# What's the relation between the sampling rate and FFT size in OFDM

In OFDM system,

Is there relationship between Sampling rate and FFT size? For example, if we downsample or oversample the received signal, Should we decrease or increase the FFT size accordingly?

thank you

• I don't think you fully understand the basics of OFDM – the FFT size is exactly the number of subcarriers your OFDM system has. Nothing ever changes that. And "Oversampling Pop Up Menu" is something that has to do with a specific software or so? You might be forgetting that we don't sit in front of the same computer using the same software as you... – Marcus Müller Jul 26 '18 at 19:35
• leave the "Oversampling Pop Up Menu" and answer about the question in the fist part. Is there relationship between Sampling rate and FFT size? For example, if we downsample or oversample the received signal, Should we decrease or increase the FFT size accordingly? – New_student Jul 27 '18 at 5:02
• Hello .. So you question is there relation between sampling rate and FFT size. Yes, there is, Let s be a data vector of length N. When oversampling is not used, the transmitted signal is x=F^H s where F is the N x N DFT matrix whose (k,l)th entry is exp(-j2¥pi k l /N). When oversampling is used, the transmitted signal is Fp^H s where Fp is the pN x N DFT matrix and p is the oversampling factor.Then, what happens if F is used is used for demodulation? .. – Zeyad_Zeyad Jul 27 '18 at 5:16
• @MarcusMüller .. It's not allowed to evaluate people if they understand or not understand. they are here to learn and help each other. I've seen some comments for you where you evaluate people themselves by negative feedback. if they understand well, they won't come here to ask and try to understand. Eng. Badr, Keep going on, – Zeyad_Zeyad Jul 27 '18 at 5:19