Re: [Discuss-gnuradio] references using in ofdm_equalizer_simpledfe

[Aditya Dhananjay]

On Thu, Sep 4, 2014 at 11:29 AM, Tiankun Hu <address@hidden> wrote:

Hi Aditya,
What's use of pilot symbols in "ofdm_equalizer_simpledfe" ?
I just found pilot symbols update their carrier's channel state, but didn't find what's the use of these pilot channel state, seems them has no relate with data symbol, data symbol also only use themselves to update channel state.


                for (int i = 0; i < n_sym; i++) {
                    for (int k = 0; k < d_fft_len; k++) {
                        if (!d_occupied_carriers[k]) {
                            continue;
                        }
                        if (!d_pilot_carriers.empty() && d_pilot_carriers[d_pilot_carr_set][k]) {
======>         //didn't find any use of these channel state in pilot carriers   <=========
                           //seems has not relate with data symbol
                            d_channel_state[k] = d_alpha * d_channel_state[k]
                                    + (1-d_alpha) * frame[i*d_fft_len + k]
                                    / d_pilot_symbols[d_pilot_carr_set][k];
                            frame[i*d_fft_len+k] = d_pilot_symbols[d_pilot_carr_set][k];
                        } else {
                            sym_eq = frame[i*d_fft_len+k] / d_channel_state[k];
                            d_constellation->
                                 map_to_points(d_constellation->
                                        decision_maker(&sym_eq),  &sym_est);
                            d_channel_state[k] = d_alpha * d_channel_state[k]
                                + (1-d_alpha) * frame[i*d_fft_len + k] / sym_est;
                            frame[i*d_fft_len+k] = sym_est;
                        }
                    }
                    if (!d_pilot_carriers.empty()) {
                        d_pilot_carr_set = (d_pilot_carr_set + 1) % d_pilot_carriers.size();
                    }
                }

​Hello Tiankun,

That's a good question. The answer depends on how you want to implement DFE -- and there are a couple of options.

a) Ignore pilots altogether. This is what I had described in my earlier email.

b) Since pilots are known symbols, if a pilot is encountered, the new channel estimate will always be correct. We know the received symbol r_i and the expected symbol E_i, so it is trivial to calculate the channel estimate H_i. However if you ignore the pilots, then Doppler or residual CFO can make the decoding on a non-pilot symbol incorrect, leading to an incorrect estimate of H_i propagating to all subsequent symbol indices on that subcarrier 'i'.

Hope that helps.

Best,

Aditya