Ep 13. Distributed and Cell-Free Massive MIMO [Wireless Future Podcast]

[@AbdulUofG]

A great and intuitive approach by both Profs, I always enjoy listening your lectures...

[@CSmaniac10012]

Hello professor, I have a quick question. I have been surveying a lot of CF-mMIMO papers and I have seen there is a dearth of papers on the detection of CFmMIMO systems, especially in downlink. Most of the receivers consider working on detecting the different users at the AP in the UL and not the other way around. Why is that so? Don't u think we should also look at how detection is done at the UEs too? Thanks

[@WirelessFuture]

The classical way to connect to a network is for the UE to send a random access sequence and then get scheduled for uplink/downlink transmission at specific resource blocks. No user identification is needed, apart from the random access procedure. However, if UEs send very little data per transmission, the random access procedure might consume too much resources, relative to the payload data. This is when the AP must also identify the user. In the downlink, if the UE isn’t scheduled but should receive data, then there is a paging procedure that can be used to contact them. There is also the concept of wake-up radio, which makes paging more energy efficient. Detection is needed at the UE in these cases as well.

[@johnaweiss]

10:04 But the system doesn't depend on interference at the other nodes, because they aren't listening at that moment, correct?

[@WirelessFuture]

The word “interference” can be confusing here since it has two different meanings. Constructive interference means that the signals arriving from different directions (different access points) are added on top of each other, so that one gets a strong signal at the desired locations. This also means that the signal will be weaker at other locations, so if there is another user that wants to simultaneously hear a different signal at that location, it will be exposed to less disturbance (less interference).

[@johnaweiss]

@@WirelessFuture Ah, so due to destructive interference at a neighbor node of the target node, the neighbor node will be in a null-ish region for the signals being sent to the target node, correct? But, depending on the transmit frequency, the quantity and positions of the transmitters, and proximity of the neighbor node, isn't it possible or, under certain circumstances, likely that whole the neighborhood will hear coherence? Won't most neighbor nodes hear sufficient coherence? For example, with 5 transmitters at random distances and angles, we'll get a pattern of nulls and peaks, correct? But most listen locations WON'T be in a null, correct? At 900 Mhz, for example, how frequent are the nulls? What's the distance between the nulls? Won't most receive positions hear sufficient coherence for proper demodulation? What must be the distance between the target node and the neighbor node to ensure the neighbor doesn't hear sufficient coherence? Half a wavelength (16.5 cm)? Half a mile? Thx!

[@johnaweiss]

I think the answer is in your other video here, correct? kzbin.info/www/bejne/gmqbZq2PqK6orcU

[@dimitrisv.1729]

Coordination of base stations increases the latency of the system, therefore channel may change during this time. Could OTFS modulation be useful at cell-free networks?

[@niravpatel3961]

As prof. erik said there are multiple antennas at user equipment to resolve number of signals coming from different transmitting antennas (access points)...so my question is, are there more than one antenna at user equipment? kindly reply...

[@WirelessFuture]

Yes, if the user device has multiple antennas, it can resolve multiple streams in that way. Starting from 4G, user devices generally have multiple receive antennas (but often only one transmit antenna).

[@niravpatel3961]

@@WirelessFuture ok thank you sir, so kindly tell me how many receive antennas are there usually in 4g hand set/user equipment...

[@WirelessFuture]

Two receive antennas. It is usually called 1T2R, in case you want search more about it.

[@johnaweiss]

This is sort of TDMA receiving, rather than TDMA sending, correct?

[@WirelessFuture]

I’m not sure of which statement in the video you are referring to. But distributed MIMO isn’t based on TDMA. Many users can transmit (or receive) data at the same time using the same frequency. When transmitting to the users, we can send multiple signals at the same time and phase-shift them in such a way that each signal is spatially focused at the desired location. So each user hears a strong version of its desired signal and a weak version of all the other signals.

[@kiyameribo2504]

thank you for your brief explanation prof emil and prof erik, what are the main challenges for scalability and power control in cell-free massive MIMO to enhance a spectral efficiency and energy efficiency?

[@WirelessFuture]

Thank you for the question! We answer it in the new Episode 14: kzbin.info/www/bejne/qJW9gH2Qicp4iZo

[@aashreyarayamajhi5909]

thank you for the podcast! what are the technical specification of cell-free massive Mimo systems?

[@WirelessFuture]

The algorithmic details are summarized in our book that you can find here: github.com/emilbjornson/cell-free-book Cell-free Massive MIMO can likely be implemented on top of the 5G standard, since it already separates the control plane and data plane. The user device doesn’t have to know if the signals come from one access points or a cluster of them.

[@williamrivera724]

Please , tell me how to buy this book?

[@WirelessFuture]

There is a link to the publisher’s website in the description. There are sometimes promotional campaigns with special prizes. The next one will be during VTC Spring 2021.

[@rjpilgrim]

Love the surf analogy lmao

[@shefudgrupa]

I like how they try and fail to pass the Turing test at the beginning of each podcast :P. Just joking, I really enjoy the thought-provoking conversations.

[@claudiarovira323]

tNice tutorials doesn't seem to work.

[@williamrivera724]

very good job 👌✍️

[@veselij85]

Sounds like BSC/RNC and Iur interface will come back in 6G :)

[@dimitrisv.1729]

A podcast about oRAN would be helpful!