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Orthogonal Frequency Division Modulation (OFDM) Orthogonal Frequency Division Modulation (OFDM) • OFDM diagram • Inter Symbol Interference • Packet detection and synchronization • Related works Motivation • Signal over wireless channel y[n] = Hx[n] • Work only for narrow-band channels, but not for wide-band channels e.g., 20 MHz for 802.11 20MHz frequency 2.45GHz (Central frequency) Capacity = BW * log(1+SNR) Orthogonal Frequency Division Multiplex (OFDM) Tutorial 1 Orthogonal Frequency Division Multiplex (OFDM) Tutorial 1 Orthogonal Frequency Division Multiplex (OFDM) Tutorial 1 Orthogonal Frequency Division Multiplex (OFDM) Tutorial 1 Intuitive Guide to Principles of Communications www.complextoreal.com Intuitive Guide to Principles of Communications www.complOrthogonalextoreal.coIntuitive Frequencym Guide toDivision Principles Multip lexingof Communications (OFDM) www.complextoreal.com Intuitive Guide to Principles of Communications Modulation - a mapping of the information on changes in the carrier phase, frequency or www.complextoreal.com amplitude or combination . Orthogonal Frequency Division Multi plexing (OFDM) Multiplexing - method of sharing a bandwidth with other independent data channels. Modulati Orthogonalon - a mapping Frequency of the informati Divisionon on chan Multiges in pthelexing carrier (phase,OFD frequencyM) or amplitudeOFDM or combination is a com. bination of modulation and multiplexing. Multiplexing generally refers to Multiplexingindependent - methodsignals, of those sharingOrthogonal produced a bandwidth by Frequency different with other sources. independDivision Soent Multiit data is a chquestionplexingannels. of(O howFDM to) share the spectru m withModulati these users. on In - OFDM a mappi theng questi of theon ofinformati multiplexingon o isn chanappliedges to inindependent the carrier phase, frequency or OFDM issignals a comamplitudeb inationbut these of m orindependentodulation combination and signals muModulatiltiplexing.. are a sub-setonMul -tiple a mappiofxing the generally ngone of m thea inrefers signal.informati to In OFDMon on chan the signalges in the carrier phase, frequency or independentitself signals, is first those splitMultiplexing produced into inde amplitudebypendent different - method channels, orsources. combination of So s mharing itodul is a atedquestion. a baby ndata ofdwidth how and to then withshare re-m theother ultiplexed independ to crenteate data channels. spectrum with these users. In OFDM the question of multiplexing is applied to independent signals butthe these OFDM independent carrier. signals are a sub-setMultiplexing of the one m -a inmethod signal. Inof OFDMsharing the a signal band width with other independent data channels. itself is first splitOFDM into inde ispendent a com channels,b ination m odulof matedodulation by data and and then m re-multiplexing.ultiplexed to Mul createtiple xing generally refers to the OFDMOFDM carrier.independent is a special casesignals, OFDMof Frequency those is a produced com Divisionbination byMult differentofip mlexodulation (FDM). sources. Asand an mSo analogy,u ltiplexing.it is a aquestion FDM Mul channeltiple of xinghow generallyto share therefers to is like water flow out of a faucet, in contrast the OFDM signal is like a shower. In a faucet all spectrum with theseindependent users. In s iOFDMgnals, those the questi producedon of by m differentultiplexing sources. is applied So it is to a independentquestion of how to share the OFDM iswater a special co mescase inof Frequencyone big streamspectru Division amnd Multwit cannothip theselex be (FDM). sub-diviusers. As In dane OFDM d.analogy, OFDM thea FDMshower questi channel ison m ofa de m upultiplexing of a lot of is applied to independent is like water flowsignals out of buta faucet, these in contindependentrast the OFD signalsM signal isare like a asub-set shower. Inof a the faucet one all main signal. In OFDM the signal little streams. signals but these independent signals are a sub-set of the one main signal. In OFDM the signal water comes initself one bi gis stream first splitand cannot into beinde sub-divipendentded. OFDM channels, shower m isodul madeated up of by a lodatat of and then re-multiplexed to create itself is first split into independent channels, modulated by data and then re-multiplexed to create little streams. the OFDM carrier. the OFDM carrier. OFDM is a specialOFDM case isof a Frequency special case Division of Frequency Mult iDivisionplex (FDM). Multi pAslex an (FDM). analogy, As ana FDM analogy, channel a FDM channel is like water flowis out like of water a faucet, flow outin cont of arast faucet, the inOFD contMrast signal the OFD is likeM signala shower. is like In a a shower. faucet allIn a faucet all (a) Basic Concept (b) of OFDM (a) water co mes in(b) one water bi gco streammes in aonend cannotbig stream be sub-diviand cannotde d.be OFDM sub-divi sdhowered. OFDM is m ashowerde up ofis ma alodet ofup of a lot of Wide-bandFig. 1little – (a) streams.A Reg channelular - FDlittleM si nglstreams.e carriMultipleer – A whol narrow-bande bunch of water c ochannelsming all in one stream. (b) Fig. 1 – (aOrt) A Regh oguonalar-lFD-FDMM si ngl– Sae cmearri amoer – uAnt w hoolf wate buncher co ofming water fr comoming a lo allt of in smallone stre stamrea. ms.(b) Orthogona l-FDM – Same amount of water coming from a lot of small streams. Think about what the advantage might be of one over the other? One obvious one is that if I put Think aboutmy what thum theb overadvantage the faucet might hole,be of oneI can over stop the the other? water On flowe obvio butus Ione cannot is that do if theI pu samet for the shower. my thumb over the faucet hole, I can stop the water flow but I cannot do the same for the shower. So althouSogh balthouoth dogh the b sameoth d thing,o the thesamey respond thing, diftheferentlyy respond to int diferference.ferently to interference. (a) (b) (a) (b) Fig. 1 – (a) A Regular-FDM single carrier – A whole bunch of water coming all in one stream. (b) Fig. 1 – (a) A RegOrtularh-ogFDonaMl -FDsinglMe –c aSarrimeer amo – Au wnth olofe wat buncher co ofming wa fterrom co aming lot of all small in one strea strems.am . (b) Orthogonal-FDM – Same amount of water coming from a lot of small streams. Think about what the advantage might be of one over the other? One obvious one is that if I put Fig. 2 – AlFil cag.rgo 2 – o Aln onel ca trurgoc ko nvs one. spl itruttincgk the vs .shipmen splittintg into the more shipmen thant o intone. more than one. Think about whatm they thum advantageb over the m ifaucetght be hole, of one I can over stop the the other? water On flowe obvio but I cannotus one dois thatthe same if I pu fort the shower. Another way tom seey thumthis intuitivelyb over istheSo to faucetusealthou the analogygh hole, both Iof dcan mo atheking stop same a shipmentthe thing, water via the flowa truck.y respond but I cannot differently do the to intsameerference. for the shower. We have twoAnother options,So althouway one to hire ghsee b athis obigth intuitively trucdok the or a samebu isnch to ofthing,use smaller the theanalogy ones.y respond Both of mmethodsa kingdiff erently acarry shipment the to intviae arference. truck. exact sameWe am haveount oftwo data. options, But in onecase hireof an a accident, big truc konl ory a1/4 bu ofnch data of on smaller the OFDM ones. trucking Both methods carry the will suffer.exact same amount of data. But in case of an accident, only 1/4 of data on the OFDM trucking will suffer. Send samples concurrently using Send a sample using the entire band multiple orthogonal sub-channels Copyright 2004 Charan Langton www.complextoreal.com Copyright 2004 Charan Langton www.complextoreal.com Fig. 2 – All cargo on one truck vs. splitting the shipment into more than one. Fig. 2 – All cargo Anotheron one tru wayck to vs see. sp lthisittin intuitivelyg the shipmen is to tuse into th moree analogy than of on me.a king a shipment via a truck. We have two options, one hire a big truck or a bunch of smaller ones. Both methods carry the Another way to seeexact this same intuitively amount isof todata. use But the in analogy case of ofan maccident,aking a onl shipmenty 1/4 of viadata a on truck. the OFDM trucking We have two options,will suffer. one hire a big truck or a bunch of smaller ones. Both methods carry the exact same amount of data. But in case of an accident, only 1/4 of data on the OFDM trucking will suffer. Copyright 2004 Charan Langton www.complextoreal.com Copyright 2004 Charan Langton www.complextoreal.com Why OFDM is better? t t 1 0 0 1 0 f 1 0 0 1 1 0 0 0 1 …........ f Wide-band Narrow-band • Multiple sub-channels (sub-carriers) carry samples sent at a lower rate Almost same bandwidth with wide-band channel • Only some of the sub-channels are affected by interferers or multi-path effect Importance of Orthogonality • Why not just use FDM (frequency division multiplexing) Not orthogonal Individual sub-channel Leakage interference from adjacent sub-channels f guard band Guard bands protect leakage interference f • Need guard bands between adjacent frequency bands à extra overhead and lower throughput 2 OFDM 6 (a) Adjacent sub-channels interfere Difference between FDM and OFDM guard band (b) Guard bands protect leakage from adjacent frequencies Figure 9: Frequency Division Multiplexingf Frequency division multiplexing f Orthogonal sub-carriers in OFDM Don’t need guard bands Figure 10: Sub-carriers in OFDM It is easy to see that these sub-carriers are orthogonal, i.e. they do not interfere with each other. N/2 1 − j2π k t j2π p t φ = e− N e N =0 (p = k) (6) ⇥ t= N/2 − Hence we can improve spectral e⇥ciency without causing interference between the sub-carriers.
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