IE 419/519Wireless Networks
Lecture Notes #6Spread Spectrum
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Introduction 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 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 Digital data
Analog data
Which option to choose? Requirements to meet Media & communications facilities
Spread Spectrum Can be used to transmit either analog or digital data,
using an analog signal
• Digital Signal
• Analog Signal
• Digital Signal
• Analog Signal
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Spread Spectrum Input is fed into a channel encoder
Produces analog signal with narrow bandwidth
Signal is further modulated using sequence of digits 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 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 What can be gained from apparent waste
of spectrum? Immunity from various kinds of noise and
multipath distortion Anti-jamming performance Interference immunity
Can be used for hiding and encrypting signals Low probability of intercept Low transmit power density
Several users can independently use the same higher bandwidth with very little interference
Multiple access communications Multiple simultaneous transmissions
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Types of Spread Spectrum Frequency Hopping Spread Spectrum
(FHSS) First type developed
Direct Sequence Spread Spectrum (DSSS) More recent technology
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Frequency Hopping SS 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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Frequency Hopping SS
Source: http://murray.newcastle.edu.au/users/staff/eemf/ELEC351/SProjects/Morris/types.htm
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Frequency Hopping SS Hopping Sequence
Channel sequence dictated by spreading code
Pseudorandom number serves as an index into a table of frequencies
Chip Period Time spent on each channel
FCC regulation maximum dwell time of 400 ms IEEE 802.11 standard 300 ms
Chipping rate Hopping rate
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Frequency Hopping SS Receiver, hopping between frequencies
in synchronization with transmitter, picks up 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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FHSS Performance Considerations Large number of frequencies used Results in a system that is quite
resistant to jamming 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 Each bit in original signal is represented
by multiple bits in the transmitted signal 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 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 In a DS system
Random binary data has a bit rate of Rb
The pseudorandom binary waveform has a rate of Rc
RequiredModulation (Eb/No)dB GdB (Eb/No)dB
PSK
BPSK
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Code-Division Multiple Access Basic Principles of CDMA
Start with a data signal with rate D Break each bit into k chips
Chips are a user-specific fixed pattern Chip data rate of new channel = kD
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Code-Division Multiple Access Advantage
Good protection against interference and tapping
Disadvantages 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 If k=6 and code is a sequence of 1s and -1s
For a ‘1’ bit, A sends code as chip pattern <c1, c2, c3, c4, c5, c6>
For a ‘0’ bit, A sends complement of code <-c1, -c2, -c3, -c4, -c5, -c6>
Receiver knows sender’s code and performs electronic decode function
<d1, d2, d3, d4, d5, d6> = received chip pattern <c1, c2, c3, c4, c5, c6> = sender’s code
665544332211 cdcdcdcdcdcddSu
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CDMA Example User A code = <1, –1, –1, 1, –1, 1>
To send a 1 bit = <1, –1, –1, 1, –1, 1> To send a 0 bit = <–1, 1, 1, –1, 1, –1>
User B code = <1, 1, –1, – 1, 1, 1> To send a 1 bit = <1, 1, –1, –1, 1, 1>
Receiver receiving with A’s code (A’s code) x (received chip pattern)
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