Re: [Discuss-gnuradio] 回复： references using in ofdm_equalizer_simpledfe

[Aditya Dhananjay]

Hi Tiankun,

To add on to what Martin said: In DFE, the channel state H_I on subcarrier i is updated everytime it receives a new symbol on that subcarrier. It doesn't really care of that symbol is a pilot or a data symbol. If it is a pilot, the channel estimate H_i is trivially calculated and updated. If it is a data symbol, then the nearest constellation point is found, and is then used to update H_i.

And as Martin mentioned, the pilot placement pattern in the packet is important to note. If I remember correctly, the reference implementation uses the "comb" type pilot patter. Look at Figure 1(b) in the link below. In the "comb" pilot pattern, the pilots are useless, as the subcarriers that contain the pilot symbols do not carry data symbols. You will need to use the "block" pilot pattern (look at Figure 1(a) below, and you will immediately see how DFE is applicable in this case.

http://scialert.net/fulltext/?doi=itj.2011.914.926&org=11

Of course, there are other types of pilot patterns, but you can ignore them for now. Good luck!

best,

aditya

On Fri, Sep 5, 2014 at 12:15 PM, Martin Braun <address@hidden> wrote:

It uses them. It will reset the channel state estimate every time it encounters a pilot symbol. See eg the qa codes for examples. If you want to make use of them in your modem, you need to put pilots on the same carriers as the data.

M

On 5 Sep 2014 11:01, "Tiankun Hu" <address@hidden> wrote:

Hi Aditya,

Thanks your time!

But how to use pilotes symbol's channel state to update data symbol's?

"ofdm_equalizer_simpledfe" is so simple, so it didn't  realize that, right?

Is there any example or paper can be for reference?

 

Thanks

Tiankun

------------------ 原始邮件 ------------------

发件人: "Aditya Dhananjay";<address@hidden>;

发送时间: 2014年9月4日(星期四) 晚上11:45

收件人: "Tiankun Hu"<address@hidden>;

抄送: "address@hidden"<address@hidden>;

主题: Re: [Discuss-gnuradio] references using in ofdm_equalizer_simpledfe

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

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