DBPSK/DQPSK CCK - Home | College of Engineering and ...cs.uccs.edu/~gsc/pub/master/pjfong/UCCS...

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Module contents

Technologies overview

Spread SpectrumDirect Sequence

Frequency Hopping

ModulationDBPSK/DQPSK

CCK

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OSI Reference Model: Phy

Network Oper. SystemNetwork LayerGuarantees delivery data

DriversLLC Layersend/receive data

LAN ControllerMAC Layerdata into/out frame

MODEMPhysical Layerframe into/out phy frame

Physical LayerPhysical Layer

IEEE: MAC LayerIEEE: MAC Layer

IEEE: LLC LayerIEEE: LLC Layer

Network LayerNetwork Layer

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Wireless LAN TechnologiesWireless LAN Technologies

InfraredInfrared Spread SpectrumSpread

SpectrumNarrow BandNarrow Band

Direct SequenceDirect

Sequence FrequencyHopping

FrequencyHopping

Wireless LAN technologies(overview)

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Wireless LAN technologies(Infrared)

low power infrared light as the carrierNo license requiredVery restricted mobility, limited coveragehigh data rate (10 Mbps, 16 Mbps)Line-of-Sight Infrared

no objects in the path between two stations

Diffuse Infrareduses reflections to set-up wireless link

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Wireless LAN technologies(Narrow Band)

Dedicated band (18 GHz)License required

ISM band (915 MHz, 2.4 GHz, 5.8 GHz)unlicensed (special modulation)extremely low output power i.e. limited coveragehigh data rate (up to 10 Mbps) on short distance

Europe - DECT band (1.8 GHz)based on voice standard

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• 915 MHz only in the Americas (region 2)• 2.4 GHz for global availability (region 1,2,3)

1 2 3 4 6 8 10 20 30 40 60 100GHz

123

ISM Frequency AllocationsWorldwide

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Wireless LAN technologies(Spread Spectrum)

Unlicensed usage (ISM band)No line of sight requirement (indoor)High link reliabilityBuilt-in transmission securityTwo techniques used:

Direct SequenceFrequency Hopping

Standard RadioTransmission

Spread SpectrumTransmission

Frequency Spectrum (MHz)

2400 2500

PowerPower

FrequencyFrequency

88 103 2400FM Band

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Module contents



Frequency Hopping


CCK

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Multiple Access MethodsMultiple users share the available spectrum

FREQUENCY

TIME

User 3User 2User 1

• Multiple users sharethe same frequencychannel sequentially

• Time slot sequencerepeats over and over

TDMA

TIME

FREQUENCY

CODE

CDMAalso known as “Spread Spectrum”

User 3User 2User 1

• Channel is “spread” over wide frequencyband

• Many users share the same frequencyband at the same time

• Each user is assigned a unique “code”to identify and separatethem

FREQUENCY

TIME

FDMA

1 2 3

Each user assigned adifferent frequency -like ordinary radio

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Spread Spectrum TechnologiesDS vs. FH

Direct SequenceEach symbol is transmitted over multiplefrequencies at the same timeVery efficient (no overhead)Higher speed than FH at comparabledistancesSystem capacity (multiple channels)higher than FH

Frequency HoppingSequential use of multiple frequenciesHop sequence and rate will vary“End hop waste time”

COMPLETE WAVEBAND ALLOCATED

Time

Time

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Spreading: Information signal (I.e. a “symbol”) is multiplied by a unique,high rate digital code which stretches (spreads) its bandwidth beforetransmission.Code bits are called “Chips”.Sequence is called “Barker Code”

Source andChannelCoding

RFModulator

CodeGenerator

X

Multiplier

Code Bits (Chips)

Digital Signal (Bits)

FrequencySpectrum

f

“Spread” FrequencySpectrum

f

Spread Spectrum TechnologiesDirect Sequence transmitter

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Spread Spectrum TechnologiesWhat happens during “spreading”

X

=

“symbol”

“Barker” sequence

Result of multiplication

Symbol time ts“1” “0”

Chiptime tc

Due to the multiplication of asymbol with Barker code, the“rate-of-change” increases witha factor 11This means that cycle rateincreases from 1 MHz to 11MHz

In terms of spectrum thismeans that after RFmodulation the signal is spreadfrom 2 MHz bandwidth to 22MHz bandwidth

2 Mhz 22 Mhz

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At the receiver, the spread signal is multiplied again by a synchronizedreplica of the same code, and is “de-spread” and recoveredThe outcome of the process is the original “symbol”

RFDemodulator

Channeland

SourceDecoding

CodeGenerator

X

Multiplied

Code Bits (Chips)

De-SpreadSignal

f

“Spread” FrequencySpectrum

f

Digital Signal (Bits)

Spread Spectrum TechnologiesDirect Sequence receiver

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Spread Spectrum TechnologiesDe-spreading

Data

:11 chip code

Direct SequenceSpread Spectrum

Signal

+11

-11

+1

-1

Symboltime

When the incoming signal is de-spread, it results in either apositive (+) or a negative (-)“spike”

These “spikes” arrive at intervalsequal to the symbol time

A positive spike represents a “1”symbol, a negative spikerepresents a “0” symbol

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Spread Spectrum TechnologiesDirect Sequence receiver - effect of echoes

echo

echo

peak

Symboltime

Echoes may arrive at thereceiver, fluctuations can benoticed at positions other than atthe symbol time boundaries

These fluctuations are ignored asthe receiver will only interpret thespike at the synchronizationpoints (separated from each otherby the symbol time)

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Module contents



Frequency Hopping


CCK

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ModulationDBPSK (Differential Binary Phase Shift Keying)

I

Q

Bit Input Phase Change (+jω)0 01 π

Table 1, 1 Mb/s DBPSK Encoding Table.

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I

Q

Dibit pattern (d0,d1)d0 is first in time Phase Change (+jω)

00 001 π/211 π10 3π/2 (-π/2)

Table 1, 2 Mb/s DQPSK Encoding Table

ModulationDQPSK (Differential Quadrature Phase Shift Keying)

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CCK = Complementary Code KeyingIEEE 802.11 standard for high speed11 and 5.5 Mbps data ratesOutstanding high multi-path performanceOutstanding low-SNR performanceSeamless interoperability with existing DSMaintains QPSK chips at 11 MHz chip rateMaintains 3 frequency channelsFCC and MKK regulations satisfied

CCKTurbo 11 Mb approach

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Data bits are encoded to a symbol which istransmitted in the form of 8 chipsFor Data-Rate = Medium Encoding means:

mapping 2 data bits to I or Q channel (in-Phase,Quaternary Phase)mapping 2 data bits to one of 4 ComplexCodewords

For Data-Rate = High Encoding means:mapping 2 data bits to I or Q channel (in-Phase,Quaternary Phase)mapping 6 data bits to one of 64 ComplexCodewords

Codewords are complex complementarycodes selected from a code set

CCKHow it works

5.5 MBpsCCK

8 chips

2 bits encoded to4 complex code words; 2-QPSK

11 MBpsCCK

8 chips


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Pick One of 4 ComplexCodes *

MUX1:8

2

DATAIN

I OUT

Q OUT

1.375 MHz8 chips clocked with 11 MHz

11 MHz

1

1

Data Rate = 4 bits/symbol * 1.375 MSps = 5.5 MBps

Scrambler

1

1 *= Code Set:747B47B78B7BB8B7see next slide

CCKOperating at medium speed

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Pick One of 64 ComplexCodes

MUX1:8

6

DATAIN

I OUT

Q OUT

1.375 MHz11 MHz

1

1

Data Rate = 8 bits/symbol * 1.375 MSps = 11 Mbps

Scrambler

1

1

Code Set is defined by formula:

},,,,

,,,{1213132141

4214314321

)()()()(

)()()(

ϕϕϕϕϕϕϕϕϕϕ

ϕϕϕϕϕϕϕϕϕϕ

jjjjj

jjj

eeeeeeeec

+++++

+++++++

−−

=

CCKHow it works

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Bit-rates:

The 11 chips Barker sequence in Standard DSSS carries one symbol clocked at1MHz, which results in a symbol rate of 1Msymbol/sec.

The 8 chips sequence in CCK clocked at 1 MHz, results in a symbol rate of 1.375Msymbol/sec (I.e. 11/8)

At date rate = medium, 4 data bits are mapped on one symbol, which results in 5.5Mbps (I.e. 1.375 * 4)

At date rate = high, 8 data bits are mapped on one symbol, which results in 11Mbps (I.e. 1.375 * 8)

CCKData rates and symbol rates

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11 MBpsCCK

5.5 MBpsCCK

802.11 DSSS QPSK2MBpsBarkerQPSK

802.11 DSSS BPSK1 MBpsBarkerBPSK

11 chips

1 MSps

1 bit used toBPSK code word

11 chips

1 MSps

2 bits used to QPSK code word

8 chips

1.375 MSps


8 chips

1.375 MSps

I, Q I, QI, QI, Q


CCKFrom DSSS BPSK to 11 Mbps CCK

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Module summary



Frequency Hopping

ModulationBQPSK/BQPSK

CCK

DBPSK/DQPSK CCK - Home | College of Engineering and ...cs.uccs.edu/~gsc/pub/master/pjfong/UCCS...

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