In mmWaves electromagnetic channels (≥30GHz), the channel models might be known, but they are too complex and/or change too fast to estimate with reasonable accuracy ?
why the channel models changes too fast ?
Dear Abdelwahab,
As the wavelength of the electromagnetic wave gets smaller the path loss for certain propagation distance d becomes much larger as the path loss is proportional to 1/lambda^2. So the path loss is very sensitive to wavelength.
More important is the multi path fading where it becomes very critical with the shorter wavelength. Assume that you have only two paths of radiation when the path difference between the two is zero the two rays will add constructively and only if the difference of the path becomes lambda/2 the two rays will interfere destructively causing very large fading of the signal.
Since one has mobile channels with say a velocity difference of v, then there will be a Doppler effect of changing the carrier frequency by df= v fc/ c , with c= lambda fc, then df= v/lambda. One defines a coherence time Tcoh= 1/df = Lambda/ v, The coherence time is the time where the channel can be considered constant. It is clear from this simple study that the multi path mobile wireless channel is subjected to channel variation with an interval Tcor= Lambd/ v. So, as lambda increases the Tcor increases and the rate of change of the channel will decrease.
In addition to the doppler shift there are the scattering objects which has more pronounce effects with decreasing the wavelength.
Best wishes
Dear Abdelwahab,
As the wavelength of the electromagnetic wave gets smaller the path loss for certain propagation distance d becomes much larger as the path loss is proportional to 1/lambda^2. So the path loss is very sensitive to wavelength.
More important is the multi path fading where it becomes very critical with the shorter wavelength. Assume that you have only two paths of radiation when the path difference between the two is zero the two rays will add constructively and only if the difference of the path becomes lambda/2 the two rays will interfere destructively causing very large fading of the signal.
Since one has mobile channels with say a velocity difference of v, then there will be a Doppler effect of changing the carrier frequency by df= v fc/ c , with c= lambda fc, then df= v/lambda. One defines a coherence time Tcoh= 1/df = Lambda/ v, The coherence time is the time where the channel can be considered constant. It is clear from this simple study that the multi path mobile wireless channel is subjected to channel variation with an interval Tcor= Lambd/ v. So, as lambda increases the Tcor increases and the rate of change of the channel will decrease.
In addition to the doppler shift there are the scattering objects which has more pronounce effects with decreasing the wavelength.
Best wishes
Dear Mr. Fawzy,
It does change very fast, this is mainly because as quoted:
"Classical result based on the well-known Clarke power angular spectrum (PAS) suggests that the coherence time is inversely proportional to the maximum Doppler frequency, i.e. Tc 1/fD. This means that by moving from a typical cellular frequency at 2 GHz to mmWave frequency at 60 GHz, the maximum Doppler frequency will be 30 times higher resulting in 30x decrease in the coherence time."
(https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7390852). Moreover, as Prof. Zekry said, the scattering effects are more obvious than sub-6GHz scenarios.
These aforementioned issues make the channel estimation more challenging.
It all depends on how fast mobility that you are considering. You need to estimate the channel once every time you have moved a particular fraction of a wavelength, for example, wavelength/4.
If you compare 3 GHz and 30 GHz systems where the users move at the same velocity, you will have to estimate the channel 10 times more often at 30 GHz. But if you are instead only supporting low-mobility users at 30 GHz (10 times slower movement) the system will work just fine.
I recommend you to read our paper:
Emil Björnson, Liesbet Van der Perre, Stefano Buzzi, Erik G. Larsson, “Massive MIMO in Sub-6 GHz and mmWave: Physical, Practical, and Use-Case Differences,” IEEE Wireless Communications, To appear. https://arxiv.org/pdf/1803.11023
As Dr Björnson explained to you, this complexity depends on the mobility and propagation environment. You could, of course, apply what is know as beam trucking algorithms or you could exploit the properties of the channel sparsity with compressing sensing method for example for the estimation. However, advance construction algorithms is needed which might be compacted in practice. I would recommend you to go to this website by Prof. Heath (http://www.profheath.org/home/videos/), which contains a lot of videos presentation and also some very effective papers in the channel estimation with MmWave. Good luck
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