## Determine the original size of the FFT used in OFDM?

Started by 6 years ago4 replieslatest reply 6 years ago364 views

Given an unknown OFDM symbol consisting of 9144 samples, interested to know the size of the FFT used in the TX/Number of bins.

Downsampling to 512,256,128,64,32 yields the following.

Do not see how can this information be derived based on this data.

As a result, have tried to analyze the time domain of the downsampled vectors, and detect samples that were removed by the downsampling process.

Seems that the third graph represents the original FFT size – 128, as the sample at 277 has not been witnessed before(fft=64,32), and is witnessed at higher sampling rates.

Wonder if this is correct and whether there is a more accurate approach to determine the FFT size used?

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little bit of an odd question the modem design is normally predetermined, i.e. you know the transmitters IFFT size. You also haven't provided any information on what it is you are receiving -standard WiFi packets? Typically its 64. Also unclear why you are downsampling at the possible FFT size, not sure what this would tell you, especially if you are not sample aligned to the start of a symbol to begin with and not sampling at the correct bandwidth.

If this is a standard WLAN packet as described by https://www.mathworks.com/help/wlan/ug/wlan-packet... then you can note the cyclic prefix will have some predetermined length in time. Try sampling at the max channel bandwidth possible for WLAN (160MHz, but probably only need to go up to 20-80MHz) and note the cyclic prefix duration which should give you the channel bandwidth being used, which should be using a standard sub-carrier spacing and from that fact you can solve for N, the fft size from:

$$\Delta_t = \frac{1}{N T}$$

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This is a propriety video link being BLINDLY demodulated.

No parameters are given/known.

There was however a progress with the problem, no bits yet though. Will update when there will be.

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