Fading: A Multipath or a Doppler issue ?

Hi all,

I entered a discussion with a colleague about the reason of fading, I always understood that the constructive and destructive interference of multipath components and moving along a spatial pattern of destructive and constructive interference will result in a time varying channel. Butmy colleague argued that fading is only due to doppler shift and that multipath only decides the frequency domain transfer function. He says that fading exists even if no multipath at all. Is this true ? i.e. Can we have a fading with a single-path channel ? What is the physical meaning of this ??

Thanks.

Dear ahmed
Hi
The feding problem in communications , has a simple justify!
First justify: the main transmitter sent a signal and one transmitter ( not main transmitter) transfered a same signal ! if those signals are in phase together , the voice of receiver (its value) will increase . if are not in phase together , the voice will decrease .
Second justify: the main transmitter , transfered the signal , and the receiver got it and the reflect of the transfered signal , ( for example influence of mountain reflect) received . mixture between them : if become in phase , voice will increase , if become in 180degree phase shift , the voice will decrease!
I hope become helpful
Best wishes
Goldsmith

For every phenomenon, we have two interpretations: one in the time domain, and one in the frequency domain. For the Doppler spread in the frequency domain we have time selectivity in the time domain. This means that the channel changing, which implies that there is motion either from the transmitter, receiver, or the environment.

So, the channel is time varying not because of the constructive and destructive addition of multipath components at a particular point in space, but because of the changes in the channel due to motions. For example, if you are talking from your mobile phone and you not moving, and the environment surrounding you is static, then the channel here is considered to be time-invariant, because the reflected components are always the same. However, if you are walking or the surrounding environment is not static, then the channel is considered time-varying.

At the end, no, we can not have fading if we don't have reflected components, but we may have distortion due to Doppler shift.

Thanks

I agree with you David, but to summarize: We cannot have fading without both Multipath and motion, right ? I mean, a single component arriving at the receiver can never fade regardless of the motion.

To have fading we have to have multiple replicas of the same signal arrived at the receiver at different times which are time dependent. In phasor forms, when we have different phasors with different phases and different amplitudes where the phases and amplitudes are time-dependent. So, yes, we can not have these things without multipath components and time variation in the channel, because otherwise, the received signal power will not vary if the channel is not time-varying, even if we have multipath components.