Spread Spectrum Communication Systems
SPREAD SPECTRUM
In spread spectrum (SS), we spread a signal over a wide
bandwidth (larger than BW in traditional communication
systems)
- These techniques are used for a variety of reasons, including
the establishment of secure communications, increasing
resistance to natural interference, noise and jamming
Direct Sequence Spread Spectrum (DSSS)
Frequency Hopping Spread Spectrum (FHSS)
Time Hopping (THSS)
Hybrid
Spread Spectrum systems:
Spread Spectrum systems:
Pseudo noise (PN) code is required in SS systems
Pseudo noise (PN) code symbols are called chips
Chip rate or PN code rate R
c
>> R
original signal
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Shannon and Hartely channel capacity theorem
C=B log
2
(1+S/N)
S/N is low for SS systems, so roughly
C/B ≈ 1.433 * S/N
or N/S ≈ B/C
To send error-free information for a given N/S in a
channel, one need only perform the fundamental
signal-spreading operation: increase B.
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Direct Sequence Spread Spectrum DSSS
DSSS phase-modulates the original signal with a continuous string of
pseudonoise (PN) code symbols, each of which has a much shorter
duration than an information bit.
DSSS example
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Receiver uses same sequence to demodulate signal
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DSSS Receiver
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Power distribution {(sinx)/x}2
…………..
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Null-to-null bandwidth is 2R
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90% of the power in the main-lobe
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Other modulations like QPSK , MFSK …………….
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Immunity from various noise and multipath
distortion
–
Including jamming
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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 multiple access (CDMA)
•
Process Gain, typically 10-60 dB
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Benefits:
Resistance to Interference and Anti-jamming Effects
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Resistance to Interception
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Resistance to Fading
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Addressing
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High resolution ranging
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Multi access technique CDMA
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Multiplexing Technique used with spread spectrum
•
It plays a critical role in building efficient, robust, and secure radio
communication systems
•
Start with data signal rate
D
–
Called bit data rate
•
Break each bit into
k
chips according to fixed pattern specific to
each user
–
User’s code
•
New channel has chip data rate
kD
chips per second
•
E.g.
k
=6, three users (A,B,C) communicating with base receiver R
•
Code for A = <1,-1,-1,1,-1,1>
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Code for B = <1,1,-1,-1,1,1>
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Code for C = <1,1,-1,1,1,-1>
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Consider A communicating with base
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Base knows A’s code
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Assume communication already synchronized
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A wants to send a 1
–
Send chip pattern <1,-1,-1,1,-1,1>
•
A’s code
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A wants to send 0
–
Send chip[ pattern <-1,1,1,-1,1,-1>
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Complement of A’s code
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Decoder ignores other sources when using A’s code to decode
–
Orthogonal codes
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n users each using different orthogonal PN
sequence
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Modulate each users data stream
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Using BPSK
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Multiply by spreading code of user
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Signal broadcast over series of frequencies
•
Receiver hops between frequencies in sync
with transmitter
•
Eavesdroppers hear unintelligible blips
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Jamming on one frequency affects only a few
bits
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Typically 2
k
carriers frequencies forming 2
k
channels
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Channel spacing corresponds with bandwidth
of input
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Each channel used for fixed interval
–
Ex. 300 ms in IEEE 802.11
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Frequency hopping spread spectrum (FHSS)
Frequency selection in FHSS
FHSS cycles
Bandwidth sharing
Spread spectrum techniques involve spreading a signal over a wide bandwidth for various reasons, such as secure communication, resistance to interference, noise, and jamming. This summary introduces Direct Sequence Spread Spectrum (DSSS), Frequency Hopping Spread Spectrum (FHSS), Time Hopping, and hybrid systems. Pseudo noise (PN) code, Shannon and Hartely channel capacity theorem, and the use of PN code symbols are discussed with illustrations of DSSS phases, a general model of spread spectrum systems, and RF bandwidth characteristics. Practical examples and explanations of DSSS implementation are provided for better understanding.
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Presentation Transcript
SPREAD SPECTRUM In spread spectrum (SS), we spread a signal over a wide bandwidth (larger than BW in traditional communication systems) - These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference, noise and jamming Spread Spectrum systems: Direct Sequence Spread Spectrum (DSSS) Frequency Hopping Spread Spectrum (FHSS) Time Hopping (THSS) Hybrid
Pseudo noise (PN) code is required in SS systems Pseudo noise (PN) code symbols are called chips Chip rate or PN code rate Rc >> Roriginal signal
Theoretical Justification Shannon and Hartely channel capacity theorem C=B log2(1+S/N) S/N is low for SS systems, so roughly C/B 1.433 * S/N or N/S B/C To send error-free information for a given N/S in a channel, one need only perform the fundamental signal-spreading operation: increase B. Dr. Samah A. Mustafa
Direct Sequence Spread Spectrum DSSS DSSS phase-modulates the original signal with a continuous string of pseudonoise (PN) code symbols, each of which has a much shorter duration than an information bit.
General Model of Spread Spectrum System Receiver uses same sequence to demodulate signal
DSSS Transmitter DSSS Receiver
RF bandwidth of DSSS Power distribution {(sinx)/x}2 .. Null-to-null bandwidth is 2Rc 90% of the power in the main-lobe Other modulations like QPSK , MFSK . Dr. Samah A. Mustafa
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 multiple access (CDMA)
Process Gain, typically 10-60 dB GP Benefits: Resistance to Interference and Anti-jamming Effects Dr. Samah A. Mustafa
More benefits Resistance to Interception Resistance to Fading Addressing High resolution ranging Multi access technique CDMA Dr. Samah A. Mustafa
Code Division Multiple Access (CDMA) Multiplexing Technique used with spread spectrum It plays a critical role in building efficient, robust, and secure radio communication systems Start with data signal rate D Called bit data rate Break each bit into k chips according to fixed pattern specific to each user User s code New channel has chip data rate kD chips per second E.g. k=6, three users (A,B,C) communicating with base receiver R Code for A = <1,-1,-1,1,-1,1> Code for B = <1,1,-1,-1,1,1> Code for C = <1,1,-1,1,1,-1>
CDMA Explanation Consider A communicating with base Base knows A s code Assume communication already synchronized A wants to send a 1 Send chip pattern <1,-1,-1,1,-1,1> A s code A wants to send 0 Send chip[ pattern <-1,1,1,-1,1,-1> Complement of A s code Decoder ignores other sources when using A s code to decode Orthogonal codes
CDMA for DSSS n users each using different orthogonal PN sequence Modulate each users data stream Using BPSK Multiply by spreading code of user
Frequency Hopping Spread Spectrum (FHSS) Signal broadcast over series of frequencies Receiver hops between frequencies in sync with transmitter Eavesdroppers hear unintelligible blips Jamming on one frequency affects only a few bits
Basic Operation Typically 2k carriers frequencies forming 2k channels Channel spacing corresponds with bandwidth of input Each channel used for fixed interval Ex. 300 ms in IEEE 802.11
