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Chap # 4

ContinuedDiffraction. Diffraction allows radio signals to propagate around the curved surface of earth and to propagate behind obstructionsRadio wave travel according to a straight line. This is an oversimplification of the true behaviour of Electro-Magnetic waves. In fact, in the shadow of obstacles, one can receive radio signals even though the straight-line model would not allow such reception. Propagation around a corner is due to the diffraction mechanism. Huygens principle A better model to understand this behaviour is Huygens principle. Radio energy is dispersed into free space. New waves can be thought to start from every point which is reached by radio energy. Every such point acts as a point source radiating energy in all directions..2ScatteringIn Practice when a surface is not perfectly smooth, a proportion of the of the incident wave is scattered (spread out) over a wide range.

The reflection coefficient can be modified for rough surfaces so that : rough= s

Where s is the scattering loss factorRoughness criteriaSo when is a surface Rough ?

The rayleigh criterion links the surface roughness to the angle of incidence and the wavelength. Consider a rough surface with a maximum variation from a flat surface of hsurf. This is considered smooth if

hsurf < /(8sin(i)) where i angle of incidence

Roughness criteriaSo if hsurf is large enough in wavelength terms then scattering is significant and the ref coefficient must be modified by rough= s

For a gaussian surface roughness of standard deviation h s=exp[-8( hsin(i)/)2]

Scattering from objects(RCS) For discrete objects distant from the antennas, we can approximate the effect by considering the radar cross section (scat), in sq meters of the object

The RCS relates to the size and shape of the object. Many objects are bigger physically then they appear to the link (trees for example)

RCS can be defined as the ratio of the power density of the signal scattered in the direction of the r/r to the power density of the radio wave incident upon the scattering object and has units of the sq.meters Scattering from objects(RCS)Once you have or can estimate, the RCS of objects the scattering power received from each object can be estimated from:

Pr,scat= Pt-Lt+Gtscat+10Log(4/2)-20Log (4 R1/ )-20 Log (4 R2/ )+Grscat-Lr Where= RCS in sq.meterR1=distance fromTx to Scattering objectR2=distance from Rx to Scattering objectGrscat= Gain of Rx antenna in dir of ScattererChannel prediction By summing up reflection, diffraction and scattering contributions and allowing for refraction and shadowing where appropriate one can derive a complex propagation model for a given environment.The disadvantages are: time consuming, input data normally imprecise or only available at certain frequencies (empirical model) etc.For initial system calculation, however approximate models do exist.Approximate ModelsFor a combination of measured data and theoretical modeling, a number of useful approximate models have been derived to allow for all loss mechanisms in a particular environment.While only of first order accuracy in complex scenarios they do allow basic channel parameters to be estimated rapidly during initial system design1)Log-distance modelIn this we havePath Loss(dB)~PL(d0)+10 n log(d/d0)Where : PL is the path loss to a reference distance d0(measured)

d is the distance between r/r and d0

n is path loss exponent and depends upon the environment Typical large-scale path lossEnvironmentPath loss exponent (n)Free space2Urban area cellular radio2.7 3.5Shadowed urban cellular radio3 5In building LOS1.6 1.8Obstructed in building4 6Obstructed in factories2 - 32)Log Normal Shadowing In practice the log distance model can be extremely inaccurate due to shadowing- the model assumes path loss is same at any given distance.For wide area coverage prediction the log distance model can be improved by assuming shadowing is Gaussian over many locations log-normal shadowing modelPath Loss(d)[dB] = PL(d) + X = =PL(d0)+10 n log(d/d0)+ X Where X is the zero mean gaussian distributed random variable (in dB) with standard deviation also in dB.Indoor ModelsDifficult to predict exactlySome statistical Models, e.g. COST 231: 800 MHz and 1.9 GHzEnvironmentExponent nPropagation Mechanism Corridors 1.4 - 1.9 Wave guidance Large open rooms2Free space lossFurnished rooms 3 FSL + multipathDensely furnished rooms 4 Non-LOS, diffraction, scattering Between different floors5Losses during floor / wall traverses Wirelesscalculate13Indoor Attenuation by Constructions900 MHz20 cm concrete 7 dB (s = 1 dB) wood and brick siding3 dB (s = 0.5 dB) Aluminum siding 2 dB (s = 0.5 dB) metal walls 12 dB (s = 4 dB) office furnishing 1 dB (s = 0.3 dB) 2.4 GHzPlasterboard wall3 dBGlass wall with metal frame6 dBCinder block wall4 dBOffice window3 dBMetal door 6 dBMetal door in brick wall12 dBHow do systems handle shadowing?GSMFrequency planning and base station locationsPower controlDECTSelect good base station locations IS95Power controlSelect good base station locations15Cell Shapes- Ideal & Real

Outdoor Propagation ModelsA number of propagation models exists to predict the path loss over irregular terrain.All these models aim to predict signal strength at a particular receiving point or area.But they widely differ in their approach, complexity and accuracy.Some of these commonly used propagation models are discussed here.Longley-Rice modelPoint to point communication systemFrequency range from 40 MHz to 100GHzTransmission loss predicted using path geometry and refractivity of troposphereTwo-ray model used to calculate signal strength with the radio horizonDiffraction loss calculated using Knife-edge diffractionAvailable as a computer software modelInput data is t/m frequency, path length, polarization, antenna height, surface refractivity, ground conductivity and climate conditions etc URBAN FACTORLongley-Rice

Urban Factor for additiona lattenuation dueto urban clutter near the receiving antenna Disadvantage: No corrections due toenvironmental factors or effects of buildingsin immediate vicinity of mobile receiver.Multipaths are also not considered.Okumaras modelOne of the widely used propagation modelFrequency range 150MHz to 1920 MHz (extrapolated to 3000 MHz)Cover distance 1 Km to 100 Km with antenna heights from 30m to 1000m.Okumara developed set of curves which gives median attenuation relative to free space in an urban area Amu. Where ht=200m & hr=3mThese curves are plotted both as a function of frequency and distance between the base station and mobile terminal.

Okumaras modelThe model can be expressed as

L50(dB)=LF + Amu(f,d) G(hte) G(hre) Garea

WhereL50 = 50th percentile or median value of total Path lossLF = Free space propagation lossAmu = median attenuation relative to free spaceG(hte) = base station antenna height gain factor G(hre) = mobile antenna height gain factorGarea = gain due to the type of environmentOkumaras modelG(hte) = 20 log (hte /200) 1000m> hte >30m G(hre) = 10 log (hre /3) 3m> hteG(hre) = 20 log (hre /3) 10m> hre >3m 22

Hatas modelHatas model is an empirical formulation of the graphical path loss data provided by okumara Valid for frequency ranges 150 MHz to 1500 MHz

The standard formulae for median path loss by hata is given by L50(urban)(dB)=69.55 + 26.16 logfc 13.82 loghte a(hre) +(44.9-6.55loghte)logd.