Introduction to Multipath

8/2/2019 Introduction to Multipath

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Direct Signal, angle of arrival and Doppler shift

Direct and reflected signal

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Multipath is a time selective phenomenon, it

gives rise to fades. In addition, the movement of atleast one of the terminals causes Doppler effects. TheDoppler shift and the angle of arrival of a given echoare interrelated, and that there is a limit to the

maximum rate of change possible, if it is only theterminal that is moving. Throughout, assuming thatmultipath gives rise to a spatial standing wave,sensed by the terminal antenna as it moves. Also,

show a possible way of generating time variationswhen the mobile terminal is stationary. Finally,introducing the case where both terminals, and eventhe scatterers, are moving.

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In the multipath propagation channel, the transmittedsignal arrives at the receiver through several echoes dueto reflection, diffraction and scattering, In general, tofeatures such as walls, rooftops, lamp posts, cars, trees,people, etc., in the near environment of the mobile.

Multipath is significant in mobile communications, since

the (MS) antenna is usually located at low heights andsurrounded by obstacles. It typically uses omnidirectionalpatterns thus picking up large numbers of echoes.Conversely, typical microwave fixed radio links are sited

above the surrounding clutter, and use very directiveantennas that discriminate against most of the existingmultipath. Only in very specific conditions whereanomalous refraction exists, rays will bend down towardthe ground giving rise to multipath.

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Figure 1: Simulation geometry for Project (side view)

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(13)

Figure 2: project 2 geometry

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(15)

The tow radio distances change in time:

(16)

(17)

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Dropping the fixed phase term:

(18)Taking into account the equality:

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:

(20)

(21)

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Project 3: Tow Scatterers

two point scatterers are assumed to be the only sourcesthrough which the transmitted signal arrives at the receiver,while the direct signal is assumed to be totallyblocked.(relating tow of aspects of multipath phenomenon:fade rate and Doppler spread)

Figure 3: project 3 simulation geometryOmnidirectional


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Project 4: Multiple scatterers

If the number of point scatterers is increased, moreintricate time series can be produced. it will be

possible to simulate many typical situations in mobilepropagation such as Rayleigh fading conditions. Theexamples reproduced here have been generated using13 rays.

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Figure 4: 2D simulation scenario made up of point scatterers

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Introduction to Multipath