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?
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" ruclips.net/video/TIcfY19dk0c/видео.html and "Amplitude Modulation AM Radio Signal Transmission Explained" ruclips.net/video/-PWg-0k2oks/видео.html
What you said is quite correct, but the Doppler shift and the specific shift frequencies of different OFDM subcarriers are slightly different (the higher the frequency, the higher the subcarrier shift df=v/c*f), so when explaining the Doppler shift, you can also introduce this situation, which will make the modeling of the Doppler shift more accurate.
So, based on my understanding for you explanation, the sensitivity to doppler effect should be critical in higher index of OFDM subcarriers, right? For example, when using the number of Subcarriers N = 1024, at subcarriers i.e. 1, ... 128 out of N, the data can be recovered at such doppler effect while data carried via subcarriers N-128, .... N will be much more affected. Is my conclusion right?
@iain_explains Yes, I agree about that. What I mean is the error percentage at data carried via subcarriers located at the beginning of the symbol is supposed to be less compared to the data carried via the subcarriers at the end of the same symbol.
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?
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.
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
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?" ruclips.net/video/LLr3-kotbz4/видео.html
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
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" ruclips.net/video/MvK3zhPrGkk/видео.html
@@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.
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" ruclips.net/video/MvK3zhPrGkk/видео.html
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.
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?" ruclips.net/video/LLr3-kotbz4/видео.html and "How Does a Doppler Shift Affect Digital Communications?" ruclips.net/video/9HqlfqBDwpY/видео.html
You might be interested in: "Why is Subcarrier Spacing Bigger in 5G Mobile Communications?" ruclips.net/video/uyDvjpPn8ms/видео.html and "What is OTFS? Orthogonal Time Frequency Space Modulation" ruclips.net/video/MvK3zhPrGkk/видео.html
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?
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" ruclips.net/video/TIcfY19dk0c/видео.html and "Amplitude Modulation AM Radio Signal Transmission Explained" ruclips.net/video/-PWg-0k2oks/видео.html
Excellent explanation! Very helpful! Thank you for talking the time to upload videos on these topics.
Glad it was helpful!
What you said is quite correct, but the Doppler shift and the specific shift frequencies of different OFDM subcarriers are slightly different (the higher the frequency, the higher the subcarrier shift df=v/c*f), so when explaining the Doppler shift, you can also introduce this situation, which will make the modeling of the Doppler shift more accurate.
Very well explained...thank you professor 👍🙏🙏
Glad you liked it
I am learning a lot from your RUclips channel !
Do you happen to take questions only through RUclips comments or is it possible to email ?
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.
So, based on my understanding for you explanation, the sensitivity to doppler effect should be critical in higher index of OFDM subcarriers, right? For example, when using the number of Subcarriers N = 1024, at subcarriers i.e. 1, ... 128 out of N, the data can be recovered at such doppler effect while data carried via subcarriers N-128, .... N will be much more affected. Is my conclusion right?
Not really. The spread of frequencies across the subcarriers is only a very small fraction of the carrier frequency.
@iain_explains Yes, I agree about that. What I mean is the error percentage at data carried via subcarriers located at the beginning of the symbol is supposed to be less compared to the data carried via the subcarriers at the end of the same symbol.
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?
Yes, that's certainly a hot topic for research at the moment. There are lots of interesting problems to be solved.
Thanks for the video, very clear and helpful as always!
Glad it was helpful!
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.
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
and study DOI: 10.1109/4234.809526. it is a very useful article
Thank you. I'll look at these and probably come back with further questions.@@yurisamoilov6128
hi! professor! Sometimes, doppler does not shift equally for all frequency, right? For the higher frequency, the doppler would cause more shift.
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?" ruclips.net/video/LLr3-kotbz4/видео.html
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
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" ruclips.net/video/MvK3zhPrGkk/видео.html
Thank you very much sir
Thanks, is it possible to give some carriers a higher transmit power than other carriers in OFDM?
Sure. You can scale the constellation in each subcarrier as you like. You can even set it to zero, if you want.
@@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.
This video might give more insights: "How are OFDM and xDSL (DMT) Related?" ruclips.net/video/CET2UuGeEqs/видео.html
Thanks Professor!
My pleasure!
So you wouldn’t use OFDM for low earth orbiting satellites (or anything that moves). But it’s okay for tv transmitters.
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" ruclips.net/video/MvK3zhPrGkk/видео.html
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 ?
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.
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?" ruclips.net/video/LLr3-kotbz4/видео.html and "How Does a Doppler Shift Affect Digital Communications?" ruclips.net/video/9HqlfqBDwpY/видео.html
great, thanks
Glad you liked it!
The problem has been presented but what is the solution?
You might be interested in: "Why is Subcarrier Spacing Bigger in 5G Mobile Communications?" ruclips.net/video/uyDvjpPn8ms/видео.html and "What is OTFS? Orthogonal Time Frequency Space Modulation" ruclips.net/video/MvK3zhPrGkk/видео.html