nomadic communications wlan – 802.11 phy...
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NomadicCommunicationsWLAN– 802.11PHYLayersRenatoLoCigno
ANSGroup– [email protected]
http://disi.unitn.it/locigno/index.php/teaching-duties/nomadic-communications
Quest’operaèprotettadallalicenza:
CreativeCommonsAttribuzione-Non commerciale-Nonoperederivate2.5ItaliaLicense
Peridettagli,consultarehttp://creativecommons.org/licenses/by-nc-nd/2.5/it/
Copyright
NomadicCommunications– [email protected]– 802.11PHYLayers 2
PHYLayers
• DetailsoftheframeformatanddetailsoftheMACdependsondetailsofthePHYlayer
• Thisiswhyintheendin802networksMACandPHYarespecifiedtogetherwhileLLCandinternetworkingareincommon
• BothISO/OSIand(tosomeextent)TCP/IPfailedtorecognizethis
• ThereisanadditionallayerbetweenMACandPHY
NomadicCommunications– [email protected]– 802.11PHYLayers 3
PHYlayersubdivision
ü PLCP:PhysicalLayerConvergenceProtocolü PMD:PhysicalMediumDependant
MAC
PLCP
PMD
MPDU
PPDU
ü PPDUcontainsthePHYlayerheadersstrippedwhenthePDUispassedtotheMAC
ü PMDdefinesthespecificelectromagneticcharacteristicsusedondifferentPHYmeans
ü PLCPHeaderü IsactuallyalreadydependentonthePMDü Includessyncpreamblesandfurtherinfoontheencodingof
theremainingpartoftheMPDU
NomadicCommunications– [email protected]– 802.11PHYLayers 4
Goalsof802.11PHY(s)
• Flexibility– Everystandardallowsmultipledata-rates
• Robustness– Againstmultipathfadingandimpairments– Againstheterogeneouschannelconditions
• Preambleand“common”informationheadertransmittedalwaysatminimumspeed
• Payloadtransmittedatthebestrate(estimatedbyandatthetransmitter)
NomadicCommunications– [email protected]– 802.11PHYLayers 5
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
NomadicCommunications– [email protected]– 802.11PHYLayers 7
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
NomadicCommunications– [email protected]– 802.11PHYLayers 8
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
NomadicCommunications– [email protected]– 802.11PHYLayers 9
DetailedMACFormat(bytes)
NomadicCommunications– [email protected]– 802.11PHYLayers 10
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.
NomadicCommunications– [email protected]– 802.11PHYLayers 11
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).
NomadicCommunications– [email protected]– 802.11PHYLayers 12
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
NomadicCommunications– [email protected]– 802.11PHYLayers 13
Infrared
• WorksintheregularIRLEDrange,i.e.850-950nm
• Usedindooronly
• Employes diffusivetransmissions,nodescanreceivebothscatteredandline-of-sightsignals
• Maxoutputpower:2W
• Neverreallyimplemented...toughcanhave“reasons”insomeenvironments,anditisverycheap
• Tx usesaLED,RxaPhotodiode
• Wavelengthbetween850and950nm
NomadicCommunications– [email protected]– 802.11PHYLayers 14
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
NomadicCommunications– [email protected]– 802.11PHYLayers 15
IRPLCPframe
NomadicCommunications– [email protected]– 802.11PHYLayers 16
• 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
NomadicCommunications– [email protected]– 802.11PHYLayers 17
802.11radios:Power
• Powerradiationislimitedto– 100mWEIRPinEU– 1000mWEIRPinUSA– 10mWEIRPinJapan
• NICcardsarethesameallovertheworld:changingpowerisjustamatteroffirmwareconfig.
• EIRP:EquivalentIsotropicRadiatedPower– Inpracticedefinesapowerdensityonairandnotatransmittedpower
• Usinghighgainantennas(inTx)canbe(legally)doneonlybyreducingthetransmittedpowerortocompensateforlossesoncables/electronics
NomadicCommunications– [email protected]– 802.11PHYLayers 18
Bandallocations
• ISM:IndustrialScientificMedical– Unlicenced bandsforgenericuse– Normallynotusedforcommunications(cfr Cellular,TV,Radio,...)– Lawdictateslimitsinuse,butdonotguaranteeinterference-freeoperations
– Similartoradio-amateursbands...butforthefactthatthoseareonlyforstudyandnotforcommercialuse
NomadicCommunications– [email protected]– 802.11PHYLayers 19
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
NomadicCommunications– [email protected]– 802.11PHYLayers 20
2.4GHzchannelsfor802.11FHSS
• 791MHzchannels• LimitsTx speedsinceTx happensononesinglechannelatatime• Thisschemeisalsousedbybluetooth
1 2 3 77 78 79
1MHz
NomadicCommunications– [email protected]– 802.11PHYLayers 21
2.4GHzchannelsfor802.11b/g
• Atmost3independent(orthogonal)FDMchannels– 1,6,11;1,7,12;2,7,12;1,7,13,...
• PartiallyoverlappingchannelsarenoxiousforCarrierSensingà exposedandhiddenterminalsresult
NomadicCommunications– [email protected]– 802.11PHYLayers 22
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
NomadicCommunications– [email protected]– 802.11PHYLayers 23
Global5GHzbandplan
Original by Martin Johnsson: http://www.hiperlan2.com/presdocs/site/whitepaper.pdf
NomadicCommunications– [email protected]– 802.11PHYLayers 24
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
NomadicCommunications– [email protected]– 802.11PHYLayers 25
Ø 802.11- FHSS
Ø 1channelisusedasguardØ 78channelsaredividedinto3orthogonalchannelsof26
subchannelseach
Ø HoppingisaPNsequenceoverthe26channelsØ TxandRxmustagreeonthehoppingsequence
1 2 3 77 7876
NomadicCommunications– [email protected]– 802.11PHYLayers 26
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
NomadicCommunications– [email protected]– 802.11PHYLayers 27
Datascrambling(whitening)
• Itisasimplefeedbackshiftregistergeneratinga127bitlongsequenceXORedwithdata– S(x)=x7+x4+1
n Every32bitsa33-rdisinsertedtosuppresseventualbiases
NomadicCommunications– [email protected]– 802.11PHYLayers 28
DSSSPHY
• DirectSpreadingthroughdigitalmultiplicationwithachipsequence
• ThescopeisfadingprotectionandnotCDMA• Max3FDMorthogonalchannels• Differentspecificationsforthe1-2and5.5-11PHYspeeds
• Differentheaders– Long for802.11and802.11bincompatibilitymode– Short for802.11bHighRatesonly(5.5-11)
NomadicCommunications– [email protected]– 802.11PHYLayers 29
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
NomadicCommunications– [email protected]– 802.11PHYLayers 30
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
NomadicCommunications– [email protected]– 802.11PHYLayers 31
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
NomadicCommunications– [email protected]– 802.11PHYLayers 32
Barkercodes
• Asequenceof+1/-1oflengthNsuchthat
forall1<v<N
• Hasverygoodautocorrelationfunction(i.e.11fort=0,<1for1<t<11
• Improvesspectrumuniformity• Increasesreflectionrejection(robustnesstofading)becauseoftheautocorrelation(upto11bittimesdelays!!)
NomadicCommunications– [email protected]– 802.11PHYLayers 33
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
NomadicCommunications– [email protected]– 802.11PHYLayers 34
Txfor5.5and11Mbit/s
• In5.5– ϕ1andϕ3donotcarryinformation– 4bitsarepairwiseDQPSKencodedonϕ2andϕ4
• In11– 8bitsarepairwiseDQPSKencodedonϕ1,ϕ2,ϕ3andϕ4
• TheresultingsignalisacomplexPSKmodulationoversinglechipswithcorrelatedevolutionovertheCCKcodes
• Inpracticethereare256(28)possiblecodewordsbutonly32(5.5Mbit/s)or64(11Mbit/s)areused– robustnesstofading
NomadicCommunications– [email protected]– 802.11PHYLayers 35
HadamardEncoding
• WecanviewthemasextensiontomultipledimensionsofBarkercodes
• Abroadsetoftransformationtechniquesusedinmanyfields– ThebasefortheMPEGvideoencoding– GeneralizationofFouriertransforms– QuantumComputing– …
NomadicCommunications– [email protected]– 802.11PHYLayers 36
TransmissionPowerMask
NomadicCommunications– [email protected]– 802.11PHYLayers 37
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
NomadicCommunications– [email protected]– 802.11PHYLayers 38
OFDMPPDU
NomadicCommunications– [email protected]– 802.11PHYLayers 39
• PLPCis12OFDMsymbolscorrespondingto48µs• RatedefinestheDATArate• Serviceisalways0andenablesscramblingsynchronization• SIGNALisprotectedwithar=1/2convolutionalcode
16
Sample16-QAMwithgraybitencoding
NomadicCommunications– [email protected]– 802.11PHYLayers 40
• Adjacentsymbolsdiffersbyonebitonly
• Makesmulti-biterrorslessprobable
• AssociatedwithinterleavingandconvolutionalencodinggreatlyreducesBERandhenceFER
Datarates,SlottimeandBW
NomadicCommunications– [email protected]– 802.11PHYLayers 41
• 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
NomadicCommunications– [email protected]– 802.11PHYLayers 42
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
NomadicCommunications– [email protected]– 802.11PHYLayers 43
• ThemodulationisdoneinthedigitaldomainwithanIFFT• Interleavingdistributes(atthereceiver)evenlyerrorsavoiding
bursts• Convolutionalcodingcorrectsmostofthe“noise”errors
– Thisjustifiesthe“observation”thatmodern802.11tendstohaveanon-offbehavior
Receiverblockscheme
NomadicCommunications– [email protected]– 802.11PHYLayers 44
• Channelestimationenablesdistortioncorrection• ViterbidecodingisanMLdecoderforconvolutionalcodes
OFDMtransmissionpowermask
NomadicCommunications– [email protected]– 802.11PHYLayers 45
802.11gspecifics
• Definestheuseof802.11aOFDMtechniquesinthe2.4GHzband
• Mandatesbackwardcompatibilitywith802.11b• Introducessomeinefficiencyforbackwardcompatibility• ManyPPDUformats
– Long/sortpreambles– AllOFDM(pureg)orCCK/DSSSHeaderswithOFDMPSDU(compatibilitymodeorb/g)
NomadicCommunications– [email protected]– 802.11PHYLayers 46