Spread Spectrum

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Spread Spectrum
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• Commercial Applications
• An example of commercial spread spectrum systems
  are systems that are designed to be used in
  so-called unlicensensed bands, such as the
  Industry, Scientific, Medical (ISM) band around
  2.4 GHz.
• Typical applications are here cordless
  telephones, wireless LANs, and cable replacement
  systems as Bluetooth.
• Since the band is unlicensed, there is no central
  control over the radio resources,
• interference is from other communication systems
  and other electrical and electronic equipment
  (e.g., microwave ovens, radars, etc.). Here the
  jamming is not intentional, but the interference

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Applications Code-division multiple access
systems (CDMA systems) use spread spectrum
techniques to provide communication to several
concurrent users. CDMA is used in one second
generation (IS-95) and several third generation
wireless cellular systems (e.g., cdma2000 and
WCDMA). One advantage of using jamming-resistant
signals in these applications is that the radio
resource management is significantly reduced.

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• Features
• This spread spectrum multiple access technique
  allow multiple signals (user) occupying the same
  RF bandwidth to be transmitted simultaneously
  without
• Interfering with one another.
• Each user is assigned a code of its own, which
  performs either direct sequence or Frequency hop
  spread spectrum modulation.
• Each code is orthogonal to all others.
• Operation is asynchronous. all users need not
  transmit simultaneously.

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Click to edit Master title style
Advantage over TDMA

• Click to edit Master text styles
• Second level
• Third level
• Fourth level
• Fifth level

• Flexibility
• CDMA does not require external synchronization
  network
• Performance degradation is gradual in case of
  increase in number of user that add to system.
• Jamming Resistance
• CDMA offers an external interference rejection
  capability like multipath rejection or resistance
  to jamming.
• Privacy
• All users can share full spectrum simultaneously
  because each user has different code .

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DS CDMA In mobile telephony DS/CDMA is used as
no two users Interfere with one other as
different long spreading codes (orthogonal to
each other) are employed . The interference is
properly designed and limited because adding
number of users maintains overall level of
interference. IS 95 CDMA employs DS SS technique.
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N users are assigned its own codes gi(t),
i1,2..N All users can share full spectrum of
resource asynchronously. Ex. Illustrating data
modulation for user 1. At channel it is linear
combination as g1(t)s1(t) g2(t) s2(t)
.gN(t)sN(t)
g2(t)s2(t)
g12(t)s1(t) g1(t)g2(t)s2(t) -
- g1(t)gN(t)sN(t)
carrier
gN(t)sN(t)
Demod
g1(t)s1(t)
S1(t)
?
Modulator
g1(t)
g1(t)
information
Tx
Rx
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FH/CDMA At each slot frequency assignment to
users are reordered. Band assignment is employs
PN code.
FH/MFSK systems employed in time synchronization.
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Multipath suppression In radio communication, say
wireless or mobile, signal can travel from
transmitter to receiver over multiple reflective
paths. i.e., one direct path and many indirect
paths or reflective paths. Reflective paths are
due to variety of scatters like buildings, moving
vehicles and other obstacles. Contributions from
these indirect paths exhibit different levels of
attenuation and delays relative to direct path
. Interference due to these contributions are
either constructive or destructive and this
interference is called Multipath Propagation
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• The fluctuations that are caused in signal
  strength is
• called as Fading.
• Amount of fading depends of whether mobile user
  is
• moving and scatters are stationary or
  vice-versa.
• To overcome this effect , spread spectrum
  modulations
• are employed.
• Direct sequence SS can be used to mitigate
  frequency
• selective ISI distortion because it is very
  effective
• in interference rejection.
• At receiver matched filters or correlators
  are used.
• The reflected signals are treated as
  uncorrelated signals
• as they have different delays.

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Frequency hop SS can also be used to improve
system performance due to multipath. It is used
to mitigate fading provided hopping rate of
transmitted signal is much faster than
reflective ones. With this is all multipath
energy will fall in frequency Slots that are
orthogonal to desired signal, suppressing
multipath signals.
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Jammers The goals of jammers are to deny
reliable communications with minimum cost. As
constraint in digital communication is managing
scarce resources Like power and bandwidth,
jammer has to create his adversary with minimum
cost, with one choice. To jam complete spectrum
with equal power or part of spectrum with large
power. Goals of communicators is to see hat jam
resistant systems are designed.
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Jammers encountered in practice are Barrage
noise jammers here jammers occupy full spread
spectrum bandwidth but it is relatively low level
noise jammers like Gaussian noise.
Example frequency hopped system V/s jammer
Wss
Wss
Wss
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Partial band noise jammers Here jammer strategy
is to trade bandwidth occupancy for greater PSD .
The total power is evenly spread only over some
frequency band which is subset of spread
bandwidth.
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Stepped tone noise jammers Strategy is to jam
only part of signal band , but jammer steps
through different region of band at random times.
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Partial band tone jammers Jammer uses group of
tones instead of continuous band of frequencies
in partial band
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Stepped tone jammers
Since jammer concentrate their resources on part
of signal band which results in error in
detection, to overcome this error correcting
codes are used prior to spreading.
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• Example1
• A pseudo random sequence is generated using a
  feed back shift register of length m4. The chip
  rate is 107 chips per second. Find the following
• PN sequence length
• Chip duration of PN sequence
• PN sequence period
• Solution
• a) Length of PN sequence N 2m-1
• 24-1 15
• b) Chip duration Tc 1/chip rate 1/107
  0.1µsec
• c) PN sequence period T NTc
• 15 x 0.1µsec
  1.5µsec

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Example2 A direct sequence spread binary phase
shift keying system uses a feedback shift
register of length 19 for the generation of PN
sequence. Calculate the processing gain of the
system. Solution Given length of shift register
m 19
Therefore length of PN sequence N
2m-1 219-1 Processing gain PG
Tb/Tc N in db 10log10N 10log10 219
57db

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Example3 A Spread spectrum communication system
has the following parameters. Information bit
duration Tb 1.024 msecs and PN chip duration of
1µsecs. The average probability of error of
system is not to exceed 10-5. calculate a) Length
of shift register b) Processing gain c) jamming
margin Solution Processing gain PG N Tb/Tc
1024 corresponding length of shift register m
10 For coherent BPSK Probability of error
Referring to error function table Eb/N0 10.8
yields average Pe approximately equal to 10-5

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Therefore jamming margin
30.10 10.33
19.8 db
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Example4 In DS/BPSK system the feedback shift
register Used to generate the PN sequence has
length m19 . The system is required to have a
probability of error due to externally generated
interfering signals that does not exceed 10-5.
calculate PG and antijam margin Solution Given
length of shiftregister m 19 PN sequence
length N 2m-1 524288 For coherent BPSK
Probability of error Referring to error function
table Eb/N0 10.8 yields average Pe
approximately equal to 10-5
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Therefore jamming margin
57.8db-10.8db 47db
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Example5 A slow FH / MFSK system has the
following parameters The number of bits per MFSK
symbol 4 The number 0f MFSK symbols per hop
5 Calculate processing gain Solution In slow FH
system symbol rate Rs is integer multiple of hop
rate Rc. Processing gain Whop / Rs PG in db
10 log 10W hop/Rs 5 / (1/4) 10
log 1020 13db

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Example5 A fast FH / MFSK system has the
following parameters The number of bits per MFSK
symbol 4 The number 0f MFSK symbols per hop
4 Calculate processing gain Solution In fast FH
system hop rate Rh is integer multiple of symbol
rate Rs. PG in db 10 log 10W hop/Rs
4 / (1/4) 10 log 1016 12db
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Spread Spectrum Conclusion
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References 1. Digital Communications by Simon
Haykins 2. Digital communications by Bernard
Sklar