Cracking the Pilot Contamination Nut in Massive MIMO

[@huynjinful]

Respect to both Marzetta and Bjomson !! Increadible and inspiring presentation !! I really hope to work or study with you someday😃😃

[@fahimejabbari5351]

me too :)

[@johnaweiss]

29:40 By "shortest to longest", are you referring to multipathing? Or are you comparing the distance between moving nodes at the start of the transmission vs the end of the transmission?

[@WirelessFuture]

Yes, it refers to multipathing. One can adjust the clock at the receiver so that it knows when the fastest/shortest path arrives, but the longer/slower paths will then arrive later. The coherence bandwidth is inversely proportional to the delay spread (time difference between slowest and fastest path).

[@johnaweiss]

13:00 To send separate data streams requires multiple transmitters at the Radio Port, and multiple receivers at the Remote Unit, correct?

[@WirelessFuture]

Yes! To send X data streams, both the transmitter and receiver must have at least X antennas. When you have very many antennas it is the propagation environment how many streams that can be transmitted since there must be sufficiently many strong propagation paths between the transmitter and receiver.

[@johnaweiss]

@@WirelessFuture Don't you need a separate RF receiver for each receive antenna, and a separate RF transmitter for each send antenna? Thx!

[@WirelessFuture]

@John Weiss Yes! When we in communication theory say “antenna”, we refer to an antenna element + a radio unit (RF chain). This what is also called an active antenna. ma-mimo.ellintech.se/2018/04/30/what-is-a-transmit-antenna/

[@johnaweiss]

@@WirelessFuture Surprising! Because you can also have certain kinds of diversity with multiple antennas and just one receiver circuit, correct? With proper isolation between the elements, you can do certain kinds of pre-processing before the signal goes to a single radio circuit, right?

[@johnaweiss]

30:09 By "number of times", you don't mean number of packets, but number of symbols. So it could be multiple packets, correct?

[@WirelessFuture]

Yes, the number of complex-valued symbols, each of them could for example be a 16-QAM symbol. A packet might contain 1000 payload symbols + header + CRC check, etc.

[@donaldviszneki8251]

Absolutely fantastic content. If I am experimenting with user radio implementations, what documentation should I refer to? Is there a layer 1/2 ping or other good "hello world" target in this space that doesn't require authentication? Thanks!

[@jagannathadas108]

Sir, this may be a stupid question. Are we not assuming here that the spatial correlation matrix is available at the AP ?

[@WirelessFuture]

Yes, the correlation matrices are assumed to be available everywhere needed. Knowing the statistics is a common assumption in capacity analysis, since it relies on transmit for infinitely long time. (Infinity might mean sending 10+ kbit of data). One can certainly question how well we can learn the statistics in practice, but the point is: it is not the pilot contamination that sets a fundamental limit on performance but other issues such as learning statistics.

[@fahimejabbari5351]

Dr.bjornson , is the figure of pilot contamination at time 52 min of the video in Your book?

[@WirelessFuture]

No, it was created directly for this presentation. It is just a few lines of code based on the formula that is shown on the slide.

[@fahimehjabbarinia401]

@@WirelessFuture thank You.

[@carolinemathieson]

My understanding is that both LTE Advanced and 5NR use orthogonal cover codes for pilots but you don't mention that here. How does that effect your analysis?

[@WirelessFuture]

This presentation focuses on sounding reference signals (SRS) and the contamination that occurs when the same sequence must be reused somewhere else in the network. SRS IS used for the channel estimation that enables precoding and combining. After the precoding has been selected, orthogonal cover codes can be transmitted in the downlink as demodulation reference signals. Contamination can occur in that case as well, when the same sequence is used at multiple places at the same time, but it is less of a problem since it does not cause coherent interference.

[@rfengineering2022]

Great presentation Dr. How to get your book and papers ?

[@WirelessFuture]

There are links to most of them here: ebjornson.com/research/

[@AdEngineer]

I'm afraid I have to watch this after no nut November :)