Why is Doppler a Problem for OFDM?

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Iain Explains Signals, Systems, and Digital Comms

10 ай бұрын

Explains how a Doppler shift affects a digital OFDM communications signal.
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Пікірлер: 36

@georgepalafox5967 10 ай бұрын

Excellent explanation! Very helpful! Thank you for talking the time to upload videos on these topics.

@iain_explains 10 ай бұрын

Glad it was helpful!

@today-nl 10 ай бұрын

Thanks for the video, very clear and helpful as always!

@iain_explains 10 ай бұрын

Glad it was helpful!

@sanjayd411 10 ай бұрын

Very well explained...thank you professor 👍🙏🙏

@iain_explains 10 ай бұрын

Glad you liked it

@ComputerScienceLessons 9 ай бұрын

Very enlightening. Thank you so much. I do have (another) query though, if I may. My mind is very much on WiFi. In your video, you said that in digital communication we send signals for a finite amount of time. I presume by that you mean one symbol at a time, for example a BPSK symbol? You went on to say that the effect of shortening the time is the same as having an infinite amount of time times a square rect function. Hence we see a sync function in the frequency domain. Are you suggesting that, after modulation, an RF symbol, when viewed in the frequency domain, is a sync function? I’m a little confused because I can visualise how a square pulse can give rise to a sync function in the frequency domain (a square wave can be constructed by adding together lots of sine waves with the appropriate frequencies and amplitudes), but how can a BPSK symbol for example also result in a sync function?

@iain_explains 9 ай бұрын

When a square wave (in the time domain) is multiplied by a sinusoid (in the time domain), it is the same as having a sinc function in the frequency domain (centred on zero frequency) and convolving it with a delta function (located at the sinusoid's frequency). When you convolve something with a delta function, it moves your function to be centred at the location of the delta function. These videos might help: "Convolution with Delta Function" kzfaq.info/get/bejne/iq-TmbxinsrPYJc.html and "Amplitude Modulation AM Radio Signal Transmission Explained" kzfaq.info/get/bejne/Y7aHmpBh0JjTm6c.html

@user-rx1hb5uv5w 9 ай бұрын

I am learning a lot from your KZfaq channel ! Do you happen to take questions only through KZfaq comments or is it possible to email ?

@iain_explains 9 ай бұрын

I'm glad you like the channel. I don't really have time to answer detailed questions, sorry. ... unless you're interested in doing a PhD with me.

@user-pg2ll9rv3v 7 ай бұрын

Hi professor, I am a student n China .I have learned a lot from your videos during this time and really like you videos. Currently, there are many studies on the design of integrated communication and radar. What do you think of this research direction?

@iain_explains 7 ай бұрын

Yes, that's certainly a hot topic for research at the moment. There are lots of interesting problems to be solved.

@Moatssim 10 ай бұрын

Thanks Professor!

@iain_explains 10 ай бұрын

My pleasure!

@pitmaler4439 10 ай бұрын

great, thanks

@iain_explains 10 ай бұрын

Glad you liked it!

@Julia-hu4xe 10 ай бұрын

Thanks, is it possible to give some carriers a higher transmit power than other carriers in OFDM?

@iain_explains 10 ай бұрын

Sure. You can scale the constellation in each subcarrier as you like. You can even set it to zero, if you want.

@pitmaler4439 10 ай бұрын

@@iain_explains do they apply that because higher frequencies have a higher path loss? Perhaps they adapt that by default, so nobody must care about different pathloss for different frequencies.

@iain_explains 9 ай бұрын

This video might give more insights: "How are OFDM and xDSL (DMT) Related?" kzfaq.info/get/bejne/eauEZbimrMupoac.html

@user-ex2iw4hi3f 9 ай бұрын

hi! professor! Sometimes, doppler does not shift equally for all frequency, right? For the higher frequency, the doppler would cause more shift.

@iain_explains 9 ай бұрын

Yes, Doppler is a function of frequency. However, across the band of an OFDM signal, the difference is not significant, from the lowest frequency to the highest - since the bandwidth is small compared to the carrier frequency. This video should help: "What are Doppler Shift, Doppler Spread, and Doppler Spectrum?" kzfaq.info/get/bejne/grKiZpCc1NrGqmg.html

@SayyedHosseinJavidHosseini 10 ай бұрын

Hi, So I have a question regarding this phenomenon. Let's assume I have an OFDM signal with 15 kHz sub-carrier spacing. I am sitting in a fast-moving train, moving at 400 kph, with an RF frequency of 1.8 GHz. Then the Doppler shift would be 700 Hz. It is around 5% of the sub-carrier spacing. Do I need to correct the Doppler in the analog domain (for example with a very accurate LO and feedback from the digital front-end)? Or do I ignore the Doppler, sample the signal as received, and account for the Doppler in the digital domain? And thank you for your insightful videos.

@yurisamoilov6128 10 ай бұрын

you can study the research "The Effect of Doppler Frequency Shift, Frequency Offset of the Local Oscillators, and Phase Noise on the Performance of Coherent OFDM Receivers" and get the answer to your question

@yurisamoilov6128 10 ай бұрын

and study DOI: 10.1109/4234.809526. it is a very useful article

@SayyedHosseinJavidHosseini 10 ай бұрын

Thank you. I'll look at these and probably come back with further questions.@@yurisamoilov6128

@deerajkumarj1408 9 ай бұрын

Sir, so, does this imply that ofdm is not efficient in High mobile environment. If that is the case what will be the solution for this. Sir, please explain this

@iain_explains 9 ай бұрын

Yes, you're right. There's not a simple answer to your question though. One thing that can be done is to consider using a new modulation format called OTFS. See this video for more details: "What is OTFS? Orthogonal Time Frequency Space Modulation" kzfaq.info/get/bejne/g9x7Zt2Ztdirm58.html

@deerajkumarj1408 9 ай бұрын

Thank you very much sir

@eswnl1 10 ай бұрын

So you wouldn’t use OFDM for low earth orbiting satellites (or anything that moves). But it’s okay for tv transmitters.

@iain_explains 10 ай бұрын

People are starting to look at using OFDM for high data rate communications on LEO satellites, and Starlink have announced that they will soon be providing 5G over their LEO network, so it's not impossible to use OFDM over LEO links, but it's not ideal. A potentially better suited modulation format is OTFS: "What is OTFS? Orthogonal Time Frequency Space Modulation" kzfaq.info/get/bejne/g9x7Zt2Ztdirm58.html

@samueldassonville9900 9 ай бұрын

But why is it a problem if the signals are shifted of 1/T Hz. If they are still orthogonal, we can still get the data no ?

@fishgrey6441 9 ай бұрын

Follow your assumption, they are actually still orthogonal. But in the receiver, you will get the wrong signal from the match filter. Like in w1 match filter, you will get a w2 signal component.

@iain_explains 9 ай бұрын

Yes, if there is only one Doppler shift in the channel (eg. if it is a line-of-sight channel), and if the receiver somehow knows what the shift is, then it can adjust it's carrier frequency and receive the data. However, if there are multiple scatterers in the channel (which is almost always the case) then there will be multiple Doppler shifts - some might be positive, some might be negative - and then it results in what we call a Doppler Spread. Then the subcarriers are definitely not orthogonal. See these videos for more details: "What are Doppler Shift, Doppler Spread, and Doppler Spectrum?" kzfaq.info/get/bejne/grKiZpCc1NrGqmg.html and "How Does a Doppler Shift Affect Digital Communications?" kzfaq.info/get/bejne/b66hn8mip6rboI0.html

@Apalion41 3 ай бұрын

The problem has been presented but what is the solution?

@iain_explains 3 ай бұрын

You might be interested in: "Why is Subcarrier Spacing Bigger in 5G Mobile Communications?" kzfaq.info/get/bejne/q990qc2htdScnac.html and "What is OTFS? Orthogonal Time Frequency Space Modulation" kzfaq.info/get/bejne/g9x7Zt2Ztdirm58.html