What are Fast Fading and Slow Fading?

[@vdashwini]

Precisely covered many aspects in such a short video. I need not go through many documents to understand the concept. Thank you.

[@vdashwini]

If you can add some examples with numbers (kind of problem and solution), that would really help to visualise the concept better.

[@iain_explains]

I'm glad you liked the video. Thanks for the suggestion of adding a numerical example. I'll add it to my "to do" list.

[@vkARTChannel1]

Excellent

[@jaiwilson8047]

Thanks mate. Doing a communications subject at uni right now, and your videos are really helping. Bloody legend

[@iain_explains]

Glad to help.

[@CSmaniac10012]

Hello Professor,I have a question about frequency selective channels and their time and frequency responses. I understand that for slow and fast fading, we primarily look at the channel's response in the time domain to determine whether the channel exhibits slow or fast fading characteristics.However, I am confused about how this relates to the frequency response of the channel. Specifically, if we have a slow or fast fading channel, do we also consider it a frequency selective channel when we examine its frequency response?I find it challenging to understand the relationship between time-selective channels and frequency-selective channels because most materials focus on either the time domain or the frequency domain, but not both simultaneously. Could you please explain the interplay between these concepts? A video explanation would be very helpful.Thank you in advance!

[@iain_explains]

This video might help: "What are Flat Fading and Frequency Selective Fading?" kzbin.info/www/bejne/gZqugXmKadenfsk

[@issaclee7114]

Could you let me know flat fading vs frequency selective fading? Thank you!

[@iain_explains]

Thanks for the suggestion. I've added it to the "to do" list.

[@iain_explains]

Hi Issac, I've just uploaded a video on this: "What are Flat Fading and Frequency Selective Fading?": kzbin.info/www/bejne/gZqugXmKadenfsk

[@RajasubramaniamBalakrishnan]

Very good. I think you can consolidate all these and make a book.

[@iain_explains]

Yes I could. But would anyone read it? In many ways I view this KZbin channel as my version of a textbook. It's much easier for people to watch the videos, rather than reading a textbook.

[@eswnl1]

Can you do a video about the FM capture effect? In my opinion, there's not enough comprehensive explanation of the phenomenon on the internet.

[@iain_explains]

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

[@girishyadav9789]

Nice explanation! Thank you!

[@iain_explains]

Glad it was helpful!

[@wendyky379]

Great explanation-your videos are helping me a lot in my research.Thank you!

[@iain_explains]

I'm so glad to hear that.

[@onepiecebarca]

Can you ad to the to do list a video about ITU pedestrian B channel?

[@iain_explains]

Thanks for the suggestion. I'd like to say yes, but in reality I've got so many topics on my "to do" list, it will be quite some time before I can get to do it.

[@earlofthearies.1982]

Universal information.

[@amir10able]

@11:12 From my professor I learned that slow fading -> no ISI and fast fading -> ISI. Where is the mistake? Best regards from Germany

[@iain_explains]

I'm not sure, sorry. Perhaps you heard your professor wrong, or perhaps your professor is wrong ... My explanations in the video justify the points I'm making, so I'm not sure why someone would think the opposite.

[@amir10able]

@@iain_explains I found this topic from his lecture script 😅. Thanks for answering

[@sarathjeeva28]

Good video man.....keep doing 👍

[@danci947]

Great thanks

[@adarshgupta9272]

nice video

[@amal717]

the best ever

[@전준화-o2v]

Thank you for your effort from Korea. Your channel has been very helpful in studying wireless communications!

[@iain_explains]

That's great to hear!

[@stringstoparadise2392]

at 6:20 what is the meaning of tau not changed but it should change since the receiver is moving with velocity v

[@iain_explains]

That's not what I said. I didn't say tau was constant. I said, _even if_ tau was not changing, then there would still be the effect of Doppler.

[@stringstoparadise2392]

u want to say that change in tou can be seen as offset in carrier frequency which is Doppler shift?

[@iain_explains]

No. I'm saying that a change in tau will result in a change in the channel gain. Also, a change in frequency (ie from Doppler) will result in a change in the channel gain. They are not the same thing. They both effect the channel gain. In most cases you get them both happening at once (due to the movement of the terminal), but this doesn't have to be the case.

[@stringstoparadise2392]

thanks sir lots of respect from India ❤️

[@fiahrose5961]

thank you Iain, that was very helpful and coherent

[@zoaz7348]

So you mean Fast & Slow fading occurs when either receiver or transmitter moves and when there are multipaths at the same time? I'm sorry that my English is bad since I'm a Korean student. Always thank you for making informative videos.

[@iain_explains]

This might help: "What is Rayleigh Fading?" kzbin.info/www/bejne/Y3eyn4x4j5yNnLM

[@satyajitdutta]

Thank you for the nice explanation. I used to think slow and fast fading are related to coherence time. Now I realise it is related only to the symbol rate.

[@iain_explains]

I'm glad it helped. Have you seen my video on Coherence Time? "What are Coherence Bandwidth and Coherence Time?" kzbin.info/www/bejne/mH-wpJywrryhgZI

[@hasnulhashim2005]

1:29 That's not wavelength; it's proportional to wavelength.

[@iain_explains]

Sorry, I'm not sure what you're saying. I show a sinusoidal waveform with a full cycle - the time duration of that cycle _is_ the wavelength - by definition.

[@jayeshgehlot9850]

Thanks, that helped me understand why slow is slow even when transmission rate if fast 👍.

[@iain_explains]

I'm glad it helped.

[@badalsoren2947]

hey professor could you please tell me whether time selectivity of channel has any affect in the coherence bandwidth of the channel ??

[@iain_explains]

Hopefully these videos will help: "What are Flat Fading and Frequency Selective Fading?" kzbin.info/www/bejne/gZqugXmKadenfsk and "Channel Estimation for Mobile Communications" kzbin.info/www/bejne/kKSvmWNno9GFsLs

[@tuongnguyen9391]

Could you make a video on how to use Matlab to illustrate or simulate fast fading ?

[@iain_explains]

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

[@xianmingfeng5134]

Thank you for the nice explanation. I have a question. If I am using some kind of analog mode like SSB or FM voice like in HAM radio, what would be the symbol rate of these voice signal?

[@iain_explains]

The definition of a "symbol" is a finite duration waveform that is used to represent digital data. Therefore, for analog communications, there are no "symbols", and hence no "symbol rate". In this case you can look at the relationship between the rate of change of the signal (related to the bandwidth), compared to the rate of change of the channel.

[@AH-zz6ei]

Thau can not be equal to half a wavelength... I got your point but saying that can be confusing...

[@iain_explains]

Tau certainly _can_ be equal to half a wavelength. It happens to my favourite AM radio station every day on my drive to work. There is a section of road between tall buildings where the radio station goes all "fuzzy" and "noisy" and I can't hear what the presenter is saying. This is precisely because there are two dominant propagation paths, and they are half a wavelength apart.

[@سلمى-ز9س]

That Was GREAT ❤

[@iain_explains]

I'm so glad you liked it.

[@banashekh]

Thank you very much Lain. I'm an M.Sc student and your videos are really helping. Can you please make a video on OFDM with Index Modulation (OFDM-IM). Many thanks in advance.

[@iain_explains]

Thanks for the suggestion. I'll add it to my "to do" list.

[@louisli8489]

Thx for your wonderful guidance in communications, may I ask somethigh which confused me, did we always talk about fast&slow fading in time domain and frequency selective&flat in frequency domain? Is Rayleigh fading a kind of tool for modelling to analyze all those above, so that we can assume and analyze channels like Rayleigh flat fading, Rayleigh fast fading. Many many thanks in advance.

[@iain_explains]

Rayleigh fading refers to the probability density function of the channel gain at a specific time, t, which is a Random Variable. When you look across time, at a whole channel gain function over time, then you are looking at the channel gain as a Random Process - and that is where the fast/slow and selective/flat characterisations come in. For more details, see the following three videos: "What is Rayleigh Fading?" kzbin.info/www/bejne/Y3eyn4x4j5yNnLM , "What is a Random Process?" kzbin.info/www/bejne/jWObXmxsds15aNU and "Autocorrelation and Power Spectral Density (PSD) Examples in Digital Communications" kzbin.info/www/bejne/jojcpYaCj7-DZ6M

[@louisli8489]

@@iain_explains Thanks a lot, Prof. Iain. Many thanks for your detailed guidance, I saw from the textbook that slow&fast fading were based on Doppler effect, I am wondering that multipath is another main reason since each Doppler shifted signal has to add up together to perform the fading in time domain. Might need your help.

[@iain_explains]

@@louisli8489 These videos should help: "What are Doppler Shift, Doppler Spread, and Doppler Spectrum?" kzbin.info/www/bejne/gn3VZGChpNmVsJY and "What are Flat Fading and Frequency Selective Fading?" kzbin.info/www/bejne/gZqugXmKadenfsk

[@rishabhkumar1050]

sir what you have explained is time domain visualisation of fast and slow fading, how can we visualise this in frequency domain

[@iain_explains]

Good question. I'll add this topic to my "to do" list. In summary though, the time variations come about because of movement. And movement results in Doppler shifts. See this video for more details on Doppler: "What are Doppler Shift, Doppler Spread, and Doppler Spectrum?" kzbin.info/www/bejne/gn3VZGChpNmVsJY

[@rishabhkumar1050]

@@iain_explains thanks professor

[@johnangelo2000]

Would be helpful if you could number the fading videos. (i.e. basics to advance as you go on explaining it). Helps make the a series to understand chronologically.

[@iain_explains]

The thing is, I don't make the videos "in order", so I can't number/label them when I make them. I choose topics based on my interest at the time, and also based on suggestions/questions from viewers. Check out my website, where all the videos are characterised and ordered: iaincollings.com

[@johnangelo2000]

@@iain_explains If somebody is going through fading episode, the suggestion to next video that youtube gives is a link list which is not aligned with concept building. Its just a observation for ease of use. The videos are as usual very informative. Thanks for the effort..

[@iain_explains]

I do have "playlists" on the KZbin channel. I'm not sure if you've seen them. I've got one on "Fading Mobile Communication Channels".

[@gpuc]

I cant' understand why if there is fast fading there will be no ISI at the receiver? The receiver may receive some copies of the transmitted signal and they wil cause the ISI

[@iain_explains]

Yes, but for large symbol times (long symbols), the delayed paths only affect a small percentage of the symbol time. So, the longer the symbol, the lower the ISI effect. Long symbols mean that there is more time for the channel to change before the next symbol. Which means the fading is fast (in relation to the symbol time).

[@gpuc]

@@iain_explains Ok, got it. Thank you

[@ahmedhusseinshatti7099]

I am very thankful for your effort in explaining the material. Your explains are more than amazing 🥰

[@iain_explains]

Thank you so much 😀 I'm glad you like the videos.

[@minhquangnguyenac1998]

I am doing about fast and slow fading, "while googling" :D ahh there you go, 15 minutes ago.

[@iain_explains]

Great timing! Glad you found this video. If you're new to the channel, you might be interested in a full list of videos on the channel, in topic categories, see: iaincollings.com

[@stringstoparadise2392]

sir I have reffered a journal paper in which it's mentioned that coherence time 500 and 35 micro sec if we are using carrier freq of 2 and 28 GHz then how can be a channel in fast fade as in order to achieve even speed of 1 Mbps we have bit duration of 1 micro sec but the condition of fast fade is symbol duration > coherence time which is never seems to be possible with such scenario? means I want to know for OFDM systems can channel be in fast fade?

[@iain_explains]

Don't forget we're not sending the 1Mbps data using binary modulation. The higher order the modulation is, the longer the "data symbol" lasts for. Also in OFDM, we are sending parallel data streams, so each sub channel is sending at a proportionally lower rate - so again, this makes the data symbol last even longer.

[@stringstoparadise2392]

@@iain_explains sir is it not possible to send our pilot bits with binary modulation scheme in order to estimate the channel , and send our data with higher modulation schemes in this way we can estimate our channel for worst channel conditions / channel is varying very fast . after all we are estimating the phase and gain of the channel has to offer so that we can equalise our channel affect

[@iain_explains]

Sure, but you asked about whether the fading rate could ever be "fast". And I was just telling you the reason that it can be "fast". I wasn't referring to anything related to how to estimate the channel. The fading "rate" relates to the actual data symbols, not the training symbols that are used to estimate the channel.

[@stringstoparadise2392]

@@iain_explains it means that channel can be estimated for any worst condition ? sir actually I am working on error performance of OFDM in presence of residual CFO which can't be estimated for fast moving vehicles as coherence time offered is very less, but sorry, what you mentioned above has confused Me as mentioned channel can be estimated for any condition if we use binary modulation for pilot carriers , then from where this residual CFO is comming , sorry prof I might be going little off topic but It has made me confused I am Ph.D student @ IIT India thanks for every reply I would be grateful to you sir I have shared your channel in my whole circle they are big fan and admirer of your teaching sir 🎉

[@iain_explains]

Actually, no, I didn't say anything at all about how to estimate the channel.

[@deepinmoutain]

Thank you very much! sir! one question about the hIgh data rate and Low data rate. if we use a longer symbol duration, why does it mean Low data rate?

[@iain_explains]

Symbols are mapped to data bits, so if the symbol takes longer to send, then the corresponding data takes longer to get to the receiver. You might like to watch: "How are Data Rate and Bandwidth Related?" kzbin.info/www/bejne/kHO2p4CYhJWghrM and "How are Throughput, Bandwidth, and Data Rate Related?" kzbin.info/www/bejne/f4qZl3ePrKhlnLc

[@deepinmoutain]

@@iain_explains Thank you very much. it is clear now!

[@mohammedhammouda6628]

Thanks, it is very simple and efficient

[@iain_explains]

Glad it helped

[@alvarodorado35]

Great video, really helping!!

[@iain_explains]

Glad it helped!

[@ahmedmuhammed6905]

Great explianation, thanx a lot

[@iain_explains]

Glad it was helpful!

[@se2702]

This is so good Iain. I like the non math intensive explanation.

[@iain_explains]

Glad you liked it!

[@suyashmisra7406]

this was very informative, thank you.

[@iain_explains]

Glad it was helpful!

[@falguni.bonaparte]

Thank you Sir!

[@iain_explains]

You are welcome!

[@Nataspin900]

well explained, thanks!

[@iain_explains]

Glad it was helpful!

[@shimelisabate8953]

Thank You Sir.

[@iain_explains]

You are most welcome