On a special classification of fading channels(?)

[commtech]

Can we classify the time-variant fading channels in terms of the following characteristics?

1. Time-variant channel with Doppler Spread.
MS, reflectors and scatters are all in motion.
2. Time-variant channel without Doppler Spread.
Reflectors and scatters are in motion, whereas the MS is stationary.

Or equivalently, my question can be put in another way, is the Doppler Spread caused only by the motion of the MS?

Thank you very much!

[charan langton]

These two effects, the Doppler and multipath are assessed separately. This is important because, both are independentl and we may want to understand the effect of one or the other. Mitigation techniques for these two effects are also different. Yes, you can classify them and often we do talk about the channel with Doppler or without but there is no one comprehensive way to treat both effects together so such classification may be academic.

Charan Langton

[charan langton]

Reply to second part of your question: The Doppler comes when there is relative motion between transmitter and receiver. It does not matter who is moving.

Charan Langton

[commtech]

According to your reply, if there is no relative motion between the transmitter and the receiver, then there is no effect of Doppler Spread.

If that is the case, what causes the channel to exhibit a time-variant characteristic?

I just think of a situation (if I am wrong, correct me please); i.e., both the transmitter and receiver are in a synchronous motion (with the same speed and same direction). Therefore, although there is no relative motion, the signal is still transmitted over different physical paths at different times.

Thank you very much!

[commtech]

When it comes to flat fading channels, and assume there is no line of sight, I think the fading amplitudes of both the channel with and without doppler spread can be modeled as Rayleigh random variables, and the only difference between them is the power spectrum density of the fading process.

Am I right?

Thank you very much!

[charan langton]

Yes, you can model it as Rayleigh if there is NO line of sight component at all, or using a Rician model which allows one main path and other weaker delayed paths. In addition, if the channel is inside a building there are numerous empirical models such as HATA that are used.

The signal variation in amplitude and phase when there is no relative motion is due to slight changes in the physical environment, obstacles such as trees which are swaying, change in direction one is pointing towards, humidity in air, temperature and thermal enviorment, EMI, emotional state of person holding the cell radio, and numerous other bizarre and amazing non-specific sources. This is what causes a channels to be time-variant .Nealry all rf channels are time-variant to some degree with LOS links such as to satellite being the most stable.

Charan Langton