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IE 419/519Wireless Networks

Lecture Notes #6

Spread Spectrum


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Introduction

In 1985, the FCC modified Part 15 of the radiospectrum 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, andMedical (ISM) bands using spread spectrummodulation

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-randomnumber generator

Effect of modulation is to increase bandwidth ofsignal to be transmitted


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Spread Spectrum

On receiving end, digit sequence is used todemodulate the spread spectrum signal

Signal is fed into a channel decoder to recoverdata


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Spread Spectrum

What can be gained from apparent waste ofspectrum? 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 randomseries of radio frequencies A number of channels allocated for the FH signal

Width of each channel corresponds to bandwidthof input signal

Signal hops from frequency to frequency atfixed intervals Transmitter operates in one channel at a time

Bits are transmitted using some encoding scheme

At each successive interval, a new carrierfrequency is selected


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Source: http://murray.newcastle.edu.au/users/staff/eemf/ELEC351/SProjects/Morris/types.htm


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Hopping Sequence

Channel sequence dictated by spreading code

Pseudorandom number serves as an index into a table offrequencies

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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Receiver, hopping between frequencies insynchronization with transmitter, picks upmessage

Advantages Eavesdroppers hear only unintelligible blips

Attempts to jam signal on one frequency succeedonly 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 tojamming

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 bymultiple 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 streamusing 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 specturmwaveforms

Used to measure the performanceadvantage of spread spectrum againstnarrowband 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

Required

Modulation (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 kchips

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 thetransmitter to apply the decoding correctly

Receiver must know the code and must separatethe channel with user data from the backgroundnoise 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

For a 0 bit, A sends complement of code

Receiver knows senders code and performselectronic decode function

= received chip pattern

= senders code

665544332211 cdcdcdcdcdcddS

u


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CDMA Example

User A code = To send a 1 bit =

To send a 0 bit =

User B code = To send a 1 bit =

Receiver receiving with As code

(As 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

Documents

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