Three Benefits of Using Multiple Antennas in Communications [Video 2]

[@rabinkanisha7503]

Wow sir, excellently explained🌷

[@steavenchede9373]

Thank you Prof Björnson for this video series. Please, could you explain more this sentence from the video (09:06) "... if outages happen very infrequently, we will also be able to fit very little information into the packets. We need to live with outages in order to get a lot of data through the channels" ? I am not sure to understand why we need to live with outage. Thank you

[@WirelessFuture]

The point is the following: We can avoid outages by communicating very slowly (few bits per packet), but if we will send a lot of data, it is better to communicate at a faster rate (more bits per packet) and allow for retransmissions whenever needed. It is like talking in a noisy room. You are either speak very slowly, or you speak in your normal way and let the listener ask you for repeating yourself when needed. Many wireless broadband communication systems are designed for 5% outage probability, which is quite high but leads to more data throughput on the average. We return to this in video 9: kzbin.info/www/bejne/b5TId3Shlst1hNk

[@steavenchede9373]

@@WirelessFuture I get your point. Thank you very much!

[@thinyuaung9333]

Thank you so much Prof Emil

[@Telecom-knowledgeBlogspot]

Very great explanation :)

[@davidhegarty9608]

Really well done very helpful

[@SandipGhosalmom]

1) @2:57 It is understandable of Options 1 and 2. Transmitted power does not follow superposition theorem actually, only voltage and current do. 2)In the outage probability of M antennas in @10:35, it is assumed that the antennas are independent..so p^M...if the antenna mutually interact, how is it going to change?

[@WirelessFuture]

2) If the channels are correlated, then you will lose some of the benefits from having multiple antennas.

[@SandipGhosalmom]

@@WirelessFuture Thank you so much. Anyway sorry for the typo in the first line of the first question: a 'not' missing.

[@WirelessFuture]

1) With beamforming, the emitted power is focused in certain directions. In those directions, one can observe M times higher power than the average over all directions.

[@abdulazizalakoub1087]

Thank you very much Prof.Emil If we have 10 users in the coverage area of the BS then we can serve them by 10 different beams simultaneously ? How can the beams recognize the users location to focus power to them ?

[@WirelessFuture]

Abdulaziz Alakoub The system needs to learn the user channels (e.g., the position if it is a line-of-sight channel) to point the beams correctly. This is done by sending known signals called pilots or reference signals to enable channel estimation.

[@abdulazizalakoub1087]

@@WirelessFuture Thank you very much

[@BD-Research-Den]

@@WirelessFuture Yes, CSIT is crucial for MIMO Multi-User

[@jyothiyeruva329]

Thank you for your session : May I know what is meant by multiple antenna here , is it with respect to logical antenna or Physical antenna elements?

[@WirelessFuture]

It refer to logical antennas.

[@niravpatel3961]

i like it sir...

[@prakhargupta1409]

Just a doubt.. with spatial multiplexing, we can spatially distribute the signals to multiple users. How does that increase the system throughput and what benefit does this give ? I understand system throughput as the no. of data bits transferred per second.

[@WirelessFuture]

With spatial multiplexing, every user can get (almost) the same number of bit/s as if they are alone in the system. The system throughput grows proportionally with the number of users (as long as we have more antennas than users)

[@davidhegarty9608]

when using spatial multiplexing for one user on the same frequency how do separate the signals !? no one talks to that do you uses different OFDM carriers ?

[@WirelessFuture]

The signals are separated by using multiple antennas at the transmitter and receiver, to transmit signals in different directions and have the ability to separate signals that are received from different directions. Video 6 in this series explains it in more detail.

[@leenhy2901]

started looking for long-range wifi routers and end up in this nice video hahha

[@mopol4400]

hi Mr what does that mean maximum spatial multiplexing gain min(nt,nr)

[@WirelessFuture]

It is the largest number of signals that can be transmitted at the same time and frequency, and still being separable at the receiver side. The separation is achieved by transmitting them in different directions and receiving them from different directions.

[@mopol4400]

@@WirelessFuture is that means the transmitter demultiplexing the input datastresm into min(nr,nt) sub stream?

[@WirelessFuture]

Yes, the transmitter divides the data stream into min(nr,nt) substreams and transmit them using different beams. The receiver extracts the substreams by utilizing that they are received from different directions. The mathematical details are found in the more technical video: kzbin.info/www/bejne/j6Oco2CsqtWgm9E

[@muhammadidrees9675]

Excellent work sir. Kindly start uploading videos regarding CST microwave studio for antenna design..

[@engAhmedRaafathosny]

Very nice technical video and great presentation. I have one question, let us consider a single tap broadcast channel, the user has one antenna and the BS does not have any knowledge abut the CSI. In this case, if we use one antenna at the BS with isotropic radiation, we could serve the user, however, if we have two antennas at the BS, due to the destructive interference, the probability of serving the user may be lower. So what is the benefit of using two antennas at the BS in this scenario ?

[@WirelessFuture]

Eng/Ahmed Ra'fat We can use space-time coding to achieve gains from having multiple antennas even if the transmitter lacks CSI. This is basically a way to spread and repeat the information over different “beams” so that the user is guaranteed to receive all of it. In the two-user case, the Alamouti code can be used.

[@saidroidp7504]

Fantastic Video. I am experimenting on Human Activty classification usign WiFi CSI. [ Spatial diversity and multipath + Denoising+ ML based activity classification]. Want to know if there is a way to tune Transmit antenna same power ? When I try to test my work , one day things work fine. After few days when I set the environment to test (my antenna orienation might not be exactly same) Average power in WiFi signals is totally different [either very high or low or totally different] and that is confusing my Machine Learnign algorithm. Any suggestions on Antenna orientation to make sure same amount of power is generated?

[@WirelessFuture]

Saidroid p I have heard from people that are doing positioning that the signal strength in WiFi can be rather unpredictable. Changes in the antenna orientation and scattering environment can have substantial impact. If you cannot get rid of this issue, you will have to design your learning algorithms to become robust to such variations.

[@gharb2282]

Greetings,thank you for this amazing lessons ,i have question if you answer me i will be so thankful to you My question is (how can i determine outage requirement in the system in this lecture you give 10^3 ?)

[@WirelessFuture]

The outage requirement depends on the application. Delay-insensitive systems might be fine with 10^(-1) as outage probability since one can afford retransmissions. While a system that require every packet to be received immediately might need an outage probability of 10^(-5). There is no simple way to determine what is the best outage probability, but there are ways to compute outage probabilities. The theory is provided in this video: kzbin.info/www/bejne/f6avinp_m9Z2d5I

[@gharb2282]

@@WirelessFuture thank you i will see it

[@jerryku]

great video, but during the spatial multiplexing explanation, you talk about multiplexing being used for "multiple users" but i believe multiplexing can also be used instead for a single user (receiver) to increase the data rate for that user.

[@WirelessFuture]

G Ku Yes, that can also be done.

[@tina__emm]

perfect thanks alot

[@aalselwi]

if we sent an orthogonal waveform associated with each antenna element ,this orthogonal waveform contains information for specific user, and in receiver side doing match filtering. can this be space-time multiplexing provided by MIMO system?

[@WirelessFuture]

The question what you mean with "orthogonal waveform". If the waveform is orthogonal in the time/frequency domain, then what you are describing is basically the same thing as TDMA, FDMA, or OFDMA. Those are orthogonal access schemes. If you want to do spatial multiplexing, then the waveforms are overlapping the time/frequency domain but are emitted in a spatially directive way so that users at different places can observe different signals.

[@aalselwi]

@@WirelessFuture thanks alot. actually, i meant by orthogonal waveforms a concept of waveform diversity.