question about ifft size in ofdm!!!!

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mai fouad

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please i want to ask if i use ifft of size 128 so we will only use 72 subcarrier in the resource grid so my question is are we will use ifft =128 at transmitter and then when we make resource element mapping we will map the first 72 to subcarriers and the others will be mapped in subcarriers but in another ofdm symbol time
 

Your question is not clear. Could you please elaborate more?
 

i want to say are we will use size=128 or 72 if 128 and if we have stream of data we will make ifft to it by size 128 regardless that we will use only 72 subcarrier and so in the resource grid (the time slot which consists of 7 ofdm symbols in time ,one symbol=128 subcarrier =128 resource element ) in the first symbol time we will use only 72 subcarrier from 128 and the rest of symbol as guard band =zeros and the last 128-72= 56 will be mapped in the second symbol .or what we do to have size =128 and only use 72 subcarrier
 

I think you should adjust your input rate to correspond the 72 channel symbols, i.e. 72 subcarriers.
Imagine, if you map 56 elements of first data frame to the second, where you take 112 elements of second frame (16 of them are added to 56 to form second symbol)? Map to third?
 

is this guard bands means that if i use 128 ifft the data entered ifft block is as following =(first 28 subcarriers=zeros +72 subcarrier of data +last 28 subcarriers zeros ) and then i make ifft for all the previous

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is this guard bands means that if i use 128 ifft the data entered ifft block is as following =(first 28 subcarriers=zeros +72 subcarrier of data +last 28 subcarriers zeros ) and then i make ifft for all the previous . but what is the advantage of all that
 

That is correct. After subcarrier mapping, you need to pad with zeros, top and bottom to match it to the FFT size. I am not sure what you mean by guard band in this context. Do you mean cyclic prefix which is added to the OFDM symbol to mitigate the effect of ISI. If it is the case, cyclic prefix is added after IFFT, which generates OFDM symbol(s) depending on the number of resource blocks that are assigned to the UE.

Hope this helps.
 

I am not sure about the practical chips of FFT/IFFT, but as far as the MATLAB is concerned you can use FFT/IFFT of any size, but it is most efficient in terms of computation when the number of subcarriers is in the form of \[2^x\].

If you have an FFT/IFFT of size 128 and you only need to transmit 72 subcarriers, then you will need to null 128-72 subcarriers. However, these 128 subcarriers still constitute one OFDM symbol. If you do not use the null subcarriers for something useful such as carrier frequency offset (CFO) estimation, your scheme is not bandwidth efficient, and it is better to increase the number of subcarriers to match that of FFT/IFFT size.
 

The addition of zero subcarriers on both sides of 72 data subcarriers is a margin for real filter implementation (transition band). The cyclic prefix is added after ifft to implement a delay between previuos channel symbol and next one so as to minimize influence of multipath components.
 

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