4/1/2018 Lecture #04: Large Scale Fading DR ABUL KHAIR BIN ANUAR Dept of Communication Engineering Faculty of Electrical and Electronic Engineering Universiti Tun Hussein Onn Malaysia, Johor MOBILE RADIO PROPAGATION: LARGE SCALE PATH LOSS Review on Radio Wave Propagation Free Space Propagation Model Relating Power to Electric Field Basic Propagation Mechanisms Reflection Ground Reflection (Two Ray) Diffraction Fresnel Zone Geometry Knife Edge Diffraction Model Scattering Practical Link Budget Design Using Path Loss Models Log distance Log normal Outdoor Propagation Models Model Okumura Model Hata Indoor Propagation Models 1 4/1/2018 Small-scale and large-scale fading Figure 4.1 Small-scale and large-scale fading. Review On Radio Wave Propagation Propagation in mobile radio follows the basic propagation theory, with some addition to compensate for moving target (mobile). Wave propagation mechanism often attributed to reflection, diffraction and scattering. Model such as Free Space Loss Model, Plane Earth Propagation Loss Model, Diffraction Model is valid to some extend of propagation scenario. Most cellular system operate where no direct line of sight path is available, causing severe diffraction loss and multipath. Other factors that need to be considered in mobile propagation are fading, Doppler effect, signal delay and indoor propagation. Some corrections is needed for the basic propagation model to be accurately represent the real propagation that took place between the transmitter and the receiver. 2 4/1/2018 Free Space Propagation Model The transmitter-receiver (T-R) path is clear from any obstruction (line of sight propagation). Mostly used for microwave link and satellite communication link. Received power decays as a function of T-R distance raised to some power (power law function). Given by Friss free space equation, or where is the system loss factor not related to propagation. Path loss, 10 log 10 log Note : You should be able to derive the Friss transmission equation. 4 Only valid when the mobile is outside the Near Field region (or in the Far Field region, the Fraunhofer region) Franhoufer distance, and - largest linear physical dimension of the antenna ≫ Power and Electric Field Power flux density, or simply power density is given as 4 / 4 The intrinsic impedance, 120 (377). / 377 where | | is the magnitude of the radiating portion of the electric field in the far field. Received power, 377 4 3 4/1/2018 Example 4.1 Find the far field distance for an antenna with maximum dimension of 1m and operating frequency of (a) 900 MHz and (b) 1800 MHz. An array of patch antennas whose dimension is 40cm by 40 cm is used at the base station. Find the far field distance of the antenna. 4 4/1/2018 Example 4.2 Transmitter power is 50W, express the power in dBW and dBm. The power is then applied to a unity gain antenna with 900 MHz carrier frequency, find received power at (a) 100 m and (b) 10km from the antenna. Also assume unity gain for the received antenna Ground Reflection (Two-Ray) Model FSPL is inaccurate when used alone as usually no single direct path from base station and mobile unit. GRM or plane earth loss model is based on geometric optics, and considers both the direct path and the reflected path between transmitter and receiver. Assume flat earth with T-R separation of few km. Total received E-field ( ) is the result of two rays, the line of sight component ( ) and ground reflected component ( ). Solving for received power yields 5 4/1/2018 Classical 2-ray ground bounce model Example 4.4 The mobile is located 5 km away from a base station and uses a vertical quaterwave monopole antenna with a gain of 2.55dB to receive cellular signals. The E-field at 1 km from the transmitter is measured to be 10-3 V/m. Carrier frequency is 900 MHz. Find (a) the length and the effective aperture of the receiving antenna (b) received power at the mobile using the two-ray ground reflection model assuming the height of the transmitting antenna is 50 m and the receiver is 1.5 meter above ground. 6 4/1/2018 Diffraction It is redistribution of energy within a wavefront as it passes the sharp edges of an object or slits The phenomenon that allows light or radio waves to propagate around corners We used the Huygen’s Principle to explain this phenomenon Huygen’s Principle states that: “Every point on a given spherical wavefront can be considered as a secondary point source of EM waves from which other secondary waves (wavelets) are radiated outward” Single-Knifed Edge Figure shows how plane wavefronts impinging on the edge from the left become curved by the edge so that, deep inside the geometrical shadow region, rays appear to emerge from a point close to the edge, filling the shadow region with diffracted rays Huygen’s principle can be applied in mathematical form to predict the actual field strength which is diffracted by the knife-edge Contributions from an infinite number of secondary sources in the region is summed, paying due regard to their relative amplitude and phases Final result can be expressed as propagation loss, which expressed the reduction in the field strength due to the knife edge diffraction in decibels 20 log 20 log 7 4/1/2018 8 4/1/2018 Fresnel diffraction geometry Figure 4.12 Illustration of Fresnel zones for different knife-edge diffraction scenarios. 9 4/1/2018 Knife edge diffraction attenuation : (-) exact (--) large approximation Another significant value is 0 6 Received power reduced by 4 when the knife edge is situated exactly on the direct path between the transmitter and receiver The parameter can be expressed in terms of the geometrical parameters defined in Figure as 2 2 where ’ is the excess height of the edge above the straight line from source to the field points For most practical case, , ≫ , so diffraction parameter can be approximated in terms of the distances measured along the ground 2 2 10 4/1/2018 Approximate solution using parameter Lee (2006); is provided by 0 1 20 log 0.5 0.62 20 log 0.5 exp 20 log 0.4 20 log . 1 0.95 0 0 0.1184 0.38 0.1 1 1 2.4 2.4 Another useful way to consider knife edge diffraction is in terms of the obstruction of Fresnel zones around the direct ray. The th Fresnel zone is the region inside an ellipsoid defined by the locus of points where the distance ( ) is larger than the direct path between the transmitter and the ) by half-wavelengths receiver ( Hence the radius of the nth zone condition If we assume that approximation ≪ and ≪ is given by applying the 2 , then to a good 11 4/1/2018 Fresnel zones 0.6 x first Fresnel zone clearance defines significant obstruction Fresnel zones can be thought of as containing the propagated energy in the wave Contribution within the first zone are all in phase, so any absorbing obstruction which do not enter this zone will have little effect on the received signal Fresnel zone clearance ( / ) can be expressed in terms of the diffraction parameter as follows 2 2 When the obstruction occupies 0.6 x the first Fresnel zone, the parameter is then approximately -0.8. Obstruction loss is the 0 dB. This clearance is often used as a criterion to decide whether an object is to be treated as a significant obstruction If this region is kept clear then the total path attenuation will be practically the same as in the unobstructed case 12 4/1/2018 Multiple knife-edge diffraction Example 4.5 Compute the diffraction loss where wavelength is 1/3 m, 1 , 2 1 and (a) 25 m (b) 0 m (c) – 25 m. 1 Compare value with approximated value given by Lee. Identify in which Fresnel zone is the tip of obstruction lies for each cases. (Hint: , excess path length can be find by ∆ and we need to find which satisfy the relation /2.) 13 4/1/2018 Example 4.6 Given the following scenario: Tx height = 50m, Rx height = 25 m, Tx – Rx distance = 12 km Knife edge height = 100 m Distance from Tx to knife-edge = 10 km f = 900 MHz Find (a)Knife edge loss (b)Height of obstacle for 6 dB loss Scattering When radio wave impinges on a rough surface (lamp posts, trees), reflected energy is spread out (diffuse) in all directions due to scattering which may provide additional radio energy at the receiver. Surface roughness is tested with Rayleigh criterion which defines a critical height, of surface protuberances for a given angle of incidence , given by 8 sin 14 4/1/2018 Path Loss Model Detail path loss model hard to factor in overall system design Most important characteristic is power fall-off with distance Radio propagation models Analytical models Empirical models Composite Applications: Predict large scale coverage for mobile systems Estimate and predict SNR Log Distance Path Loss Model Average path loss for an arbitrary T-R separation is expressed as a function of distance using a path loss exponent, ∝ or 10 log indicates the rate at which the path loss increases with distance, 0 is the close-in reference which is determine from measurements close to the transmitter, and d is the T-R separation distance. The bars indicate the ensemble average of all possible path loss values for a given value of . Free space reference value chosen according to the propagation environment, i.e. 1 km for large cell, or 1 or 100 m for smaller cells. 15 4/1/2018 Typical large-scale path loss Log Normal Shadowing Log distance model fails to address the fact that environmental clutter may be vastly different at two location having the same T-R separation. Measured signals vastly different from the average value. Path loss at any value d is random and distributed lognormally (normal) about the mean distance dependent value that is 10 thus is a zero mean Gaussian distributed random variable (in dB) with standard deviation (also in dB). 16 4/1/2018 Log normal distribution describe the random shadowing effect over large number of measurement locations which have the ame T-R separation, but have different levels of clutter on the propagation path, refers to log normal shadowing. Close in reference d0, path loss exponent n, and the standard deviation statistically describe the path loss model for an arbitrary location having specific T-R separation. Values of n and are computed from measured data, the difference between measures and estimated path loss is minimized in a mean square error sense over wide range of measurement locations and T-R separations. Rayleigh and Log-normal 17 4/1/2018 Example 5.7 4 received power measurement were taken at distance 100m, 200m, 1km and 3km and given in the table where it is assume that these values follow log normal shadowing model where d0 = 100m. Distance from Tx Received Power 100 m 0 dBm 200 m – 20 dBm 1000 m – 35 dBm 3000 m - 70 dBm Find the minimum mean square error (MMSE) estimate for the path loss exponent, n Calculate the standard deviation about the mean value Estimate the received power at d = 2km using the resulting model Predict the likelihood that the signal level at 2 km will be greater than -60dBm Predict the percentage of area within a 2 km radius cell that receives signals greater than – 60 dBm, given the result in (d) 18 4/1/2018 MMSE estimate may be found using the following method. Let pi be the received power at a distance and let ̂ be the estimate for using the / 0 path loss model. The sum of square errors between the measure and estimated values is given by ̂ The value of n which minimizes the mean square error can be obtained by equating the derivative of to zero, and then solving for . Outdoor Propagation Model Radio transmission often takes place over irregular terrain Terrain profile may vary from simple curved to a mountainous profile. Trees, buildings and other obstacles cannot be neglected and must be taken in the estimation. The purpose of the outdoor model is to predict the average received signal strength at a given distance from the TX, as well as the variability of the signal strength in close spatial proximity to a particular location. 19 4/1/2018 Classifications: Computer based models: Longley-Rice model Durkin’s model Measurement model Okumura model Empirical model Hata model PCS extension and wideband PCS microcell models Walfish and Bertoni Model Longley-Rice Model (LR, ITM) In 1965 & 1968, Rice and Longley propose the model to be applicable to point-to-point communication systems in the frequency range from 40MHz to100GHz. In 1978, Longley-Rice model is also available as a computer program to calculate large-scale median transmission loss relative to free space loss over irregular terrain for frequencies between 20MHz and 10GHz Longley Rice has been adopted as a standard by the FCC Many software implementations are available commercially Includes most of the relevant propagation modes [multiple knife & rounded edge diffraction, atmospheric attenuation, tropospheric propagation modes (forward scatter etc.), precipitation, diffraction over irregular terrain, polarization, specific terrain data, atmospheric stratification, different climatic regions, etc. etc. …] 20 4/1/2018 Operate in two mode: Point-to-Point mode prediction: A detail terrain path profile is available, the path specific parameters can be easily determined. Area mode prediction: Once the terrain path profile is not available, the Longley-Rice method provides techniques to estimate the path-specific parameters. Shortcoming: Does not providing a way of determining corrections due to environmental factors in the immediate vicinity of the mobile receiver No consideration of correlation factors to account for the effects of buildings and foliage No consideration of multipath 21 4/1/2018 Durkin’s Model In 1969 & 1975, Durkin propose a computer simulator for predicting field strength contours over irregular terrain. Durkin path loss simulator consists of two parts: Access a topographic database of a proposed service area Reconstruct the ground profile information along the radial joining the TX to RX. Assumption: the receiving antenna receives all of its energy along that radial No multipath propagation is considered Simply LOS and diffraction from obstacles along the radial, and excludes reflections from other surrounding objects and local scatterers Refer to Rappaport, pp 146. Okumura Model Most widely used model in urban areas, obtained by extensive measurements, no analytical explanation – based on measurement in the city of Tokyo, Japan. Represented by charts (curves) giving median attenuation relative to free space attenuation Valid under: Frequency band: 150-1920 MHz T-R distance: 1-10 km, = 1m to 3m BS antenna height: 30-1000 m Quasi-smooth terrain (urban & suburban areas) among the simplest & best for in terms of path loss accuracy in cluttered mobile environment disadvantage: slow response to rapid terrain changes common std deviations between predicted & measured path loss 10dB - 14dB 22 4/1/2018 Okumura developed a set of curves in urban areas with quasi-smooth terrain effective antenna height: base station hte = 200m mobile: hre = 3m gives median attenuation relative to free space (Amu) developed from extensive measurements using vertical omni-directional antennas at base and mobile measurements plotted against frequency Estimating path loss using Okumura Model 1. determine free space loss, Amu(f,d), between points of interest 2. add terrain , and correction factors to account for , – – – L50 = 50% value of propagation path loss (median) LF = free space propagation loss Amu(f,d) = median attenuation relative to free space G(hte) = base station antenna height gain factor G(hre) = mobile antenna height gain factor GAREA = gain due to environment 23 4/1/2018 , & frequencies have been plotted for wide range of antenna gain varies at rate of 20dB per decade or 10dB per decade 20 log 10 log 20 log 200 3 3 , 10 , , 1000 3 3 10 model corrected for = terrain undulation height isolated ridge height average terrain slope mixed land/sea parameter 24 4/1/2018 Okumura and Hata’s model Hata Model empirical model of graphical path loss data from Okumura predicts median path loss for different channels valid over UHF/VHF band from 150MHz-1.5GHz charts used to characterize factors affecting mobile land propagation standard formulas for approximating urban propagation loss correction factors for some situations compares closely with Okumura model as mobile systems 1 large 25 4/1/2018 Parameter Comment L50 50th % value (median) propagation path loss (urban) fc frequency from 150MHz-1.5GHz hte, hre Base Station and Mobile antenna height (hre) correction factor for hre , affected by coverage area d Tx-Rx separation log 69.55 26.16 log – 13.82 log – represents fixed loss – approximately 2.6 power law dependence on dependence on antenna heights is proportional to 44.9 6.55 log . represents path loss exponent, worst case ≈ 4.5 69.55 26.16log 44.9 – 6.55log – 13.82log log – 26 4/1/2018 Mobile Antenna Height Correction Factor for Hata Model (hre) Comment (1.1log10 fc - 0.7)hre – (1.56log10 fc - 0.8)dB Medium City 8.29(log10 1.54hre)2 – 1.1 dB Large City (fc 300MHz) 3.2(log10 11.75hre)2 – Large City (fc > 300MHz) 4.97 dB Hata Model for Rural and Suburban Regions represent reductions in fixed losses for less demanding environments L50 (dB) Comment L50 (urban) - 2[log10 (fc/28)]2 – 5.4 Suburban Area L50 (urban) - 4.78(log10 fc)2 - 18.33log10 fc - 40.94 Rural Area Valid Range for Parameters 150MHz < fc < 1GHz 30m < hte < 200m 1m < hre < 10m 1km < d < 20km Propagation losses increase with frequency in built up areas 27 4/1/2018 hte = 30, hre = 2 -- f = 1500MHz -- f = 900 MHz -- f = 700MHz hte = 30, hre = 2, f = 900MHz -- large city -- small to medium sized city 28 4/1/2018 PCS Extension to Hata Model European Co-operative Scientific & Technical (EUROCOST) formed COST-231, extend Hata’s model to 2GHz 46.3 44.9 33.9 log – 13.82 log 6.55 log defined in the Hata model For medium sized city, CM = 0 dB Metropolitan centre, CM = 3 dB Limits 1500 30 1 1 2 200 10 20 Indoor Propagation Model The indoor environment differs from outdoor smaller Tx-Rx separation distances than outdoors higher environmental variability for much small Tx-Rx separation conditions vary from: doors open/closed, antenna position, variable far field radiation for receiver locations & antenna types strongly influenced by building features, layout, materials Dominated by same mechanisms as outdoor propagation (reflection, refraction, scattering) Classified as either LOS or OBS, with varying degrees of clutter Some key models are Partition Losses – Same Floor Partition Losses – Different Floor Log-distance path loss model Ericsson Multiple Breakpoint Model Attenuation Factor Model 29 4/1/2018 Partition losses Partition Losses (same floor) Variety of obstacles and partitions Hard partitions – part of the building structure Soft partitions – do not span to the ceiling Vary widely in their physical and electrical characteristic Database for various type of partitions, here or refer to Rappaport pp 158-159. Partition Losses between Floors Structural dimension, materials, type of construction, windows Floor attenuation factor, FAF for three buildings in San Fransisco. 30 4/1/2018 31 4/1/2018 Log Distance Path Loss Model n depends on surroundings and building type = normal random variable in dB having std deviation identical to log normal shadowing mode, typical value in Table aa d PL(dB) PL(d0 ) 10n log d0 Ericsson Multiple Breakpoint Model measurements in multi-floor office building uses uniform distribution to generate path loss values between minimum & maximum range, relative to distance 4 breakpoints consider upper and lower bound on path loss assumes 30dB attenutation at d0 = 1m accurate for f = 900MHz & unity gain anntenae provides deterministic limit on range of path loss at given distance 32 4/1/2018 Ericsson’s indoor model Attenuation Factor Model includes effect of building type & variations caused by obstacles reduces std deviation for path loss to 4dB std deviation for path loss with log distance model 13dB d PL(d ) (dB) PL(d0 ) (dB) 10nSF log FAF(dB) PAF(dB) d0 nSF = exponent value for same floor measurement – must be accurate FAF = floor attenuation factor for different floor PAF = partition attenuation factor for obstruction encountered by primary ray tracing PAF(1) FAF Tx PAF(2) Rx 33 4/1/2018 Replace FAF with nMF = exponent for multiple floor loss d PL(d ) (dB) PL(d0 ) (dB) 10nMF log PAF(dB) d0 decreases as average region becomes smaller-more specific Building Path Loss obeys free space + loss factor () (Dev90b) loss factor increases exponentially with d (dB/m) = attenuation constant for channel d PL(d ) (dB) PL(d0 ) (dB) 20log d FAF (dB) PAF(dB) d0 Measured indoor path loss 34 4/1/2018 Measured indoor path loss Measured indoor path loss 35 4/1/2018 Devasirvatham’s model Path Loss Exponent & Standard Deviation for Typical Building Location n σ (dB) number of points same floor 2.76 12.9 501 through 1 floor 4.19 5.1 73 through 2 floor 5.04 6.5 30 through 3 floor 5.22 6.7 30 36 4/1/2018 Signal Penetration into Buildings no exact models signal strength increases with height lower levels are affected by ground clutter (attenuation & penetration) higher floors may have LOS channel stronger incident signal on walls RF Penetration affected by frequency height within building antenna pattern in elevation plain Penetration loss decreases with increased frequency loss in front of windows is 6dB greater than without windows penetration loss decreases 1.9dB with each floor when < 15th floor increased attenuation at >15 floors – shadowing affects from taller buildings metallic tints result in 3dB to 30dB attenuation penetration impacted by angle of incidence Penetration Loss vs Frequency for two different building Frequency (MHz) Attenuation (dB) 441 896.5 1400 Frequency (MHz) Attenuation (dB) 16.4 900 14.2 11.6 1800 13.4 7.6 2300 12.8 37 4/1/2018 Ray Tracing & Site Specific Models rapid acceleration of computer & visualization capabilities SISP – site specific propagation models GIS – graphical information systems support ray tracing augmented with aerial photos & architectural drawings Example A BS of 30m height is operating at 900 MHz and transmits 20W power. The transmitter and receiver antenna gains are 6dB and 2dB respectively. A MS of 2m height is located at 5km from the BS. If other losses is 5dB and fading =6dB due to log-normal fading, compare the minimum power received by the MS in dBm if the following propagation models are used: Free space propagation loss, FSPL Plane earth propagation loss, PEPL log-distance with do=1km, =4 and PL(do)=FSPL, and diffraction model if an obstacle of 30m height is located at 2km from the BS Comment on the practical minimum power received by the MS if an obstacle stated in (d) exists. Write a MATLAB program to plot the path loss for each model from 1km to 5 km. 38