I want to clarify my understanding of OFDM training fields. Please correct me if I am wrong as I am getting 2 different information from online resources.

For 802.11a standard the preamble contains 10 short training Field (STF) and 2 Long Traning Field (LTF). Each of the STF (A1,...A10) contains 52 subcarriers that contain symbols as shown in the picture.

And In some other document, I am seeing that there is 2 short training field instead of 10, the 2 LTF is there as same. Which one is true, or both are true in a different perspective? I have also seen in many implementation 2 STF are used. enter image description here

In addition:

a) Is the same packet detection method used for both 802.11a and 802.11n? (i.e with STF)?

b) The standard says each of the STF symbols has 16 samples. Can anyone explain this? How this 16 sample is achieved or comes from?


Typically both are used, coming from an 802.11p perspective. The short preamble (STF) is used for packet detection and coarse frequency offset. The long training preamble (LTF) is used for timing synchronization and fine frequency offset. The LTF can also be used for channel equalization. Both are used from my understanding to accomplish total synchronization.

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  • $\begingroup$ Thanks for the reply. But I am not sure if I was able to make my question clear. Definitely, both STF and LTF are needed for time/frequency offset. But how many STF should be there altogether? The standard 802.11a says 10 STFs, but some implementation says 2 STF. $\endgroup$ – Nusrat Oct 9 '18 at 17:50
  • $\begingroup$ Can you post links of papers that show 10STFs vs 2 STF? As for question 2) The standard says each of the STF symbols has 16 samples. Can anyone explain this? How this 16 sample is achieved or comes from? The symbol is 160 samples long / 10 repetitions = 16 samples each $\endgroup$ – spet Oct 9 '18 at 18:14
  • $\begingroup$ page 89 (says 10 repetitive symbols) : pdfs.semanticscholar.org/1c2c/… and page 21 (saying 2) : cnrood.com/en/media/solutions/… $\endgroup$ – Nusrat Oct 9 '18 at 18:23
  • $\begingroup$ Ah, I see. They don't mean 2 symbols vs 10 'symbols'. They mean the short and long preamble are both 2 symbols long because the ifft length is 64 samples and the cyclic prefix is 16 samples long which makes one ofdm symbol 80 samples long. The long and short preamble are both 160 samples or 2 symbols in length. The short preamble is 2 ofdm symbols long but contains 10 repetitions in time. The long preamble is 2 ofdm symbols long and just happens to have 2 (well 2.5) repetitions. $\endgroup$ – spet Oct 9 '18 at 18:28
  • $\begingroup$ Ahh! Now it makes some sense. So, I didn't include the CP which has a length of 16 symbols. Therefore 64 OFDM symbols + 16 CP symbols = 80 symbols. Now, from the document & also what you have mentioned, STF+LTF = 80+80 = 160 samples. But why are we taking 2 STF together into account? And saying it 2 OFDM symbols ? $\endgroup$ – Nusrat Oct 9 '18 at 19:28

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