Spread spectrum is a modulation technique where a signal occupies a bandwidth much larger than the minimum required to send information. It spreads the signal by multiplying it with a spreading code that is independent of the data. At the receiver, despreading occurs by correlating the received signal with a synchronized copy of the spreading code. Spread spectrum provides benefits like resistance to interference and jamming, ability for multiple users to share bandwidth, and covert communication capabilities.

1. Spread Spectrum
Module IV
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2. Principles
 Spread spectrum is a modulation method applied
to digitally modulated signals.
 The signal occupies a bandwidth much larger than
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.
 Despreading at the receiver is done by correlating
the received signal with a synchronized copy of
the spreading code
 Set of linearly independent signals si(t), i = 1, . .
.,M of bandwidth B and time duration T can be
written
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3.  The interference signal I(t)
 Total energy over [0, T] given by
 The signal si(t) is transmitted , the
received signal is the sum of the
transmitted signal plus interference
,ie x(t) = si(t) + I(t).
 The correlator output is the signal-to-
interference (SIR) power ratio of this
signal is ,where G=N/M
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4. Basics
 Increasing signal bandwidth beyond the
minimum necessary for data communication
 Spread spectrum techniques can hide a signal below
the noise floor, making it difficult to detect
 Spread spectrum can provide coherent combining of
different multipath components
 Spread spectrum also allows multiple users to share
the same signal bandwidth
 Resistance to narrowband jamming and interference ,
and its low probability of detection and interception
 The ISI rejection and bandwidth sharing capabilities
of spread spectrum are very desirable in cellular
systems and wireless LANs.
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5. 5
The effect of multiplication is to spread the
base band bandwidth Rs of dt to a base band
bandwidth of Rc
Bwinfo = Rs << BWss = Rc
Processing gain Gp=BWss/BWinfo = Rc/Rs =
Tb/Tc =Nc
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6. What is Spread Spectrum?
 Spread spectrum techniques are
methods by which electromagnetic
energy generated in a particular
bandwidth is deliberately spread in the
frequency domain, resulting in a signal
with a wider bandwidth
 Spread spectrum methods:
 Frequency hopping spread spectrum
 Direct sequence spread spectrum
 Time hopping spread spectrum
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7. General Model of Spread Spectrum
System
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8. 8
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)
= 1
Where : for all t
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9. Forms of spread spectrum
 Frequency-hopping spread
spectrum(FHSS)
 Direct-sequence spread spectrum
(DSSS)
 Time-Hopping Spread Spectrum
(THSS)
 Chirp Spread Spectrum (CSS)
and combinations of these techniques
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10. Gains
 Immunity from various noise and
multipath distortion
 Including jamming
 Can hide/encrypt signals
 Only receiver who knows spreading code
can retrieve signal
 Several users can share same higher
bandwidth with little interference
 Cellular telephones
 Code division multiplexing (CDM)
 Code division multiple access (CDMA)
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11. Types
 Direct Sequence Spread
Spectrum(DSSS)- the modulated data
signal s(t) is multiplied by
a wideband spreading signal or code
sc(t), where sc(t) is constant over a
time duration Tc and has amplitude
equal to 1 or -1, The spreading code
bits are usually referred to as chips,
and 1/Tc is is called the chip rate.
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12.  Frequency Hoping Spread Spectrum(FHSS)-The basic
premise of frequency hopping spread spectrum
(FHSS) is to hop the modulated data signal over a
wide bandwidth by changing its carrier frequency
according to a spreading code sc(t).
 The time duration over which the modulated
data signal is centered at a given carrier frequency fi
before hopping to a new carrier frequency
1.Tc = kTs for some integer k, which is called slow
frequency hopping (SFH)
2. Tc = Ts/k for some integer k, which is called fast
frequency hopping(FFH)
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13. Spread Spectrum
 What can be gained from apparent waste of
spectrum?
 Immunity from various kinds of noise and
multipath distortion
 Can be used for hiding and encrypting signals
 Multiple access capability
 Several users can independently use the same wider
bandwidth with very little interference
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14. Frequency Hopping Spread Spectrum
(FHSS)
 Signal is broadcast over seemingly random series
of radio frequencies
 A number of channels allocated for the FH signal
 Width of each channel corresponds to bandwidth of
input signal
 Signal hops from frequency to frequency at fixed
intervals
 Transmitter operates in one channel at a time
 Bits are transmitted using some encoding scheme
 At each successive interval, a new carrier frequency is
selected
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15. Frequency Hoping Spread Spectrum
 The frequency sequence is dictated by the
spreading code
 Receiver should hop synchronously with the
transmitter to be able to recover the message
 Advantages
 Eavesdroppers hear only unintelligible blips
 Attempts to jam signal on one frequency succeed only at
knocking out a few bits
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16. Frequency Hoping Spread Spectrum
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17. w
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18. Symbol Error for FHSS
 Ps is defined as symbol error
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19. FHSS
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20. Frequency Hopping Spread
Spectrum
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21. Frequency Hopping Spread
Spectrum
 Slow-frequency-hop spread spectrum
 The hopping duration is larger or equal
to the symbol duration of the modulated
signal
Tc >= Ts
 Fast-frequency-hop spread spectrum
 The hopping duration is smaller than the
symbol duration of the modulated signal
Tc < Ts
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22. Slow Frequency-Hop SS
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23. Fast Frequency-Hop SS
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24. FHSS Performance Considerations
 Large number of frequencies used
 Results in a system that is quite resistant to
jamming
 Jammer must jam all frequencies
 With fixed power, this reduces the jamming power
in any one frequency band
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25. Multiuser FHSS
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26. Direct Sequence Spread Spectrum
(DSSS)
 The modulated signal is spread by a spreading
waveform (spreading code)
 The spreading code spreads the signal over a wider
frequency band
 Spread is in direct proportion to number of bits per
symbol used
 The spreading code is usually periodic with a
period larger or equal to the symbol duration of the
modulated signal
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27. DSSS Using BPSK
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29. w
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30. Direct Sequence Spread Spectrum
(DSSS)
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31. Spectrum of DS Spread
Spectrum Signal
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32. Code-Division Multiple Access
(CDMA)
 CDMA is multiple access scheme that allows
many users to share the same bandwidth
 3G (WCDMA), IS-95
 Basic Principles of CDMA
 Each user is assigned a unique spreading code
 The processing gain protects the useful signal and
reduces interference between the different users
PG = (Bandwidth after spreading)/(Bandwidth before spreading)
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33. CDMA for Direct Sequence Spread
Spectrum
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34. CDMA Example
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36. w
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37. Spreading Sequences
 Spreading sequences are very important in the
design of spread spectrum communication
 Two categories of Spreading Sequences
 PN sequences
 Orthogonal codes
 FHSS systems
 PN sequences most common
 DSSS CDMA systems
 PN sequences
 Orthogonal codes
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38. PN Sequences
 PN sequences are periodic but appear random within
one period
 PN sequences are very easy to generate
 Generated using LFSR
 PN sequences are easy to re-generate and
synchronize at the receiver
 PN sequences have good random properties
 PN sequences converge to a Gaussian process when
the period tends to infinity
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39. Spreading in Cellular CDMA Systems
 Cellular CDMA systems use two layers of
spreading
 Channelization codes (orthogonal codes)
 Provides orthogonality among users within the same
cell
 Long PN sequences (scrambling code)
 Provides good randomness properties (low cross
correlation)
 Reduces interference from other cells
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40. DSSS
X(t)=∑sl g(t-lTs), where g(t) is the shaping pulse and Ts is
the symbol time and Sl is the symbol transmitted over the
lth symbol time.
Basics of DSSS is
The channel introduces several multipath components:
h(t) = α0δ(t − τ0) + α1δ(t − τ1) + . .
The spreading code generator then outputs the spreading
code sc(t − τ ), where τ = τi if the synchronizer is perfectly
aligned with the delay associated with the ith multipath
component.
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41. DSSS System Model
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42. Synchronisation
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44. RAKE Recievers
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45. Multiuser DSSS System
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46. Spreading Codes
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47. A Comparison of Frequency
Hopping and Direct Sequence
Spread Spectrum
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48. Multiuser Detection
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49. Multicarrier CDMA
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50. Why we need to compare the
performance of this technology?
(WLAN’s)
 possibility to collocate systems
 noise and interference immunity
 operation in environments generating
radio reflections
 data transfer capacity (throughput)
 size
 power consumption (relevant for
battery based note books)
 price.
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51. Direct Sequence Spread
Spectrum
• Modulation technique ,Also known as
direct sequence code division multiple
access (DS-CDMA)
• The name 'spread spectrum' comes from
the fact that the carrier signals occur over
the full bandwidth (spectrum) of a
device's transmitting frequency.
• A RF carrier and pseudo-random pulse
train are mixed to make a noise like
wide-band signal.
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52. • Direct sequence contrasts with the other
spread spectrum process, known as
frequency hopping spread spectrum, in
which a broad slice of the bandwidth
spectrum is divided into many possible
broadcast frequencies.
• In general, frequency-hopping devices
use less power and are cheaper, but the
performance of DS-CDMA systems is
usually better and more reliable.
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53. DS Modulation
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54. Frequency Hopping spread
spectrum
• It is the repeated switching of frequencies
during radio transmission, often to
minimize the effectiveness of "electronic
warfare" - that is, the unauthorized
interception or jamming of
telecommunications.
• It also is known as frequency- hopping
code division multiple access (FH-CDMA).
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55. • Transmitting on one frequency for a
certain time, then randomly jumping to
another, and transmitting again.
• FH-CDMA devices use less power and are
generally cheaper, but the performance
of DS-CDMA systems is usually better
and more reliable.
• The biggest advantage of frequency
hopping lies in the coexistence of several
access points in the same area,
something not possible with direct
sequence.
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56. FH Modulation
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57. • Frequency hopping has two benefits.
Electrical noise—random electromagnetic
signals which are not part of any
communications signal—will only affect a
small part of the signal. Also, the effects
of any other forms of radio
communications operating in narrow
bands of the spectrum will be minimized.
Any such interference that occurs will
result in only a slightly reduced quality of
voice transmission, or a small loss of
data. Since data networks acknowledge
successful receipt of data, any missing
pieces will trigger a request to transmit
the lost data.
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58. Hybrid DS/FH Spread Spectrum
Communication System
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59. Hybrid DS/FH Spread Spectrum
• The DS/FH Spread Spectrum technique
is a combination of direct-sequence and
frequency-hopping. One data bit is
divided over frequency-hop channels
(carrier frequencies). In each frequency-
hop channel one complete PN-code of
length is multiplied with the data signal
(see figure, where is taken to be 5).
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60. • As the FH-sequence and the PN-codes are
coupled, an address is a combination of
an FH-sequence and PN-codes. To bound
the hit-chance (the chance that two users
share the same frequency channel in the
same time) the frequency-hop sequences
are chosen in such a way that two
transmitters with different FH-sequences
share at most two frequencies at the
same time (time-shift is random).
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61. Hybrid System: DS/(F)FH
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62. Additional
CDMA use technique Spread
Spectrum
How spread spectrum
generated
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63. Additional
occupies a larger bandwidth
than necessary
use of a code which is
independent of the transmitted
data
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64. Additional
Direct Sequence
Data signal multiplied by Pseudo
Random Noise Code(PN Code)
 Low cross-correlation value
 Anti-jamming
 Main problem: Near-Far effect
 In cellular, it can do power control
by BS
 In non-cellular, it need Frequency
Hopping
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65. Additional
Frequency Hopping
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66. Additional
Disadvantage:
 a high processing-gain is
hard. There is need for a
frequency-synthesizer able
perform fast-hopping over
the carrier-frequencies.
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