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UNIT 4 Spread Spectrum

This document discusses spread spectrum signals for digital communication. Spread spectrum modulation spreads the signal across a wide frequency band by modulating it with a pseudorandom code. This provides advantages like increased security, preventing jamming, and allowing multiple users to share the same channel. There are three main types of spread spectrum: direct sequence, time hopping, and frequency hopping. Frequency hopping spread spectrum transmits data by modulating a carrier frequency that is then randomly changed or "hopped" between different frequencies. Slow frequency hopping transmits multiple symbols per hop while fast frequency hopping transmits portions of a single symbol across multiple hops.

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141 views26 pages

UNIT 4 Spread Spectrum

This document discusses spread spectrum signals for digital communication. Spread spectrum modulation spreads the signal across a wide frequency band by modulating it with a pseudorandom code. This provides advantages like increased security, preventing jamming, and allowing multiple users to share the same channel. There are three main types of spread spectrum: direct sequence, time hopping, and frequency hopping. Frequency hopping spread spectrum transmits data by modulating a carrier frequency that is then randomly changed or "hopped" between different frequencies. Slow frequency hopping transmits multiple symbols per hop while fast frequency hopping transmits portions of a single symbol across multiple hops.

Uploaded by

Sana Bajpai
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PPTX, PDF, TXT or read online on Scribd
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UNIT 7

SPREAD SPECTRUM SIGNALS


FOR DIGITAL COMMUNICATION:

What is Spread Spectrum


signal?
In communication till now we had
discussed how to utilize the channel BW
efficiently & how to minimize the amount
of transmitted power?
But beside this some other problems are
there they are as follows:
1. Security
2. Jam the transmission by others
3. Same channel is used by others

Remedy:
SPREAD SPECTRUM MODULATION
It uses some kind of coding ,spreading at
transmitter & despreading at the receiver
obtained with the code word
Code word is independent of the
information carried by the signal
SS signal is pseudorandom in nature
Specially designed receiver is required

Advantages &
Disadvantages:

Classification:

Model of Spread Spectrum Digital


Communication:

Informatio
Channe
n seq
l
Encode
r

Spread
spectrum
signal
Modulat
or

Pseudorando
m pattern
generator

Transmit
ter

Channe
l

Demod
ulator

Channe
Outpu
l
Decode
r

Pseudorando
m pattern
generator

Receiv
er

Direct Sequence:

Time Hopped spread


spectrum:

Frequency Hopping spread spectrum:

Idealized model of baseband spread


spectrum system:

Waveforms in the transmitter of DSSS

Power Spectral Density:

PSD after Despreading:

DSSS with Coherent BPSK:

Frequency Hop Spread Spectrum


(FH-SS) System:

Data is used to modulate a carrier


The data modulated carrier is then
randomly hopped from one frequency to
other
Due to this spectrum of transmitted
signal is spread sequentially rather than
instantaneously
M-ary fsk is known as FH/MFSK
Based on rate of hopping FH/MFSK
system has been classified into two
categories

Slow frequency hopping:


Each frequency hop=several symbols
2. Fast frequency hopping:
Each symbol Transmission=Several
frequency hop
. Frequency shifted every T seconds
c
1.

Duration of signal element is Ts seconds

Slow FHSS has Tc Ts

Fast FHSS has Tc < Ts

Generally fast FHSS gives improved performance


in noise (or jamming)

Basic Operation:

Typically 2k carriers frequencies forming


2k channels
Channel spacing corresponds with
bandwidth of input

FHSS Example:

FHSS Transmitter:

FHSS Receiver:

Slow Frequency Hop Spread Spectrum


Using MFSK (M=4, k=2):

Fast Frequency Hop Spread Spectrum


Using MFSK (M=4, k=2):

Chip Rate(Rc):

Individual tone of shortest duration is


defined as a chip in FHSS
Rc=max(Rh ,Rs)
where Rh-hop rate
Rs-symbol rate

Rc is higher than Rh or Rs for slow


frequency hopping
Rc = Rs = (Rb/K)>= Rh
Where K=log2(M) or M=2^K

Process Gain:
PG=BW(spread spectrum)/BW(unspread
signal)
BW(spread spectrum)=2^n*fs
BW(unspread signal)=fs
PG=2^n
n is no of bits at the output of PN code
generator

Difference:
Slow Frequency
Hopping

Fast Frequency
Hopping

1) More than one symbols are


transmitted per frequency hop

1) More than one frequency hops


are required to transmit one
symbol

2) Chip rate is equal to symbol


rate

2) Chip rate is equal to hop rate

3) Symbol rate is higher than hop 3) Hop rate is higher than symbol
rate
rate
4) Same carrier frequency is used 4) One symbol is transmitted
to transmit one or more symbols over multiple carriers in different
hops
5) A jammer can detect this
signal if the carrier frequency in
one hop is known

5) A jammer cant detect this


signal because one symbol is
transmitted using more than one

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