spread spectrum 2

8/2/2019 Spread Spectrum 2

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DE NAYER INSTITUUT -mec :) 1

Spread Spectrum (SS)

1. Definition of Spread Spectrum (SS)A transmission technique in which a pseudo-noise

code, independant of the information data, is employed as amodulation waveform to spread the signal energy over abandwidth much greater than the signal informationbandwidth. At the receiver the signal is despread using asynchronized replica of the pseudo-noise code.

2. Basic Principle of Spread Spectrum Systems:DSSS and FHSS

Pseudorandom Shift of the Phase Coherent Demodulation

Pseudorandom Shift of the Frequency Non-coherentDemodulation

Direct Sequence Spread Spectrum

A pseudo-noise sequence pnt generated at themodulator, is used in conjunction with an M-ary PSKmodulation to shift the phase of the PSK signapseudorandomly, at the chipping rate Rc (=1/Tc) a rate that isan integer multiple of the symbol rate Rs (=1/Ts).

The transmitted bandwidth is determined by the chiprate and by the baseband filtering. The implementation limitsthe maximum chiprate Rc (clock rate) and thus the maximumspreading.

The PSK modulation scheme requires a coherendemodulation. A short-code system uses a PN code lengthequal to a data symbol. A long-code system uses a PN codelength that is much longer than a data symbol, so that adifferent chip pattern is associated with each symbol.

Frequency Hopping Spread Spectrum

A pseudo-noise sequence pnt generated at themodulator is used in conjunction with an M-ary FSK modulationto shift the carrier frequency of the FSK signa

pseudorandomly, at the hopping rate Rh. The transmittedsignal occupies a number of frequencies in time, each for aperiod of time Th (=1/Rh), referred to as dwell time. FHSSdivides the available bandwidth into N channels and hopsbetween these channels according to the PN sequence. Aeach frequency hop time the PN generator feeds the frequencysynthesizer a frequency word FW (a sequence of n chips

which dictates one of 2n

frequency positions fhi. Transmitteand receiver follow the same frequency hop pattern.

The transmitted bandwidth is determined by thelowest and highest hop positions and by the bandwidth per hopposition (fch). For a given hop, the instantaneous occupiedbandwidth is identical to bandwidth of the conventional M-FSKwhich is typically much smaller than Wss. So the FSSS signal is

a narrowband signal, all transmission power is concentrated onone channel. Averaged over many hops, the FH/M-FSKspectrum occupies the entire spread spectrum bandwidthBecause the bandwidth of an FHSS system only depends onthe tuning range, it can be hopped over a much wider bandwiththan a DSSS system.

Since the hops generally result in phase discontinuity(depending on the particular implementation) a noncoherendemodulation is done at the receiver.

With slow hopping there are multiple data symbolsper hop and with fast hopping there are multiple hops per datasymbol.


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3. Basic Principle of Direct Sequence SpreadSpectrum

Input:

Binary data dt with symbol rate Rs = 1/Ts (= bitrate Rb fo

BPSK)

Pseudo-noise code pnt with chip rate Rc = 1/Tc (an intege

of Rs)

Spreading:

In the transmitter, the binary data d t (for BPSK, I and

Q for QPSK) is directly multiplied with the PN sequence pn t

which is independant of the binary data, to produce the

transmitted baseband signal txb:

txb = dt pnt

The effect of multiplication of dt with a PN sequence is

to spread the baseband bandwidth Rs of dt to a baseband

bandwidth of Rc.

Despreading:The spread spectrum signal cannot be detected by a

conventional narrowband receiver. In the receiver, the receivedbaseband signal rxb is multiplied with the PN sequence pn r.

If pnr = pnt and synchronized to the PN sequence inthe received data, than the recovered binary data isproduced on dr. The effect of multiplication of thespread spectrum signal rxb with the PN sequence pnused in the transmitter is to despread the bandwidthof rxb to Rs.

If pnr pnt , than there is no despreading action. Thesignal dr has a spread spectrum. A receiver noknowing the PN sequence of the transmitter canno

reproduce the transmitted data.

To simplify the description of modulation anddemodulation, the spread spectrum system is considered fobaseband BPSK communication (without filtering) over an ideachannel.


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Modulation:

Spread spectrum systems are spreading the information signal dt which has a BWinfo, over a much larger bandwidth BWSS:

BWinfo Rs


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Demodulation:

pnr = pnt

To demodulate, the received signal is multiplied by

pnr, this is the same PN sequence as pnt (the pseudo-noisecode used in the transmitter), synchronized to the PNsequence in the received signal rxb. This operation is called(spectrum) despreading, since the effect is to undo thespreading operation at the transmitter.

The multiplier output in the receiver is then (since pnr= synchronized pnt) :

dr = rxb pnr = (dt pnt) pnt

The PN sequence pnt alternates between the levels -1and +1, in the example:

pnt = +1 +1 +1 -1 +1 -1 -1

The alternation is destroyed when the PN sequence

pnt is multiplied with itself (perfectly synchronized), because:

pnt . pnt = +1 for all tThus:

autocorrelation Ra ( =0) = average (pnt . pnt) = +1

The data signal is reproduced at the multiplier output:dr = dt

If the PN sequence at the receiver is not synchronizedproperly to the received signal, the data cannot be recovered.

pnr pntIf the received signal is multiplied by a PN sequence

pnr, different from the one used in the modulator, the multiplieoutput becomes:

dr = rxb . pnr = (dt . pnt ). pnr

In the receiver, detection of the desired signal isachieved by correlation against a local reference PNsequence. For secure communications in a multi-useenvironment, the transmitted data dt may not be recovered by

a user that doesnt know the PN sequence pnt used at thetransmitter. Therefore:

is required. This orthogonal property of the allocated spreadingcodes, means that the output of the correlator used in thereceiver is approximately zero for all except the desiredtransmission.

4. Performance in the Presence of Interference

To simplify the influence of interference, the spreadspectrum system is considered for baseband BPSKcommunication (without filtering). The received signal rxbconsists of the transmitted signal txb plus an additive

interference i (noise, other users, jammer, ):

To recover the original data d t , the received signal rxis multiplied with a locally generated PN sequence pnr that isan exact replica of that used in the transmitter (that is pnr= pnand synchronized). The multiplier output is therefore given by:

The data signal dt is multiplied twice by the PNsequence pnt , whereas the unwanted interference i ismultiplied only once. Due to the property of the PN sequence:

The multiplier output becomes:

The data signal dt is reproduced at the multiplieoutput in the receiver, except for the interference representedby the additive term i pnt. Multiplication of the interference by the locally generated PN sequence, means that thespreading code will affect the interference just as it did with theinformation bearing signal at the transmitter. Noise andinterference, being uncorrelated with the PN sequencebecome noise-like, increase in bandwidth and decrease inpower density after the multiplier.


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After despreading, the data component dt is narrowband (Rs) whereas the interference component is wideband(Rc). By applying the dr signal to a baseband (low-pass) filterwith a bandwidth just large enough to accommodate therecovery of the data signal, most of the interferencecomponent i is filtered out. The effect of the interference isreduced by the processing gain (Gp).

Narrowband Interference

The narrowband noise is spread by the multiplicationwith the PN sequence pnr of the receiver. The power density ofthe noise is reduced with respect to the despread data signal.Only 1/Gp of the original noise power is left in the informationbaseband (Rs). Spreading and dispreading enables a bandwithtrade for processing gain against narrow band interferingsignals. Narrowband interference would disable conventionalnarrowband receivers.

The essence behind the interference rejectioncapability of a spread spectrum system: the useful signal (data)gets multiplied twice by the PN sequence, but the interferencesignal gets multiplied only once.

Wideband Interference

Multiplication of the received signal with the PNsequence of the receiver gives a selective despread of thedata signal (smaller bandwidth, higher power density). Theinterference signal is uncorrelated with the PN sequence and isspread.

Origin of wideband noise:

Multiple Spread Spectrum users: multiple accessmechanism.

Gaussian Noise: There is no increase in SNR withspread spectrum. The larger channel bandwidth (Rinstead of Rs) increases the received noise powewith Gp:

The spread spectrum signal has a lower power

density than the directly transmitted signal.

5. Pseudo-Noise Sequences PN

Random White Gaussian Noise

Zero-mean White Gaussian Noise (WGN) has thesame power spectral density GWGN(f) for all frequencies. Theadjective white is used in the sense that white light containsequal amounts of all frequencies within the visible band oelectromagnetic radiation.

The autocorrelation function of WGN is given by theinverse Fourier transform of the noise power spectral densityGWGN(f):


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Pseudorandom Noise

A Pseudo-Noise (PN) code sequence acts as a noise-like (but deterministic) carrier used for bandwidth spreading ofthe signal energy. The selection of a good code is important,because type and length of the code sets bounds on thesystem capability.

The PN code sequence is a Pseudo-Noise orPseudo-Random sequence of 1s and 0s, but not a realrandom sequence (because periodic). Random signalscannot be predicted. The autocorrelation of a PN code hasproperties similar to those of white noise.

Pseudo-Random:

Not random, but it looks randomly for the user whodoesnt know the code.

Deterministic, periodical signal that is known to boththe transmitter and the receiver. The longer theperiod of the PN spreading code, the closer will thetransmitted signal be a truly random binary wave,

and the harder it is to detect. Statistical properties of sampled white-noise.

Length:

Short code: The same PN sequence for each datasymbol (Nc.Tc = Ts).

Long code: The PN sequence period is much longerthan the data symbol, so that a different chip patternis associated with each symbol (Nc.Tc >> Ts).

Properties of PN Sequences

Balance PropertyIn each period of the sequence the number of binary

ones differs from the number of binary zeros by at most onedigit (for Nc odd).

When modulating a carrier with a PN coding

sequence, one-zero balance (DC component) can limit thedegree of carrier suppression obtainable, because carriesuppression is dependent on the symmetry of the modulatingsignal.

Run-length DistributionA run is a sequence of a single type of binary digits

Among the runs of ones and zeros in each period it is desirablethat about one-half the runs of each type are of length 1, abouone-fourth are of length 2, one-eighth are of length 3, and so on

AutocorrelationThe origin of the name pseudo-noise is that the digita

signal has an autocorrelation function which is very similar tothat of a white noise signal: impulse like.

The autocorrelation function for the periodicsequence pn is defined as the number of agreements less thenumber of disagreements in a term by term comparison of one

full period of the sequence with a cyclicshift (position ) of the

sequence itself:

It is best if Ra() is not larger than one count if no

synchronized ( =0).


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For PN sequences the autocorrelation has a largepeaked maximum (only) for perfect synchronization of twoidentical sequences (like white noise). The synchronization ofthe receiver is based on this property.

Frequency Spectrum

Due to the periodic nature of the PN sequence thefrequency spectrum has spectral lines which become closer to

each other with increasing sequence length Nc. Each line isfurther smeared by data scrambling, which spreads eachspectral line and further fills in between the lines to make thespectrum more nearly continuous. The DC component isdetermined by the zero-one balance of the PN sequence.

Cross-correlation:

Cross-correlation describes the interference betweencodes pn and pnj :

Cross-correlation is the measure of agreement

between two different codes pn i and pnj. When the cross-correlation Rc() is zero for all , the codes are called

orthogonal. In CDMA multiple users occupy the same RFbandwidth and transmit simultaneous. When the user codesare orthogonal, there is no interference between the users afterdespreading and the privacy of the communication of eachuser is protected.

In practice, the codes are not perfectly orthogonal;hence the cross-correlation between user codes introducesperformance degradation (increased noise power afterdespreading), which limits the maximum number ofsimultaneous users.

Types

m-sequenceA Simple Shift Register Generator (SSRG) has all the

feedback signals returned to a single input of a shift register(delay line). The SSRG is linearif the feedback function can beexpressed as a modulo-2 sum (xor).

The feedback function f(x1,x2, ,xn) is a modulo-2sum of the contents xi of the shift register cells with c i being thefeedback connection coefficients (ci=0=open, ci=1=connect).

An SSRG with L flip- flops produces sequences thatdepend upon register length L, feedback tap connections andinitial conditions. When the period (length) of the sequence is

exactly Nc = 2L-1, the PN sequence is called a maximum-

length sequenceor simply an m-sequence.An m-sequence generated from a linear SSRG has

an even number of taps. If an L-stage SSRG has feedback

taps on stages L, k, m and has sequence , ai, ai+1, ai+2,than the reverse SSRGhas feedback taps on L, L-k, L-m andsequence , ai+2, ai+1, ai, .

In the following table the feedback connections (evennumber) are tabulated for m-sequences generated with a lineaSSRG (without image set).

For every set [L, k, , p] feedback taps listed in thetable, there exists an image set (reverse set) of feedback taps[L, L-k, , L-p] that generates an identical sequence reversedin time.

Properties

balanceFor an m-sequence there is one more one than

zero in a full period of the sequence. Since all states but theall-zero state are reached in an m-sequence, there must be

2L-1

ones and 2L-1

-1 zeros.

run-length distributionFor every m-sequence period, half the runs (of all 1s

or all 0s) have length 1, one-fourth have length 2, one-eigh


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have length 3, etc. For each of the runs there are equally manyruns of 1s and0s.

autocorrelationThe autocorrelation function of the m-sequence is 1

for all values of the chip phase shift except for the [-1, +1] chipphase shift area, in which correlation varies linearly from the -1

value to 2L-1 = Nc (the sequence length).

The autocorrelation peak increases with increasinglength Nc of the m-sequence and approximates theautocorrelation function of white noise. Other codes can do nobetter than equal this performance of m-sequences.

Cross-correlationCross-correlation is the measure of agreement

between two different codes. Unfortunately, cross- correlationis not so well behaved as autocorrelation. When large numbersof transmitters, using different codes, are to share a commonfrequency band (multi-user environment), the code sequencesmust be carefully chosen to avoid interference between users.

securityThe m-sequence codes are linear, and thus not

usable to secure a transmission system. The linear codes are

easily decipherable once a short sequential set of chips (2L+1)from the sequence is known. (The overall system could still besecure if the information itself where encoded by acryptographically secure technique).

Barker CodeThe number of stages L in the SSRG also determines

the length (period) Nc =2L1 of the m sequence codes. The

Barker code gives codes with different lengths and similarautocorrelation properties as the m-sequences.

The autocorrelation function of the balanced 11 chipBarker code is shown in the next figure.

Gold CodesThe autocorrelation properties of the m-sequences

cannot be bettered. But a multi-user environment (CodeDevision Multiple Access) needs a set of codes with the samelength and with good cross-correlation properties.

Gold code sequences are usefull because a largenumber of codes (with the same length and with controlledcrosscorrelation) can be generated, although they require onlyone pair offeedback tap sets.

Gold codes are product codes achieved by theexclusive or-ing (modulo-2 adding) of two maximum-lengthsequences with the same length (factor codes). The codesequences are added chip by chip by synchronous clockingBecause the m-sequences are of the same length, the twocode generators maintain the same phase relationship, and thecodes generated are of the same length as the two basecodes which are added together, but are non-maximal (so

the autocorrelation function will be worse than that of msequences). Every change in phase position between the twogenerated m-sequences causes a new sequence to begenerated.

Any 2-register Gold code generator of length L can

generate 2L

- 1 sequences (length 2L

- 1) plus the two base m

sequences, giving a total of 2

L

+ 1 sequences.In addition to their advantage in generating large

numbers of codes, the Gold codes may be chosen so that ovea set of codes available from a given generator theautocorrelation and the crosscorrelation between the codes isuniform and bounded. When specially selected m-sequencescalled preferred m-sequences, are used the generated Goldcodes have a three valued crosscorrelation.


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This important subset of Gold codes are the PreferredPair Gold codes.

Predictable cross-correlation properties are necessaryin an environment where one code must be picked fromseveral codes which exist in the spectrum. Only part of thegenerated Gold codes are balanced.

Hadamard-Walsh CodesThe Hadamard-Walsh codes are generated in a set o

N = 2n

codes with length N = 2n. The generating algorithm is

simple:

The rows (or columns) of the matrix HN are theHadamard-Walsh codes.

I n each case the first row (row 0) of the matrix consisentirely of 1s and each of the other rows contains N/2 0s andN/2 1s. Row N/2 starts with N/2 1s and ends with N/2 0s.

The distance (number of different elements) betweenany pair of rows is exactly N/2. For H8 the distance betweenany two rows is 4, so the Hamming distance of the Hadamardcode is 4. The Hadamard-Walsh code can be used as a blockcode in a channel encoder: each sequence of n bits identifies

one row of the matrix (there are N =2n

possible rows). All rowsare mutually orthogonal:

for all rows i and j. The cross-correlation between any twoHadamard-Walsh codes of the same matrix is zero, whenperfectly synchronized. In a synchronous CDMA system thisensures that there is no interference among signals transmittedby the same station.

Only when synchronized, these codes have goodorthogonal properties. The codes are periodic, which results inless spreading efficiency and problems with synchronizationbased on autocorrelation.


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6. Transmitter ArchitectureA typical architecture of a Direct Sequence Spread Spectrum (DS-SS) transmitter:

7. Receiver Architecture

A typical architecture of a Direct Sequence Spread Spectrum (DS-SS) receiver:

The basic building blocks of a DS-SS (digital) receiver are:

coherent IQ vector-demodulator with waveform synthesizer (Direct Digital Synthesis) at the IF-carrier frequency (f IF) and

chip matched filters (usually Square Root Raised Cosine) despreading (correlation of the received symbols with the locally generated PN-sequence(s) pnI and pnQ)

decorrelated IQ to data demodulator mapping

synchronization loops for the IF-carrier (fIF, phase error IF measured after despreading to reduce the influence of noiseand chip frequency (fchip)


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8. PN Decorrelators

Two PN decorrelator architectures can be used fordespreading spread spectrum signals: the matched filter andthe active correlator. They are optimum from a SNR point ofview.

PN Matched FilterA typical matched filter implements convolution using

a finite impulse response filter (FIR) whose coefficients are the

time reverse of the expected PN sequence, to decode thetransmitted data.

For the given example:

The output of the FIR filter is the convolution of thereceived IQ-demodulated and filtered signal iqc (Ichip or Qchip on

the receiver architecture block diagram) with the FIR impulsereponse h=[ h0 h1 hNc-1]. Due to the time reversion, theoutput of the filter is the correlation of rxb with the local PNsequence.

In the shown example, 1 sample of the receivedsignal per chip is taken (fsample = fchip). To increase the

accuracy of synchronization oversampling with a factor s canbe used. In this case there are s samples per chip (fsample =s.fchip). The dimensions of the matched filter are also increasedwith a factor s (each filter coefficient h i is used s time).

If the receiver is not synchronized, then the receivedsignal will propagate through the matched filter, which outputsthe complete correlation function. The large peak confirms thatthe correct code is indeed being received and providesaccurate timing information for the synchronization of thereceived signal. The output R= of the FIR PN matched filter isimmediately the decorrelated data: the polarity of the large

correlation peaks indicates the data value.

PN Active Correlator (Integrate and Dump)

When timing information is already available, then the simpleactive correlator receiver can be used. This receiver onlyoperates correctly when the local PN sequence pnr isaccurately matched and correctly timed, with respect to thespreading code within the received signal rxb. Synchronizationcan be obtained by sliding the reference signal through thereceived signal. This can be an extremely slow processhowever, for large spreading waveforms (long codes).

9. PN SynchronizationFor its proper operation, a SS communication systemrequires that the locally generated PN sequence (pnr used inthe receiver Rx to despread the received signal) issynchronized to the PN sequence of the transmitter generato(pnt used to spread the transmitted signal in the transmitter Txin both its rate and its position. Due to the sharp peak in theautocorrelation function, a misalignment in the PN sequence oTc/2 gives a loss of a factor 2 in processing gain.


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Sources of Synchronization UncertaintyTime uncertainty:

Uncertainty in distance between Tx-Rx (propagationdelay)

Relative clock shifts

Different phase between Tx-Rx (carrier, PNsequence)

Frequency uncertainty:

Relative velocity vr between Tx-Rx (Dopplerfrequency shift) affects the carrier frequency fcarrier(with c the speed of light in the propagation medium):

For a carrier frequency of 2.4 GHz this gives afrequency shift fcarrier = 2.2 Hz/km/hr. For a relative velocity v r= 100 km/hr this gives fcarrier = 220Hz.

The process of synchronizing the locally generatedPN sequence with the received PN sequence is usuallyaccomplished in two steps. The first step, called acquisition,consists of bringing the two spreading signals into coarsealignment with one another. Once the received PN sequence

has been acquired, the second step, called tracking, takes overand continuously maintains the best possible waveform finealignment by means of a feedback loop. This is essential toachieve the highest correlation power and thus highestprocessing gain (SNR) at the receiver.

Acquisition Phase (Coarse Synchronization)The acquisition problem is one of searching

throughout a region of timeand frequency (chip, carrier) inorder to synchronize the received spread-spectrum signal withthe locally generated PN sequence. Since the despreadingprocess typically takes place before carrier synchronization,and therefore the carrier is unknown at this point, mostacquisition schemes utilize noncoherent detection.

A common feature of all acquisition methods is thatthe received signal and the locally generated PN sequence arefirst correlatedwith a coarse time step (mostly Tc/2) to producea measure of simularity between the two. This measure is thencompared to a threshold to decide if the two signals are insynchronism. If they are, a verification algorithm is started. Toprevent false locking, it is necessary to dwell for some time totest synchronism. Than the tracking loop takes over. Forproper synchronization, a peaked autocorrelation is requiredfrom the PN sequence.

Matched Filter (parallel)A matched filter calculates the correlation function at

each sample timestep Tsample. This gives the shortestacquisition time but the fully parallel implementation requires a

lot of hardware. The hardware increases with the PNcodelength and and oversampling factor s. Therefore it ismostly used for short codes.

Active Correlator (serial)An active correlator needs an integration over a total

period Nc.Tcof the PN sequence to calculate one point of thecorrelation function. Less hardware is needed, but a largeracquisition time is required. This can be reduced by usingparallelism as explained below.

Serial Synchronization (Sliding Correlator)The sliding correlator is based on the correlation

result of one active correlator. The correlator cycles throughthe time uncertainty, usually in discrete time intervals of Tc/2seconds or less. The correlation is performed over the periodof the PN sequence Ts = Nc.Tc. After each integration intervathe correlator output is compared with a threshold todetermine if the known PN sequence is present. If thethreshold is not exceeded, the known PN sequence of thereceiver (pnr) is advanced by Tc/2 seconds and the correlationprocess is repeated. These operations are performed until a

signal is detected or until the search has been performed overthe time uncertainty interval Tu. For a coarse time step of T c/2the worst case acquisition time is (Tu = NcTc):

This becomes unacceptable long for long codes (large Nc).

Serial/Parallel Synchronization

More active correlators are placed in parallel (3 in thisexample) with PN sequences spaced one half chip (Tc/2) apartAfter the integration period Nc.Tc the results of the correlatooutputs are compared. The correlation function is thuscalculated in 3 successive points (spaced one half chip apart)


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When no comparator output exceeds the threshold thesequences are advanced over 3Tc /2 seconds. When thethreshold is exceeded, the correlator output with the largestoutput is chosen. For a search with 3 parallel correlators overthe time uncertainty Tu interval in time steps of Tc/2 the worstcase acquisition time is(Tu = NcTc):

The search time is reduced at the expense of a morecomplex and costly implementation.

Tracking Phase (Fine Synchronization)

The tracking maintains the PN code generator at the receiverin synchronism with the received signal. This is needed toachieve maximum processing gain. For a PN sequence phaseerror of Tc/2 the processing gain is reduced with a factor 2.

10. Multiple Access

Code Division Multiple Access (CDMA) is a method omultiplexing (wireless) users by distinct (orthogonal) codes. Alusers can transmit at the same time, and each is allocated theentire available frequency spectrum for transmission. CDMA isalso known as spread-spectrum multiple access SSMA.

CDMA does not require the bandwidth allocation oFDMA, nor the time synchronization of the individual usersneeded in TDMA. A CDMA user has full time and fulbandwidth available, but the quality of the communicationdecreases with an increasing number of users (BER).

In CDMA each user: has its own PN code

uses the same RF bandwidth

transmits simultaneously (asynchronous osynchronous)

Correlation of the received baseband spread spectrum signarxb with the PN sequence of user 1 only despreads the signaof user 1. The other users produce noise Nu for user 1.


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Only that portion of the noise produced by the otherusers falling in the information bandwidth [-Rs, Rs] of thereceiver, will cause interference with the desired signal.

The set of PN codes must have the followingproperties:

autocorrelation for good synchronization

low crosscorrelation (orthogonal codes) for low MAI

Useful codes are:

Gold codes, Kasami codes (asynchronous CDMA)

Hadamard-Walsh codes (synchronous CDMA)

Multiple Access Interference (MAI)

The detector receives a signal composed of the sumof all users signals, which overlap in time and frequency.Multiple access interference (MAI) refers to the interferencebetween direct- sequence users and is a factor which limits thecapacity and performance of DS-CDMA systems.

In a conventional DS-CDMA system, a particularusers signal is detected by correlating the entire receivedsignal with that users code waveform. The conventionaldetector does not take into account the existence of MAI.Because of the interference among users, however, a betterdetection strategy is one of multi-user detection. Informationabout multiple users is used jointly to better detect eachindividual user.

Near-Far problem

Suppose:

Wireless channel

Multi-users (transmitters) using the same channel

One receiver

Each user is a source of interference for the otherusers, and if one is received with more power, than that usergenerates more interference for the other users. It is importantthat the receiver gets the same power from each transmitter.The use of power controlensures that all users arrive at aboutthe same power Prx at the receiver, and therefore no user isunfairly disadvantaged relative to the others. The signal-to-

noise interference power ratio at the receiver input for Nusimultaneous users is:

11. Multipath Channels

In wireless channels there exists often multiple pathpropagation: there is more than one path from the transmitterto the receiver. Such multipaths may be due to:

Atmospheric reflection or refraction

Reflections from ground, buildings or other objects

Multipaths may result in fluctuations in the receivedsignal level (fading). Each path has its own attenuation andtime delay. It is important to keep the direct path and reject theothers.

Assume that the receiver is synchronized to the timedelay and RF phase of the direct path. The signals at thereceiver can be from: the direct path, other paths, white noise

interference.Suppose two discrete paths: a direct path and only

one non-direct path (delayed by a time compared to thedirect path).


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The signal at the receiver can be expressed as:

For the receiver, synchronized to the direct path signal, theoutput of the correlator, can be written as:

The PN sequence has an autocorrelation function with theproperty:

Multipath signals that are delayed by a chip period orlonger relative to the desired signal (outdoor reflections) areessentially uncorrelated and do not contribute to multipathfading. The SS System effectively rejects (mitigation) the

multipath interference like in the case of CDMA.

with n0 = noise and multipath interference

The PN code that arrives from the non-directchannel(s) is not synchronized to the PN code of the directpath and is rejected.

12. JammingThe goal of a jammer is to disturb the communication of

his adversary. The goals of the communicator are to develop a jam-resistant communication system under the followingassumptions:

Complete invulnerability is not possible The jammer has a priori knowledge of most system

parameters, frequency bands, timing, traffic, ...

The jammer has no a priori knowledge of the PNspreading code

Protection against jamming waveforms is provided bypurposely making the information-beating signal occupy abandwidth far in excess of the minimum bandwidth necessaryto transmit it. This has the effect of making the transmittedsignal assume a noise-like appearance so as to blend intobackground.

The transmitted signal is thus enabled to propagatethough the channel undetected by anyone who may belistening. Spread spectrum is a method of camouflaging the

information-bearing signal.

13. ISM Bands

ISM (Industrial, Scientific, and Medical) frequency bands arereserved for (unlicensed) spread spectrum applications.

Properties for higher frequencies:- higher path loss, shorter distance

- higher implementation cost+ less interference+ more channels, higher throughput

Regulations for the 2.4 GHZ ISM band:

USA FCC ( Federal Communications Commission)

EuropeETS (European Telecommunication Standard):

FHSS = Frequency Hopping Spread Spectrum

20 non-overlapping channels (hopping positions)

dwell time/channel 400 ms

each channel occupied at least once during 4.(#channels).(dwell time/hop)

DSSS = Direct Sequence Spread SpectrumSpread spectrum modulation that does not satisfy the

constraints of the FHSS specification.

14. Evaluation SS

Positive1. Signal hiding (lower power density, noise-like), non-

interference with conventional systems and other SSsystems

2. Secure communication (privacy)3. Code Division Multiple Access CDMA (multi-user)4. Mitigation (rejection) of multipath, hold only the direc

path5. Protection to intentional interference (Jamming)6. Rejection of unintentional interference (narrowband)7. Low probability of detection and interception (LPI)8. Availability of licence-free ISM (Industrial, Scientific

and Medical) frequency-bands

Negative9. No improve in performance in the presence of

Gaussian noise10. Increased bandwidth (frequency usage, wideband

receiver)11. Increased complexity andcomputational load

spread spectrum 2

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