What are Flat Fading and Frequency Selective Fading?
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- čas přidán 21. 08. 2024
- Explains Flat Fading and Frequency Selective Fading in digital communication systems, and the relationship between the fading characteristics and the data rate.
Related videos: (see: iaincollings.com)
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For a full list of Videos and Summary Sheets, goto: www.iaincolling...
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Again, a life saver.
Amazing analysis. Going through the signal and its F.T. gives the viewer such a good understanding as opposed to general functions of narrowband and wideband which are not grounded in the physics/mathematics behind it. Keep it up and your channel will grow!
Thanks for your nice comment. I'm glad you liked the approach I took in the video.
That is so good! I am from china and now preparing a course for undergraduate students. I have been struggling for several days on how to explain delay spread and coherence bandwidth for them. Now, I will follow your way and let students follow your channel! Thank you so much!
I'm so glad to hear that the video is helpful. I guess you have already noticed that there is a fully categorised listing of all the videos on the channel at the website: iaincollings.com Hopefully this helps your students. Please let me know if you think there are other topics I should cover.
Hi dear Professor, recently I have started to check out your videos and they are very interesting. Every complicated theory is explained clearly and it is easy to follow the main concepts. Thank you very much for explaining your knowledge and I hope we will be able to receive further videos from you.
That’s great to hear. I’m glad you like the videos.
woow you are genious and amazing . you explain in very clever way . better than any famous authors . thank u a lot sir
Thanks and welcome
I had a homework problem on Frequency Selective Fading, so I knew just where to go to learn about it! Fantastic video as always, really made things clear.
Glad it was helpful!
Amazing thank you so much. You always make me motivated to go further.
I'm so glad you're finding the videos useful and motivating. I'm enjoying making them.
I will watch every single video uploaded by you. Great videos
I'm glad you like the videos. If you haven't spotted it yet, I've got a website that lists all the videos in categorised order: iaincollings.com
Hello Sir, I refer to your lectures. All the videos are concise and yet so much informative at the same time. So, there is no chance of getting bored at all. I really love the way you combine the various fundamental aspects of communication and signal processing in just a single tutorial session. The explanation is clear and easy to understand. Thanks for taking your valuable time in making our lives simpler. I'm a dedicated subscriber and follower of your channel (I mean CZcams channel and not flat fading channel :-)) and I would surely recommend your lectures to others as well. Thank you once again and keep up the great work!
That's great to hear. I'm so glad you like the videos. I really enjoy hearing from people who are finding the videos useful.
Thanks for the conceptual lecture
Glad you found it useful.
Amazing video! Thank you, man. Like and comment as usual
Glad you liked it! And thanks for your support of the channel.
Perfect explanation ! Answered exactly to the questions I was having, thank you !
Great to hear!
Brilliant description
Glad you liked it
Enjoyed it a lot...
Thanks for the fantastic explanation. Please, keep going )
Thanks, will do!
good infromative video
very good conceptual explanation sir, thank you very much
Thanks. I'm glad it was helpful.
Thank you sir as always
Very welcome. Glad you like the videos.
Thank you so much, these videos are really helpful !!!!
That's great to hear. I'm glad you've found them useful.
Thank you very much
Great explanation. Concepts are clear. In general, Fading is effect of multicomponent on same symbol and ISI is effect of multicomponent on next symbol.
Am i correct ?
Yes, that's right.
Great explanation! Thanks a lot.
Glad it was helpful!
Great explanation sir, In the above explanation, 2 sine signals cancel each other but the first half wavelength of the signal that was received at (time = 0 )still exists right? It shouldn't be a completely zero outcome. In that case, the dip from frequency selective fading shouldn't be 0. Please correct me if i am wrong
In those pictures I've only shown a short segment of the sin waves, so that I could highlight the phase offset. In practice, the waveforms are much higher frequencies, and they are continuous/on-going (modulated by the digital data waveform), so the "first half wavelength" that you mentioned, is not "on it's own".
Thank you so much Sir!!
Most welcome!
Amazing explanation! so for a given time dispersive channel with more than one dirac delta functions other than at time t=0, there exists a value of wT=-π, that will cancel the carrier x(t) causing complete cancellation and a sharp dip in frequency response of the channel for that particular value of w. This is inevitable in a multipath channel when operating at higher bandwidth and no channel equalizer can undo frequency selective fading, there may be some remedy for inter symbol interference but not for this...please comment since we are having a raging debate here on OFDM vs Single carrier solution for a particular use case. I am all in for OFDM despite the scenario can be modeled using a rician fading model with significantly high K factor, the frequency selective fading challenge remains
I'm not sure I understand what you're getting at, exactly. If the channel is frequency selective, then you've got two options: 1. hope that it remains constant for long enough to be able to measure the channel and feed it back to the transmitter, so that the transmitter can avoid using the "nulled" portion of the frequency band, using precoding (as happens in DSL), or 2. accept that there will be nulls in the frequency domain, and try to equalise them at the receiver and use error correction coding to effectively spread each data bit over the whole frequency band.
U are mircle ❤❤
I'm glad you like the video.
sir angle of delayed version shod be -90 degree always. as there is only imaginary component in exponential term. Please reply anyone who have understood
Hi, Mr lain, thanks for your video which helps me a lot！I have a question that what's the meaning of h(t) impulse signal. You used sin wave as an example and I don't understand the relationship between the sin signal and the h(t) impulse signal clearly. Could you please explain it more? thanks a lot!
h(t) is the impulse response of the channel. For a description of what an "impulse response" is, see: "What is an Impulse Response?" czcams.com/video/WTmelRV_Yyo/video.html
Hi Iain. Thanks a lot for this explanation. I read in one 4G book: "transmitted signal is only subject to non-frequency-selective fading and white noise. that is, there is no radio-channel time dispersion. What they exactly mean by radio-channel time dispersion? Maybe that signal won't be changed (spread out) in time domain? Thanks in advance. Alex
They would be referring to the OFDM modulation format that avoids inter symbol interference (ISI). Here are a couple of videos that give more details: "How does OFDM Overcome ISI?" czcams.com/video/xcQ6rtIXv6M/video.html and "What is Intersymbol Interference ISI?" czcams.com/video/I087FUvW2ys/video.html
Hi, thank you very much on this video,
So this is the main motivation to use Equalizers?
If a channel is frequency selective across the band, then it has inter symbol interference, and you will either need to use an equaliser at the receiver, or use OFDM modulation at the transmitter. See: "What is Intersymbol Interference ISI?" czcams.com/video/I087FUvW2ys/video.html and "How does OFDM Overcome ISI?" czcams.com/video/xcQ6rtIXv6M/video.html
Great explanation.
Assuming MIMO where spatial channel correlation exists, is it "right" in OFDM systems to assume a flat fading channel for each subcarrier?
How the channel gain frequency response (between subcarriers or among different paths) differs in correlated scenario? Is it generally assumed flat fading channel for each subcarrier regardless of correlation and the effect of correlation seems at similar frequency response between different channel coeffients for example h_11 h_12?
Generally, spatial correlation occurs from fixed reflectors close to the antennas. For base stations, this can arise from the metal in the support tower (for example), and for user equipment it can arise from the metal case of a laptop (for WiFi signals for example), or the metal structure of a vehicle (for example if someone is using their mobile phone in a car). In these situations, since the reflectors are close to the antennas, there is very little difference in the distances of the resulting paths. Therefore these reflectors will not add significant ISI. In other words, close scatterers have the biggest effect on the spatial correlation, while non-close scatterers have the biggest effect on the frequency response.
Is it possible to calculate the length or difference in length of an indirect path which WON'T cause fading at a given transmit frequency (say 900 Mhz)?
Any path that differs in length by an integer multiple of the wavelength will add up constructively.
@@iain_explains Ok, so if we're sitting in a null, then moving the distance of half a wavelength (based in frequency) is the surest way to step out of the null. Correct?
Yes, that's right.
Thanks for the visual understanding. I have a question that at the selected frequency position OFDM solves the problem of frequency selective fading ?
Sorry, I don't know what you mean by "selected frequency position". OFDM converts a high rate data sequence into a set of parallel lower rate data sequences transmitted over orthogonal "sub channels". The lower rate means the symbols are longer (in time), and are therefore less affected by ISI. In most OFDM systems, the sub channels are narrow enough to be considered "flat". I'm planning to make a video on this, so keep an eye out for it.
@@iain_explains Sorry I confused you, in your example in the "pi/T" position, I see an abnormality happening when H (jw) -> 0 so I imagine if there are any subcarriers in OFDM appears at that position, I imagine it will lose the signal at that frequency
Can i have wideband flat fading, which is separated into N (N=subcarriers) narrowband channels? Also these N subchannels could be all equal in terms of fading? Is it realistic or ofdm is assumed only in frequency-selective wideband channels?
If the delay spread of the various propagation paths is small, then the channel will be flat over a wide frequency range (bandwidth). This is a property of the channel, and is independent of the modulation format used to transmit data. The OFDM modulation format has advantages in frequency selective channels, compared to other modulation techniques, but it is not restricted to frequency selective channels.
@@iain_explains Thanks, your answers are really helpful. So i understand that scattering environment determines the type of fading but it is no prohibitive to assume a flat channel over a wide frequency range for a MIMO system, even if this does not happen often.
Alsο does the spatial correlation affect the delay spread?
The spatial correlation is a measure of how similar the paths are in different directions. If the spatial correlation was 1, then all paths in all directions would have the same gain/loss and phase shift, and therefore they would all have the same delay.
Sir can you please help me to model a doubly selective fading channel in matlab?