Dear All,
I’ve been trying to investigate how channel estimation works in OFDM
based
on the implementation provided in Gnuradio for OFDM transmission. I
found
that it was done in the block digital_ofdm_frame_acquisition.cc/h. As I
understand, the digital_ofdm_frame_acquisition::calculate_equalizer does
the job: based on known transmitted symbols (stored in d_known_symbol)
and
received symbols (stored in symbol), the inverse of the channel
coefficient
can be obtained according an equation similar to the following:
channel_coefficient = symbol/d_known_symbol
However, there are two differences in the implementation in relation to
this equation: 1) the block computes the inverse of the channel
coefficient
(d_known_symbol/symbol), and 2) there is a frequency compensation term
(coasre_freq_comp) which basically rotates the complex samples by phase
corresponding to the frequency deviation and obtained via the preceding
sync block. One final note is that channel coefficient is obtained in
every
other tab, and the coefficient in the intertaps are obtained via linear
interpolation of the acquired channels.
Now, my questions are as follows:

Does this explanation seems correct?

By modifying the VERBOSE variable to be equal to 1 in
digital_ofdm_frame_acquisition.cc, the block also plots the estimated
channel coefficients in the following order: transmitted symbol →
known
symbol → estimated channel inverse → output). I noticed that when
using
a file source/sink to store/receive packets from OFDM benchmark
transmitter
and receiver, the channel coefficients are still not equal to 1, despite
the fact that no receive noise nor wireless fading occurs. What do these
coefficients represent? 
Are the obtained coefficients eligible to be used in further
precoding
of transmitted packets, assuming that the channel between Tx/Rx is
reciprocal, and that a receiver can switch roles with the transmitter?
Thank you
–

Mohammed Hassan Karmoose
Teaching Assistant, Electrical Engineering Dept.
Faculty of Engineering, Alexandria University
AlHoreya Rd, ElHadara, Alexandria, Egypt  21544
*Tel: *(++203)5921852  *Fax: *(++203)5921853
Email: m [email protected]_h_karmoose@a [email protected]
lexu.edu.eg