Nomadic Communications WLAN – 802.11 PHY...

NomadicCommunicationsWLAN– 802.11PHYLayersRenatoLoCigno

ANSGroup– [email protected]

http://disi.unitn.it/locigno/index.php/teaching-duties/nomadic-communications

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Peridettagli,consultarehttp://creativecommons.org/licenses/by-nc-nd/2.5/it/

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NomadicCommunications– [email protected]– 802.11PHYLayers 2

PHYLayers

• DetailsoftheframeformatanddetailsoftheMACdependsondetailsofthePHYlayer

• Thisiswhyintheendin802networksMACandPHYarespecifiedtogetherwhileLLCandinternetworkingareincommon

• BothISO/OSIand(tosomeextent)TCP/IPfailedtorecognizethis

• ThereisanadditionallayerbetweenMACandPHY


PHYlayersubdivision

ü PLCP:PhysicalLayerConvergenceProtocolü PMD:PhysicalMediumDependant

MAC

PLCP

PMD

MPDU

PPDU

ü PPDUcontainsthePHYlayerheadersstrippedwhenthePDUispassedtotheMAC

ü PMDdefinesthespecificelectromagneticcharacteristicsusedondifferentPHYmeans

ü PLCPHeaderü IsactuallyalreadydependentonthePMDü Includessyncpreamblesandfurtherinfoontheencodingof

theremainingpartoftheMPDU


Goalsof802.11PHY(s)

• Flexibility– Everystandardallowsmultipledata-rates

• Robustness– Againstmultipathfadingandimpairments– Againstheterogeneouschannelconditions

• Preambleand“common”informationheadertransmittedalwaysatminimumspeed

• Payloadtransmittedatthebestrate(estimatedbyandatthetransmitter)


802.11PHY:Earlystandards

NomadicCommunications– [email protected] – 802.11PHYLayers 6

st—year Freq/Bandw DataRates(Mbit/s) SStechnique

Maxdistin—out

- —97 2.4GHz/20MHz 1,2 FHSS 20-100

- —97 THz/Baseband 1,2 none 10-??

b—99 2.4GHz/20MHz 5.5,11 DSSS 25-150

a/h—99 5.0GHz/20MHz 6,9,12,18,24,36,48,54 OFDM 20-150

g—03 2.4GHz/20MHz 6,9,12,18,24,36,48,54 OFDM 20-150

802.11PHY:CurrentStandards&Evolution


st—year Freq/Bandw DataRates(Mbit/s) SStechniqueMaxdistin—out

n– 092.4or5GHz/20-40MHz

15,30,45,60,90,120,135,150(40MHz);divideby2for20MHz

OFDM 40-250

ac– 12+ 5GHz/160MHz toomany...upto1.69Gbit/s OFDM 40-250?

ad– 12+60GHz/2.16GHz

toomany...upto7Gbit/s UWB/OFDM Meters

ah/aj/ax/ay

900MHz/60GHz

Futurestandards ??? ???

GenericDSSS(802.11b)packet

SYNC -56 bits SFD-16 bits

SFD – Start Frame Delimiter

PHY – 1Mbps

PLPC – Physical Layer Convergence Protocol

MAC – 2Mbps

CONTROL8 bytes

ADDR12-24 bytes

CRC4 bytes

PSDU – 2, 5.5, 11 Mbps

min 96 µs

Signal Tx. speed ... 48 bits


SomeNumericalValues…

802.11b

• PHYHDR:16bytes,transmitted@1Mbps

• MACHDR:34bytes,transmitted@1/2Mbps

– Ifslot=20μs,PHYHDR+MACHDR=20slots

• ACK=PHYHDR+14bytes,transmitted@1/2Mbps

– Ifslot=20μs,ACK=12slots


DetailedMACFormat(bytes)


Sequence Control

Address4(tx node)

Data FCS

2 6 0 - 2,312 4

FrameControl

Duration ID

Address1(source)

Address2(destination)

Address3(rx node)

2 2 6 6 6

MACFormatfields

Field Bits Notes/Description

FrameControl 15 - 14 Protocol version. Currently 0

13 - 12 Type

11 - 8 Subtype

7 To DS. 1 = to the distribution system.

6 From DS. 1 = exit from the Distribution System.

5 More Frag. 1 = more fragment frames to follow (last or unfragmentedframe = 0)

4 Retry. 1 = this is a re-transmission.

3 Power Mgt. 1 = station in power save mode, 0 = active mode.

2 More Data. 1 = additional frames buffered for the destination address (address x).

1 WEP. 1 = data processed with WEP algorithm. 0 = no WEP.

0 Order. 1 = frames must be strictly ordered.


MACFormatfields

Field Bits Notes/Description

Duration ID 15 - 0 For data frames = duration of frame. For Control Frames the associated identity of the transmitting station.

Address 1 47 - 0 Source address (6 bytes).

Address 2 47 - 0 Destination address (6 bytes).

Address 3 47 - 0 Receiving station address (destination wireless station)

Sequence Control 15 - 0

Address 4 47 - 0 Transmitting wireless station.

Frame Body 0 - 2312 octets (bytes).

FCS 31 - 0 Frame Check Sequence (32 bit CRC).


PhysicalLayer

A collection of differentaccess techniques:

• Infrared (IR), never really used

• Frequency hopping spread spectrum (FHSS), 1-2 Mbit/s nowobsolete

• Direct sequence spread spectrum (DSSS), 1,2,5.5 and 11 Mbit/s,the most diffused till 3-4 years ago

• Orthogonal Frequency Division Multiplexing (OFDM), nothing todo with FDM, this is a modulation technique 6 to 54 Mbit/s nowthe most used, and beyond

• Four different standards: 802.11; /b; /a/h/g; /n


Infrared

• WorksintheregularIRLEDrange,i.e.850-950nm

• Usedindooronly

• Employes diffusivetransmissions,nodescanreceivebothscatteredandline-of-sightsignals

• Maxoutputpower:2W

• Neverreallyimplemented...toughcanhave“reasons”insomeenvironments,anditisverycheap

• Tx usesaLED,RxaPhotodiode

• Wavelengthbetween850and950nm


Infrared

• Modulationis“baseband”PPM(PulsePositionModulation),similartoon-offkeyingwithManchesterencodingtoensureconstantsynctransitions

• 1Mbit/s:16/4PPM• 0000à 0000000000000001• 0001à 0000000000000010• 0010à 0000000000000100• 0011à 0000000000001000• 0100à 0000000000010000• ...

• 2Mbit/s:4/2PPM• 00à 0001• 01à 0010• 10à 0100• 11à 1000

• Pulsesare250ns


IRPLCPframe


• SYNC:variablelength,synchronizationandoptionalfieldsongaincontrolandchannelquality

• SFD(StartFrameDelimiter):4L-PPMslotswithahexsymbolof1001.ThisfieldindicatesthestartofthePLCPpreampleandperformsbitandsymbolsynchronization

• DR(DataRate):3L-PPMslotsandindicatesthespeedused:– 1Mbps:000;2Mbps:001

• DCLA(DCLevelAdjustment):usedforDClevelstabilization,32L-PPMslotandlookslikethis:– 1Mbps:00000000100000000000000010000000– 2Mbps:00100010001000100010001000100010

• LENGTH:numberofoctetstransmittedinthePSDU:16-bitinteger• CRC:headerprotection– 16bits• PSDU:actualdatacomingfromtheMAClayer;Max2500octets,Min0

SYNC SFD DR DCLA LENGTH CRC PSDU

802.11radios:SpreadSpectrum

Ø Allradio-basedPHYlayersemploySpreadSpectrum

Ø FrequencyHopping:transmitoverrandomsequenceoffrequencies

Ø DirectSequence:randomsequence(knowntobothsenderandreceiver),calledchippingcode

Ø OFDM:spreadthesignalovermanysubcarrierswithFFTbasedtechniques


802.11radios:Power

• Powerradiationislimitedto– 100mWEIRPinEU– 1000mWEIRPinUSA– 10mWEIRPinJapan

• NICcardsarethesameallovertheworld:changingpowerisjustamatteroffirmwareconfig.

• EIRP:EquivalentIsotropicRadiatedPower– Inpracticedefinesapowerdensityonairandnotatransmittedpower

• Usinghighgainantennas(inTx)canbe(legally)doneonlybyreducingthetransmittedpowerortocompensateforlossesoncables/electronics


Bandallocations

• ISM:IndustrialScientificMedical– Unlicenced bandsforgenericuse– Normallynotusedforcommunications(cfr Cellular,TV,Radio,...)– Lawdictateslimitsinuse,butdonotguaranteeinterference-freeoperations

– Similartoradio-amateursbands...butforthefactthatthoseareonlyforstudyandnotforcommercialuse


Bandallocations

• 2.4—2.5GHz– Actually83.5MHzofbandwidthinEU(13channels)and71.5inUS(11channels)

• 4.9—5.9GHz– Actualbandwidthassigneddependsoncountries,inUSandEUtherearenormally20-25channels(about120-150MHzofbandwidth)

• 3.5GHz– CurrentlyallottedonlyintheUS,veryusefulforextendedrange(upto5kmwith1Wpower)

• 60GHz– Oxigen absorption,verysmallBSS...alotofbandwidth


2.4GHzchannelsfor802.11FHSS

• 791MHzchannels• LimitsTx speedsinceTx happensononesinglechannelatatime• Thisschemeisalsousedbybluetooth

1 2 3 77 78 79

1MHz


2.4GHzchannelsfor802.11b/g

• Atmost3independent(orthogonal)FDMchannels– 1,6,11;1,7,12;2,7,12;1,7,13,...

• PartiallyoverlappingchannelsarenoxiousforCarrierSensingà exposedandhiddenterminalsresult


5GHzchannelsfor802.11a

• Overlappingchannelsareavoided– inUS12non-overlappingchannelscenteredat

• 5.180,5.200,5.220,5.240,5.260,5.280,5.300,5.320

• 5.745,5.765.5.785,5.805– inEUthefrequenciesaboveareforhyperlan2(licensed)thusintermediatefrequenciesareused• 5.35—5.47GHz6nonoverlappingchannels


Global5GHzbandplan

Original by Martin Johnsson: http://www.hiperlan2.com/presdocs/site/whitepaper.pdf


802.11 - FHSS

Ø 1or2Mbit/s only@ 2.4GHz

Ø GFSK modulation:basewaveforms aregaussian shaped,bitsareencoded shifting frequency,but thetechnique is such that it canalso beinterpreted as

Ø BPSK(2GFSKà 1Mbit/s)

Ø QPSK(4GFSKà 2Mbit/s)

Ø SlowFrequency Hopping SS

Ø 20to400ms dwell time⇒max 50 hop/s,min2.5hop/s


Ø 802.11- FHSS

Ø 1channelisusedasguardØ 78channelsaredividedinto3orthogonalchannelsof26

subchannelseach

Ø HoppingisaPNsequenceoverthe26channelsØ TxandRxmustagreeonthehoppingsequence

1 2 3 77 7876


FHPLCPframe

• Alwaystransmittedat1Mbits/s• SYNC:80bitsalternating01010101...• SFD:16bits(0000110010111101)• PLW:numberofoctetstransmittedinthePSDU:12-bitinteger• PSF:4bits,indicatestherateusedinthePSDU• CRC:headerprotection– 16bits

– GeneratingPolinomialG(x)=x16+x12+x5+1• PSDU:actualdatacomingfromtheMAClayer;Max4095octets,

Min0– Scrambledto“whiten”it

SYNC SFD PSFPLW HEC PSDU


Datascrambling(whitening)

• Itisasimplefeedbackshiftregistergeneratinga127bitlongsequenceXORedwithdata– S(x)=x7+x4+1

n Every32bitsa33-rdisinsertedtosuppresseventualbiases


DSSSPHY

• DirectSpreadingthroughdigitalmultiplicationwithachipsequence

• ThescopeisfadingprotectionandnotCDMA• Max3FDMorthogonalchannels• Differentspecificationsforthe1-2and5.5-11PHYspeeds

• Differentheaders– Long for802.11and802.11bincompatibilitymode– Short for802.11bHighRatesonly(5.5-11)


802.11bLongPreamblePLCPPDU

• CompatiblewithlegacyIEEE802.11systems• Preamble(SYNC+StartofFrameDelimiter)allowsreceivertoacquirethe

signalandsynchronizeitselfwiththetransmitter• Signalidentifiesthemodulationscheme,transmissionrate• LengthspecifiesthelengthoftheMPDU(expressedintimetotransmitit)• CRCsameasHECofFHSS

SYNC SFD Signal Service Length CRC MPDU

128 16 8 8 16 16

PLCPPreamble1Mbit/s

PLCPPDU(PPDU)

PLCPHeader1Mbit/s

1– 2– 5.5– 11Mbit/s


802.11bShortPreamblePLCPPDU

• NotcompatiblewithlegacyIEEE802.11systems• Fieldsmeaningisthesame

SYNC SFD Signal Service Length CRC MPDU

58 16 8 8 16 16

PLCPPreamble1Mbit/s

PLCPPDU(PPDU)

PLCPheader2Mbit/s 2– 5.5– 11Mbit/s


Txfor1-2Mbit/s

• Spreadingisobtainedwithan11bitsBarkercode– +1,–1,+1,+1,–1,+1,+1,+1,–1,–1,–1

• 1Mbit/susesabinarydifferentialPSK(DBPSK)– 0à jω =0;1à jω =π

• 2Mbit/susesaquadraturedifferentialPSK(DQPSK)– 00à jω =0;01à jω =π/2– 10à jω =π ;11à jω =3π/2


Barkercodes

• Asequenceof+1/-1oflengthNsuchthat

forall1<v<N

• Hasverygoodautocorrelationfunction(i.e.11fort=0,<1for1<t<11

• Improvesspectrumuniformity• Increasesreflectionrejection(robustnesstofading)becauseoftheautocorrelation(upto11bittimesdelays!!)


Txfor5.5and11Mbit/s

• UsesacomplexmodulationtechniquebasedonHadamardTransformsandknownasComplementaryCodeKeyingCCK

• Itisasequenceof8PSKsymbolswiththefollowingformula

c={ej(ϕ1+ϕ2+ϕ3+ϕ4);ej(ϕ1+ϕ3+ϕ4);ej(ϕ1+ϕ2+ϕ4);–ej(ϕ1+ϕ4);ej(ϕ1+ϕ2+ϕ3);ej(ϕ1+ϕ3);–ej(ϕ1+ϕ2);jϕ1 }

ϕi aredefineddifferently for5.5and11Mbit/s

n Theformuladefines8differentcomplexsymbolsat11Mchip/sn At11Mbit/s1bitismappedon1chip,at5.5themappingis1à2


Txfor5.5and11Mbit/s

• In5.5– ϕ1andϕ3donotcarryinformation– 4bitsarepairwiseDQPSKencodedonϕ2andϕ4

• In11– 8bitsarepairwiseDQPSKencodedonϕ1,ϕ2,ϕ3andϕ4

• TheresultingsignalisacomplexPSKmodulationoversinglechipswithcorrelatedevolutionovertheCCKcodes

• Inpracticethereare256(28)possiblecodewordsbutonly32(5.5Mbit/s)or64(11Mbit/s)areused– robustnesstofading


HadamardEncoding

• WecanviewthemasextensiontomultipledimensionsofBarkercodes

• Abroadsetoftransformationtechniquesusedinmanyfields– ThebasefortheMPEGvideoencoding– GeneralizationofFouriertransforms– QuantumComputing– …


TransmissionPowerMask


802.11a/gOFDMPHY

• 6,9,12,18,24,36,48,and54Mb/s• 6,12,24mandatory• 52subcarriersover20MHz,312.5kHzapart• AdaptiveBPSK,QPSK,16-QAM,64-QAM• OFDMsymbolduration4µs• Providesalso“halfed”and“quarter”over10and5MHzbydoubling(X4)theOFDMsymboltime

• Convolutionalencodingwithdifferentratesforerrorprotection– EncodingisembeddedwithintheOFDMMoDem


OFDMPPDU


• PLPCis12OFDMsymbolscorrespondingto48µs• RatedefinestheDATArate• Serviceisalways0andenablesscramblingsynchronization• SIGNALisprotectedwithar=1/2convolutionalcode

16

Sample16-QAMwithgraybitencoding


• Adjacentsymbolsdiffersbyonebitonly

• Makesmulti-biterrorslessprobable

• AssociatedwithinterleavingandconvolutionalencodinggreatlyreducesBERandhenceFER

Datarates,SlottimeandBW


• 802.11a achieves data rates 6,9,12,18,24,36,48, and 54 MB/s. • One OFDM symbol is sent every 4us, of which 0.8μs is the cyclic prefix (guard time)

64-QAM example:• 250ksymbols/s, 48 data carriers.• 64-QAM modulation = 64 = 26

• a convolutional code of rate 3/448 * 0.75 * 250k *6 = 54 Mbit/s

SLOT TIME• Slot time = RX-to-TX turnaround time + MAC processing delay + CCA < 9μswhere CCA = clear channel assessment

Typical times: • RX-to-TX turnaround time < 2μs• MAC processing delay < 2μs• CCA < 4μs

BPSK example:• 250k symbols sent every second.• One symbol uses 48 data carriers.• BPSK modulation with a convolutional code of rate 1/248 * 0.5 * 250k = 6 Mb/s

Bandwidth• One OFDM is 20 MHz and inludes 64 carriers:=> One carrier = 20MHz/64 = 312 kHz.

802.11a/gmodulations


Mod. Net(Mbit/s)

Gross(Mbit/s)

FECrate

Efficiency(bit/sym.)

T1472 B

(µs)

BPSK 6 12 1/2 24 2012

BPSK 9 12 3/4 36 1344

QPSK 12 24 1/2 48 1008

QPSK 18 24 3/4 72 672

16-QAM 24 48 1/2 96 504

16-QAM 36 48 3/4 144 336

64-QAM 48 72 2/3 192 252

64-QAM 54 72 3/4 216 224

Transmissionblockscheme


• ThemodulationisdoneinthedigitaldomainwithanIFFT• Interleavingdistributes(atthereceiver)evenlyerrorsavoiding

bursts• Convolutionalcodingcorrectsmostofthe“noise”errors

– Thisjustifiesthe“observation”thatmodern802.11tendstohaveanon-offbehavior

Receiverblockscheme


• Channelestimationenablesdistortioncorrection• ViterbidecodingisanMLdecoderforconvolutionalcodes

OFDMtransmissionpowermask


802.11gspecifics

• Definestheuseof802.11aOFDMtechniquesinthe2.4GHzband

• Mandatesbackwardcompatibilitywith802.11b• Introducessomeinefficiencyforbackwardcompatibility• ManyPPDUformats

– Long/sortpreambles– AllOFDM(pureg)orCCK/DSSSHeaderswithOFDMPSDU(compatibilitymodeorb/g)


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