What is OTFS? Orthogonal Time Frequency Space Modulation ("Best video in youtube for OTFS")

Explains OTFS from a fundamental intuitive perspective in relation to other modulation schemes such as OFDM and Coded-OFDM.
For more detailed insights, see our two recently published papers:
• “A New Micro-Subcarrier OFDM Based Waveform for Delay Doppler Domain Communication” ieeexplore.iee...
• “Zak-OTFS Implementation via Time and Frequency Windowing” ieeexplore.iee...
If you would like to support me to make these videos, you can join the Channel Membership, by hitting the "Join" button below the video, and making a contribution to support the cost of a coffee a month. It would be very much appreciated.
Check out my 'search for signals in everyday life', by following my social media feeds:
Facebook: www.facebook.c...
Instagram: / iainexplains
Website: www.iaincolling...
Related videos: (see: iaincollings.com)
• Why is Subcarrier Spacing Bigger in 5G Mobile Communications? • Why is Subcarrier Spac...
• How are OFDM Sub Carrier Spacing and Time Samples Related? • How are OFDM Sub Carri...
• Why is the OFDM Symbol Prefix Shorter in 5G Mobile and 802.11ac WiFi? • Why is the OFDM Symbol...
• OFDM and the DFT • OFDM and the DFT
• What is Intersymbol Interference ISI? • What is Intersymbol In...
• How are the Fourier Series, Fourier Transform, DTFT, DFT, FFT, LT and ZT Related? • How are the Fourier Se...
• What is SC-FDMA? And why is it used for the Uplink of 4G/5G Mobile? • What is SC-FDMA? And w...
• Why is SC-FDMA called "Single Carrier"? • Why is SC-FDMA called ...
• How are Different Equalization Methods Related? (DFE, ZF, MMSE, Viterbi, OFDM) • How are Different Equa...
• What are Doppler Shift, Doppler Spread, and Doppler Spectrum? • What are Doppler Shift...
• What is Multi-User MIMO Communications (MU MIMO)? • What is Multi-User MIM...
• What is MIMO OFDM? • What is MIMO OFDM?
• What is a Cyclic Prefix in OFDM? • What is a Cyclic Prefi...
• How does OFDM Overcome ISI? • How does OFDM Overcome...
• Modelling mmWave MIMO Channels • Modelling mmWave MIMO ...
• What are Doppler Shift, Doppler Spread, and Doppler Spectrum? • What are Doppler Shift...
• What are Flat Fading and Frequency Selective Fading? • What are Flat Fading a...
Full categorised list of videos with pdf Summary Sheets: iaincollings.com

[@jagadeeshkota3564]

I am an aerospace student with zero background in communications, I have failed twice in communication during my master's in space engineering, for the third time I have discovered your channel, and I have passed my test by just watching your videos for 2 days before the exam. I cannot thank you enough for that. I was joking with my friends if I pass my test I will fly to Australia :)). I could not find your email to thank you formally. T hank you very much for doing gods work.

[@iain_explains]

I'm really glad to hear that my videos helped you to pass your exam. Hopefully someday you will make a trip to Australia, and I can claim to have boosted tourism! 🤣

[@fahedelabbar4773]

more videos about OTFS, please . The way you explain concepts is too good

[@iain_explains]

Thanks for the suggestion. I've put it on my "to do" list do do more on OTFS.

[@biikih1641]

Thank you so much for making this so clear. I applaud you for explaining OFTS without once mentioning the phrase: “quasi-periodic”. Seriously, thank you for making this very clear to understand.

[@iain_explains]

Glad it was helpful!

[@bh6351]

I have watched almost all videos in your channel over years. It has become a habit to watch your new uploads. As a person who has worked on wireless communications for years from academic to industry, I would like to say your videos are one of the best to explain abstract concepts. Great appreciation to you!

[@iain_explains]

That's great to hear. Thanks for your nice comment. I'm so glad you like the videos.

[@13_suryatejareddy11]

The way you explain concepts is too good...that helped me to crack my placement....

[@iain_explains]

I'm so glad to hear that! All the best for your placement.

[@suhailfarhaan5580]

could'nt thank you enough for this! best video in youtube for OTFS

[@iain_explains]

Glad you liked it!

[@basilkjeemon196]

The best explanation in youtube. Thank you.

[@iain_explains]

Thanks for your endorsement. Glad it was helpful!

[@zhou6486]

Happy to learn about the communication research trend from your video. Thank you so much for sharing!

[@iain_explains]

Glad it was helpful!

[@mysticnate5936]

hello professor, i think at 14:00 , the explanation is not correct ( i may be wrong) but when we represent a DD signal in time domain we get a "pulsone". and the one which you are showing " 2D sinc pulse" is dd domain representation only, but it happens when we band limit and time limit the pulsone in time domain, otherwise it would have been an impulse. which is not possible. plz clearify if possible.. I love your videos, they always help Thankyou professor.❤

[@iain_explains]

Sorry, I don't see what you're saying is wrong about what I said. As you say, the time-domain waveform that corresponds to a "2D sinc" in the DD domain, will be time limited and band limited. That's exactly the case for the OFDM-based time-domain waveform that is shown. For more general insights, you might be interested in our two recently published papers: “A New Micro-Subcarrier OFDM Based Waveform for Delay Doppler Domain Communication” ieeexplore.ieee.org/document/10504818 and “Zak-OTFS Implementation via Time and Frequency Windowing” ieeexplore.ieee.org/document/10436682

[@Edwards129-sss]

I nearly give up finding videos of OTFS but I find your video, it's amazing ! The others can't explain OTFS without plenty of formula. Besides I wonder if the matrix in DD domain must be sparse? If the matrix can't be sparse, will the signal in DD domain be orthogonal? Thx for answering my problems :-）

[@iain_explains]

I'm glad you found the video helpful. As I mentioned, it is not clear that practical channels will have sparse DD representations in reality. That's an assumption that research papers make, but there is no clear measurement data to confirm it, that I've seen yet at least. And in terms of your question about orthogonality, as I mention in the video, the input data DD representation is of course orthogonal, but the output of the channel is not (ie. never) orthogonal, even if the channel is sparse (due to the twisted convolution effect of the channel).

[@ranchordaschancad3410]

thankyou very much sir for the video I was struck on one of the concepts it got cleared from this video 👍

[@iain_explains]

That's great to hear. I'm glad you found the video helpful.

[@stringstoparadise2392]

great video sir as always thanks 👍

[@iain_explains]

Thanks 👍

[@qqwer6898]

can u explain with practical example taking a x[n,m]matrix...its been confusing how FT,IFFT applied on input data we have

[@dr.jimmyyiminghuo4956]

Thanks a lot, Professor Collings. I am just wondering if you could consider giving some lectures on DFT-s-OFDM in the future.

[@iain_explains]

Hi Jimmy, have you seen my webpage? iaincollings.com It's got lots of videos on OFDM (on the Digital Communications page, including how it's implemented with the DFT) and on the DFT more generally (on the Signals and Systems page).

[@Verbatino]

Thanks for this excellent video.🙂 IMHO naming it "orthogonal time frequency space modulation" was littlebit confusing. There is no orthogonality in time or space as the name could suggest if we consider that we are working on delay-doppler as you explained in video...

[@iain_explains]

Yes, I agree, I'm pretty sure I said the same thing in the video ... I wasn't the one who named it.

[@Ben-zc1bw]

This is very interesting! I took a look at some of the papers, but as someone interested in comms but without professional experience, I'm a bit confused about the high level advantages. I'd thought that 1) one of the key benefits of OFDM is that it turns the ISI problem experienced by time-localized carriers in multipath channels into a much easier to compensate per-carrier equalization problem, and 2) high doppler spread channels for OFDM are challenging primarily because of the ISI they create. It seems that what OTFS does is turn delay/doppler spread into purely an ISI problem, with flat gain regardless of carrier? The flat gain is clearly attractive, but this seems like the exact opposite of the trade that was made when going to OFDM. At the point that OTFS carriers are spaced far enough apart to avoid ISI, it's not intuitive to me why that's better than doing the same in OFDM. Same for any de-spreading scheme. I'm sure I'm missing something here, and would appreciate any insight!

[@iain_explains]

It depends on the form of ISI. The links between OTFS and OFDM are the topic of current research. I've actually got a paper submitted to a journal at the moment on the topic.

[@DN-pz4wv]

Thank you for the video! Has this paper already been published?

[@wuwu1011]

wow!!Thank u so much for making this video!!! It really helps me!!

[@iain_explains]

Glad it helped!

[@OveysJavanmardtilaki]

At ''9:95'' you changed the equation from frequency-time domain to delay-doppler and you changed multiplication with convolution. the question is that, from time to doppler we use FT but from frequency to delay we use IDFT. I mean at the same time there are 2 transform. But when you changed multiplication with convolution, you just considered FT. Could you please explain it. thanks

[@jonesy3516]

Dear Ian, thank you very much for your explanations. They illustrate OTFS really well. However, it is not entirely clear to me why the robustness of an OTFS signal should not be dependent on Doppler shifts. With several paths with different Doppler shifts, the symbols still interfere in the Delay Doppler plane, don't they? Can I calculate this interference back by “de-convolution”, in contrast to the intercarrier interference in an OFDM signal? Could you please explain this a little? Thank you very much!

[@iain_explains]

Yes, exactly. That’s right, the channel is not orthogonal, and the received signal needs to be equalised in the delay-Doppler domain. I plan to make a video about this in the near future.

[@bekryad5362]

Thank you very much sir you are the best you helped us a lot please we want a video about Channel estimation and thanks a lot.

[@iain_explains]

I'm glad you liked the video. I do have some videos on channel estimation, but perhaps you are asking about channel estimation specifically for OTFS? "Channel Estimation for Mobile Communications" kzbin.info/www/bejne/kKSvmWNno9GFsLs and "Quick Introduction to MIMO Channel Estimation" kzbin.info/www/bejne/i4HKdWh9o9SUb5I

[@bekryad5362]

@@iain_explains yes sir please we want vidéos about Channel estimation for otfs and thanks à lot.

[@EyosiasHabtemariam]

I was given an assignment on OFTS picking a topic like performance analysis or characteristics. I got the concept of it from you. Thank you for that. which part of this modulation should be analyzed to see its drawbacks? Thanks again

[@iain_explains]

As I mention in the video, there is a challenge to estimate the channel.

[@Kinghhn]

Thank you so much for making this video. I'm currently studying on wireless communication, so I'm feeling my understanding is lacking. I want to be clear about the concept of FFT on H[n,m]. As I understood, n is the index of time and m is the index of frequency. Then, why cannot m represent Doppler and why cannot n represent delay? Why IFFT and FFT change them to delay and Doppler, respectively?

[@iain_explains]

You're right - in principle, n and m can represent whatever you want them to. But if you have a matrix of data that you are going to send out into a channel, then the order in which you send it out defines the time direction. It gives meaning to that axis of the matrix. It's hard to explain, but I've added this topic to my "to do" list, so keep an eye out for a future video.

[@Kinghhn]

@@iain_explains Thank you so much !! I'm really looking forward to watching that video !!

[@basics7930]

Good video..Thanks

[@rishabhkumar1050]

hello sir, the loss of the orthogonality of the subcarriers is occurring due to dopplers shift in the carrier frequency ie(fc+(dopplers freq)), which means when we observe in the frequency domain at receiver we will receive an instantaneous frequency shifted by dopplers and which can be observed as the time rate of change of phase with time(1/2*pi *(phase/time)) sir is not possible to somehow cancel that effect of dopplers by varying the phase of basis function in the opposite direction so that overall effect of the rotation in phase due to dopplers will be canceled by rotation in phase of the basis function in opposite direction.

[@iain_explains]

Sorry, it's not clear if you are asking a question, or making a statement. The first part of your comment appears to be a re-statement of what I said in the video - so yes, I agree with that. In the second part of your comment, you raise an interesting point, that I think many people think is what's going on in OTFS - but it isn't. In principle it is tempting to think that it would be possible to 'precode' the input matrix - for example as is done in MIMO with a zero-forcing precoder. But this is not possible in OTFS for two reasons: 1. the delay-doppler of the channel might be able to be assumed to be constant over the length of an OTFS symbol, but it is unlikely to remain constant over longer time-scales of multiple OTFS symbols - so it won't be possible to know the channel ahead of time at the transmitter. 2. It is not a simple case of pre-multiplying by a 'precoding' matrix, since the channel operation is 'convolution-like' in the "delay-doppler" domain, so it's not clear what sort of precoding could be done - certainly not just matrix-multiplication precoding.

[@vinayakbhat7205]

very useful. thanks

[@iain_explains]

Glad it was helpful!

[@rudrasingh9501]

hello lain , @ 3:07 you mentioned one of the subchannel is getting knocked out means what may be the reasons behind it?

[@iain_explains]

The channel is frequency selective, due to factors such as multipath. More details here: "What are Flat Fading and Frequency Selective Fading?" kzbin.info/www/bejne/gZqugXmKadenfsk

[@scienteer3562]

Have heard of FBMCM? What are the advantages of this compared to OFDM or this? Thanks for the content BTW.

[@iain_explains]

You'll have to spell out the acronym, sorry. I'm guessing the MCM stands for Multi-Carrier Modulation, but what's the FB? There are so many proposals of slightly varying schemes!

[@Super_Lizard]

@@iain_explains I think he means Filter Bank Multicarrier, FBMC

[@iain_explains]

Ah yes, I guess so. I'll add it to my "to do" list.

[@codingmarco]

What would be a prectical example of a channel with a time-varying frequency response?

[@iain_explains]

Any channel that is wide-band and has a moving transmitter and/or receiver. For example LEO satellite channels, UAV communications, communications to fast moving trains, ...

[@davebordenkircher8131]

Hi professor, I had an OFDM question. I understand that OFDM is popularly used in downlink, but it is not particularly favorable in mobile uplink due to OFDM symbols having a relatively high PAPR which is not favorable for mobile devices with low power resources (approx. 12 dB is the figure I saw on Wiki). I see that this is due to the central limit theorem but am having difficulty conceptualizing why OFDM symbols have high PAPR. Could you provide some details on this topic? Dave

[@iain_explains]

This video will hopefully help: "What is PAPR? and its relationship to OFDM" kzbin.info/www/bejne/fGWvco2KmdKSmJo

[@scottshi2909]

Hi professor, does that mean for OFDM, we are actually looking at OFDM sym by sym, that our inputs to OFDM system are only subcarriers (frequency domain data), while for OTFS our inputs to the system have both delay and Doppler data. Because in delay/Doppler domain we don't need that many data compared to frequency/Time domain, we reduce the complexity when performing channel estimates (estimate H[k,j] by deconvoluion Y[k,j] with X[k,j]) which can jointly cover two domains. Is my understanding correct?

[@iain_explains]

It's not really clear what you're asking, sorry. But overall it sounds like you've got the right idea.

[@林智賢-o5x]

Hi professor, thank you so much professor for making so much videos. Howerer, I had a question that still couldn't figure out. That is, how to understand the resolution of Delay-Doppler Domain? I don't know why the resolution of Delay & Doppler are (1/M*delta_f) & 1/NT, respectly.

[@iain_explains]

This video might help: "How does the Discrete Fourier Transform DFT relate to Real Frequencies?" kzbin.info/www/bejne/pnqpq2tqpM9smaM

[@faust-cr3jk]

There is a fair amount of buzz about OTFS in context of 6G, similarly as it was with NOMA and 5G. However, I am not sure if this is the right direction. IHMO, the key for next generation waveforms would be increasing spectral efficiency and, as far as I am aware, OTFS does not address this issue. Accuracy of channel estimation and quality of CSIT / CSIR are well-explorer research issues. And yes, this could be really problematic since for instance it could limit number of available degree of freedom when it comes to MIMO. But again, I am not sure how OTFS targets those problems or if it even addresses MIMO at all.

[@iain_explains]

Yes indeed. Those are all good points. OTFS is perhaps best suited to situations with a small number of scatterers and fast moving transmitters and/or receivers (eg. LEO Satellite comms). These are not good conditions for MIMO. OTFS could conceivably be combined with beamforming or MU-MIMO.

[@pitmaler4439]

Thanks a lot. (I believe it is "Heisenberg" with e, after Werner Heisenberg)

[@iain_explains]

Thanks for that. Spelling has never been my strong point.

[@santiprom69]

Hin where is the qam box. cp,pilots ones?

[@iain_explains]

The "x-vector" (top left) contains complex numbers that are drawn from a QAM constellation.

[@asdrty258]

F freq (n,m) L doppler (k,l) my mnemonic aid

[@suhailfarhaan5580]

How is OTFS modulation orthogonal in Time ?

[@iain_explains]

Yes, as I explain in the video, it is not actually orthogonal in time, frequency or space, but as I say at the 14:23 min mark it is orthogonal in the "delay" variable and in the "Doppler" variable. Personally, I don't think that OTFS is a good name for this modulation format - especially since it doesn't actually use the "space" resource (since it only has a single antenna at the transmitter and the receiver). It might be better to call it Doubly Spread Modulation, or perhaps Delay-Doppler Modulation (but I don't like that either, since it's the channel that imparts the delay and Doppler, not the transmitter).

[@TurbidSugar19]

Interesting why it wasn't used in LTE with freedom of pilots allocation. 5G don't support it in any case. Still looks very interesting topic. What problem should it address, high speed dopler?

[@iain_explains]

It hadn't been developed when the 5G standard was being written. And it still hasn't been fully developed! Yes, it's proponents claim that it is best suited to high Doppler channels.

[@PeggySolomon-f4x]

Percy Mall

[@stephensu4371]

video: FT for the data input （square wave in the case) me: yep video: OFDM me:yep video: coded me: no problem video: convolution is multiple when FFT me: yep, that was what my signal system told me video: the graph just next to the graph of multi path delay and doppler me:??. (?o?)

[@iain_explains]

If there are only a small number of scatterers in the channel, then the channel impulse response will only have that same small number of non-zero delays. And if the terminal and/or basestation are moving, then each of those Delay components will have a Doppler value associated with it. So they will appear as isolated spikes in the Delay-Doppler domain.

[@AnthonyCarrick-t5y]

Reta Heights