Multichannel fading

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CONTENTSWireless communicationRadio wave propagationLOS Wireless Transmission ImpairmentsMulti path propagationfadingFactor influencing multipath propagationTypes of fadingDoppler shift


Wireless communication

Transmitting signalReceived signal satelliteTransmitting antenna Receivingantenna3


Radio wave propagation

Line-of-Sight PropagationSky Wave PropagationGround Wave Propagation4

LOS Wireless Transmission ImpairmentsAttenuationFree space lossNoiseAtmospheric absorptionMultipath propagation


Multi path propagation effect6

A: free spaceB: reflectionC: diffractionD: scattering

fadingFading signals occur due to reflections from ground & surrounding buildings (clutter) as well as scattered signals from trees, people, towers, etc


Factor influencing fadingMultipath propagation Multiple versions of the signal arrives at the receiver. Can cause signal smearing due to inter symbol interference.Speed of the Mobile Results in random frequency modulation due to Doppler shifts on each of the multipath components. It can be positive or negative depending on the movement of mobile receiver.Speed of surrounding objectsIf the surrounding objects move at a greater rate than the mobile, then this effect dominates the small-scale fading.Transmission bandwidth of the signal The bandwidth of the multipath channel can be quantified by the coherence bandwidth.The coherence bandwidth is a measure of the maximum frequency difference for which signals are still strongly correlated in amplitude.


Practical examples of small scale multipath fadingCommon examples of multipath fading are temporary failure of communication due to a severe drop in the channel signal to noise ratio (You may have also experienced this. And you moved a steps away & noted that reception is better. It is due to small scale fading effects. )FM radio transmission experiencing intermittent loss of broadcast when away from station9


The most common types of fading areSmall scale fading: Small scale fading or simply fading is used to describe the rapid fluctuations of the amplitude, phases or multipath delays of a radio signal over a short period of time or travel distance, Large scale fading : Large-scale fading is the result of signal attenuation due to signal propagation over large distances and diffraction around large objects in the propagation path.10



Frequency selective fading

Frequency selective fading, where the bandwidth of the signal is greater than the coherence bandwidth of the channel or the delay spread is greater than the symbol period. 12

Flat fading

Flat fading is caused by absorbers between the two antenna and is countered by antenna placement and transmit power level. Here all frequency component of the received signal fluctuate in the same proportion simultaneouslyFlat fading, where the bandwidth of the signal is less than the coherence bandwidth of the channel or the delay spread is less than the symbol period


Fast fading

Fast Fading is a kind of fading occurring with small movements of a mobile or obstacle. Depending upon how rapidly the transmitted base band signal changes as compared to the rate of change of the channel.14

Slow fading

Slow Fading is a kind of fading caused by larger movements of a mobile or obstructions within the propagation environment. This is often modelled as log normal distribution with a standard deviation according to the log distance path loss modelled.15

Small-scale fading due to movements: Doppler shiftHow rapidly the channel fades will be affected by how fast the receiver and/or transmitter are moving Motion causes Doppler shift in the received signal components the change in frequency of a wave for a receiver moving relative to the transmitter


Doppler shiftSay a mobile phone moving at velocity v km/hr in the x direction and the radio wave impinging on the receiver at an angle kThe motion introduces a Doppler frequency shift, fk = vcos k/

If the ray is directed opposite to the mobiles motion (=0), then fk=v/The frequency of the signal has increased by the Doppler spread, fk


Mitigation Techniques for the Multipath Fading ChannelDiversity techniqueAdaptive equalizationForward error correction


Mitigation Techniques for the Multipath Fading Channel19Space diversity techniques involving physical transmission pathFrequency diversity techniques where the signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriersTime diversity techniques aimed at spreading the data out over time