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