How does OFDM Overcome ISI?

[@CarnaticSriram]

The series on OFDM is some of the best I have seen! Presents a clear and practical view! Thank you sir.

[@iain_explains]

Glad it was helpful!

[@Eche1972]

I have been looking for a presentation like these series for months, thanks so much Iain

[@iain_explains]

Glad the videos are helpful!

[@jabhad4591]

My utmost gratitude goes to you.

[@oussamaraoudi1249]

You are a true hero with those video of yours!

[@iain_explains]

Thanks. I'm glad you've found them helpful.

[@gill6335]

Thanks for this wonderful video! I want to clarify something. Does it mean that when we sample longer OFDM symbols at receiver, we will get ISI only for a very small [ insignificant] part of the symbol whereas without ofdm, the ISI might span all of the next symbol?

[@iain_explains]

That's right. The tradeoff is that with longer OFDM symbols there is more delay, and more chance for the channel to be "time varying" within a symbol period.

[@gill6335]

@@iain_explains Thanks for taking time to reply! In this case where time variation happens within the symbol, is there a concern of intra symbol interference? My understanding is that the received symbol will be summation of slightly delayed versions of multi-path symbols while will make the symbol shape deviate from the original transmitted symbol shape..does that make sense?

[@usamazahid1]

really amazing....great work sir

[@iain_explains]

Thank you! Cheers!

[@aarzoosingh8316]

hello professor, why there is a need for equalization in the case of OFDM when there is no ISI effect is there as it has already been resolved by making our channel a frequency flat ??

[@iain_explains]

There is still ISI, but OFDM overcomes it by using sub channels and adding a cyclic prefix. Even then, each sub channel needs to have it's gain "equalised" (scaled) at the receiver since the sub-channel gains (which are flat in frequency) are not the same for all sub channels (due to the non-flat overall channel).

[@amitbora6336]

Hi professor, once you are free, can you make a video on ICI effects in OFDM and how OTFS overcomes better than OFDM in high Doppler scenarios? Thank you.

[@iain_explains]

Thanks for the suggestion Amit, it's a good one, and it's already on my "to do" list. The problem is finding the time (as you say). Hopefully I'll have a bit more time over our upcoming Southern Hemisphere Summer.

[@lutzvonwangenheim9682]

hey Prof Lain got a question for you if the OFDM systems are only solving the problem of ISI by converting a big chunk of channel in to small chunk of channel which are frequency flat then what new in this OFDM we ca even elliminate this problem of frequency selective fading/ISI by using Equilizers as simple as that problem solved . I mean why there is big hype around OFDM technique ????

[@iain_explains]

It's not totally clear what you're asking, sorry. It sounds like you think that equalisers are easy to implement, and you're wondering why there is any need for OFDM? Well, that's not the case. Equalisers are _not_ easy to implement. Optimal equalisers, such as the Viterbi algorithm, are very complex, and Linear equalisers are not optimal and don't remove all of the ISI. See: "Viterbi Algorithm Explained with an Example" kzbin.info/www/bejne/rqnTc3t5oN6jiZY and "How are Different Equalization Methods Related? (DFE, ZF, MMSE, Viterbi, OFDM)" kzbin.info/www/bejne/noqxqIeKjNejapo

[@lutzvonwangenheim9682]

@@iain_explains ok lain first of all thanks for the video, I think you got irritated by my question just asked out of curiosity as i was not getting the answers the way you explained 😊

[@iain_explains]

No I wasn't irritated. I was just wanting to clear up any confusion. I am always very keen to encourage curiosity, so please keep asking questions.

[@DataWiseDiscoveries]

Great explanation, loved it!!

[@iain_explains]

Glad you liked it!

[@bunty55555]

Very simply explained , Brilliant ....! thank you

[@iain_explains]

Glad you liked it

[@stringstoparadise2392]

sir is there any diffrence between the practical delay spread of a stationary channel mobile phone at rest and non stationary channel like a mobile phone in train ( moving at 180- 200 km|hr )

[@iain_explains]

The delay spread depends on where the reflectors are. This is related to the environment you're in. If the terminal is moving, then the delay spread value will change (since the relative locations of the reflectors will change). You might be interested in this video: "What are Doppler Shift, Doppler Spread, and Doppler Spectrum?" kzbin.info/www/bejne/gn3VZGChpNmVsJY

[@yasserothman4023]

Thanks but why is there no mentioning for the cyclic prefix which should be greater than the maximum delay spread to avoid isi ?

[@iain_explains]

Ah yes, good point. I should have mentioned that too. I have another video on that topic: "What is a Cyclic Prefix in OFDM?" kzbin.info/www/bejne/d3vKZmp3eqenhNk

[@rationalthinker9612]

I am taking communication engineering right now and we are going over OFDM. My question is the following, you want to hold each data bit for a longer period of time in order to compensate for channel delay spread, but isn't the channel also changing over that period of time?

[@iain_explains]

Great question. It depends on how fast the user is moving. Here are a few related videos: "What are Fast Fading and Slow Fading?" kzbin.info/www/bejne/ip6QhqyXn8iop9U , "Why is Doppler a Problem for OFDM?" kzbin.info/www/bejne/o3OTeHlvqrB2apo and "What is OTFS? Orthogonal Time Frequency Space Modulation" kzbin.info/www/bejne/g6euZK2ehdd6oc0

[@rishabhkumar1050]

does bit rate in case of OFDM and single carrier modulation remains same if we put aside the effect of the intersymbol interference . if so can you please spread a little light on this it would be very helpful

[@iain_explains]

Sorry, I'm not sure what you're asking exactly.

[@rishabhkumar1050]

@@iain_explains sir i am asking if we have same alloted bandwidth for both OFDM scheme as well as single carrier scheme than weather the data rate will be same in both the case (the data rate for OFDM = DATA rate of single carrier scheme ) bandwidth allotted same for both scheme condition - neglecting the ISI effect means channel is frequency flat channel

[@iain_explains]

Across a band limited frequency-flat AWGN channel, the maximum error-free data rate is given by the Shannon Capacity equation (see: "What are Channel Capacity and Code Rate?" kzbin.info/www/bejne/hmG6imxsjKeIr6M ). Whether you can achieve this in practice depends on many factors, including what type of coding you're using. It's not just about the modulation scheme.

[@RaviKumar-nt9ge]

Amazing explanation.

[@iain_explains]

Glad it was helpful!

[@georgepalafox5967]

Hi Sir! Hope ur well! Why is the OFDM symbol duration equal to 1/subcarrier spacing? Let’s say you have 64 subcarriers in one OFDM symbol which would where the subcarrier spacing is 312kHz. Wouldn’t the symbol duration be 1/64*312kHz? I am probably missing something basic here. Thanks!

[@iain_explains]

This video might help: "OFDM and the DFT" kzbin.info/www/bejne/kGWvepqEnLN0oqs

[@alikhrizat7108]

Great Explanation! i really need to know how can we calculate the symbol period and the amount of delay spread for a single carrier transmission that uses 32-QAM where the delay spread is not more than 10% of the symbol period, and NO Cyclic prefix is used? appreciate the help!

[@iain_explains]

You'd need to know what the bandwidth is, and what pulse shaping filter is being used.

[@alikhrizat7108]

Okay so can we find the symbol period if the bit rate is 50 Mbit/sec and the symbol rate is 10 M symbols/ sec ? Thanks again

[@iain_explains]

Perhaps this video will help: "How are Data Rate and Bandwidth Related?" kzbin.info/www/bejne/kHO2p4CYhJWghrM

[@georgepalafox5967]

Sir, are you sending the entire bit stream (8 bits) in the carrier to the right or is that just for one of the bits? For this example, your skipping the modulation of the bits. Should I interpret each bit to be a symbol?

[@iain_explains]

I mention at the 38 sec mark of the video that I am showing the frequency plot for the case of modulating the input digital sequence by a carrier. In other words direct amplitude modulation. I drew a situation with a baseband digital signal that only has two levels, so this corresponds to binary modulation. So yes, each symbol represents one bit.

[@surajv1986]

May I know what is meant by delay spread? Thanks

[@iain_explains]

Transmission paths that reflect off buildings etc., will arrive with different delays. The spread of delays across multiple paths, is called 'delay spread'. More details here: "What is Intersymbol Interference ISI?" kzbin.info/www/bejne/f2GbaHmLq7xlr9U

[@surajv1986]

@@iain_explains Thanks for the clarification.

[@mihailomarjanovic4354]

Hi professor, thank you for this very educational video. I would like to ask one question: Is the subcarrier modulated per bit in data stream or it could be "several bits"? Meaning, if we are using for ex 64QAM each subcarrier is modulated per 6 bits, or to be more precise each subcarrier is modulated based on the 64QAM constellation point among with phase and amplitude, frequency of each subcarrier is selected based on Ts = 6*Tb, deltaf = 1/Ts. Thanks in advance for answering.

[@iain_explains]

Yes, it could be any constellation you like, on each subcarrier.

[@undertaker7523]

Thank you for this!

[@iain_explains]

My pleasure!

[@georgepalafox5967]

This a great explanation sir, thanks for putting this together.

[@iain_explains]

Glad it was helpful!

[@oggamer2244]

Absolutely amazing 🤗, thank you.

[@iain_explains]

Thanks for your nice comment. Glad you found it useful.

[@sukhdevsajwan8495]

Thanks you sir 🙏 for for great explanation

[@iain_explains]

Most welcome

[@bhanu3178]

SIR can you do videos on filters.

[@iain_explains]

Thanks for the suggestion, I've added it to my list.

[@umarhayat459]

love it .

[@iain_explains]

Thanks. Glad you liked it.

[@ElsaGrover]

1. I have a fundamental question that relates to Dual Connectivity i.e. N-DC refers to the dual connection of 4G wireless access network and 5G NR. 2. Let’s say that we are using Band-X in 4G and another similar band in 5G and are attempting to communicate with both RATs to take advantage of EN-DC. 3. The question is what will happen if there is overlap in terms of the “spectrum occupied”. 4. Let’s say that the 4G component has been assigned spectrum in Band 3 from 1830 MHz through to 1850 MHz (i.e. a chunk of 20 MHz Bandwidth). 5. Let’s say that the 5G component has been assigned spectrum in Band n3 from 1820.45 MHz through to 1830.45 MHz (i.e. a chunk of 10 MHz Bandwidth). 6. In such a scenario, there is an overlap of 450 KHz between these two components i.e. the assigned 5G-NR band component eats into the 4g spectrum component by 450 KHz. 7. Will such a deployment work in light of the designated configuration based on existing product implementations? There are bound to be issues in decoding such transmissions at the UE because of the overlap in the spectrum. The reason, I am asking this question was because an argument (however erroneous) can be made that the 5G transmissions would potentially be orthogonal to the 4G transmissions (even if they have an SCS of 15 kHz or 30 kHz) and hence the decoding at the mobile will work because orthogonality will prevent any decoding issues. I do not agree with this argument i.e. that the orthogonality provided by the OFDM structure of the waveforms would protect them against each other (i.e. from the UE perspective in the sense that the UE can successfully decode the two transmissions). I would be very grateful if you could enlighten me with your perspective with references (if applicable). 8. If the answer is in the negative, from an R&D implementation, perspective - what changes would one need to implement to make such a configuration (i.e. characterized by overlapping assigned spectrums) workable in realistic network implementations? 9. It would also be immensely helpful if you could further substantiate your arguments with references from the relevant 3GPP specifications (or other relevant specifications) or draw my attention to any specific specifications or publications which can further clarify my doubt?

[@iain_explains]

Sorry, I'd like to help, but I just don't have time to go through such a long comment.

[@user-qh4uv5dz7i]

Thanks!

[@iain_explains]

You're welcome!

[@amrwael8348]

Awesome

[@iain_explains]

Thanks. I'm glad you liked it.