IE 419/519
Wireless Networks
Lecture Notes #6
Spread Spectrum
Introduction
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In 1985, the FCC modified Part 15 of the radio
spectrum regulation
 Governs unlicensed devices
 Attempt to stimulate the production and use of
wireless network products
The modification authorized wireless network
products to operate in the Industrial, Scientific, and
Medical (ISM) bands using spread spectrum
modulation
 902 - 928 MHz
 2.4 - 2.4835 GHz
 5.725 - 5.850 GHz
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Introduction
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FCC allows users to operate wireless products
without obtaining licenses if the products meet certain
requirements
 e.g., Operation under 1 watt transmitter output
power
This deregulation of the frequency spectrum
eliminates
 Need to perform costly and time-consuming
frequency planning to avoid interference with
existing radio systems
 Need to license product again at a new location (if
equipment is moved)
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Spread Spectrum Encoding
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Digital data
• Digital Signal
• Analog Signal
• Digital Signal
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Analog data
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Which option to choose?
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• Analog Signal
Requirements to meet
Media & communications facilities
Spread Spectrum
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Can be used to transmit either analog or digital data, using
an analog signal
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Spread Spectrum
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Input is fed into a channel encoder
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Produces analog signal with narrow bandwidth
Signal is further modulated using sequence of
digits
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Spreading code or spreading sequence
Generated by pseudonoise, or pseudo-random
number generator
Effect of modulation is to increase bandwidth of
signal to be transmitted
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Spread Spectrum
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On receiving end, digit sequence is used to
demodulate the spread spectrum signal
Signal is fed into a channel decoder to recover
data
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Spread Spectrum
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What can be gained from apparent waste of
spectrum?
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Immunity from various kinds of noise and
multipath distortion
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Can be used for hiding and encrypting signals
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Anti-jamming performance
Interference immunity
Low probability of intercept
Low transmit power density
Several users can independently use the same
higher bandwidth with very little interference
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Multiple access communications
Multiple simultaneous transmissions
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Types of Spread Spectrum
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Frequency Hopping Spread Spectrum (FHSS)
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First type developed
Direct Sequence Spread Spectrum (DSSS)
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More recent technology
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Frequency Hopping SS
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Signal is broadcast over seemingly random
series of radio frequencies
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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
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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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Frequency Hopping SS
Source: http://murray.newcastle.edu.au/users/staff/eemf/ELEC351/SProjects/Morris/types.htm
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Frequency Hopping SS
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Hopping Sequence
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Channel sequence dictated by spreading code
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Chip Period
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Time spent on each channel
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Pseudorandom number serves as an index into a table of
frequencies
FCC regulation  maximum dwell time of 400 ms
IEEE 802.11 standard  300 ms
Chipping rate
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Hopping rate
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Frequency Hopping SS
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Receiver, hopping between frequencies in
synchronization with transmitter, picks up
message
Advantages
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Eavesdroppers hear only unintelligible blips
Attempts to jam signal on one frequency succeed
only at knocking out a few bits
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FHSS Performance Considerations
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Large number of frequencies used
Results in a system that is quite resistant to
jamming
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Jamming signal must jam all frequencies
With fixed power, this reduces the jamming power
in any one frequency band
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Direct Sequence SS
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Each bit in original signal is represented by
multiple bits in the transmitted signal
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Spreading code spreads signal across a wider
frequency band
Spread is in direct proportion to the number of
bits used
One technique combines digital information
stream with the spreading code bit stream
using exclusive-OR
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Direct Sequence SS
Source: http://www.sss-mag.com/primer.html
Source: http://murray.newcastle.edu.au/users/staff/eemf/ELEC351/SProjects/Morris/types.htm
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Direct Sequence SS
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Processing Gain
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Unique property of spread specturm
waveforms
Used to measure the performance
advantage of spread spectrum against
narrowband forms
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Processing Gain in FHSS
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Processing Gain in DHSS
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In a DS system
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Random binary data has a bit rate of Rb
The pseudorandom binary waveform has a rate of
Rc
Modulation
(Eb/No)dB
GdB
Required
(Eb/No)dB
PSK
BPSK
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Code-Division Multiple Access
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Basic Principles of CDMA
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Start with a data signal with rate D
Break each bit into k chips
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Chips are a user-specific fixed pattern
Chip data rate of new channel = kD
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Code-Division Multiple Access
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Advantage
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Good protection against interference and tapping
Disadvantages
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Receiver must be precisely synchronized with the
transmitter to apply the decoding correctly
Receiver must know the code and must separate
the channel with user data from the background
noise composed of other signals and
environmental noise
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CDMA Example
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If k=6 and code is a sequence of 1s and -1s
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For a ‘1’ bit, A sends code as chip pattern
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For a ‘0’ bit, A sends complement of code
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<c1, c2, c3, c4, c5, c6>
<-c1, -c2, -c3, -c4, -c5, -c6>
Receiver knows sender’s code and performs
electronic decode function
Su d   d1 c1  d 2  c2  d 3  c3  d 4  c4  d 5  c5  d 6  c6
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<d1, d2, d3, d4, d5, d6> = received chip pattern
<c1, c2, c3, c4, c5, c6> = sender’s code
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CDMA Example
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User A code = <1, –1, –1, 1, –1, 1>
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User B code = <1, 1, –1, – 1, 1, 1>
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To send a 1 bit = <1, –1, –1, 1, –1, 1>
To send a 0 bit = <–1, 1, 1, –1, 1, –1>
To send a 1 bit = <1, 1, –1, –1, 1, 1>
Receiver receiving with A’s code
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(A’s code) x (received chip pattern)
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User A ‘1’ bit: 6 -> 1
User A ‘0’ bit: -6 -> 0
User B ‘1’ bit: 0 -> unwanted signal ignored
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CDMA for DSSS
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Spread Spectrum
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