Mobile Communications

Transcription

1 Mobile Communications Part IV- Propagation Characteristics Professor Z Ghassemlooy School of Computing, Engineering and Information Sciences University of Northumbria U.K.

2 Contents Radiation from Antenna Propagation Model (Channel Models) Free Space Loss Plan Earth Propagation Model Practical Models Summary

3 Wireless Communication System Source Message Signal Source Encoder Channel code word Channel Encoder Demodulator Modulator Modulated Transmitted Signal Wireless Channel User Estimate of Message signal Source Decoder Channel Decoder Estimate of channel code word Received Signal

4 Antenna - Ideal Isotropics antenna: In free space radiates power equally in all direction. Not realizable physically H P t d E EM fields around a transmitting antenna, a polar coordinate d Distance d Area d- distance directly away from the antenna. φ is the azimuth, or angle in the horizontal plane. θ is the zenith, or angle above the horizon.

5 Antenna - Real Not isotropic radiators, but always have directive effects (vertically and/or horizontally) A well defined radiation pattern measured around an antenna Patterns are visualised by drawing the set of constant-intensity surfaces

6 Antenna Real - Simple Dipoles Not isotropic radiators but, e.g., dipoles with lengths λ/4 on car roofs or λ/ as Hertzian dipole λ/4 λ/ y y z x z x simple dipole side view (xy-plane) side view (yz-plane) top view (xz-plane) Example: Radiation pattern of a simple Hertzian dipole shape of antenna proportional to wavelength

7 Antenna Real - Sdirected and Sectorized Used for microwave or base stations for mobile phones (e.g., radio coverage of a valley) y y z x z x Directed side view (xy-plane) side view (yz-plane) top view (xz-plane) z z x x Sectorized top view, 3 sector top view, 6 sector

8 Antenna - Ideal - contd. The power density of an ideal loss-less antenna at a distance d away from the transmitting antenna: P a = PG t 4πd t W/m Note: the area is for a sphere. G t is the transmitting antenna gain The product P t G t : Equivalent Isotropic Radiation Power (EIRP) which is the power fed to a perfect isotropic antenna to get the same output power of the practical antenna in hand.

9 Antenna - Ideal - contd. The strength of the signal is often defined in terms of its Electric Field Intensity E, because it is easier to measure. P a = E /R m where R m is the impedance of the medium. For free space R m = 377 Ohms.. E P R P R = and E = V/m t m t m 4πd 4πd

10 Antenna - Ideal - contd. The receiving antenna is characterized by its effective aperture A e. The effective aperture A e is related to the gain G r as follows: A e = P r / P a => A e = G r λ /4π which is the equivalent power absorbing area of the antenna. G r is the receiving antenna gain and λ = c/f

11 Signal Propagation (Channel Models)

12 Channel Models High degree of variability (in time, space etc.) Large signal attenuation Non-stationary, unpredictable and random Unlike wired channels it is highly dependent on the environment, time space etc. Modelling is done in a statistical fashion The location of the base station antenna has a significant effect on channel modeling Models are only an approximation of the actual signal propagation in the medium. Are used for: performance analysis simulations of mobile systems measurements in a controlled environment, to guarantee repeatability and to avoid the expensive measurements in the field.

13 Channel Models - Classifications System Model - Deterministic Propagation Model- Deterministic Predicts the received signal strength at a distance from the transmitter Derived using a combination of theoretical and empirical method. Stochastic Model - Rayleigh channel Semi-empirical (Practical +Theoretical) Models

14 Channel Models Is almost always linear, and also time-variant because of its mobility. Thus, fully described by its impulse response h(τ, t), where τ is the delay parameter and t is the time. The complex impulse response h(τ, t) is a low-pass equivalent model of the actual real band-pass impulse response. Equivalently, the channel is characterized by its transfer function which is the Fourier transform of the h(τ, t): H ( f, t) = h( τ, t)exp( jπfτ ) dτ The magnitude H(f, t) is changing randomly in time, so the mobile radio channel is described as a fading channel. The phase arg H(f, t) is also a random function of time.

15 Channel Models Multi-path channel impulse response )) ( ( ))], ( ) ( ( )exp[, ( ), ( 1 0 t t t f j t a t h i i i N i c i b τ τ δ τ φ τ π τ τ + = =

16 Propagation Path Loss The propagation path loss is L PE = L a L lf L sf where L a is average path loss (attenuation): (1-10 km), L lf - long term fading (shadowing): 100 m ignoring variations over few wavelengths, L sf - short term fading (multipath): over fraction of wavelength to few wavelength. Metrics (dbm, mw) [P(dBm) = 10 * log[ P(mW) ]

17 Propagation Path Loss Free Space Power received at the receiving antenna P r = PG G t t r 4 λ πd Thus the free space propagation path loss is defined as: P r GtGrλ Lf = 10Log10 = 10Log10 P t (4πd) Isotropic antenna has unity gain (G = 1) for both transmitter and receiver.

18 Propagation - Free Space contd. The difference between two received signal powers in free space is: ΔP = P r1 d1 log10 = 0log Pr d db If d = d 1, the ΔP = -6 db i.e 6 db/octave or 0 db/decade

19 Propagation - Non-Line-of-Sight Generally the received power can be expressed as: P r d -v For line of sight v =, and the received power P r d - For non-line of sight with no shadowing, received power at any distance d can be expressed as: d P r ( d) = 10log10[ Pr ( dref )] + 10v log10 dref 100 m< d ref < 1000 m

20 Propagation - Non-Line-of-Sight Log-normal Shadowing d Pr ( d) = 10log Pr d v 10[ ( ref )] + 10 log10 + d ref X σ Where X σ : N(0,σ) Gaussian distributed random variable

21 Received Power for Different Value of Loss Parameter v v =, Free space Received power (dbm) v = 3 Rural areas v = 4, Distance (km) City and urban areas

22 Propagation Model- Free Space In terms of frequency f and the free space velocity of electromagnetic wave c = 3 x 10 8 m/s it is: L f c / f = 0log10 4πd db Expressing frequency in MHz and distance d in km: L f = 0log10( c / 4π) + 0log10( f ) + 0log10( d) = 0 log (0.3/ 4π) + 0log ( f ) + 0log ( d) db L f = log10( f ) + 0log10( d) db

23 Propagation Model- Free Space (non-ideal, path loss) Non-isotropic antenna gain unity, and there are additional losses L ad, thus the power received is: P r Pλ t = GtGr d > 0 and L 0 (4πd ) Lad Thus for Non-isotropic antenna the path loss is: L BS f ni = 10log log ( G 10 t ( ) 10log 10 f ) + 0log ( G 10 r 1 ) 0log 10 ( d) + 10log ( c / 4π) 10 ( L MU Note: Interference margin can also be added ad ) db

24 Propagation Model - Mechanisms Reflection Diffraction Scattering Source: P M Shankar

25 Channel Model- Plan Earth Path Loss - Ray Reflection In mobile radio systems the height of both antennas (Tx. and Rx.) << d (distance of separation) d d d Direct path (line of sight) h b h m d r Ground reflected path From the geometry d d = [d + (h b -h m ) ]

26 Channel Model- Plan Earth Path Loss - contd. Using the binomial expansion Note d >> h b or h m. d d d h b d h m Similarly d r d 1 + hb d h m The path difference Δd = d r - d d = (h b h m )/d The phase difference Δφ = π λ h b d h m = 4πh b λd h m

27 Channel Model- Plan Earth Path Loss contd. Total received power P r = PG G t t r λ 4πd 1+ ρe jδφ Where ρ is the reflection coefficient. For ρ = -1 (low angle of incident) and. 1 e jδφ = 1 cos Δφ + j sin Δφ Hence 1 e jδφ = (1 cos Δφ) + sin Δφ = (1 cos Δφ) = 4sin ( Δφ / )

28 Channel Model- Plan Earth Path Loss contd. Therefore: λ π π λ = d h h d PG G P m b r t t r 4 sin Assuming that d >> h m or h b, then 1 << λ π d h h m b sin x = x for small x Thus = d h h PG G P m b r t t r which is 4 th power law

29 Channel Model- Plan Earth Path Loss contd. Propagation path loss (mean loss) P r L PE = 10log = 10log GtG Pt Compared with the free space = P r = 1/ d r hbh d m In a more general form (no fading due to multipath), path attenuation is L PE = 10log 10 G 0log t 10 10log h m + 10 G r 40log 10 0log d 10 h b dβ L PE increases by 40 db each time d increases by 10

30 Channel Model- Plan Earth Path Loss contd. Vertical Horizontal P r ~ 1/d 4 P r ~ 1/d Free space S Loyka

31 LOS Channel Model - Problems Simple theoretical models do not take into account many practical factors: Rough terrain Buildings Refection Moving vehicle Shadowing Thus resulting in bad accuracy Solution: Semi- empirical Model

32 Sem-iempirical Model Practical models are based on combination of measurement and theory. Correction factors are introduced to account for: Terrain profile Antenna heights Building profiles Road shape/orientation Lakes, etc. Okumura model Hata model Saleh model SIRCIM model Outdoor Indoor Y. Okumura, et al, Rev. Elec. Commun. Lab., 16( 9), M. Hata, IEEE Trans. Veh. Technol., 9, pp , 1980.

33 Okumura Model Widely used empirical model (no analytical basis!) in macrocellular environment Predicts average (median) path loss Accurate within db in urban and suburban areas Frequency range: MHz Distance: > 1 km BS antenna height: > 30 m. MU antenna height: up to 3m. Correction factors are then added.

34 Hata Model Consolidate Okumura s model in standard formulas for macrocells in urban, suburban and open rural areas. Empirically derived correction factors are incorporated into the standard formula to account for: Terrain profile Antenna heights Building profiles Street shape/orientation Lakes Etc.

35 Hata Model contd. The loss is given in terms of effective heights. The starting point is an urban area. The BS antennae is mounted on tall buildings. The effective height is then estimated at 3-15 km from the base of the antennae. P M Shankar

36 Hata Model - Limits Frequency range: MHz Distance: 1 0 km BS antena height: m MU antenna height: 1 10 m

37 Hata Model Standard Formula for Average Path Loss for Urban Areas L pl u = log 10 ( f ) + ( log h ) 10 b log 10 d 13.8 log 10 h b a ( h ) (db) mu Correction Factors are: Large cities a a ( ) [ ( )] h = 3. log 11.75h 4.97 ( f 400MHz) db a ( ) [ ( )] h = 8.3 log 1.5h 1.1 ( f 00MHz) db mu mu Average and small cities 10 mu ( h ) = [.1log ( f ) 0.7] h [ 1.56 log ( f ) 0.8] db mu 10 mu 1 10 mu 10

38 Hata Model Average Path Loss for Urban Areas contd. Carrier frequency 900 MHz, BS antenna height 150 m, MU antenna height 1.5m. P M Shankar

39 Hata Model Average Path Loss for Suburban and Open Areas Suburban Areas L pl su = Lpl u Log 10 f Open Areas L pl o = Lpl u 4.78(Log10 f ) 18.33Log f 40.94

40 Hata Model - Average Path Loss S. Loyka, 003, Introduction to Mobile Communications

41 Improved Model Hata-Okumura model are not suitable for lower BS antenna heights ( m), and hilly or moderate-to-heavy wooded terrain. To correct for these limitations the following model is used [1]: For a given close-in distance d ref. the average path loss is: L pl = A + 10 v log10 (d / d ref ) + s for d > d ref, (db) where A = 0 log10(4 π d ref / λ) v is the path-loss exponent = (a b hb + c / hb) hb is the height of the BS: between 10 m and 80 m d ref = 100m and a, b, c are constants dependent on the terrain category s is representing the shadowing effect [1] V. Erceg et. al, IEEE JSAC, 17 (7), July 1999, pp

42 Improved Model Terrains Model Type A Type B Type C parameter a b c The typical value of the standard deviation for s is between 8. And 10.6 db, depending on the terrain/tree density type Terrain A: The maximum path loss category is hilly terrain with moderate-to-heavy tree densities. Terrain B: Intermediate path loss condition Terrain B: The minimum path loss category which is mostly flat terrain with light tree densities

43 Summary Attenuation is a result of reflection, scattering, diffraction and reflection of the signal by natural and human-made structures The received power is inversely proportional to (distance) v, where v is the loss parameter. Studied channel models and their limitations

44 Questions and Answers Tell me what you think about this lecture Next lecture: Multi-path Propagation- Fading