Chapter 4 Signal Degradation for Line-of-Sight ... - DESCANSO

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Chapter 4 Signal Degradation for Line-of-Sight Communications 4.1

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Figure4.1: Best fits powercurve cumulativefade distributionsof form (4.1) for line of sight distributionsin whichmultipathfadingdominatesfor mountainousterrain.

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4.3 MultiPath Due to Roadside‘Ikees

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Table 4.1: Coefficientsa and bin formulation(4.1) describingbest fit cumulativefade distributionfor multipathin mountainousterrain Frequency(GHz) 0.870 1.5

El = 30° El = 45° dB Range b dB Range a b 2-7 2-4 34:52 1.855 31.64 2.464 33.19 1.710 2-8 39.95 2.321 2-5

elevationangles. We note from Figure4.1 that over the percentagerangeof 1% to 10%, the fades due to multipathvary between2 and 5 dB at 45°, and 2 and 8 dB at 30° elevation. The higher frequency (L-Band) exhibits slightly larger fades which are generallywithin 1 dB or less relativeto UHF. The slightlylargerfades at L-Band can be attributedto the smallamount of tree fading whichmay have been present. Theremay also have been a presenceof more reflectingfacetson the canyonwallswith sizescomparableto 20 cm (L-Band) or largerthan does exist for the UHF case (34 cm). Such facets (L-Band case) would offer larger cross sections(Mie scattering)than facets whose dimensionswere small relativeto a wavelength (UHF case) whereRayleighscatteringis applicable.

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Largerfadesat the 30° elevationrelativeto 45° can be attributedto sometree shadowing wherethe smallerelevationangleoffersmorepropagationpath throughthe foliageand hence greaterattenuation. It can also be attributedto the fact that multipathis dominatedby illuminatedsurfacescloser to the vehicle; which implieslower reflectingheightsand more shallowelevationangles.

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Multipath Due to Roadside ‘lYees

Similartypes of line-of-sightmeasurementswereperformedby the authorsin centralMarylandalongtreelinedroads IGoldhirshandVogel, 1989]as weredescribedfor the mountainous terraincase in Section 4.2. That is, repeatedmeasurementruns at 30°, 45°, and 60° were implementedwith the helicopterfollowingthe vehicleand cumulativedistributionswerederived at both UHF and L-Band. The distributionswereobservedto be relativelyinsensitive

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Multbath Due to Roadside!LYees

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Table4.2: Coefficientsu and v in formulation(4.2) describingbest exponentialfit cumulative fade distributionsfor multipathfor tree-linedroads. Fkequency(GHz) 0.870 1.5

Fade Range (dB) “ u 127.7 0.8~73 1-4.5 125.6 1.116 1-6

to path elevation. The three runs were combined into one distributionat each frequency comprising75 km of driving. The resultantdistributionswerefound to follow an exponential form given by, P = 1% to 50~ P = uexp (–vA)

. (42)

whereu and v are tabulatedin Table 4.2 and the correspondingdistributionsare plotted in Figure4.2. The fades at the two frequenciesfit very well an exponentialfunction from 1 dB (at an exceedanceof 40% to 50%) to approximately4.5 to 5.5 dB (at an exceedanceof 1%). We note a slight dependenceexhibiteddue to frequencywith L-Band giving approximately 1.5 dB greaterfades at 1%. The indicated best fit exponential were found to agree with each of the original measuredcumulativedistributionsto within 0.2 dB. Fading due to multipathis presumedto emanatefrom scatteringoff of tree canopieswhichreradiate,more or less,isotropicallyin elevationangle. Suchan explanationis consistentwith the fact that the distributionswere relativelyinsensitiveto path elevationangle in the angularinterval between30° and 60°.

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Enhancedfading due to multipatheffects are expected for antennaspointed at lower angles(e.g., 5° to 20°) wherescatteringfrom tree canopiesand trunks,other vehicles,and the road itselfmay be receivedwith smallerantennagain filtering. Also, a greaterlikelihood existsfor shadowingto occur at the lowerelevationangles.

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