Wireless Communication Channels Lecture 6: Channel Models EITN85, FREDRIK TUFVESSON ELECTRICAL AND INFORMATION TECHNOLOGY Content Modelling methods Okumura-Hata path loss model COST 231 model Indoor models Wideband models COST 27 (GSM model) ITU-R model for 3G Directional channel models Multiantenna (MIMO) models Ray tracing & Ray launching HT218 Wireless Communication Channels 2 1
Channel measures HT218 Wireless Communication Channels 3 Modeling methods Stored channel impulse responses realistic reproducible hard to cover all scenarios Deterministic channel models based on Maxwell s equations site specific computationally demanding Stochastic channel models describes the distribution of the field strength etc mainly used for design and system comparisons HT218 Wireless Communication Channels 4 2
Narrowband models Review of properties Narrowband models contain only one attenuation, which is modeled as a propagation loss, plus largeand small-scale fading. Path loss: Often proportional to 1/d n, where n is the propagation exponent (n may be different at different distances). Large-scale fading: Log-normal distribution (normal distr. in db scale) Small-scale fading: Rayleigh, Rice, Nakagami distributions... (of amplitudes and not in db-scale) HT218 Wireless Communication Channels 5 Okumura s measurements Extensive measurement campaign in Japan in the 196 s. Parameters varied during measurements: Frequency Distance Mobile station height Base station height Environment 1 3 MHz 1 1 km 1 1 m 2 1 m medium-size city, large city, etc. Propagation loss is given as median values (5% of the time and 5% of the area). Results from these measurements are displayed in figures 7.12 7.14 in the appendix. HT218 Wireless Communication Channels 6 3
Okumura s measurements excess loss FIGURE 7.12 in appendix Excess loss [db] Distance [km] These curves are only for h b =2 m and h m =3 m Frequency [MHz] 9 MHz and 3 km distance HT218 Wireless Communication Channels 7 The Okumura-Hata model Background In 198 Hata published a parameterized model, based on Okumura s measurements. The parameterized model has a smaller range of validity than the measurements by Okumura: Frequency Distance Mobile station height Base station height 15 15 MHz 1 2 km 1 1 m 3 2 m HT218 Wireless Communication Channels 8 4
The Okumura-Hata model How to calculate prop. loss Metropolitan areas Small/mediumsize cities Suburban environments Rural areas ( ( hm )) ( ( hm )) ( 1.1log ( f MHz ).7) h ( 1.56 log( f MHz ) -.8) 2 2 - - m ( km ) LO - H = A+ Blog d + C A= 69.55 + 26.16 log ( f MHz) -13.82 log ( hb) -a( hm) B = 44.9-6.55 log ( hb ) a( h m) = C = 8.29 log 1.54-1.1 for f 2 MHz 3.2 log 11.75-4.97 for f 4 MHz ( f ) ø 2 MHz -2 Ø º log / 28 ß -5.4 2 ( f ) ø MHz ( f MHz ) h b and h m in meter - 4.78 Ø º log ß + 18.33log -4.94 HT218 Wireless Communication Channels 9 The COST 231-Walfish-Ikegami model The Okumura-Hata model is not suitable for micro cells or small macro cells, due to its restrictions on distance (d > 1 km). The COST 231-Walfish-Ikegami model covers much smaller distances, is better suited for calculations on small cells and covers the 18 MHz band as well. Frequency Distance Mobile station height Base station height 8 2 MHz.2 5 km 1 3 m 4 5 m HT218 Wireless Communication Channels 1 5
The COST 231-Walfish-Ikegami model How to calculate prop. loss L= L + Lmsd + Lrts Free space Building multiscreen Roof-top to street BS MS Details about calculations can be found in the appendix. d HT218 Wireless Communication Channels 11 Motley-Keenan indoor model For indoor environments, the attenuation is heavily affected by the building structure, walls and floors play an important rule distance dependent path loss sum of attenuations from walls, 1-2 db/wall sum of attenuation from the floors (often larger than wall attenuation) site specific, since it is valid for a particular case HT218 Wireless Communication Channels 12 6
Wideband models Tapped delay line model often used N h t, t = a t exp jq t d t -t ( ) () () i= 1 ( ) ( ) i i i Often Rayleigh-distributed taps, but might include LOS and different distributions of the tap values Mean tap power determined by the power delay profile HT218 Wireless Communication Channels 13 Power delay profile Often described by a single exponential decay P ( t) = sc exp( - t / S ) t t otherwise log( Psc( t )) t delay spread though often there is more than one cluster P( t) = k P S c k c t, k P ( t - t ) t sc c, k otherwise log( Psc ( t )) t HT218 Wireless Communication Channels 14 7
arrival time If the bandwidth is high, the time resolution is large so we might resolve the different multipath components Need to model arrival time The Saleh-Valenzuela model: L K hb=> > J k,l bnb? T l?b k,l l= k= cluster arrival time (Poisson) ray arrival time (Poisson) Double-exponential ray power: HT218 Wireless Communication Channels 15 Wideband models COST 27 model for GSM The COST 27 model specifies: FOUR power-delay profiles for different environments. FOUR Doppler spectra used for different delays. It does NOT specify propagation losses for the different environments! HT218 Wireless Communication Channels 16 8
-1-2 Wideband models COST 27 model for GSM P [ db] P [ db] -3 t [ ms -3 1 ] 1 2 3 4 5 6 7 t [ ms] -1-2 Four specified power-delay profiles P [ db] RURAL AREA BAD URBAN HT218 Wireless Communication Channels 17-1 -2 P [ db] -3 t [ ms -3 5 1 ] 1 2t [ ms] -1-2 TYPICAL URBAN HILLY TERRAIN Wideband models COST 27 model for GSM Four specified Doppler spectra s (, ) P nt CLASS GAUS1 t.5 ms.5 ms < t 2 ms i i i (, ) P nt s i -n max +n max (, ) P nt s i -n max +n max (, ) P nt s i GAUS2 t 2 ms i RICE > Shortest path in rural areas -n max +n max -n max +n max HT218 Wireless Communication Channels 18 9
Wideband models COST 27 model for GSM Doppler spectra: CLASS GAUS1 GAUS2-1 -2 P [ db] -3 1 RURAL AREA First tap RICE here -1-2 P [ db] TYPICAL URBAN -3 t [ ms] 1 2 3 4 5 6 7 t [ ms] -1-2 P [ db] BAD URBAN P [ db] -3 t [ ms -3 5 1 ] 1 2t [ ms] HT218 Wireless Communication Channels 19-1 -2 HILLY TERRAIN Transfer function, Typical urban HT218 Wireless Communication Channels 2 1
Wideband models ITU-R model for 3G : The ITU-R model specifies: SIX different tapped delay-line channels for three different scenarios (indoor, pedestrian, vehicular). TWO channels per scenario (one short and one long delay spread). TWO different Doppler spectra (uniform & classical), depending on scenario. THREE different models for propagation loss (one for each scenario). The standard deviation of the log-normal shadow fading is specified for each scenario. The autocorrelation of the lognormal shadow fading is specified for the vehicular scenario. HT218 Wireless Communication Channels 21 Wideband models ITU-R model for 3G ns HT218 Wireless Communication Channels 22 11
Directional channel models The spatial domain can be used to increase the spectral efficiency of the system Smart antennas MIMO systems Need to know directional properties How many significant reflection points? Which directions? Model incoming angle (direction of arrival) and outgoing angle (direction of departure) to scatterers Model independent of specific antenna pattern HT218 Wireless Communication Channels 23 Double directional impulse response TX position RX position N r h t, r TX, r RX,b,I,H => =1 number of multipath components for these positions h t, r TX, r RX,b,I,H delay direction-of-departure direction-of-arrival h t, r TX, r RX,b,I,H = a e jj Nb?b NI?I NH?H HT218 Wireless Communication Channels 24 12
Double directional impulse response with slightly different notation: Time and location is omitted here! HT218 Wireless Communication Channels 25 Physical interpretation W t l Y HT218 Wireless Communication Channels 26 13
Angular spread double directional delay power spectrum DDDPSI,H,b =XP s H,I,b,X dx angular delay power spectrum ADPSI,b =XDDDPSH,I,b G MS H dh tl I H angular power spectrum APSI =XAPDSI,b db power P =XAPSIdI HT218 Wireless Communication Channels 27 Geometry-Based Stochastic Channel Model (GSCM) Assign positions for scatterers according to given distributions Derive impulse response given the scatterers and distributions for the signal properties. Used in the COST 259 model, COST 273, COST 21, WINNER 3GPP/3GPP2 HT218 Wireless Communication Channels 28 14
Geometry-Based Stochastic Channel Model (GSCM) Create an imaginary map for radio wave scatterers (clusters) Cluster Local cluster MS 2 MS 1 Local cluster BS Courtesey: K. Haneda, Aalto Uni. HT218 Cluster Wireless Communication Channels 29 MIMO channel channel matrix Ø h11( t ) Œ h ( ) = Œ 21 t H ( t ) Œ M Œ Œº hm ( ) Rx 1 t h h h 12 22 M Rx 2 ( t ) ( t ) M ( t ) L L O L h h h 1M 2M M Rx ( t ) ø Tx ( t ) œ Tx œ M œ œ M ( t ) Tx œß D-1 signal model y() t = H( t) x( t-t) t= HT218 Wireless Communication Channels 3 15
Deterministic modeling methods Solve Maxwell s equations with boundary conditions Problems: Data base for environment Computation time Exact solutions Method of moments Finite element method Finite-difference time domain (FDTD) High frequency approximation All waves modeled as rays that behave as in geometrical optics Refinements include approximation to diffraction, diffuse scattering, etc. HT218 Wireless Communication Channels 31 Ray launching TX antenna sends out rays in different directions We follow each ray as it propagates, until it either Reaches the receiver, or Becomes too weak to be relevant Propagation processes Free-space attenuation Reflection Diffraction and diffuse scattering: each interacting object is source of multiple new rays Predicts channel in a whole area (for one TX location) HT218 Wireless Communication Channels 32 16
Ray tracing Determines rays that can go from one TX position to one RX position Uses imagining principle Similar to techniques known from computer science Then determine attenuation of all those possible paths HT218 Wireless Communication Channels 33 Example: Ray tracing Required base station power to connect to a WCDMA cell phone. Example from Stuttgart. Courtesey: Awecommunications HT218 Wireless Communication Channels 34 17
Example: Ray tracing Coverage for a WCDMA cell phone. Example from Stuttgart. Courtesey: Awecommunications Propagation Models HT218 Wireless Communication Channels 35 18