Wireless Communications

Transcription

1 Wireless Communications Cannel Moeling Large Scale Hami Barami Electrical & Computer Engineering

2 EM Spectrum

3 Raio Wave l Raio wave: a form of electromagnetic raiation, create wenever a carge object accelerates wit a frequency l How are EM waves prouce? Carge particle à E-fiel an moving carge particle à B-feil

4 Review: Raio Wave Propagation l Electric fiel an magnetic fiels are ortogonal l Te irection of propagation of te EM wave is ortogonal to bot te electric an magnetic fiels l EM wave is propagating in te z irection l E fiel in orientate along te x axis l B fiel in orientate along te y axis

5 Raio Wave Propagation l Statement of te problem l Pat loss, reflection, iffraction, an scattering l Lack of irect line-of sigt pat between te Tx an Rx l Multipat faing l Large-scale faing: transmission over large T-R separation istance (unres or tousans meters) l Small-scale faing: transmission over sort travel istance (a few wavelengts) or sort time uration (secons)

6 Free Space Propagation Moel l To preict receive signal strengt wen te Tx an Satellite Rx ave a line-of-sigt (LOS) pat l Satellite communication systems l Microwave LOS raio links l Friis free space equation Receive power λ = c f = Transmitte power πc ω c P PG G λ t t r r ( ) = Transmitter antenna gain ( ) 4π L Eart Station Transceiver Receiver antenna gain Wavelengt in meters System loss factor L=1: no loss Distance between te Tx an Rx

7 Example: page 109, 4. l If 50W is applie to a unity gain antenna wit a 900M Hz carrier frequency, fin te receive power in Bm at a free space istance of 100 m from te antenna. Assume unity gain for te receiver antenna. P ( ) = PG G λ t t r r ( 4π ) L 8 6 ( / ) (50)(1)(1) P 3 r (100) = = mw (100) (1) ( 4π ) 3 ( 10 ) = 4. Bm mw 10 log 3.5 5

8 Free Space Propagation Moel l Pat loss l l l l P ( ) = PG G λ t t r r ( ) L 4π Signal attenuation Te ifference between te effective transmitte power an te receive power May or may not inclue te effect of te antenna gains Measure in B PL( B) = 10 log Pt P r = 10 log GtG (4π ) rλ PL( B) = 10 log Pt P r = 10 log λ (4π )

9 Free Space Propagation Moel l Review: near-fiel l Te close-in region of an antenna were te angular fiel istribution is epenent upon te istance from te antenna l Te region close to a source l Review: far-fiel l Te close-in region of an antenna were te angular fiel istribution is inepenent upon te istance from te antenna

10 Free Space Propagation Moel l Far-fiel (Fraunofer region) l Te region beyon te far-fiel istance f f = D λ f Te largest pysical linear imension of te antenna >> D f >> λ Reference point: P ( ) r = P ( r ) f P ( ) Bm r P ( ) r 0 0 = 10 log + 0 log 0 f

11 Example: page 109, Ex. 4.1 l Fin te far-fiel istance for an antenna wit max. imension of 1m an operating frequency of 900 MHz Answer: D f = λ Largest imension of antenna, D=1m Operating frequency, λ = c f = = 0.33m Te far-fiel istance D f = = = 6m f >> D f >> λ λ 0.33

12 Example: page 109, 4. l If 50W is applie to a unity gain antenna wit a 900M Hz carrier frequency, fin te receive power in Bm at a free space istance of 100 m from te antenna. Wat is Pr(10km)? Assume unity gain for te receiver antenna. Reference point: P ( ) r = P ( r ) f Pr ( 0) 0 Pr ( ) Bm = 10 log + 0 log P r ( ) Bm = log = 64. 5Bm f

13 Review: Raio Wave Propagation l Statement of te problem l Reflection, iffraction, an scattering l Lack of irect line-of sigt pat between te Tx an Rx l Multipat faing l Large-scale faing: transmission over large T-R separation istance (unres or tousans meters)

14 Take a look of tis case first

15 Reflection l Wen a raio wave propagating in one meium impinges upon anoter meium ave ifferent electrical properties l Te wave is partially reflecte l Te reflection coefficient l Te material properties l Te wave polarization l Te angle of incience l Te wave frequency

16 Groun Reflection Moel l Two-Ray moel Taking notes in class t E LOS E i E g E TOT = E LOS + E g θ θ r Te pat istance between te LOS an te groun reflecte pat Δ = ʹʹ ʹ = Δ t r ( + ) + ( ), wen >> t r t + r t r + 10( t + r )

17 1 t t r t + r t - r t r t r t r ; ; >> + >> >> an t t t r + << << 1 1 ( ) 1 r t t t r r r t r t Δ n n n n x n n n x = = + 0! 4 ) (1 )! ( 1) ( 1 Taylor Series = = )(4) ( 1)() ( 1 1 r t r t r t r t r t r t

18 Groun Reflection Moel l Te receive E-fiel E00 π tr ETOT λ l Te receive power at a istance >> t + r t r 4 Pr = PG t tgr PL( B) = 40 log ( 10 logg + 10 logg + 0 log + 0 log ) t r t r

19 Groun Reflection Moel l Avantage l Consier bot te irect pat an a groun reflecte propagation pat between te Tx an Rx l Disavantage l Oversimplifie: oes not inclue factors like terrain profile an surrounings l Weter te two-ray moel coul be applie? l Case 1: t =35 m, r =3m, =50 m l Case 1: t =30 m, r =1.5m, =450 m

20 Review: Raio Wave Propagation l Statement of te problem l Reflection, iffraction, an scattering l Lack of irect line-of sigt pat between te Tx an Rx l Multipat faing l Large-scale faing: transmission over large T-R separation istance (unres or tousans meters)

21 Diffraction l Signals propagate aroun te curve surface to te eart, beyon te orizon an to propagate bein obstructions l Cause by te propagation of seconary wavelets into a saowe region Diffracte power Tx Obstruction (e.g. mountain) Saowe faing Rx

22 Fresnel Zone Geometry β α γ o t r Tx Excess pat lengt a ( a + b) ( ) ( + ) Δ + = a b a b πδ π Δ θ = λ λ Fresnel-Kircoff iffraction parameter a b a v b = ( + ) a λ a b b b Rx Pase ifference: Heigt of te obstruction Position of te obstruction Position of Tx an Rx

23 Fresnel Zone Geometry α β ' γ t o r Tx a b Rx β θ b θ a α o - r t - r γ Tx a b Rx

24 Fresnel Zone Geometry l Diffraction Loss l An function of te pat ifference aroun an obstruction l An obstruction causes a blockage of energy from some of te Fresnel zones, tus allowing only some of transmitte energy to reac te receiver l Design LOS microwave links: 55% of te first Fresnel zone is clear l Preiction of te iffraction loss is not easy in a real life ue to complex an irregular terrain

25 Knife-ege Diffraction Moel l Te simplest iffraction moel l Wen saowing is cause by a single object l In practice, grapical or numerical solutions are relie upon to compute iffraction gain 10 ( ) G ( B ) = 0log F v G 0 v 1 1 0log ( 0. 6v ), 1 v v 0log ( ) ( e ), 0 v 1 B = ( ) 0log ( ), 1 v log ( v ), v >. 4

26 Multiple Knife-ege Diffraction l More tan one obstructive object l Te total iffraction loss ue to all of te obstacles must be compute l Replace all obstacles by a single equivalent one β α γ o t r Tx a b Rx

27 Scattering l Wen a raio wave impinges on a roug surface, te reflecte energy is sprea out (iffuse) in all irections ue to scattering l Resulting in te stronger receive signal

28 Example 4.8, pp. 133 l Given te following geometry, etermine (a) te loss ue to knife-ege iffraction, an (b) te eigt of te obstacle require to inuce 6 B iffraction loss. f=900 MHz. 100m 50m 10km km 5m

29 Practical Pat Loss Moel l Pat loss is te loss in signal strengt as a function of istance l Terrain epenent l Site epenent l Frequency epenent l May or may not epen on line of sigt (LOS) l Commonly use to estimate link bugets, cell sizes an sapes, capacity, anoff criteria, etc. l Moels are approximations of losses erive from measurements

30 Practical Pat Loss Moel l Log-istance pat loss moel l Te average receive signal power ecreases logaritmically wit istance PL( ) PL( B) = 0 n PL( 0 ) + 10n log 10 0 n: te pat loss exponent 0 : te close-in reference istance : te T-R separation istance

31 Practical Pat Loss Moel cont. l Log-istance pat loss moel l Pat loss exponents for ifferent environments Free space Urban area cellular raio.7 to 3.5 Saowe urban cellular raio 3 to 5 In builing LOS 1.6 to 1.8 Obstructe builings 4 to 6 Obstructe factories to 3

32 Practical Pat Loss Moel l Log-normal saowing l Consiering te ifferent surrouning environmental clutter wit te same T-R separation l Te pat loss PL() at a particular location is ranom an istribute log-normally (B) PL ( B) = PL( 0) + 10n log10 Xσ + 0 X :a zero - mean Gaussian istribute RV (B) σ wit stanar eviation σ (B)

33 Review: Gaussian Distribution l Q function or error function (erf) ) ( 1 ) ( 1 1 exp 1 ) ( z Q z Q x erf x x z Q z = = = π [ ] [ ] = > = > σ γ γ σ γ γ ) Pr( ) Pr( Pr ) Pr( ) Pr( Pr Q Q

34 Problem 4.1 l l l l During te first mont of work, you get an assignment to perform a measurement campaign to estimate te cannel pat loss exponent for a new wireless prouct. You performe fiel measurements an collecte te following ata: l reference pat loss: PL ( 0 ) l Pat loss measurements: PL ( 1 ), PL ( ), Using te pat loss exponent moel, fin an expression for te optimum value of te pat loss exponent n, wic minimizes te mean square error between measurements an te moel. Hint: te optimum value of n soul minimize te mean square error (MSE) between your preicte pat loss an measure pat loss. Take notes

35 Outoor Propagation Moels l Longley-Rice moel [ITS Irregular Terrain Moel] l Point-to-point communication systems l Frequency range: 40 MHz ~ 100 GHz l Tecniques l Te two-ray groun reflection moel l Te Fresnel-Kircoff knife-ege moel l Forwar scatter teory over long istances l Sortcomings l Does not provie corrections ue to environment factors l Does not consier multipat

36 Outoor Propagation Moels l Durkin s moel l Consiering te nature of propagation over irregular terrain an losses cause by obstacles in a raio pat l Typically use for te esign of moern wireless systems l Durkin pat loss simulator l Non-LOS l LOS, but wit inaequate first Fresnel-zone clearance l Sortcomings l Does not consier man-mae structures l Does not consier multipat

37 Outoor Propagation Moels l Okumura moel l One of te most wiely use moels in urban areas l Frequency range: 150 MHz ~ 190 MHz l Distance: 1km ~ 100 km l Station antenna eigts: 30m ~ 1000m l Wolly base on measure ata an oes not provie any analytical explanation l Sortcomings l Slow response to rapi canges in terrain

38 Outoor Propagation Moels l Hata moel l l An empirical formulation of te grapical pat loss ata Frequency range: 150 MHz ~ 1500 MHz L50( urban)( B) = log fc 13.8 log te a( re) + ( log te ) log 30m ~ 00m 1m ~ 10m l Well suite for large cell mobile systems, but ot personal communication systems (PCS) l Cost-31 l PCS extension to Hata moel

39 Inoor Propagation Moels l Differ from te traitional mobile raio cannels l Te istance covere are muc smaller l Te variability of te environment is muc greater l Relatively new researc fiel l Partition losses (same floor) l Har partition: partitions are forme as part of te builing structure l Soft partition: partitions may be move an o not span to te ceiling

40 Inoor Propagation Moels l Partition losses between floors l Te external imensions l Materials of te builings l Te type of construction use to create te floors l Te external surrounings l Te number of winows

41 Inoor Propagation Moels l Log-istance pat loss l Ericsson multiple breakpoint moel l Measurements in a multiple floor office builing l Four breakpoints PL ( B) = PL( 0 ) + 10n log + X σ l Bot upper an lower boun on te pat loss are consiere 0

42 Inoor Propagation Moels l Attenuation factor moel l Accurately eploy inoor an campus networks l Reuce te stanar eviation between measure an preicte pat loss to aroun 4 B PL( B) = PL( + n + + FAF + 0 ) 10 log PAF FAF: a floor attenuation factor for a specifie number of builing floors PAF: te partition attenuation factor for a specific obstruction encountere by a ray rawn between te TX an Rx in 3-D 0