Dear Neeraj,

It could not understand what do said in your question because as the wavelength decreases the electromagnetic waves will be subjected to scattering by even smaller scattering object. So, it is expected that as the frequency increases the transmission will be nonline of sight for the same scattering path.

It may be that in the mm-wave one can intentionally establish line of sight communication by decreasing the length of the path and using antennas which concentrate the radiated radiation into narrow beams.

However in multi user MIMO this will help increase spatial multiplexing. It is so that one can easily direct a beam by specific user.

Best wishes

I agree with Dr. Abdelhalim abdelnaby Zekry and I would like to add this:

in mmWave frequencies, it has been proven (through measurements) that you can get usually one LoS and between (3-8) NLoS beams because of the use of directional antennas with narrow beams and high gain and because mmWave don't reflect well from a lot of objects (they are easily absorbed and vanish when hitting trees, humans, walls,....etc.).

Hope this explains why we say that the mmWaves have small spatial multiplexing

Dear Neeraj,

All what you mentioned in your question is correct ! The mmWave channel is sparse (LoS dominated) . Therefore, although we can use Massive MIMO with very large number of antennas at both the transmitter and receiver, we can only serve a limited number of data streams which is smaller than (min(Nt,Nr) as in conventional MIMO).

Concerning Multi-user case, I want to add to the previous replies that it is not straightforward or simple ! Because, given that the channel is sparse and LoS dominated, the spatial correlation is a BOTTLENECK specially for clustered users and it is not simple as in rich scattering channels in microwave that can be simply modeled as an iid rayleigh channel. Therefore, multi-user scenarios at mmWave are only possible with MASSIVE MIMO in order to have a high spatial resolution and break the spatial correlation deadlock. In this case you can separate between users in the beam space as previously mentioned in the comments.

Kind regards

Hi there, I suppose what you and other researchers have proposed and discussed are very what the 5G community has been trying to aim and effectively deal with.

Moreover, at mmWave bands, whether spatial multiplexing or beamforming (or both) schemes should be adopted, it really depends on the specific application scenarios, such as indoor/outdoor, hardware resources availability, scattering status, etc., that is also why various beamforming techniques/architectures exist here, and some of which are suitable for spatial multiplexing, and some are more suitable for pure analog beamforming, or hybrid. On the other hand, the sparsity of mmWave channels also motivates researcher to take good advantage of massive MIMO to harvest and push up the performance of spatial multiplexing to compensate its sparsity characteristics. There is a classic paper about how to do precoding under such challenging environments: Article Spatially Sparse Precoding in Millimeter Wave MIMO Systems