Nplooj ntawv no piav qhia txog Fading hauv paus thiab hom fading hauv kev sib txuas lus wireless. Cov hom Fading tau muab faib ua cov nplai loj fading thiab me me fading (multipath ncua kis thiab doppler kis).
Flat fading thiab zaus xaiv fading yog ib feem ntawm multipath fading qhov twg sai fading thiab qeeb fading yog ib feem ntawm doppler kis fading. Cov fading hom no yog siv raws li Rayleigh, Rician, Nakagami thiab Weibull kev faib khoom lossis qauv.
Taw qhia:
Raws li peb paub wireless kev sib txuas lus system muaj transmitter thiab receiver. Txoj kev los ntawm transmitter mus rau lub receiver tsis du thiab lub teeb liab kis tau los ntawm ntau yam attenuations nrog rau txoj kev poob, multipath attenuation thiab lwm yam. Lub teeb liab attenuation los ntawm txoj kev nyob ntawm ntau yam. Lawv yog lub sij hawm, xov tooj cua zaus thiab txoj kev los yog txoj hauj lwm ntawm transmitter/receiver. Cov channel ntawm transmitter thiab receiver tuaj yeem yog lub sijhawm sib txawv lossis kho nyob ntawm seb lub transmitter / receiver tau kho lossis txav mus los ntawm ib leeg.
Dab tsi yog fading?
Lub sij hawm variation ntawm tau txais lub zog hluav taws xob vim muaj kev hloov pauv hauv kev sib kis nruab nrab lossis txoj hauv kev yog hu ua fading. Fading nyob ntawm ntau yam raws li tau hais los saum toj no. Nyob rau hauv tas li scenario, fading nyob ntawm atmospheric tej yam kev mob xws li los nag, lightening thiab lwm yam. Nyob rau hauv mobile scenario, fading nyob ntawm obstacles hla txoj kev uas txawv raws li lub sij hawm. Cov teeb meem no tsim kev sib kis tau zoo rau cov teeb liab kis.
Daim duab-1 qhia txog qhov amplitude piv rau daim ntawv qhia kev deb rau qeeb fading thiab ceev fading hom uas peb yuav tham tom qab.
Fading hom
Xav txog ntau yam channel cuam tshuam txog kev puas tsuaj thiab txoj hauj lwm ntawm transmitter / receiver hauv qab no yog cov hom kev ploj hauv kev sib txuas lus wireless.
➤ Loj Scale Fading: Nws suav nrog txoj kev poob thiab cov duab ntxoov ntxoo.
➤Small Scale Fading: Nws muab faib ua ob pawg loj viz. multipath ncua kis thiab doppler kis. Lub multipath ncua kis tau muab faib ua fading fading thiab zaus xaiv fading. Doppler kis tau muab faib ua nrawm nrawm thiab qeeb fading.
➤ Fading qauv: Saum toj no fading hom yog siv nyob rau hauv ntau yam qauv los yog faib uas muaj xws li Rayleigh, Rician, Nakagami, Weibull thiab lwm yam.
Raws li peb paub, fading signals tshwm sim vim kev xav ntawm hauv av thiab ib puag ncig cov tsev nrog rau cov teeb liab tawg los ntawm cov ntoo, cov neeg thiab cov yees nyob hauv thaj chaw loj. Muaj ob hom fading viz. loj nplai fading thiab me me fading.
1.) Loj Scale Fading
Loj loj fading tshwm sim thaum muaj kev cuam tshuam los ntawm transmitter thiab receiver. Hom kev cuam tshuam no ua rau muaj qhov tseem ceeb ntawm cov teeb liab lub zog txo. Qhov no yog vim EM nthwv dej yog duab ntxoov ntxoo los yog thaiv los ntawm kev cuam tshuam. Nws muaj feem xyuam nrog kev hloov pauv loj ntawm lub teeb liab hla kev deb.
1.a) Txoj kev poob
Qhov chaw dawb txoj kev poob tuaj yeem qhia tau raws li hauv qab no.
➤ Pt/Pr = {(4 * π * d)2/ λ2} = (4 * π * f * d)2/c2
Qhov twg,
Pt = Xa lub zog
Pr = Txais lub zog
λ = wavelength
d = Qhov deb ntawm kev xa xov thiab txais kav hlau txais xov
c = speed of light ie 3 x 108
Los ntawm qhov sib npaug nws txhais tau hais tias cov teeb liab kis tau los ntawm qhov kev ncua deb raws li lub teeb liab tau kis mus rau thaj tsam loj thiab loj dua los ntawm kev xa mus rau qhov kawg ntawm qhov kawg.
1.b) Duab ntxoov ntxoo nyhuv
• Nws raug pom nyob rau hauv kev sib txuas lus wireless. Duab ntxoov ntxoo yog qhov sib txawv ntawm qhov tau txais lub zog ntawm EM teeb liab los ntawm tus nqi nruab nrab.
• Nws yog tshwm sim los ntawm obstacles hla txoj kev ntawm transmitter thiab receiver.
• Nws nyob ntawm qhov chaw nyob raws li xov tooj cua zaus ntawm EM (ElectroMagnetic) nthwv dej.
2. Me me fading
Cov nplai me me fading yog txhawj xeeb nrog kev hloov pauv sai ntawm cov teeb liab tau txais lub zog dhau qhov kev ncua deb thiab lub sij hawm luv luv.
Raws limultipath ncua kev sib kismuaj ob hom me me fading viz. tiaj fading thiab zaus xaiv fading. Cov multipath fading hom nyob ntawm qhov chaw nthuav tawm.
2.a) Pas fading
Lub wireless channel tau hais tias yuav tsum tau fading yog tias nws muaj qhov nce tsis tu ncua thiab cov lus teb theem tawm dhau lub bandwidth uas ntau dua qhov bandwidth ntawm cov teeb liab kis.
Nyob rau hauv no hom ntawm fading tag nrho cov zaus Cheebtsam ntawm lub tau txais teeb liab fluctuate nyob rau hauv tib proportions ib txhij. Nws tseem hu ua non-selective fading.
• Teeb liab BW << Channel BW
• Lub sij hawm cim >> Ncua kis
Cov nyhuv ntawm tiaj tus fading yog pom tias txo qis hauv SNR. Cov fading fading channel no yog hu ua amplitude varying channels los yog narrowband channels.
2.b) Zaus Xaiv fading
Nws cuam tshuam cov spectral sib txawv ntawm lub xov tooj cua teeb liab nrog txawv amplitudes. Yog li lub npe xaiv fading.
• Teeb liab BW > Channel BW
• Lub sij hawm cim < ncua kis
Raws lidoppler kis taumuaj ob hom fading viz. ceev fading thiab qeeb fading. Cov doppler kis fading hom nyob ntawm qhov ceev ntawm lub xov tooj ntawm tes piv txwv li ceev ntawm receiver nrog kev hwm rau transmitter.
2.c) Fast fading
Qhov tshwm sim ntawm kev nrawm nrawm yog sawv cev los ntawm kev hloov pauv sai ntawm cov teeb liab hla thaj chaw me me (piv txwv li bandwidth). Thaum cov teeb liab tuaj txog ntawm tag nrho cov lus qhia hauv lub dav hlau, ceev fading yuav raug pom rau txhua qhov kev taw qhia.
Fast fading tshwm sim thaum channel impulse teb hloov sai heev nyob rau hauv lub cim lub sij hawm.
• Siab doppler kis
• Lub sij hawm cim > Lub sij hawm sib txuas
• Teeb liab variation < Channel variation
Cov kev txwv no ua rau muaj qhov sib txawv ntau zaus lossis lub sijhawm xaiv fading vim doppler kis. Fast fading yog tshwm sim los ntawm kev xav ntawm cov khoom hauv zos thiab kev txav ntawm cov khoom txheeb ze rau cov khoom.
Nyob rau hauv ceev fading, tau txais cov teeb liab yog suav nrog ntau cov cim qhia uas tau cuam tshuam los ntawm ntau qhov chaw. Cov teeb liab no yog qhov sib npaug lossis qhov sib txawv ntawm ntau lub teeb liab uas tuaj yeem tsim kho lossis puas tsuaj raws li cov txheeb ze hloov pauv ntawm lawv. Phase kev sib raug zoo nyob ntawm qhov ceev ntawm kev txav, zaus ntawm kev sib kis thiab cov txheeb ze txoj kev ntev.
Fast fading distors cov duab ntawm lub baseband mem tes. Qhov no distortion yog linear thiab tsimISI(Inter Symbol Interference). Adaptive equalization txo ISI los ntawm kev tshem tawm cov kab sib txawv ntawm cov channel.
2.d) Ua qeeb qeeb
Kev fading qeeb yog tshwm sim los ntawm kev ntxoov ntxoo los ntawm cov tsev, toj, roob thiab lwm yam khoom hla txoj kev.
• Tsawg Doppler kis
• Lub sijhawm cim <
• Teeb liab Variation >> Channel Variation
Kev siv ntawm Fading qauv los yog fading distributions
Kev siv cov qauv fading lossis fading distributions suav nrog Rayleigh fading, Rician fading, Nakagami fading thiab Weibull fading. Cov kev faib tawm los yog cov qauv no yog tsim los koom nrog fading nyob rau hauv cov ntaub ntawv baseband raws li fading profile yuav tsum tau.
Rayleigh fading
• Hauv Rayleigh qauv, tsuas yog cov khoom tsis yog Kab pom (NLOS) yog simulated ntawm transmitter thiab receiver. Nws yog xav tias tsis muaj LOS txoj hauv kev nyob nruab nrab ntawm transmitter thiab receiver.
• MATLAB muab "rayleighchan" muaj nuj nqi los simulate rayleigh channel qauv.
• Lub hwj chim yog exponentially faib.
• Cov theem yog sib npaug faib thiab ywj siab los ntawm lub amplitude. Nws yog hom Fading feem ntau siv hauv kev sib txuas lus wireless.
Rician fading
• Hauv cov qauv rician, ob qho tib si Line of Sight (LOS) thiab non Line of Sight (NLOS) Cheebtsam yog simulated ntawm transmitter thiab receiver.
• MATLAB muab "ricianchan" muaj nuj nqi los simulate rician channel qauv.
Nakagami fading
Nakagami fadding channel yog ib qho qauv siv los piav qhia txog kev sib txuas lus wireless uas tau txais sgnal undergoes multipath fading. Nws sawv cev rau ib puag ncig nrog rau nruab nrab mus rau loj fading xws li hauv nroog lossis suburban cheeb tsam. Cov kab zauv hauv qab no tuaj yeem siv los simulate Nakagami fading channel qauv.
• Hauv qhov no peb qhia h = r*ej Φthiab lub kaum sab xis Φ yog sib npaug ntawm [-π, π]
• Qhov sib txawv r thiab Φ yog suav tias yog kev ywj pheej.
• Nakagami pdf yog qhia raws li saum toj no.
• Hauv Nakagami pdf, 2σ2= E{r2}, Γ (.) yog Gamma muaj nuj nqi thiab k >= (1/2) yog tus fading daim duab (degrees ntawm kev ywj pheej hais txog tus naj npawb ntawm Gaussion random variables ntxiv).
• Nws yog Ameslikas tsim empirically raws li kev ntsuas.
• Gamma tau txais tam sim ntawd. • Nrog k = 1 Rayleigh = Nakagami
Weibull fading
Cov channel no yog lwm tus qauv siv los piav qhia txog kev sib txuas lus wireless. Weibull fading channel feem ntau yog siv los sawv cev ib puag ncig nrog ntau hom kev fading nrog rau ob qho tib si tsis muaj zog thiab mob hnyav.
Qhov twg,
2 s ib2= E{r2}
• Weibull faib sawv cev rau lwm qhov kev nthuav dav ntawm Rayleigh kev faib tawm.
• Thaum X thiab Y yog iid xoom txhais tau tias gaussian variables, lub hnab ntawv ntawm R = (X2+ Y2)1/2yog Rayleigh faib. • Txawm li cas los lub hnab ntawv txhais tau tias R = (X2+ Y2)1/2, thiab qhov sib thooj pdf (faiv fab faib profile) yog Weibull faib.
• Cov kab zauv hauv qab no tuaj yeem siv los simulate Weibull fading qauv.
Hauv nplooj ntawv no peb tau dhau los ntawm ntau lub ntsiab lus ntawm fading xws li dab tsi yog fading channel, nws hom, fading qauv, lawv cov ntawv thov, kev ua haujlwm thiab lwm yam. Ib tus tuaj yeem siv cov ntaub ntawv muab rau ntawm nplooj ntawv no txhawm rau sib piv thiab muab qhov sib txawv ntawm cov nplai me me fading thiab cov nplai loj fading, qhov sib txawv ntawm lub tiaj fading thiab zaus xaiv fading, qhov sib txawv ntawm ceev fading thiab qeeb fading, qhov sib txawv ntawm rayleigh fading thiab rician fading thiab yog li ntawd.
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Post lub sij hawm: Aug-14-2023
