Spread Spectrum

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Spread Spectrum
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Objective Concept Benefits Coding sequence
Different techniques of SS Application Jammers
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• Spread Spectrum
• Initially developed for military applications
  during II world war,
• less sensitive to intentional interference or
  jamming by third parties.
• Spread spectrum technology has blossomed into one
  of the fundamental building blocks in current and
  next-generation wireless systems
• DEFINITION
• A transmission technique wherein PN code
  independent of information bits is used as
  modulation waveform to spread the signal energy
  over bandwidth much greater than signal bandwidth
  .
• At the receiver despread is done using
  synchronised version of same P N code.

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• Problem of radio transmission
• Narrow band can be wiped out due to interference
  
• To disrupt the communication, the adversary needs
  to do two things,
• to detect that a transmission is taking place and
  
• (b) to transmit a jamming signal which is
  designed to confuse the receiver.
• .

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Solution A spread spectrum system is therefore
designed to make these tasks as difficult as
possible. Firstly, the transmitted signal
should be difficult to detect by an
adversary/jammer, i.e., the signal should have a
low probability of intercept (LPI). Secondly,
the signal should be difficult to disturb with a
jamming signal, i.e., the transmitted signal
should possess an anti-jamming (AJ)
property spread the narrow band signal into a
broad band to protect against interference
.
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Principle of Spread Spectrum

• Bandwidth occupancy of signal is excess of
  minimum bandwidth needed to send the information.
  
• Because of much larger bandwidth the power
  spectral density is lower, in the channel the
  signal just looks like noise.
• The Spreading is done by combining the data
  signal with a code which is independent of the
  transmitted data message.
• At the receiver, despreading is accomplished by
  the correlation of the received spread signal
  with a synchronised replica of the spreading
  signal used to spread the information.

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BENEFITS Low power spectral density. As the
signal is spread over a large frequency-band, the
Power Spectral Density required is very small,
measured in watts per hertz, than narrow band
transmitters. Spread-spectrum and narrowband
signals can occupy the same band, with little or
no interference. This capability is the main
reason for all the interest in spread spectrum
today. Multiple Access number of independent
users possible
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Interference limited operation. In all
situations the whole frequency-spectrum is
used. Privacy due to unknown random codes. The
applied codes are - in principle - unknown to a
hostile user. This means that it is hardly
possible to detect the message of an other user.
Reduction of multi-path effects. Random
access possibilities. Users can start their
transmission at any arbitrary time. Good
anti-jam performance.
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Benefits towards interference or jammers

• Main feature of SS is Anti-Jam characteristics.
• Many orthogonal signal coordinates or dimensions
  are
• available .
• -At any point of time only small subset is used.
• -It is difficult to jammer to track the subset in
  use.
• Jammer has two choice
• To Jam All signal coordinates with equal power
• little power in all coordinates
• To Jam a few signal coordinates with increased
  power .
• poor choice of coordinates

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G (f)
G (f)
Before spreading
Gss(f)
Before spreading
Gss(f)
Jammer choice 1
Gss(f)
after spreading
Jammer choice 2
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Direct sequence and frequency hopping commonly
used methods for the spread spectrum technology

• Direct sequence spread spectrum
• Narrowband information is spread out into a
  much larger bandwidth by using a pseudo-random
  chip sequence.
• 2. Frequency Hop Spread spectrum
• The data modulated carrier is made to hop in
  pseudo random manner rapidly from one frequency
  to the next .
• it is nearly impossible for someone to jam
  the signal

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• CodingTechniquesPseudo Noise Sequence
• Are class of sequences of 1s and 0s which are
  periodic and possess autocorrelation property.
• Are of much greater length since it is intended
  for bandwidth spreading.
• Protection against interference Coding enables
  a bandwidth trade for processing gain against
  interfering signals.
• Provision for privacy Coding enables protection
  of signals
• Noise-effect reduction codes can reduce the
  effects of noise and interference.

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• Maximal length codes are commonly used PN codes
• generated by a given shift register of given
  length in conjunction with the appropriate logic,
  which feeds back a logical combination of the
  state of two or more of its stages to its input.
• In binary shift register, the maximum length
  sequence is
• N 2m-1
• chips, where m is the number of stages of
  flip-flops in the shift register. N is the period
  .

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output
X1
X2
X3
X4

Modulo -2 adder

• Linear Feedback Shift Register with modulo two
  adder
• At each clock pulse
• contents of register shifts one bit right.
• Contents of required stages are modulo 2 added
  and fed back
• to input.

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0
0
0
1

output
X1
X2
X3
X4

Modulo -2 adder
Let initial status of shift register be 1000
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1 0 0 0 0 1 0 0 0 0 1 0 1 0 0 1 1 1 0 0 0 1 1 0 1
0 1 1 0 1 0 1 1 0 1 0 1 1 0 1 1 1 1 0 1 1 1 1 0 1
1 1 0 0 1 1 0 0 0 1 1 0 0 0

• We can see for shift Register of length m .At
  each clock the change in state of flip-flop is
  shown.
• Feed back function is modulo two of X3 and X4.
• After 15 clock pulses the sequence repeats.
• Output sequence is
• 0 0 0 1 0 0 1 1 0 1 0 1 1 1 1

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• Properties of PN Sequence
• Randomness of PN sequence is tested by following
  properties
• Balance property
• Run length property
• Autocorrelation property

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• Balance property
• In each Period of the sequence , number of
• binary ones differ from binary zeros
• by at most one digit .
• Consider output of shift register
• Seven zeros and eight ones -meets balance
  condition.

0 0 0 1 0 0 1 1 0 1 0 1 1 1 1
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• Run length property
• Among the runs of ones and zeros in each period,
• it is desirable that about one half the runs of
  each type are of length 1, one- fourth are of
  length
• 2 and one-eighth are of length 3 and so-on.
• Consider output of shift register
• Number of runs 8

0 0 0 1 0 0 1 1 0 1 0 1 1 1 1
3 1 2 2 1 1
1 4
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• Auto correlation property
• Auto correlation function of a maximal length
  sequence is periodic and binary valued.
• Autocorrelation sequence of binary sequence in
  polar format is given by

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Where N is length or period of the sequence and
k is the lag of the autocorrelation Where
l is any integer.
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Or we can state Autocorrelation function as
No. of agreements No.
of disagreements in
comparison of one full period

Consider output of shift register for l1

• 0 0 0 1 0 0 1 1 0 1 0 1 1 1 1
• 0 0 0 1 0 0 1 1 0 1 0 1 1 1
• d a a d d a d a d a d d a a a

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Yields PN autocorrelation as
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Rc(?)
1.0
-Tc
Tc
NTc
PN autocorrelation function.
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Range of PN Sequence Lengths
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• A notion of spread spectrum
• - SS provides protection against external jamming
  .
• Made Possible by spreading BW.
• Transmitted signal propagate like background
  noise
• through the channel.
• - Mathematically this attribute can be explained
• b(t) is data sequence is used to modulate a
  wide band
• PN sequence c(t)

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b(t)
Tb
c(t)
Tc
NTc
m(t)
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• m(t) c(t) . b(t)
• Multiplication of two unrelated signal in time
  domain yields convolution of spectrum in
  frequency domain.
• b(t) a narrow band signal multiplies c(t) a wide
  band PN code to give m(t) whose spectrum is
  nearly equal to that of PN code.
• When b(t) multiplies c(t ) each information bit
  is chopped into small intervals called chips.

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m(t) c(t) . b(t) Received signal r(t) m(t)
i(t) c(t) . b(t) i(t) r(t) is
applied to demodulator to recover b(t) Receiver
is synchronised with transmitter and replica of
PN code is available. Demodulated signal z(t)
c(t) . r(t)
c(t) c(t) . b(t) i(t)
c2(t)b(t) c(t)i(t)
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• c2(t) 1 for all t
• z(t) b(t) c(t) . i(t)
• Data sequence can be recovered if effect if
  interference is cancelled.
• Interference signal i(t) is multiplied by c(t)
  which is now wideband.
• Using Low-pass filter b(t ) can be recovered

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Transmitted signal
Received signal after demodulation
signal
interference
spread signal
interference
spread signal
power
spread interference
Narrow band interference
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1
0
0

output
X1
X2
X3
0 0 1 1 1 0 1
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• Balance property
• . 0 0 1 1 1 0 1
• Run length property

0 0 1 1 1 0 1 Out of 4 runs 2 are of
length 1 ,1 is of length 2 ..

• Auto correlation property

No. of agreements No. of disagreements in
comparison of one full period

• 0 0 1 1 1 0 1
• 0 0 1 1 1 0 I/7(3-4) -1/7
• d a d a a d d