1、Radio Propagation,CSCI 69424 September 1999Lewis Girod,17 March 1999,Radio Propagation,2,Outline,Introduction and terminologyPropagation mechanismsPropagation models,17 March 1999,Radio Propagation,3,What is Radio?,Radio Xmitter induces E&M fieldsElectrostatic field components 1/d3Induction field co
2、mponents 1/d2Radiation field components 1/dRadiation field has E and B componentField strength at distance d = EB 1/d2Surface area of sphere centered at transmitter,17 March 1999,Radio Propagation,4,General Intuition,Two main factors affecting signal at receiverDistance (or delay) Path attenuation M
3、ultipath Phase differences,Green signal travels 1/2 farther than Yellow to reach receiver, who sees Red. For 2.4 GHz, (wavelength) =12.5cm.,17 March 1999,Radio Propagation,5,Objective,Invent models to predict what the field looks like at the receiver. Attenuation, absorption, reflection, diffraction
4、.Motion of receiver and environmentNatural and man-made radio interference.What does the field look like at the receiver?,17 March 1999,Radio Propagation,6,Models are Specialized,Different scalesLarge scale (averaged over meters)Small scale (order of wavelength)Different environmental characteristic
5、sOutdoor, indoor, land, sea, space, etc.Different application areasmacrocell (2km), microcell(500m), picocell,17 March 1999,Radio Propagation,7,Outline,Introduction and some terminologyPropagation MechanismsPropagation models,17 March 1999,Radio Propagation,8,Radio Propagation Mechanisms,Free Space
6、propagationRefractionConductors & Dielectric materials (refraction)DiffractionFresnel zonesScattering“Clutter” is small relative to wavelength,17 March 1999,Radio Propagation,9,Free Space,Assumes far-field (Fraunhofer region) d D and d , whereD is the largest linear dimension of antenna is the carri
7、er wavelengthNo interference, no obstructions,17 March 1999,Radio Propagation,10,Free Space Propagation Model,Received power at distance d iswhere Pt is the transmitter power in Wattsa constant factor K depends on antenna gain, a system loss factor, and the carrier wavelength,17 March 1999,Radio Pro
8、pagation,11,Refraction,Perfect conductors reflect with no attenuationDielectrics reflect a fraction of incident energy“Grazing angles” reflect max*Steep angles transmit max*,q,qr,qt,Reflection induces 180 phase shift,*The exact fraction depends on the materials and frequencies involved,17 March 1999
9、,Radio Propagation,12,Diffraction,Diffraction occurs when waves hit the edge of an obstacle“Secondary” waves propagated into the shadowed regionExcess path length results in a phase shiftFresnel zones relate phase shifts to the positions of obstacles,17 March 1999,Radio Propagation,13,Fresnel Zones,
10、Bounded by elliptical loci of constant delayAlternate zones differ in phase by 180Line of sight (LOS) corresponds to 1st zoneIf LOS is partially blocked, 2nd zone can destructively interfere (diffraction loss),Fresnel zones are ellipses with the T L1 = L2+l,Path 1,Path 2,17 March 1999,Radio Propagat
11、ion,14,Power Propagated into Shadow,How much power is propagated this way?1st FZ: 5 to 25 dB below free space prop.,Obstruction of Fresnel Zones ,1st 2nd,0-10-20-30-40-50-60,0o,90,180o,dB,Tip of Shadow,Obstruction,LOS,Rappaport, pp. 97,17 March 1999,Radio Propagation,15,Scattering,Rough surfacescrit
12、ical height for bumps is f(,incident angle) scattering loss factor modeled with Gaussian distribution.Nearby metal objects (street signs, etc.)Usually modelled statisticallyLarge distant objectsAnalytical model: Radar Cross Section (RCS),17 March 1999,Radio Propagation,16,Outline,Introduction and so
13、me terminologyPropagation MechanismsPropagation modelsLarge scale propagation modelsSmall scale propagation (fading) models,17 March 1999,Radio Propagation,17,Propagation Models: Large,Large scale models predict behavior averaged over distances Function of distance & significant environmental featur
14、es, roughly frequency independentBreaks down as distance decreasesUseful for modeling the range of a radio system and rough capacity planning,17 March 1999,Radio Propagation,18,Propagation Models: Small,Small scale (fading) models describe signal variability on a scale of Multipath effects (phase ca
15、ncellation) dominate, path attenuation considered constantFrequency and bandwidth dependent Focus is on modeling “Fading”: rapid change in signal over a short distance or length of time.,17 March 1999,Radio Propagation,19,Large Scale Models,Path loss modelsOutdoor modelsIndoor models,17 March 1999,R
16、adio Propagation,20,Free Space Path Loss,Path Loss is a measure of attenuation based only on the distance to the transmitterFree space model only valid in far-field; Path loss models typically define a “close-in” point d0 and reference other points from there:,What is dB?,17 March 1999,Radio Propaga
17、tion,21,Log-Distance Path Loss Model,Log-distance generalizes path loss to account for other environmental factorsChoose a d0 in the far field.Measure PL(d0) or calculate Free Space Path Loss.Take measurements and derive empirically.,17 March 1999,Radio Propagation,22,Log-Distance 2,Value of charact
18、erizes different environments,Rappaport, Table 3.2, pp. 104,17 March 1999,Radio Propagation,23,Log-Normal Shadowing Model,Shadowing occurs when objects block LOS between transmitter and receiverA simple statistical model can account for unpredictable “shadowing” Add a 0-mean Gaussian RV to Log-Dista
19、nce PLMarkov model can be used for spatial correlation,17 March 1999,Radio Propagation,24,Outdoor Models,“2-Ray” Ground Reflection modelDiffraction model for hilly terrain,17 March 1999,Radio Propagation,25,2-Ray Ground Reflection,For d hrht, low angle of incidence allows the earth to act as a refle
20、ctorthe reflected signal is 180 out of phasePr 1/d4 (=4),17 March 1999,Radio Propagation,26,Ground Reflection 2,Intuition: ground blocks 1st Fresnel zoneReflection causes an instantaneous 180 phase shiftAdditional phase offset due to excess path lengthIf the resulting phase is still close to 180, th
21、e gound ray will destructively interfere with the LOS ray.,180,17 March 1999,Radio Propagation,27,Hilly Terrain,Propagation can be LOS or result of diffraction over one or more ridgesLOS propagation modelled with ground reflection: diffraction lossBut if there is no LOS, diffraction can actually hel
22、p!,17 March 1999,Radio Propagation,28,Indoor Path Loss Models,Indoor models are less generalizedEnvironment comparatively more dynamicSignificant features are physically smallerShorter distances are closer to near-fieldMore clutter, scattering, less LOS,17 March 1999,Radio Propagation,29,Indoor Mode
23、ling Techniques,Modeling techniques and approaches:Log-Normal, 1/T),For a system with bw W and symbol time T.,17 March 1999,Radio Propagation,38,Qualitative Delay Spread,Noise threshold,Delay,Power(dB),Typical values for :Indoor: 10-100 nsOutdoor: 0.1-10 s,17 March 1999,Radio Propagation,39,Characte
24、rizing Fading 2*,Characterizing Time-variation: How does the impulse response change with time?Coherence time (tc): for what value of are responses at t and t+ uncorrelated? (How quickly is the channel changing)Doppler Spread (fd): How much will the spectrum of the input be spread in frequency?fd1/t
25、c,17 March 1999,Radio Propagation,40,Effect of Coherence Time*,Is the channel constant over many uses?if T tc: “Fast fading”Frequent adaptation requiredFor typical systems, symbol rate is high compared to channel evolution,For a system with bw W and symbol time T.,17 March 1999,Radio Propagation,41,
26、Statistical Fading Models,Fading models model the probability of a fade occurring at a particular locationUsed to generate an impulse responseIn fixed receivers, channel is slowly time-varying; the fading model is reevaluated at a rate related to motionSimplest models are based on the WSSUS principl
27、e,17 March 1999,Radio Propagation,42,WSSUS*,Wide Sense Stationary (WSS)Statistics are independent of small perturbations in time and positionI.e. fixed statistical parameters for stationary nodesUncorrelated Scatter (US)Separate paths are not correlated in phase or attenuationI.e. multipath componen
28、ts can be independent RVsStatistics modeled as Gaussian RVs,17 March 1999,Radio Propagation,43,Common Distributions,Rayleigh fading distributionModels a flat fading signalUsed for individual multipath componentsRicean fading distributionUsed when there is a dominant signal component, e.g. LOS + weak
29、er multipathsparameter K (dB) defines strength of dominant component; for K=-, equivalent to Rayleigh,17 March 1999,Radio Propagation,44,Application of WSSUS,Multi-ray Rayleigh fading:The Rayleigh distribution does not model multipath time delay (frequency selective)Multi-ray model is the sum of two
30、 or more independent time-delayed Rayleigh variables,Rappaport, Fig. 4.24, pp. 185.,17 March 1999,Radio Propagation,45,Saleh & Valenzuela (1987),Measured same-floor indoor characteristicsFound that, with a fixed receiver, indoor channel is very slowly time-varyingRMS delay spread: mean 25ns, max 50n
31、sWith no LOS, path loss varied over 60dB range and obeyed log distance power law, 3 n 4Model assumes a structure and models correlated multipath components.,Rappaport, pp. 188,17 March 1999,Radio Propagation,46,Saleh & Valenzuela 2,Multipath modelMultipath components arrive in clusters, follow Poiss
32、on distribution. Clusters relate to building structures. Within cluster, individual components also follow Poisson distribution. Cluster components relate to reflecting objects near the TX or RX.Amplitudes of components are independent Rayleigh variables, decay exponentially with cluster delay and w
33、ith intra-cluster delay,17 March 1999,Radio Propagation,47,References,Wireless Communications: Principles and Practice, Chapters 3 and 4, T. Rappaport, Prentice Hall, 1996.Principles of Mobile Communication, Chapter 2, G. Stber, Kluwer Academic Publishers, 1996.Slides for EE535, K. Chugg, 1999.Sprea
34、d Spectrum Systems, Chapter 7, R. Dixon, Wiley, 1985 (there is a newer edition).Wideband CDMA for Third Generation Mobile Communications, Chapter 4, T. Ojanpera, R. Prasad, Artech, House 1998.Propagation Measurements and Models for Wireless Communications Channels, Andersen, Rappaport, Yoshida, IEEE
35、 Communications, January 1995.,17 March 1999,Radio Propagation,48,The End,17 March 1999,Radio Propagation,49,Scattering 2,hc is the critical height of a protrusion to result in scattering.RCS: ratio of power density scattered to receiver to power density incident on the scattering objectWave radiate
36、d through free space to scatterer and reradiated:,17 March 1999,Radio Propagation,50,Free Space 2a,Free space power flux density (W/m2)power radiated over surface area of spherewhere Gt is transmitter antenna gainBy covering some of this area, receivers antenna “catches” some of this flux,17 March 1
37、999,Radio Propagation,51,Free Space 2b,Fraunhofer distance: d 2D2/ Antenna gain and antenna apertureAe is the antenna aperture, intuitively the area of the antenna perpendicular to the fluxGr is the antenna gain for a receiver. It is related to Ae.Received power (Pr) = Power flux density (Pd) * Ae,1
38、7 March 1999,Radio Propagation,52,Free Space 2c,where L is a system loss factorPt is the transmitter powerGt and Gr are antenna gains is the carrier wavelength,17 March 1999,Radio Propagation,53,LNSM 2,PL(d)dB = PL(d0) +10nlog(d/d0)+ Xwhere X is a zero-mean Gaussian RV (dB) and n computed from measu
39、red data, based on linear regression,17 March 1999,Radio Propagation,54,Ground Reflection 1.5,The power at the receiver in this model isderivation calculates E field; Pr = |E|2Ae; Ae is ant. apertureThe “breakpoint” at which the model changes from 1/d2 to 1/d4 is 2hthr/where hr and ht are the receiv
40、er and transmitter antenna heights,17 March 1999,Radio Propagation,55,Convolution Integral,Convolution is defined by this integral:,Indexes relevant portion of impulse response,Scales past input signal,17 March 1999,Radio Propagation,56,Partition Losses,Partition losses: same floorWalls, furniture, equipmentHighly dependent on type of material, frequencyHard partitions vs soft partitionshard partitions are structuralsoft partitions do not reach ceiling“open plan” buildings,
