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Introduction to

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Contents:

DEFINITION OF SPREAD SPECTRUM ( SS ) CHARACTERISTICS OF SPREAD SPECTRUM

BASIC PRINCIPLES OF DIRECT SEQUENCE

SPREAD SPECTRUM ( DSSS ) BASIC PRINCIPLES OF FREQUENCY HOPPING

SPREAD SPECTRUM ( FHSS )

PERFORMANCE IN THE PRESENCE OFINTERFERENCE

PSEUDO-NOISE SEQUENCES ( PN )

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Definition of Spread Spectrum :

Spread spectrum is a modulation method applied todigitally modulated signals that increases the transmit

signal bandwidth to a value much larger than is needed

to transmit the underlying information bits.

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Spread Spectrum Signal Characteristics :

1. They are difficult to intercept for unauthorizedperson.

2. They are easily hidden, it is difficult to even

detect their presence in many cases.3. They are resistant to jamming.

4. They have an asynchronous multiple-access

capability.5. They provide a measure of immunity to

distortion due to multipath propagation.

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Spread Spectrum Conditions :

The signal occupies a bandwidth much largerthan is needed for the information signal.

The spread spectrum modulation is done

using a spreading code, which is

independent of the data in the signal.

Dispreading at the receiver is done bycorrelating the received signal with a

synchronized copy of the spreading code.

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Processing Gain :

The spread spectrum increases the bandwidth

of the message signal by a factorN, called

the processing gain where b is the messagesignal bandwidth, bss is the corresponding

SS signal bandwidth.

, N> 1

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Spread Spectrum Techniques :

There are several forms of spreadSpectrums :

1. Direct sequence spread spectrum (DS/SS)

2. Frequency hopping spread spectrum

(FH/SS)

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Direct Sequence Spread Spectrum

Building block of DSSS system.

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The channel output given by:

y(t) = x(t) + j(t)

= c(t) s(t)+ j(t)

The Coherent detector input u(t) : u(t) =c(t) y(t)=s(t)+ c(t) j(t)

= 1Where : forall t

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Spreading

Input:

Binary data dtwith symbol rate Rs = 1/Ts( = Bit rate Rbfor BPSK ).

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

Spreading :

The binary data is multiplied with the PN sequence which

is independent on the binary data to produce thetransmitted signal txb.

txb = dt . pnt

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The effect of multiplication is to spread the base bandbandwidth Rs ofdt to a base band bandwidth ofRc

Bwinfo = Rs

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Modulation

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Dispreading

The spread spectrum signal cannot be detected by a narrowband receiver. In the receiver, the received base band

signal is multiplied with the PN code Pnr .

If Pnt = Pnr and synchronized to the PN code in thereceived data, then the recovered binary data is producedon dr. the effect of multiplication of the spread spectrumsignal rxb with the PN sequencepnt used in the transmitterto dispread the bandwidth ofrxb to Rs.

If then there is no dispreading action.

A receiver not knowing the PN code of transmitter cannotproduce the transmitted data.

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Demodulation

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At :

Pnt = PnrAutocorrelationRa (t=0)= average ( Pnt . Pnt)

= +1

At :

Cross correlationRc(t= 0) = average ( Pnt . Pnt)

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The operating principle of DS-SS multiple access. Two

users are sending two separate messages m1(t) and m2(t)through the same channel in the same frequency band at

the same time.

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Pseudo-Noise Sequence

A pseudo-noise ( PN ) sequence is a periodic binary

sequence with a noise like waveform that is usuallygenerated by a means of a feed back shift register. It

consists of a shift registermade up of m flip-flops and a

logic circuitto form a multiloopfeedbackcircuit.

Feedback shift register.

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Properties of the PN sequences :

An m-bit codeword produces a sequence of length

The peak values are

The autocorrelation function is equal to1 other than atthe peaks.

The O/P sequence contains ones &

Zeros.

Their power density spectrum is uniform so they may

used as white noise sources.

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The autocorrelation function of a bipolar

waveform three-stage pseudo noise generator

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We use a correlation receiver to determine whether

a +1 or a

1 was transmitted at time t

DecorrelatorsPN

PN Matched Filters

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A typical matched filter implements convolution using FIR

filter whose coefficients are the time inverse of the

expected PN sequence to decode the transmitted data.

If the receiver is not synchronized, then the received signal

will propagate through the matched filter, which outputs

the complete correlation function. The large peak confirmsthat the correct code is being received providing accurate

synchronization. The output of the FIR filter is the

decoded data.

The polarity of the large correlation peaks indicates the

data value.

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

interference.

2. Secure communications (Privacy).

3. Code division multiple access CDMA.

4. Mitigation of multi path effect.

5. Protection to international interference (jamming)

6. Rejection of unintentional interference (narrow

band)

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Negative

1. No improve in performance in the presence of

Gaussian noise.

2. Increase bandwidth (frequency usage, wideband

receiver).

3. Increase complexity and computational load.

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