Transmission Radio.2P 2
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Les transmissions radioLes transmissions radio
Bernard Cousin
Les pionniers du sans fil (1800Les pionniers du sans fil (1800--1900)1900)
Claude Chappe, 1793 : Inventeur du premier systme de
transmission base de smaphores(mcanique et visuel) et d'un code
Samuel Morse, 1837 : Inventeur de l'alphabet deux
signaux qui porte son nom et dutlgraphe lectrique
Thomas Edison, 1878 : Pionnier de la transmission longue
distance filaire et inventeur du
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phonographe (Enregistrement etrestitution de la voix)
Guglielmo Marconi, 1895 : Premire transmission par radio
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L'histoireL'histoire du Wireless Networkingdu Wireless Networking
1900 : Premire tlcommunication sans fil en France (Marconi
partir de la tour Eiffel)
1948 : Claude Shannon et la thorie de linformation
1986 : Naissance de lInternet
Large diffusion publique de linternet sans fil
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Architecture d'interconnexionArchitecture d'interconnexion
Le modle OSI Chaque couche met en uvre
un (ou plusieurs) protocole(s)de communications entreentits distantes
Afin de rendre un service lacouche suprieure
En s'appuyant sur le service
couche infrieure
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Les couches radiosLes couches radios
Les couches Liaison de donnes Physique
Exemples : Ethernet (IEEE 802.3), Wifi
(IEEE 802.11), Bluetooth(IEEE 802.15.4), Zigbee,HDLC (ISO 3309), GSM,
yper an Organismes de
normalisation: IEEE, ITU-T, 3GPP, ETSI
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Les couches radiosLes couches radios
La couches Liaison de donnes Transmission de trames entre entits
Identification et distribution Fragmentation
Contrle d'erreur La couche Physique : Adaptation l'environnement Codage Modulation
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BibliographieBibliographie
Wireless Communications and Networks. William Stallings.Prentice Hal, 2002 (traduit)
, ,Guy Pujolle, Guillaume Vivier. Eyrolles, 2001.
Wi-Fi par la pratique. Guy Pujolle, Davor Males. Eyrolles,2004.
Le guide de Wi-Fi et du Bluetooth. Guy de Lussigny,Joanna Truffaut, Bertrand Grossier, Eska interactive, 2004.
802.11 Rseaux sans fil : La rfrence. Matthew Gast' '
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. , . Certaines figures sont issues de :
Toufik AHMED (Universit de Bordeaux), Bndicte LEGRAND(Universit Pierre et Marie Curie), Yassine HADJADJ (Universit deRennes 1), Philippe JACQUET (INRIA), ou W. STALLINGS
PlanPlan
Introduction la transmission sans fil Prsentation des rseaux sans fil
Le spectre
Le multiplexage La transmission radio : les problmes
L'attnuation
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La propagation multi-trajet
Les solutions La transmission par talement de spectre
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Caractristiques des rseaux sans filCaractristiques des rseaux sans fil
Avantages Trs flexibles (souplesse) dans la zone de rception Pas ou peu de planification ncessaire (c.--d. rseaux
ad-hoc ) Presque pas de difficults de cblage (par ex. btiments
historiques, ) Plus robuste en situation de dsastre et dconnexion
de cble !
uppor e a mo
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Caractristiques des rseaux sans filCaractristiques des rseaux sans fil
Dsavantages Faible bande passante (c.--d. 1-54 Mbit/s)
bande passante partage
Beaucoup de solutions propritaires tablissement de normes lent (voir IEEE 802.11, et encore plus
Hiperlan) Contraintes nationales (agence de rgulation des
tlcoms)
difficiles e.g., IMT-2000 (technologies d'accs radio des systmes
cellulaires de la troisime gnration de lITU-T incluantWIMAX)
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Objectifs des rseaux locauxObjectifs des rseaux locaux
sans filsans fil Communication de donnes numriques sans fil entre
entits adjacentes :
Itinrance naturelle Coopration spontane dans les runions (rseaux ad-hoc) Technologie de transmission doit tre adapte et robuste Faible consommation de puissance (dure de batterie) Pas de licence dutilisation (pas toujours le cas ) Scurit (des donnes), respect de la vie prive (pas de collectes
' ' ,faible)
Possibilit de localisation (services lis lendroit o on se trouve) Transparence vis--vis des applications et des couches suprieures
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Exemples dapplication desExemples dapplication desrseaux sans filrseaux sans fil
Mise en place dun rseau dans un btiment historique Mise en place dun rseau de courte dure (chantiers,
expos ons, ocaux empora res, zones vas es
Interconnexion automatique et constante de tous lesparticipants dune runion
Accs sans fil aux infos enregistres sur chaque patientpendant les visites mdicales au lit des patients
Vhicules souvrant lapproche de leur propritaire, ou
Porte dentre se dverrouillant automatiquement, systmedalarme se mettant en veille et les lumires sallumant
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General Frequency RangesGeneral Frequency Ranges
Ondes lumineuses Infrared frequency range
Rou hl 3x1011 to 2x1014 Hz Useful in local point-to-point multipoint applications within
confined areas Microwave frequency range
1 GHz to 40 GHz Directional beams possible Suitable for point-to-point transmission Used for satellite communications
Radio fre uenc ran e
30 MHz to 1 GHz Suitable for omnidirectional applications Ondes sonores
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Spectre lectromagntiqueSpectre lectromagntique
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Terrestrial MicrowaveTerrestrial Microwave
Description of common microwave antenna Parabolic dish (3 m in diameter) Fixed rigidly and focuses a narrow beamAchieves line-of-sight transmission to receiving antenna Located at substantial heights above ground level
Applications Long haul telecommunications service - -
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Satellite MicrowaveSatellite Microwave
Description of communication satellite Microwave relay station Used to link two or more ground-based microwave
transmitter/receivers Receives transmissions on one frequency band (uplink),
amplifies or repeats the signal, and transmits it onanother frequency (downlink)
Applications Television distribution Long-distance telephone transmission Private business networks
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Broadcast RadioBroadcast Radio
Description of broadcast radio antennas Omnidirectional Antennas not required to be dish-shapedAntennas need not be rigidly mounted to a precise
alignment
Applications Broadcast radio
VHF and part of the UHF band; 30 MHZ to 1GHz
Covers FM radio and UHF and VHF television
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MultiplexingMultiplexing
Capacity of transmission medium usually exceedscapacity required for transmission of a singles gna
Multiplexing - carrying multiple signals on a single
medium More efficient use of transmission medium
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Reasons for Widespread Use ofReasons for Widespread Use of
MultiplexingMultiplexing Cost per kbps of transmission facility declines with
an increase in the data rate Cost of transmission and receiving equipment
declines with increased data rate Most individual data communicating devices
require relatively modest data rate support
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Multiplexing TechniquesMultiplexing Techniques
Frequency-division multiplexing (FDM) Takes advantage of the fact that the useful bandwidth of
the medium exceeds the required bandwidth of a givens gna
Time-division multiplexing (TDM)
Takes advantage of the fact that the achievable bit rateof the medium exceeds the required data rate of a digitalsignal
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AttnuationAttnuation
Attnuation : 4 ( d/ )^2
Distance : d Longueur d'onde :
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Categories of NoiseCategories of Noise
Thermal noise Intermodulation noise Crosstalk Impulse noise
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Thermal NoiseThermal Noise
Thermal noise due to agitation of electrons Present in all electronic devices and transmission
media Cannot be eliminated Function of temperature Particularly significant for satellite communication
Amount of thermal noise to be found in a bandwidth of 1 Hzin any device or conductor is:
N0 = noise power density in watts per 1 Hz of bandwidth k = Boltzmann's constant = 1.3803 * 10-23 J/K T = temperature, in kelvins (absolute temperature)
W/Hzk0 TN
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Thermal NoiseThermal Noise
Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz
(in watts):
or, in decibel-watt
TBN k
BTN log10log10klog10
BT log10log10dBW6.228
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Noise TerminologyNoise Terminology
Intermodulation noise occurs if signals withdifferent frequencies share the same medium Interference caused by a signal produced at a
frequency that is the sum or difference of originalfrequencies
Crosstalk unwanted coupling betweensignal paths
Impulse noise irregular pulses or noise
spikes Short duration and of relatively high amplitude Caused by external electromagnetic disturbances,
or faults and flaws in the communications system
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ExpressionExpression EEbb//NN00
Ratio of signal energy per bit to noise powerdensity per Hertz
The bit error rate for digital data is a function ofEb/N0 Given a value for Eb/N0 to achieve a desired error rate,
TR
S
N
RS
N
b
k
/
00
As bit rate R increases, transmitted signal power must
increase to maintain required Eb/N0
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Other ImpairmentsOther Impairments
Atmospheric absorption water vapor and oxygen contribute to attenuation
Multipath obstacles reflect signals so that multiple copies with
varying delays are received
Refraction bending of radio waves as they propagate through the
atmos here
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Propagations des ondesPropagations des ondes
Par ondes de surface < 2 Mhz
Ionosphrique [2-30] Mhz
En vue directe > 30 Mhz
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Cause of MultipathCause of Multipath PropagationPropagation
Reflection occurs when signal
encounters a surface thats arge re a ve o ewavelength of the signal
Diffraction occurs at the edge of an
impenetrable body that islarge compared towavelength of radio wave
Scattering
signal hits an objectwhose size in the order ofthe wavelength of thesignal or less
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EffectsEffects of Multipath Propagationof Multipath Propagation
Multiple copies of a signal may arrive at differentphases If phases add destructively, the signal level relative to noise
declines, making detection more difficult Intersymbol interference (ISI)
One or more delayed copies of a pulse may arrive at the
same time as the primary pulse for a subsequent bit
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Types of FadingTypes of Fading
Evanouissement de la longueur d'onde (m) Fast fading (plus petit/rapide que la longueur d'onde)
' 1 Ghz =/= 30 cm
Flat fading Indpendant de la longueur d'onde
Selective fading Fonction de la longueur d'onde
Evanouissement de canal
Additive White Gaussian Noise (AWGN) Rayleigh fading
Plusieurs chemins mais pas de chemin prpondrant
Rician fading Plusieurs chemins et un chemin prprondrant (d'un facteur K)
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Solutions: ErrorSolutions: Error CompensationCompensationMechanismsMechanisms
Forward error correction Adaptive equalization Diversity techniques
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Forward Error CorrectionForward Error Correction
Transmitter adds error-correcting code to datablock Code is a function of the data bits
Receiver calculates error-correcting code fromincoming data bits If calculated code matches incoming code, no error
occurred If error-correctin codes dont match, receiver attem ts
to determine bits in error and correct
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Adaptive EqualizationAdaptive Equalization
Can be applied to transmissions that carry analogor digital information Analog voice or video Digital data, digitized voice or video
Used to combat intersymbol interference Involves gathering dispersed symbol energy backinto its original time interval
Lumped analog circuits Sophisticated digital signal processing algorithms
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Diversity TechniquesDiversity Techniques
Diversity is based on the fact that individualchannels experience independent fading events
Space diversity Techniques involving physical transmission path Antennes multiples (et directives)
Frequency diversity Techniques where the signal is spread out over a larger
carriers Time diversity
Techniques aimed at spreading the data out over time
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Spread SpectrumSpread Spectrum
Input is fed into a channel encoder Produces analog signal with narrow bandwidth
Signal is further modulated using sequence of digits Spreading code or spreading sequence Generated by pseudonoise, or pseudo-random number
generator Effect of modulation is to increase bandwidth of signal to be
transmitted
,demodulate the spread spectrum signal
Signal is fed into a channel decoder to recover data
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Spread SpectrumSpread Spectrum
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Spread SpectrumSpread Spectrum
What can be gained from apparent waste ofspectrum? Immunity from various kinds of noise and multipath
distortion Can be used for hiding and encrypting signals Several users can independently use the same higher
bandwidth with very little interference
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Frequency Hoping Spread SpectrumFrequency Hoping Spread Spectrum
(FHSS)(FHSS) Signal is broadcast over seemingly random series
of radio frequencies A number of channels allocated for the FH signal Width of each channel corresponds to bandwidth of
input signal
Signal hops from frequency to frequency at fixedintervals Transmitter o erates in one channel at a time
Bits are transmitted using some encoding schemeAt each successive interval, a new carrier frequency is
selected
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Frequency Hoping Spread SpectrumFrequency Hoping Spread Spectrum
Channel sequence dictated by spreading code Receiver, hopping between frequencies in
synchronization with transmitter, picks up message Advantages
Eavesdroppers hear only unintelligible blips Attempts to jam signal on one frequency succeed only
at knocking out a few bits
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Frequency Hoping Spread SpectrumFrequency Hoping Spread Spectrum
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Frequency Hoping Spread SpectrumFrequency Hoping Spread Spectrum
vitement d'uneinterfrence avec unep age e r quencesbruite
v emen une au retransmission"orthogonale"
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Schma FHSSSchma FHSS
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FHSSFHSS usingusing MFSKMFSK
MFSK signal is translated to a new frequencyevery Tc seconds by modulating the MFSK signalw e carr er s gna
For data rate of R:
duration of a bit: T = 1/R seconds duration of signal element: Ts = LT seconds
Tc Ts - slow-frequency-hop spread spectrum c s - - -
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FHSSFHSS usingusing MFSKMFSK
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FHSSFHSS usingusing MFSKMFSK
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FHSS Performance ConsiderationsFHSS Performance Considerations
Large number of frequencies used Results in a system that is quite resistant to
jamming Jammer must jam all frequencies With fixed power, this reduces the jamming power in any
one frequency band
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Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum(DSSS)(DSSS)
Each bit in original signal is represented bymultiple bits in the transmitted signal
Spreading code spreads signal across a widerfrequency band
Spread is in direct proportion to number of bits used One technique combines digital information stream
with the spreading code bit stream using-
See next Figure
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Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum
(DSSS)(DSSS)
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DSSSDSSS usingusing BPSKBPSK
Multiply BPSK signal,sd(t) =A d(t) cos(2 fct)
by c(t) [takes values +1, -1] to gets(t) =A d(t)c(t) cos(2 fct)
A = amplitude of signal fc = carrier frequency d(t) = discrete function [+1, -1]
At receiver, incoming signal multiplied by c(t) Since, c(t) * c(t) = 1, incoming signal is recovered
See next Figure
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DSSSDSSS usingusing BPSKBPSK
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DSSSDSSS usingusing BPSKBPSK
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CodeCode--Division Multiple Access (CDMA)Division Multiple Access (CDMA)
Basic principles of CDMA D = rate of data si nal
Break each bit into k chips Chips are a channel-specific fixed pattern
Chip data rate of new channel = k * D
See next Figure
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CodeCode--Division Multiple Access (CDMA)Division Multiple Access (CDMA)
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CDMA ExampleCDMA Example
If k=6 and channel code is a sequence of '1's and '-1's For a 1 bit, A sends code as chip pattern
For a 0 bit, A sends complement of code
Receiver shares the sender's channel code and performselectronic decoding function
= received chip pattern
= user u's channel code
u
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CDMA ExampleCDMA Example User A's channel code =
To send a 1 bit = < 1, 1, 1, 1, 1, 1> To send a 0 bit =
User B's channel code = User C's channel code =
Receiver X receiving A's chip code for '1' (Xs channel code) x (received chip pattern)
SA (< 1, 1, 1, 1, 1, 1>) = 6 => "1" SB (< 1, 1, 1, 1, 1, 1>) = 0 => "signal ignored"
(A and B channel codes are orthogonal)
SC(< 1, 1, 1, 1, 1, 1>) = 2 => "signal ignored"
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Categories of Spreading SequencesCategories of Spreading Sequences
Spreading Sequence Categories PN sequences
Orthogonal codes
For FHSS systems PN sequences most common
For DSSS systems not employing CDMA
sequences mos common For DSSS CDMA systems
Orthogonal codes (or PN sequences)
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PN SequencesPN Sequences
Pseudo-random generator produces periodic sequencethat appears to be random
PN Sequences Generated by an algorithm using initial seed
Sequence isnt statistically random but will pass many test ofrandomness
Unless algorithm and seed are known, the sequence isimpractical to predict
Sequences referred to as pseudorandom numbers orpseudonoise sequences
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Important PN PropertiesImportant PN Properties
Randomness Uniform distribution
Balance property: P("1") =
Run property : P("111") = 1/8
Independence
Correlation property
Unpredictability
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Linear Feedback Shift RegisterLinear Feedback Shift Register
ImplementationImplementation
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Properties of MProperties of M--SequencesSequences
Property 1: Has 2n-1 ones and 2n-1-1 zeros
For a window of length n slid along output forN(=2n-1)
shifts, each n-tuple appears once, except for the all zerossequence
Property 3: Sequence contains one run of ones, length n
One run of zeros, length n-1
One run of ones and one run of zeros, length n-2 Two runs of ones and two runs of zeros, length n-3
2n-3 runs of ones and 2n-3 runs of zeros, length 1
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Properties of MProperties of M--SequencesSequences
Property 4: The periodic autocorrelation of a 1 m-sequence is
otherwise
...2N,N,0,1
1
R
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DefinitionsDefinitions
Correlation The concept of determining how much similarity one set of
data has with another
Range between 1 and 1 1 The second sequence matches the first sequence
0 There is no relation at all between the two sequences
-1 The two sequences are mirror images
Auto correlation
e compar son etween a s te copy o a sequence w t tse Cross correlation
The comparison between two sequences from different sources, one ofthem being shifted
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Advantages of Cross CorrelationAdvantages of Cross Correlation
The cross correlation between an m-sequence and noiseis low This property is useful to the receiver in filtering out noise
The cross correlation between two different m-
sequences is low This property is useful for CDMA applications
Enables a receiver to discriminate among spread spectrumsi nals enerated b different m-se uences
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Gold SequencesGold Sequences
Gold sequences constructed by the XOR of two m-sequences with the same clocking
Codes have well-defined cross correlation ro erties Only simple circuitry needed to generate large number of
unique codes Plus performant que les M-sequences pour DSSS avec
CDMA
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Orthogonal CodesOrthogonal Codes
Orthogonal codes All pairwise cross correlations are zero
Fixed- and variable-length codes used in CDMA systems
For CDMA application, each mobile user uses one sequencein the set as a spreading code
Provides zero cross correlation among all users
Types of Orthogonal codes Walsh codes (fixed length)
Variable-Length Orthogonal codes Permet de gnrer des dbits diffrents
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Walsh CodesWalsh Codes
Set of Walsh codes of length n consists of the nrows of an n n Walsh matrix:
W1 = (0)
n = dimension of the matrix
Every row is orthogonal to every other row and tothe lo ical not of ever other row
nn
nn
nWW
WWW
2
2
Requires tight synchronization Cross correlation between different shifts of Walsh
sequences is not zero
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Typical Multiple Spreading ApproachTypical Multiple Spreading Approach
Spread data rate by an orthogonal code (channelizationcode) Provides mutual orthogonality among all users in the same
cell
Further spread result by a PN sequence (scramblingcode) Provides mutual randomness (low cross correlation) between
users in different cells
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ConclusionConclusion
Les rseaux sans fil offrent une communicationflexible
L'environnement n'est pas sre Les techniques d'talement de spectre permettent
une adaptation efficace
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