time-varying.

§ There is also Doppler-shift in the carrier frequency.



Time-Varying Multipath Channel



Wireless Spectrum



Cellular Wireless System§ A large geographical region

is segmented into smaller “cells”. Ø Transmit power limitation Ø Facilitates frequency spectrum


Ø Facilitates frequency spectrum re-use

§ Cellular network design issues Ø Inter-cell synchronization Ø Handoff mechanism Ø Frequency planning

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Cellular Wireless System

§ Frequency reuse


Frequency re-use = 1-Higher spectral efficiency

-Higher interference for cell-edge users

Frequency re-use = 7- lower interference for cell-edge users

-Lower spectral efficiency



§ Sectorized Cells




§ Frequency reuse = 3



What is Spread Spectrum?§ A transmission technique in which a pseudo-

noise code, independent of the information data, is employed as a modulation waveform to “spread” the signal energy over a bandwidth much greater than the signal


bandwidth much greater than the signal information bandwidth.

§ At the receiver the signal is “despread” using a synchronized replica of the pseudo-noise code.

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Spread Spectrum

§§ In spread spectrum (SS), we combine signals In spread spectrum (SS), we combine signals from different sources to fit into a larger from different sources to fit into a larger bandwidth, but our goals are to prevent bandwidth, but our goals are to prevent eavesdropping and jamming. eavesdropping and jamming.


eavesdropping and jamming. eavesdropping and jamming. §§ To achieve these goals, spread spectrum To achieve these goals, spread spectrum techniques add redundancy.techniques add redundancy.

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Why Spread Spectrum?§ Hide a signal below the noise floor§ Resistance to narrowband jamming and interference§ Mitigate performance degradation due to inter-symbol

and narrowband interference§ In conjunction with RAKE receiver, SS can provide


§ In conjunction with RAKE receiver, SS can provide coherent combining of different multipath components

§ Allow multiple users to share the same signal bandwidth

§ Wide bandwidth of SS signals is useful for location and timing acquisition

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Spreading



Spreading methods§ Frequency Hopping

ØApplied in GSM, Military, ISM bands, Blue tooth

§ Direct sequenceØApplied in IS-95 IS-136 Cellular CDMA, GPS, UMTS,

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ØApplied in IS-95 IS-136 Cellular CDMA, GPS, UMTS, W-CDMA, Military

§ Multi-Carrier CDMAØ4G Networks

§ Ultra Wide BandØRADAR & Short range communications

Applications of spread spectrum are in Military: This is the oldest known application. It is popular for security reasons. (Satellite) Positioning Systems: High bandwidth signals allow accurate measurements of propagation delays. This is used to estimate the the distance to a transmitter. (Note: the location of a strong spread spectrum can be estimated much more accurately that a that of a narrowband radio emitter. This can be a disadvantage in military applications) Cellular telephony: It is mainly used to combat dispersion and to provide multiple access. Wireless LANs: If conventional modulation would be used frequency management for many coexisting links can be very difficult. Moreover, narrowband transmission could be impaired by deep local fades.


Spread Spectrum Properties

§ Signal occupies a bandwidth much larger than is needed for the information signal

§ Spread spectrum modulation is done using a spreading code independent of the data in the


spreading code independent of the data in the signal

§ Despreading at the receiver is done by correlating the received signal with a synchronized copy of the spreading code

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A Short History§ Spread-spectrum communications technology was first described on

paper by an actress and a musician! In 1941 Hollywood actress HedyLamarr and pianist George Antheil described a secure radio link to control torpedos. Ø 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


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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Multiplexing § Frequency Division Duplexing (FDD)

ØMobile users use two different channels to transmit and receive information simultaneously.

ØEach frequency channel is called simplex channel (forward or reverse). A duplexer is used in MS/BS to allow bi-directional transmission.


allow bi-directional transmission.§ Time Division Duplexing (TDD)

ØMobile users transmit and receive information in different assigned time slots. No Duplexer is needed if single channel is used.

ØThe time separation between forward and reverse time slots needs to be small for real time systems.

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Differences between Multiplexing & Multiple Access

§ A communication resource (CR) represents the time and bandwidth that is available for communication signaling associated with a given system.

§ The terms “multiplexing” and “multiple access” refers to the sharing of Communication resource.

§ With multiplexing, the sharing of CR is fixed, or at most, slowly changing. The resource allocation is assigned a priori, and the


changing. The resource allocation is assigned a priori, and the sharing is usually a process that takes place within the confines of a local site (e.g., a circuit board).

§ Multiple access, however, usually involves the remote sharing of a resource.

§ Multiplexing involves an algorithm that is known a priori; usually, it is hard-wired into the system. Multiple-access, on the other hand, is generally adaptive, and may require some overhead to enable the algorithm to operate.

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Allocation of Communication Resource

§ There are three basic ways to increase the throughput (total data rate) of a communication resource. ØTo increase the transmitter’s effective isotropic


ØTo increase the transmitter’s effective isotropic radiated power (EIRP), or to reduce the system losses so that the received Eb/N0 is increased.

ØTo provide more channel bandwidth. ØTo make the allocation of CR more efficient –

domain of multiple access.

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Multiple Access?

§ Users share a common pool of radio channels,common pool of radio channels,and user can access to any channel.

§ A channelchannel is portion of limited radio resourceradio resourcewhich is temporarily allocated for a specific


which is temporarily allocated for a specific purpose (e.g. phone call).

§ It is a technique to divide radio spectrumradio spectrum into channelschannels, and to allocate channels to number number of usersof users, simultaneously.

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Multiple Access Techniques§ Three major MA techniques:

Ø Frequency Division Multiple Access (FDMA) – specified bands of frequency are allocated.

Ø Time Division Multiple Access (TDMA) – periodically recurring time slots are allocated.

Ø Code Division Multiple Access (CDMA) - specified members of a set of orthogonal or nearly orthogonal spread spectrum codes (each using


orthogonal or nearly orthogonal spread spectrum codes (each using the full channel bandwidth) are allocated.

§ Other techniques include:Ø Space Division Multiple Access (SDMA) or multiple beam frequency

reuse – spot beam antennas are used to separate radio signals by pointing in different directions. It allows for reuse of same frequency.

Ø Polarization Division (PD) or dual polarization frequency reuse –Orthogonal polarizations are used to separate signals, allowing for reuse of the same frequency band.

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Narrowband MA Systems

§ Narrowband MA Systems:Ø The transmission BW of single channel is same as

of the coherence BW of the channelØ In FDMA/FDD, each user is assigned a distinct


Ø In FDMA/FDD, each user is assigned a distinct duplex channel for transmission (Tx) and reception (Rx)

Ø In TDMA/FDD (TDMA/TDD), each user is assigned a unique time slot for Tx and Rx on a different (same) radio channel

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Wideband MA Systems

§ Wideband MA SystemsØThe transmission BW of single channel is much

larger than the coherence BW of the channelØMany users can transmit on same radio channel


ØMany users can transmit on same radio channelØIn CDMA/FDD (TDMA/TDD), spread spectrum is

used to allow all transmitters to access the channel simultaneously with FDD (TDD) multiplexing techniques

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FDMA§ The whole frequency band is divided into C non-overlapping channels§ Each user is allocated a dedicated channel to use upon request§ Selection of the channel is based on particular channel assignment

scheme§ Transmission is continuous (FDD)



TDMA§ The whole frequency band is divided into C channels. Each channel is divided into

N time slots which comprises a frame. § Each user is allowed to transmit Tx or Rx in a particular time slot in each frame and

the user repeats transmission in the frame.§ Share a channel with several users.§ Can be user with FDD and TDD.


§ Can be user with FDD and TDD.§ Transmission is not continuous.§ Synchronization is required between MS and BS.

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TDMA



CDMA§ The narrowband signal is multiplied

by a very large BW signal called spreading signal, which is a pseudo-noise (PN) code sequence.

§ Each user can transmit information on same frequency channel at any time.

§ The receiver needs to know the exact


§ The receiver needs to know the exact codeword of the desired transmitter.

§ Can be user with FDD and TDD.§ Soft capacity and soft handover are

two features.§ Near-far problem occurs

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Orthogonal Signals§ The key to all multiplexing or multiple access schemes is that various

signals sharing a CR does not create unmanageable interference.§ Orthogonal signals on separate channels will avoid interference. § Signal waveforms xi(t), where i = 1, 2, …, are defined to be orthogonal if

they can be described as:


§ where K is a nonzero constant.

§ Where the function Xi(f) are the Fourier transform of the signal waveform xi(t).

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Time domain

frequency domain


A Comparison Power Density

FDMA

Power Density

TDMA


Frequency Frequency

Power Density

Frequency

CDMA


Frequency Hopping Spread Spectrum (FHSS)§ Signal broadcast over seemingly random

series of frequencies§ Receiver hops between frequencies in sync

with transmitter


with transmitter§ Eavesdroppers hear unintelligible blips§ Jamming on one frequency affects only a few

bits

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Basic Operation§ Typically 2k carriers frequencies forming 2k

channels§ Channel spacing corresponds with bandwidth of

inputEach channel used for fixed interval


§ Each channel used for fixed intervalØ300 ms in IEEE 802.11ØSome number of bits transmitted using some

encoding scheme• May be fractions of bit (see later)

ØSequence dictated by spreading code

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Frequency Hopping Example



Bluetooth uses FH

§ Bluetooth is a FH-SS system, which achieves a (coded) bit rate of 1 Mbps (potentially up to 3 Mbps), but uses 80 MHz of spectrum, in 79 different center frequencies, with a hopping


different center frequencies, with a hopping period Th = 1/1600 s/hop.

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Benefits of FHSS§ Three benefits of FH-SS are:

Ø 1. Interference avoidance: There may be significant interference at a few of the center frequencies. But even if we totally lose all bits during those hops, we will be able to recover using the bits received during successful (non-interfered) hops. We also avoid being an interferer to someone else’s signal for too long.

Ø 2. Multiple Access: Two devices can occupy the same spectrum


Ø 2. Multiple Access: Two devices can occupy the same spectrum and operate without coordinating medium access at all. Their transmissions will “collide” some small fraction of the time, but not often enough to cause failure.

Ø 3. Stealth: There is an advantage to switching randomly among frequencies when an eavesdropper doesn’t know your hopping pattern – they will not be able to easily follow your signal.

• This was the original reason for the discovery and use of FHSS (by actor and inventor Hedy Lamarr, in 1940).

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Bandwidth Sharing



Direct Sequence Spread Spectrum (DSSS)

§ Each bit represented by multiple bits using spreading code§ Spreading code spreads signal across wider frequency band

Ø In proportion to number of bits usedØ 10 bit spreading code spreads signal across 10 times bandwidth of 1

bit code


bit code

§ One method:Ø Combine input with spreading code using XORØ Input bit 1 inverts spreading code bitØ Input zero bit doesn’t alter spreading code bitØ Data rate equal to original spreading code

§ Performance similar to FHSS

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Direct Sequence

§ User data stream is multiplied by a high rate (fast) code sequence

§ Example:

EXORUser Bits

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§ Example: ØUser bits 101 (+ - +)ØCode 1110100 (+ + + - + - -); spead factor = 7

Code Sequence

1 -1 -1-111 1 -1 1 11-1-1 -1 1 -1 -1-111 1

User bit-1 = 1 User bit0 = -1 User bit+1= 1

In Direct Sequence Spread Spectrum transmission, the user data signal is multiplied by a fast code sequence. Mostly, binary sequences are used. The duration of an element in the code is called the "chip time". The ratio between the user symbol time and the chip time is called the spread factor. The transmit signal occupies a bandwidth that equals the spread factor times the bandwidth of the user data. Different CDMA users use different codes. In this example the receiver sees the signal from user 1, while the signal from user 2 is heavily attenuated by the correlator (multiplier and integrator) in the receiver. Despreading the signal requires knowledge of the user's code. In military systems these codes are kept secret, so it is very difficult for an unauthorized attacker to tap into or transmit on another user's channel. Often it is even difficult to detect the presence of a spread-spectrum signal because it is below the noise that is present in the transmit bandwidth. Note that in cellular systems, the codes are fully described in publicly available standards. In digital systems, security against eavesdropping (confidentiality) is obtained through encryption. This is a highly desirable alternative to the analog FDMA cellular phone system in wide use today, where with an inexpensive scanner one can tune in to the private conversations of unwary neighbours.


Direct Sequence Spread Spectrum Example



Processing Gain (PG)

§ It is the number of chips per bit.

§ Where Rc = 1/Tc is the chip rate. ØFor example, 802.11b has a chip rate of 11 M-cps


ØFor example, 802.11b has a chip rate of 11 M-cps (chips per second) and a symbol rate of 1 M-sbs(symbols per second).

ØAs another example, in IS-95, the “short code” has PG = 215+1 = 32768.

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