Ofdm plateau_detector_fb_impl//ofdm_sync_sc_cfb_impl

GNU Radio
1

Hi all,
I have a question on the synchronization of ofdm
example,namely the sch&cox source code.

       The plateau_detector_fb_impl.cc can find the plateau,and then

header_payload_demux_impl can find the head data.
STEP 1: plateau_detector_fb_impl find the plateau.
flank_start = i;
while(i < noutput_items && in[i] >= d_threshold)
i++;
if((i - flank_start) > 1) { // 1 Sample is not a plateau
out[flank_start + (i-flank_start)/2] = 1;
i = std::min(i+d_max_len, noutput_items-1);
}
STEP 2:header_payload_demux_impl can find the head data
case STATE_HEADER:
if (check_items_available(d_header_len, ninput_items,
noutput_items, nread)) {
copy_n_symbols(in, out_header, PORT_HEADER,
d_header_len);
STEP 3:
memcpy((void *) out, (void *) (in + d_gi * d_itemsize),
d_items_per_symbol * d_itemsize);

        I think may be the code have some errors.For example,the

data is cdef+abcdef,the cyclic prefix is cdef,and the data is abcdef.
According step1,the output will ouput 0010000000.Because the sch&cox
will output a plateau in the cyclic prefix,namely the index
corresponding to the cdefa will be the plateau.The source code output 1
in the middle.Namely the index corresponding to the middle “e” output 1.
According to the step 2 and step3, we copy the
d_items_per_symbol * d_itemsize bytes after (in + d_gi * d_itemsize) to
the output.In the example, cdef+abcdef,the d_gi is 4. (in + d_gi *
d_itemsize) will correspond to the c.Then output d_items_per_symbol is
6,then output cdef…But the true head data is abcdef,so I think maybe
the source code have some problems.Is my understanding right?
Thank you.
Best regards,
zswx

2

On 31.08.2015 07:15, zs wrote:

       According to the step 2 and step3, we copy the

d_items_per_symbol * d_itemsize bytes after (in + d_gi * d_itemsize) to
the output.In the example, cdef+abcdef,the d_gi is 4. (in + d_gi *
d_itemsize) will correspond to the c.Then output d_items_per_symbol is
6,then output cdef…But the true head data is abcdef,so I think maybe
the source code have some problems.Is my understanding right?

Not sure I’m understanding you correctly, but maybe these factoids help
you:

  • If you check out ofdm_rx.grc, you’ll see a couple of delays that try
    and align things.
  • The qa_ofdm_*.py files check the OFDM stuff pretty exhaustively, and
    you might be interested in qa_ofdm_sync_sc_cfb.py in particular.
  • If we start copying at the first ‘e’ in your example, the receiver
    will still work fine (minus a phase offset).

Cheers,
Martin