In IEEE 802.11a standards, there are many fields. Of which is the Data field which has 52 available sub-carriers used for transmission of coded bits. If the subcarrier locations are numbered from 32,-31,...0,1,...31.
I have read in the book that
that 48 out of 52 subcarriers are used for transmission of data bits. While 4 pilots are used as pilots for phase and frequency tracking and training. The pilot subcarriers are located at -21,-7,7,21 with values 1 (at -21,-7,7) and -1 at 21.
I am just wondering, usually the preamble (fields before the data field) is used to estimate the channel by sending a pilot symbol (using ALL subcarriers). Then my questions are
1) what is the purpose of sending these 4 pilot tones?
2) I am guessing phase and frequency tracking is different from channel estimations?
3) My last question is do we send the pilot values on the predefined pilot subcarriers ALONG with the data bits? so basically is each OFDM symbol made up of pilot and data tones? This is different than the procedure in preamble where ALL subcarriers are used to send one pilot symbol, right?