lect 08- spread spectrum technologies.pdf

8/11/2019 Lect 08- Spread Spectrum Technologies.pdf

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Spread SpectrumTechnologies

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Define spread spectrum technologies and howthey are used

Describe modulation and the different data rates

Explain and compare FHSS, DSSS and OFDM

List the factors that impact signal throughput andrange

ObjectivesUpon completion of this chapter you will be able to:


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Spread Spectrum

Spread spectrum is a communication technique thatspreads a narrowband communication signal over a wide rangeof frequencies for transmission then de-spreads it into theoriginal data bandwidth at the receive.

Spread spectrum is characterized by:

wide bandwidth and

low power

Jamming and interference have less effect on Spreadspectrum because it is:

Resembles noise

Hard to detect

Hard to intercept

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Narrowband vsSpread Spectrum

Frequency

Power

Spread Spectrum(Low Peak Power)

Narrowband(High Peak Power)


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Narrow Band vs Spread Spectrum

Narrow Band Uses only enough frequency spectrum to carry the signal

High peak power

Easily jammed

Spread Spectrum

The bandwidth is much wider than required to send to thesignal.

Low peak power

Hard to detect

Hard to intercept

Difficult to jam

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Spread Spectrum Use

In the 1980s FCC implemented a set of rules making SpreadSpectrum available to the public.

Cordless Telephones

Global Positioning Systems (GPS)

Cell Phones

Personal Communication Systems

Wireless video cameras

Local Area Networks

Wireless Local Area Networks (WLAN)

Wireless Personal Area Network (WPAN)

Wireless Metropolitan Area Network (WMAN)

Wireless Wide Area Network (WWAN)


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FCC Specifications

The Code of Federal Regulations (CFR) Part 15 originally

only described two spread spectrum techniques to be used inthe licensed free Industrial, Scientific, Medical (ISM) band,2.4 GHz, thus 802.11 and 802.11b.

Frequency Hopping Spread Spectrum (FHSS)and

Direct Sequence spread Spectrum (DSSS)

Orthogonal Frequency Division Multiplexing (OFDM)wasnot covered by the CFR and would have required licensing.

802.11a, employing OFDM, was created to work in the 5GHzUnlicensed National Information Infrastructure (UNII)

In May, 2001 CFR, Part 15 was modified to allow alternative"digital modulation techniques".

This resulted in 802.11gwhich employs OFDM in the 2.4GHz range

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Wireless LAN Networks Wireless LANs RF spread spectrum management techniques

Frequency Hopping Spread Spectrum (FHSS). Operates in the 2.4 Ghz range

Rapid frequency switching 2.5 hops per second w/ a dwell time of 400ms.

A predetermined pseudorandom pattern

Fast Setting frequency synthesizers.

Direct Sequence Spread Spectrum (DSSS) Operates in the 2.4 GHz range

Digital Data signal is inserted into a higher data ratechipping code.

A Chipping code is a bit sequence consisting of a redundant bit pattern.

Barker, Gold, M-sequence and Kasami codes are employed

Orthogonal Frequency Division Multiplexing (OFDM)

Operates in both the 5 Ghz and 2.4 GHz range with a data rate of between 6and 54 Mbps. 802.11a divides each channel into 52 low-speed sub-channels 48 sub-channels are for data while the other 4 are pilot carriers.

The modulation scheme can be either BPSK, QPSK or QAM dependingupon the speed of transmission.


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FCC Radio Spectrum

VLF 10 kHz - 30 kHz Cable Locating Equipment

LF 30 kHz - 300 kHz Maritime Mobile Service.

MF 300 kHz - 3 MHz Aircraft navigation, ham radio andAvalanche transceivers.

HF 3 MHz - 30 MHz CB radios, CAP, Radio telephone,and Radio Astronomy.

VHF 30 MHz - 328.6 MHZ Cordless phones, Televisions, RCCars, Aircraft, police and business radios.

UHF 328.6 MHz - 2.9 GHz police radios, fire radios, businessradios, cellular phones, GPS, paging,wireless networks and cordless phones.

SHF 2.9 GHz - 30 GHz Doppler weather radar, satellitecommunications.

EHF 30 GHz and above Radio astronomy, military systems,vehicle radar systems, ham radio.

Band Name Range Usage

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ISM Frequency Bands

UHF ISM 902 - 928 MhzS-Band 2 - 4 Ghz

S-Band ISM (802.11b) 2.4 - 2.5 Ghz

C-Band 4 - 8 Ghz

C-Band Satellite downlink 3.7 - 4.2Ghz

C-Band Radar (weather) 5.25 - 5.925 Ghz

C-Band ISM (802.11a) 5.725 - 5.875 Ghz

C-Band satellite uplink 5.925-6.425 Ghz

X-Band 8-12 Ghz

X-Band Radar (police/weather) 9.5-10.55 Ghz

Ku-band 12-18 Ghz

Ku-band Radar (Police) 13.5-15 Ghz

15.7-17.7 Ghz

ISM - Industrial, Scientific and Medical


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FHSS

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Frequency Hopping Spread Spectrum

Carrier changes frequency (HOPS)according to a pseudorandom Sequence.

Pseudorandom sequence is a list of frequencies. Thecarrier hops through this lists of frequencies.

The carrier then repeats this pattern.

During Dwell Timethe carrier remains at a certainfrequency.

During Hop Timethe carrier hops to the next frequency.

The data is spread over 83 MHz in the 2.4 GHz ISMband.

This signal is resistant but not immune to narrow bandinterference.


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Channel 1 Channel 2 Channel 78

Elapsed Time in Milliseconds (ms)

200 400 600 800 1000 1200 1400 1600

2.401

2.479

TransmissionFrequency(GHz)

Dividedinto79

1MHzChannels

Frequency Hopping Spread Spectrum

An Example of a Co-located Frequency Hopping System

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FHSS Contd

The original 802.11 FHSS standard supports 1 and2 Mbps data rate.

FHSS uses the 2.402 2.480 GHz frequency range in the ISM band.

It splits the band into 79 non-overlapping channels with each channel1 MHzwide.

FHSS hops between channels at a minimum rate of 2.5 times persecond. Each hop must cover at least 6 MHz

The hopping channels for the US and Europe are shown below.


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FHSS Contd

Dwell Time The Dwell time per frequency is around 100 ms(The FCC specifies a dwell time of 400 ms per carrierfrequency in any 30 second time period).

Longer dwell time = greater throughput.

Shorter dwell time = less throughput

Hop Time

Is measured in microseconds (us) and isgenerally around 200-300 us.

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FHSS Contd

Gaussian Frequency Shift Keying

The FHSS Physical sublayer modulates the data stream usingGaussian Frequency Shift Keying (GFSK).

Each symbol, a zero and a one, is represented by a differentfrequency (2 level GFSK)

two symbols can be represented by four frequencies (4 levelGFSK).

A Gaussian filter smoothes the abrupt jumps betweenfrequencies.

fc + fd2fc + fd1fc - fd1fc fd2

10110100

fc


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FHSS Disadvantages

Not as fast as a wired Lan or the newer WLANStandards

Lower throughput due to interference.

FHSS is subject to interference from other frequencies inthe ISM band because it hops across the entire frequencyspectrum.

Adjacent FHSS access points can synchronizetheir hopping sequence to increase the number of co-located systems, however, it is prohibitively expensive.

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DSSS


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Direct Sequence Spread Spectrum

Spread spectrum increases the bandwidth of the signal

compared to narrow band by spreading the signal.

There are two major types of spread spectrum techniques:

FHSS and DSSS.

FHSS spreads the signal by hopping from one frequency to

another across a bandwidth of 83 Mhz.

DSSS spreads the signal by adding redundant bits to the

signal prior to transmission which spreads the signal across 22

Mhz.

The process of adding redundant information to the

signal is calledProcessing Gain .

The redundant information bits are calledPseudorandom

Numbers (PN).

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Direct Sequence Spread Spectrum

DSSS works by combining informationbits (data signal) withhigher data rate bit sequence (pseudorandom number (PN)).

The PN is also called a Chipping Code (eg., the Barker chipping

code)

The bits resulting from combining the information bits with the

chipping code are calledchips - the result- which is then

transmitted.

The higherprocessing gain (more chips) increases the signal's

resistance to interference by spreading it across a greater number of

frequencies.

IEEE has set their minimum processing gain to 11. The number

of chips in the chipping code equates to the signal spreading ratio.

Doubling the chipping speed doubles the signal spread and the

required bandwidth.


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Signal Spreading

The Spreaderemploys an encoding scheme (Barker or

Complementary Code Keying (CCK).

The spread signal is then modulated by a carrier employing either

Differential Binary Phase Shift Keying (DBPSK), or DifferentialQuadrature Phase Shift Keying (DQPSK).

The Correlatorreverses this process in order to recover the original

data.

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Fourteen channels are identified, however, the FCC specifies only 11

channelsfor non-licensed (ISM band) use in the US.

Each channels is a contiguous band of frequencies 22 Mhz wide with each

channel separated by 5 MHz.

Channel 1 = 2.401 2.423 (2.412 plus/minus 11 Mhz).

Channel 2 = 2.406 2.429 (2.417 plus/minus 11 Mhz).

Only Channels 1, 6 and 11 do not overlap

DSSS Channels


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Spectrum Mask

A spectrum Mask represents the maximum power output for thechannel at various frequencies.

From the center channel frequency, 11 MHz and 22 MHZ the signalmust be attenuated 30 dB.

From the center channel frequency, outside 22 MHZ, the signal isattenuated 50 dB.

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DSSS Frequency Assignments

Channel 12.412 GHz

Channel 62.437 GHz

Channel 112.462 GHz

25 MHz25 MHz

The Center DSSS frequencies of each channel are only 5 Mhz apart but

each channel is 22 Mhz wide therefore adjacent channels will overlap.

DSSS systems with overlapping channels in the same physical space

would cause interference between systems.

Co-located DSSS systems should have frequencies which are at least5 channels apart, e.g., Channels 1 and 6, Channels 2 and 7, etc.

Channels 1, 6 and 11 are the only theoretically non-overlappingchannels.


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2.401 GHz 2.473 GHz

Channel 1 Channel 6 Channel 11

22 MHz

3 MHz

f

P

DSSS Non-overlapping Channels

Each channel is 22 MHz wide. Inorder for two bands not to overlap(interfere), there must be five channels

between them.

A maximum of three channels may

be co-located (as shown) withoutoverlap (interference).

The transmitter spreads the signalsequence across the 22 Mhz wide

channel so only a few chips will beimpacted by interference.

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DSSSEncoding and Modulation


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DSSS Encoding and Modulation

DSSS (802.11b) employs two types of encoding schemes

and two types of modulation schemes depending upon thespeed of transmission.

Encoding Schemes

Barker Chipping Code: Spreads 1 data bit across 11 redundantbits at both 1 Mbps and 2 Mbps

Complementary Code Keying (CCK):

Maps 4 data bits into a unique redundant 8 bits for 5.5 Mbps

Maps 8 data bits into a unique redundant 8 bits for 11 Mbps.

Modulation Schemes

Differential Binary Phase Shift Keying (DBPSK):Two phaseshifts with each phase shift representing one transmitted bit.

Differential Quadrature Phase Shift Keying (DQPSK):Fourphase shifts with each phase shift representing two bits.

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DSSS Encoding


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Barker Chipping Code

802.11 adopted an 11 bit Barker chipping code.

Transmission.

The Barker sequence, 10110111000, was chosen to spreadeach 1 and 0 signal.

The Barker sequence has six 1s and five 0s.

Each data bit, 1 and 0, is modulo-2 (XOR) added to theeleven bit Barker sequence.

If a one is encoded all the bits change.

If a zero is encoded all bits stay the same.

Reception.Azero bit corresponds to an eleven bit sequence of six 1s.

Aone bit corresponds to an eleven bit sequence ofsix 0s.

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Barker Sequence

One Bit

1 0

1 0 1 1 0 1 1 1 0 0 0 1 0 1 1 0 1 1 1 0 0 0

Chipping Code(Barker Sequence)

Original Data

Spread Data

0 1 0 0 1 0 0 0 1 1 1 1 0 1 1 0 1 1 1 0 0 0

Six 0s = 1 Six 1s = 0

One Bit

10110111000


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Direct Sequence Spread Spectrum Contd

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Complementary Code Keying (CCK)

Barker encoding along with DBPSK and DQPSK modulationschemes allow 802.11b to transmit data at 1 and 2 Mbps

Complementary Code Keying (CCK)allows 802.11b totransmit data at 5.5 and 11 Mbps.

CCK employs an 8 bit chipping code.

The 8 chipping bit pattern is generated based upon thedata to be transmitted.

At 5.5 Mbps, 4 bits of incoming data is mapped into a

unique 8 bit chipping pattern.

At 11 Mbps, 8 bits of data is mapped into a unique 8bit chipping pattern.


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Complementary Code Keying (CCK) Contd

To transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits.. The unique 8 chipping bits is determined by the bit pattern of the 4 data

bits to be transmitted. The data bit pattern is:

b0, b1, b2, b3

b2 and b3determine the unique pattern of the 8 bit CCK chippingcode.

Note: j represents the imaginary number, sqrt(-1), and appears on the imaginaryor quadrature axis of the complex plane.

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To transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits..

The unique 8 chipping bits is determined by the bit pattern of the 4 data

bits to be transmitted. The data bit pattern is:

b0, b1, b2, b3

b0 and b1 determine the DQPSK phase rotation that is to beapplied to the chip sequence.

Each phase change is relative to the last chip transmitted.


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To transmit 11 Mbps 8 data bits is mapped into 8CCK chipping bits.

The unique 8 chipping bits is determined by thebit pattern of the 8 data bits to be transmitted. Thedata bit pattern is:

b0, b1, b2, b3, b4, b5, b6 ,b7

b2, b3, b4 ,b5, b6 and b7selects one unique

pattern of the 8 bit CCK chipping code out of 64

possible sequences.

b0 and b1 are used to select the phase rotation

sequence.

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DSSSModulation


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Differential Binary Phase Shift Keying (DBPSK)

0 Phase

ShiftA Zero phase shift from theprevious symbol is interpreted as

a 0.

A 180 degree phase shift fromthe previous symbol is interpreted

as a 1.

180 degree

Phase Shift

180 degreePhase Shift

Previous

carrier symbol

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Differential Quadrature Phase Shift Keying (DQPSK)

A Zero phase shift from the previoussymbol is interpreted as a 00.

Previous

carrier symbol

0 PhaseShift

A 90 degree phase shift from the previoussymbol is interpreted as a 01.



90 PhaseShift

180 PhaseShift

270 PhaseShift


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DSSS Summary

1 Barker Coding 11 chips encoding 1 bit DBPSK

2 Barker Coding 11 chips encoding 1 bit DQPSK

5.5 CCK Coding 8 chips encode 8 bits DQPSK

11 CCK Coding 8 chips encode 4 bits DQPSK

Data Rate Encoding Modulation

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FHSS vs DSSS

DSSS is more susceptible to narrow band noise.

DSSS channel is 22 Mhzwide whereas

FHSS is 79 Mhzwide.

The FCC regulated that DSSS use a maximum of 1 watt

of transmitter power in Pt-to-Multipoint system.

DSSS costs less then FHSS

FHSS can have more systems co-located than

DSSS.

DSSS systems have the advantage in throughput The Wi-Fi alliance tests for DSSS compatibility

No such testing alliance exists for FHSS.


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802.11a Network Channel AssignmentsArea Frequency Band Channel Center Frequency

USA U-NII Lower Band 36 5.180 Ghz

(5.150-5.250 Ghz) 40 5.200 Ghz

44 5.220 Ghz

48 5.240 Ghz

USA U-NII Middle Band 52 5.260 Ghz

(5.250 5.350 Ghz) 56 5.260 Ghz

60 5.280 Ghz

64 5.320 Ghz

USA U-NII Upper Band 149 5.745 Gh

(5.725 5.825) 153 5.765 Ghz

157 5.785 Ghz

161 5.805 Ghz

NOTE: 1. U-NII : Unlicensed National Information Infrastructure.2. 802.11a is specific to the US.

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OFDM

A mathematical process that allows52 channels to overlap withoutlosing their orthogonality (individuality).

48 sub-channel are used for data

Each sub-channel is used to transmit data

4 sub-channel are used as pilot carriers.

The pilot sub-channels are used to monitor path shift and

shifts in sub-channel frequencies (Inter Carrier Interference

(ICI)).

OFDM

OFDM selects channels that

overlap but do not interferewith one another.

Channels are separated

based upon orthogonality.


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802.11a Channels

Lower UNII Band Middle UNII Band

802.11a use the lower and middle UNII 5 GHz bands to create 8 channels.

Each Channel is 20 MHz each.

Each channel is broken into 52 sub-channels with each sub-channel300 KHz each.

48 Sub-channels are used to transmit data

4 sub-channels are used as Pilot carriers to monitor the channel

8Channels

52Sub-Channels

for each 8channels

Each channel is20 MHz wide

Lower andMiddle UNII

frequency band

48

OFDM

Modulation


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Modulation Background

In order to properly understand OFDM modulation we need to do

a quick review of various modulation techniques.

James Clark Maxwell, 1864, first developed the idea thatelectromagnetic magnetic waves arose as a combination electriccurrent and magnetic field an electromagnetic wave.

Heinrich Hertz , in 1880s, developed the first RadioFrequency device that sent and received electromagnetic wavesover the air

The name Hertz (Hz)was given to the unit of frequencymeasurement representing one complete oscillation of anelectromagnetic wave. This is also called cycle per second.

Kilohertz = thousands of cycles per second

Megahertz = millions of cycles per second

Gigahertz = billions cycles per second

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Modulation Background Contd

The oscillating electromagnetic wave, also called a sine wave, is shown below.

This wave can be used as a carrier signalto carry information.

The information can be imposed upon the carrier through a process calledmodulationwhich is accomplished by modifying one of three physical wavecharacteristic. These physical characteristics are:

Amplitude The height of the wave

Frequency the number of oscillation (cycles) per second.

Phase the starting point of the wave (when compared to the starting point ofthe previous wave.

The are two major types of modulation schemes: Analog and Digital

Amplitude

Frequency

Phase

Sine Wave


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Analog Modulation

Amplitude Modulationvaries the

height of the carrier wave.

Frequency Modulationvaries thenumber of oscillation (waves) persecond

Phase Modulationchanges thestarting point of the wave.

Change inPhase

Change inFrequency

Change inAmplitude

1 = 1800 Phase Change0 = No Phase Change

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Digital Modulation


Amplitude Shift Keying (ASK)changes the amplitude of the carrier

wave to represent a 0 or 1.

Frequency Shift Keying (FSK)changes the frequency of the carrier

wave to represent a 0 or 1.

Phase Shift Keying (PSK)changes

the phase of the carrier wave torepresent a 0 or 1.

180 degreephase change


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Phase Modulation Extended

Phase Modulationchangesthe starting point of the wave.

Change inPhase


900

2700

180o

0o

1 0

Phase shift can also be represented on an x/y axisconstellation such that:

In this instance we can transmit 1 bit for every phaseshift.

This is called Binary Phase Shift Keying (BPSK) in802.11a

radians)1 = 1800 Phase Change (0 = No Phase Change

radians)1 = 1800 Phase Change (0 = No Phase Change

BPSK

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QUADRATURE AMPLITUDE MODULATION (QAM)

90

0

2700

00

135o

01

11 10

35o

315o

225o

180o

0o

2 bits/phase

Quadrature Phase Shift Keying (QPSK)extends this technique to transmit two bits for

every phase shift.

0000

0001

0011

00100110

0111

01010100

1100

1101

1111

1110 1010 1011

1001 1000

900

2700

180o

0o

4 bits/phase

Quadrature Amplitude Modulation

(QAM)generalizes these techniques to

encode information in both phase (by

employing PSK techniques such as BPSKand QPSK) with amplitude.

For example, in the diagram a right, eachquadrature contains 4 amplitudes (16 levels)

and can therefore transmit 4 bits per phase.

00 = 350 Phase Change




QPSK

QAM


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QAM Extended

In the diagram at right, eachquadrature contains 8 amplitudes (64levels) and can therefore transmit 6 bits

per phase.

900

2700

180o

0o

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Summary of OFDM Encoding/Modulation

64 Phase shifts can encode 6 bits /phase shift resulting is a transmission rate of

either 48 or 54 Mbps depending upon the number of sub-channels (R) used for error

correction.

Coding Rate (R) is the ratio of sub-channels carrying data to sub-channels

carrying error correction code. E.G., 1/2 would indicate that 24 sub-channels (1/2 X48 = 24) are being used for error correction while the remaining 24 sub-channels are

used for data transmission.

The Length of the each Symbol is equal to number of sub-carriers times the bits/transition. e.g., 48 X 6 = 288.


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Summary of OFDM Encoding/Modulation

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The end

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