Fading y Shadowing

Universidad Politcnica Salesiana Sede Guayaquil
Carrera de Ingeniera Electrnica

Redes Inalmbricas Grupo 4060
Ronald Moreira Sandoval
Fecha de entrega: 5 de noviembre de 2013
Fading
El trmino fading o desvanecimiento se refiere a que dicha seal es atenuada debido a la prdida en el espacio, los
obstculos y resistencia que debe traspasar durante toda su trayectoria hasta su destino. Tambin lo podemos definir
como una variacin temporal de la amplitud, fase y polarizacin de la seal recibida con relacin a la seal nominal
debido al trayecto de Propagacin: multitrayecto, conductos, reflexin, difraccin y dispersin.
El desvanecimiento de la seal es cuando una seal se atena debido a la prdida en el espacio, los obstculos y
resistencia que debe traspasar durante toda su trayectoria hasta que llega a su destino. Dicho desvanecimiento se
refleja como baja o mal potencia recibida en el terminal receptor. El desvanecimiento tiene carcter probabilstico.
Se puede presentar bajo condiciones de densas nieblas rastreras, o cuando el aire extremadamente fro se mueve sobre
un terreno clido. El resultado en cada caso es un aumento sustancial en la prdida de trayectoria dentro de una amplia
banda de frecuencia. La magnitud y la rapidez de este tipo de desvanecimiento lento y plano se pueden reducir, en
general, usando mayores alturas de antena.
Desvanecimiento en gran escala

El desvanecimiento a gran escala representa un promedio de la atenuacin de la potencia de la seal en grandes reas.
Los modelos de propagacin a gran escala predicen el comportamiento medio para distancias >> . Corresponden a
cambios del valor medio de la seal cuando la distancia del transmisor al receptor vara significativamente.
Desvanecimiento en pequea escala

El desvanecimiento a pequea escala, o simplemente desvanecimiento, es un trmino para describir la rpida
fluctuacin de la amplitud y la fase de una seal de radio en un corto periodo o durante el viaje de sta en una
distancia corta en relacin a su longitud de onda .
Desvanecimiento multitrayectoria
Causado por mltiples Reflexiones, Difraccin, Dispersin de la seal transmitida y que llegan al receptor, crea
efectos de desvanecimiento a pequea escala, de los cuales los tres ms importantes son:
Rpidos cambios en la intensidad de la seal sobre una corta distancia de viaje o en intervalos.
Modulacin de frecuencia aleatoria debido a cambios en la Dispersin Doppler de diferentes seales
multitrayectoria. (Movimiento)
Dispersiones en el tiempo (ecos) causadas por retardos en la propagacin multitrayectoria.
Desvanecimiento lento
Se deben a la presencia de algn obstculo de grandes dimensiones, es decir, son provocados por Zonas de
Sombra (Shadowing) que impiden la visin directa entre el emisor y receptor. Esto contribuye con una
atenuacin
adicional
que
se
suma
a
las
prdidas
por
distancia.
La duracin real del desvanecimiento lento est sujeta a la velocidad de desplazamiento del terminal mvil.
Estas prdidas tienen una variacin temporal lenta.
Shadowing
Shadowing is the effect that the received signal power fluctuates due to objects obstructing the propagation
path between transmitter and receiver. These fluctuations are experienced on local-mean powers, that is,
short-term averages to remove fluctuations due to multipath fading.
Experiments reported by Egli in 1957 showed that, for paths longer than a few hundred meters, the received
(local-mean) power fluctuates with a 'log-normal' distribution about the area-mean power. By 'log-normal' is
meant that the local-mean power expressed in logarithmic values, such as dB or neper, has a normal (i.e.,
Gaussian) distribution.
We distinguish between:
Local means: average over about 40l, to remove multipath fading

denoted by a single overline
Area means: average over tens or hundreds of meters, to remove multipath fading and shadowing
denoted by a double overbar.
Depth of Shadowing
Egli studied the error in a propagation model predicting the path loss, using only distance, antenna heights
and frequency. For average terrain, he reported a logarithmic standard deviation of about s = 8.3 dB and 12
dB for VHF and UHF frequencies, respectively. Such large fluctuations are caused not only by local shadow
attenuation by obstacles in the vicinity of the antenna, but also by large-scale effects (hills, foliage, etc.)
along the path profile, which cause attenuation. Hence, any estimate of the area-mean power which ignores
these effects may be coarse.
This log-normal fluctuation was called 'large-area shadowing' by Marsan, Hess and Gilbert. They measured
semi-circular routes in Chicago, thus fixing distance to the base station, antenna heights and frequency, but
measuring different path profiles. The standard deviation of the path loss ranged from 6.5 dB to 10.5 dB,
with a median of 9.3 dB. This 'large-area' shadowing thus reflects shadow fluctuations if the vehicle moves
over many kilometers.
In contrast to this, in most papers on mobile propagation, only 'small-area shadowing' is considered: lognormal fluctuations of the local-mean power are measured when the antenna moves over a distance of tens
or hundreds of metres. Marsan et al. reported a median of 3.7 dB for small area shadowing. Preller and Koch
measured local-mean powers at 10 m intervals and studied shadowing over 500 m intervals. The maximum
standard deviation experienced was about 7 dB, but 50% of all experiments showed shadowing of less than
4 dB.
Mawaira of the Netherlands' PTT Research modelled large-area and small-area shadowing as two
independent superimposed Markovian processes:
3 dB with coherence distance over 100 m, plus
4 dB with coherence distance 1200 m

Implications for Cell Planning
Shadowing makes practical cell planning complicated. To fully predict local shadow attenuation, up-to-date
and highly detailed terrain data bases are needed. If one extends the distinction between large-area and
small-area shadowing, the definition of shadowing covers any statistical fluctuation of the received localmean power about a certain area-mean power, with the latter determined by (predictable) large-scale
mechanisms. Multipath propagation is separated from shadow fluctuations by considering the local-mean
powers. That is, the standard deviation of the shadowing will depend on the geographical resolution of the
estimate of the area-mean power. A propagation model which ignores specific terrain data produces about 12
dB of shadowing. On the other hand, prediction methods using topographical data bases with unlimited
resolution can, at least in theory, achieve a standard deviation of 0 dB. Thus, the standard deviation is a
measure of the impreciseness of the terrain description. If, for generic system studies, the (large-scale) path
loss is taken of simple form depending only on distance but not on details of the path profile, the standard
deviation will necessarily be large. On the other hand, for the planning of a practical network in a certain
(known) environment, the accuracy of the large-scale propagation model may be refined. This may allow a
spectrally more efficient planning if the cellular layout is optimised for the propagation environment.
How do systems handle shadowing?

System
GSM
DECT
Countermeasure
Frequency planning and base station location
Adaptive transmit power control
Base station location
Power control. Power control in IS95 is also needed to achieve

sufficient performance of the CDMA receiver.
Base station locations
Single frequency networks. Shadow fades are filled in by signals from

other co-channel transmitters.
IS95 Cellular CDMA
Digital Audio
Broadcasting

Fading y Shadowing

Cargado por

Copyright:

Formatos disponibles

Fading y Shadowing

Cargado por

Información del documento

Descripción original:

Derechos de autor

Formatos disponibles

Compartir este documento

Compartir o incrustar documentos

Opciones para compartir

¿Le pareció útil este documento?

¿Este contenido es inapropiado?

Copyright:

Formatos disponibles

Fading y Shadowing

Cargado por

Copyright:

Formatos disponibles

Universidad Politcnica Salesiana Sede Guayaquil

Carrera de Ingeniera Electrnica

Desvanecimiento en gran escala

Desvanecimiento en pequea escala

Local means: average over about 40l, to remove multipath fading

3 dB with coherence distance over 100 m, plus

4 dB with coherence distance 1200 m

How do systems handle shadowing?

Frequency planning and base station location

Adaptive transmit power control

Base station location

Power control. Power control in IS95 is also needed to achieve

Base station locations

Single frequency networks. Shadow fades are filled in by signals from

IS95 Cellular CDMA

También podría gustarte