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What are Spatial Diversity and Spatial Multiplexing in MIMO?
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- Опубліковано 13 сер 2024
- Explains the difference between Diversity and Multiplexing in MIMO wireless digital communication systems. Discusses when to use each of them, in an Adaptive MIMO approach, based on the channel characteristics.
For more details see: C.-B. Chae, A. Forenza, R.W. Heath Jr., M.R. McKay and I.B. Collings, "Adaptive MIMO Transmission Techniques for Broadband Wireless Communication Systems", IEEE Communications Mag., Vol. 48, No. 5, pp. 112-118, May 2010. ieeexplore.ieee.org/document/...
* Note that most universities in the world subscribe to the IEEE Explore system, so on-line access to this paper can generally be found by going to a university library website and searching for the IEEE Explore data base access.
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Full categorised list of videos and PDF Summary Sheets: iaincollings.com
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Hi Ian, I am watching your videos since 3rd grade bachelor, now i finished my first year in master. Still there are lots of contents in you channel:)
Thanks for the video, it was a clean one
That's so great to hear. I'm glad you're still finding interesting topics on the channel. It only seems like yesterday that I started the channel. Time flies!
You are a god, sir. My lecturer didn't explain this well, and the prescribed textbook explanation was hard to understand. This was very concise and easy to digest.
I'm so glad it helped!
My God, I had no clue what my professor was talking of. He seems to be proficient in his subject, but if he only he uttered that Beam forming means: "changing the phases, so that the signals from these different antennas, all add up in phase in a particular direction"
But, my prof just scrambled up some equations with no motivation whatsoever
Imagine if I didn't understand why it's even called Spatial Diversity...
Thanks a lot, you have no idea how much of help you are. A few of my professors together have killed all my inquisitiveness and curiosity. I am having to force myself to ask doubts. And most of my fellow batch mates don't ask any real doubts either. I end up thinking I am the who one is dumb and insane, but in the end even though I got a single digit score in the exam, I was among the top 3 in the mid-sem exam.
I wish I could do something to show my gratitude...
I'm sorry to hear that your professors haven't inspired you. I'm so glad my videos have helped. Have you seen my webpage? There are lots more videos listed there, in categorised order, which might also help you and your class mates: iaincollings.com
Thanks for sharing the website, I have actually told about your channel to quite a few of my friends. But at this point most of us are doing it only for clearing the exam and not for the fun of it.
Sorry to take your time, I just took the liberty to rant cause of exam stress...
@@iain_explains Also, what is Space Time Block Coding ? Is it hard to get an intuition for the different coding schemes in 2, 3 and 4 transmit antennas
STBC is sitting on my "to do" list for a future video.
Hello Iain,
very educational video as always.
Once you described spatial multiplexing, you added that data coming at higher rates, than being demultiplexed and sent over different antenna elements at lower rates. It is a little bit confusing, as spatial multiplex increases throughout, but If I understood that sending data at slower rates means our symbol duration is longer, then SNR is better, and we can increase the input data rate (by using higher order modulation). Did I catch the point?
Multiple parallel "lower rate" signals result in an end-to-end "high data rate".
Thanks!
Could you please give an example?
If we take an example for 20mb/s data rate, in case we are using spatial diversity there will be at each branch copy of 20mb/s, even if we add an additional branch.
In case we have 4 branches in Spatial multiplexing each branch will carry 5 MB/s? If we increase the number of branches, again we have the same date rate (20 mb/s) just branches carry lower rates? How we then achieve a higher date rate? @@iain_explains
20Mbps over a channel (sent in 4 parallel streams - that interfere with each other) is higher than 5Mbps over the same channel (if you only sent one stream - that doesn't experience any interference)
Like and comment to support the channel:)
Very neatly explained.
Thanks. Glad you liked it.
In the last example, if there is only LoS, i don't think you'll have diversity even if you do beamforming. You'll only get an array gain
Thanks Professor, great lectures.
Glad you like them!
Wonderful video!!
Glad you liked it!
Dear Prof. Iain
I have a little bit confused:
1. Both of two techniques which are Spatial Diversity and Spatial Multiplexing can boost the data rates?
2. What are differences between Spatial Diversity and Transmit Diversity?
Thank you for your valuable videos.
Answers:
1. Yes. "Space" is a dimension (similar to "time" and "frequency"). If you use more of it (eg. by using more antennas) then you can send more data.
2. The term "spatial diversity" applies at both the transmitter and the receiver.
There is sth about Digital beamforming that I can't digest. Basically, with Digital beamforming, we create different "beams" pointing to the specific spatial directions for different streams. That means:
1) Two nearby users cannot be scheduled at the same time, correct?
2) More than one stream will be transmitted over the same antenna. If I understood correctly, we set a complex gain (phase and amplitude) for each stream and each antenna in the baseband. Assuming, we have two streams, x_1 and x_2 can interfere with each other over the same antenna. This is the bit that I can't visualize. How can we send two signals over the same frequency and at the same time without interfering with each other?
The answer to both questions is "yes". Perhaps this video will help you to visualise what's going on: "What is Multi Channel Beamforming?" ua-cam.com/video/e6scR0C4RAc/v-deo.html
Hello 👋 👋 👋
What is the difference between spacial diversity and multiplexing and when we use this technique
Thanks 😊
It's clear in the video, "we send the same signal over multiple paths to get spatial diversity", when we send multiple streams (data sequences) over the multiple paths we have spatial multiplexing. In short, the main difference is one stream vs multiple streams.
When to use which? Prof Iain also explains how the H matrix (channel coefficients) would influence the choice. To get the real benefits of spatial multiplexing, uncorrelated channel coefficients scenario is desirable.
I would like to add that spatial diversity improves SNR (array gain) and reliability (diversity order). Therefore, its real advantage comes at lower SNR levels, where improving the SNR is the priority. Spatial multiplexing, on the other hand, is particularly useful at higher SNR levels, where the capacity is bandwidth limited. Basically, we send more data using the same bandwidth.
Glad you liked the video.
Hi,
From what I understood it is 2 technics that can not be used at the same time right ? However we could imagine a system that is "agile" between 2 modes (1 mode reliable : spatial diversity and 1 mode capacity increase: multiplexing diversity) ?
Thanks !!
Yes, exactly.
5:13 If signal is getting scattered, then won't that mean that any one connection in spatial multiplexing will be unreliable, thus causing dropped data? Ie, it seems that spatial diversity is a form of redundancy, which seems useful when there's a lot of multipathing, whereas spatial multiplexing is the opposite of redundancy.
The descriptions can be confusing, it's true (... and by the way, it wasn't be who named them). The term "spatial diversity" does not refer to the channel having multiple paths, it refers to using multiple antennas to send/receive the same signal (data stream) at multiple different physical (antenna) locations, and hence _providing_ a diverse set of paths ... or if you think of the receiver end of things, having multiple antennas receiving (the same) signal (data stream) at different locations with different (independent) antenna noise values (ie. a "diversity" of received values). The term "spatial multiplexing" refers to sending multiple data streams at the same time. The different paths through the channel do not cause "dropped data". They are simply different versions of the same signal (different amplitude and phase) that add up at the receiver. If you can measure the overall effect of all the paths at each antenna, then you can "invert" their effect (if you have more antennas than data streams).
To the left Case : is it really beneficial to vary the single amplitudes? You get the singe antenna signals to be in phase and they add together. So why should I mitigate an amplitude? I assume, I should put the maximal amplitude in every antenna. Thanks.
Yes you're right, if you are only trying to form a beam in a single direction. However you might also be wanting to avoid sending energy in specific other directions too (for example in the direction of other users who do not want your interference). In that case there will be benefits to reducing power on particular antennas (in the extreme case, it could be that N-1 antennas are deconstructively fully cancelling out in the "other" user's direction, and then it would be necessary to turn off the N-th antenna in order to maintain a null in that other user's direction).
Hi, Could you please explain the Full Duplex MIMO, and and what are the challenges in practical implementation?
Thanks for the topic suggestion. I've added it to my "to do" list.
Do the beamforming vector (left) and the beamforming matrix (right) have complex entries?
With just a real number I can just adjust the gain and not the phase - so it must be complex(?)
Yes, the entries are complex. They represent amplitude scaling and phase rotations.
Many thanks for the explanation! So did you mean that when we do the spatial diversity, at the receiver, the decoding is done per antenna element, whereas, the spatial multiplexing combines the received signals from all the antenna elements and then perform the channel inverse operation?
In the spatial diversity (beamforming) case there is either only a single antenna at the receiver (which is the case I showed), or the receiver has multiple antennas and performs receive-beamforming. Either way, there is only a single "stream" of data being sent (one modulated symbol at a time). The M data symbols need to be sent one-after-another (beamformed from the multiple transmit antennas). In the spatial multiplexing case, each transmit antenna sends a different modulated symbol in each symbol-period, and the receiver needs to have multiple antennas, and perform the channel inversion.
@@iain_explains Thanks for the reply! So this means we don't require phase shifters attached to each antenna element when we are doing spatial multiplexing.
Yes, that's right.
what is the link between Rich scattering and inversion of matrix ? I think to inverse the determinant of the matrix has to be not zero but I can't understand why a rich scattering environment would imply a NON zero determinant.
Thanks !!
Good question. If there was only a line of sight path, (and no scatterers providing alternate delayed paths with different departure and arrival angles), then the gain between each of the transmit antennas and each of the receive antennas would be (almost exactly) the same. This would mean that every row of the channel matrix would be the same. And in turn, this means that the matrix would not be able to be inverted (because there would only be one non-zero Eigen value).
1:35 It seems strange that a multiplexer is based on a demultiplexer. Is a demultiplexer based on a multiplexer?
Sorry, but I don't understand what you're trying to say. What do you mean: "a multiplexer is based on a demultiplexer"?
@@iain_explains 1:35 Your spatial multiplexor appears to contain a demultiplexor. That sounds weird.
Sorry John, I don't know what "appears to contain" means. What "appears" to one person, will not necessarily "appear" to another person. Sorry, but I don't have time to try to interpret your interpretation. If you can try to be more precise, then I'll try to answer.
@@iain_explains I see a section on the right-hand side of your drawing called "Spatial Multiplexing". In the center of that section is a block called "Demux", which i believe means "Demultiplexer". So, a system called "Multiplexing" contains a block called "Demultiplexer".
OK, I see what you're saying. In that situation the _channel_ is a "multiplexing channel" (when it's used in that way) because multiple (different) data streams are all transmitted at the same time, through the same channel (one from each antenna). In this way they can be viewed as being "multiplexed" in the channel. But when you've only got a single (high rate) data sequence to send from a single (multi-antenna) transmitter, then you need to "create" these seperate data streams (for each of your transmit antennas), and you do this by demultiplexing your high rate data stream into multiple low rate streams. To reiterate: the _channel_ is the "multiplexer".
Can we say that the left technique (spatial diversity) is analogue beamforming and the right is digital beamforming? Many thanks.
No, the RHS shows parallel data streams, where each one is simply being multiplied by a scalar (the respective element of the diagonal matrix W). But if you replaced the diagonal matrix with a digital beamforming matrix, then it would be.
I am sorry for disturb you sir. But in 1:25, when we change the phase and amplitude of input signal X, so I think that the copied signals should be different. Why you still call this as same signals ? Thank you sir
They are all signals that represent the same digital data (they have different phase and amplitudes, but they all represent the same "data symbol").
Nice explanation
Thanks. I'm glad you liked it.
@@iain_explains Hi, I am not able to get the link of research paper that you mentioned in the video. Can you please share the link
Hi teacher,
what is the meaning of full diversity ? I saw this term in OTFS.
Thanks for the question. I've put it on my "to do" list for a future video.
Great stuff ian. I could not open the IEEE paper since it was asking for login. Do you have any other link where I can see the paper. ,?
Unfortunately I can't share it on an open platform, due to IEEE copyright. If you have access to a university library, most universities in the world subscribe to the IEEE Explore system, so you should be able to get on-line access by going through your university library.
@@iain_explains Thanks a lot Iain. Will check for acess in the library
non , c'est le contraire , multiplexage permer d'augmenter le debit par contre le diversiter ameliorer la qualite
When the quality is improved (from diversity), then the bit rate can be higher! It's not as simple as you suggest.
Clearest picture anywhere........
That's great to hear. I'm glad you found it helpful.