decode_mib.m

function peak_out=decode_mib(peak,tfg);

% Try various combinations of antenna ports and frame timings to attempt
% to decode the MIB.

% Copyright 2012 Evrytania LLC (http://www.evrytania.com)
%
% Written by James Peroulas <james@evrytania.com>
%
% This program is free software: you can redistribute it and/or modify
% it under the terms of the GNU Affero General Public License as published by
% the Free Software Foundation, either version 3 of the License, or
% (at your option) any later version.
%
% This program is distributed in the hope that it will be useful,
% but WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
% GNU Affero General Public License for more details.
%
% You should have received a copy of the GNU Affero General Public License
% along with this program.  If not, see <http://www.gnu.org/licenses/>.

peak_out=peak;

% Local shortcuts
n_id_1=peak.n_id_1;
n_id_2=peak.n_id_2;
cp_type=peak.cp_type;

% Derive some values
n_ofdm=size(tfg,1);
if (strcmpi(cp_type,'normal'))
  n_symb_dl=7;
elseif (strcmpi(cp_type,'extended'))
  n_symb_dl=6;
else
 error('Check code...');
end
n_id_cell=n_id_2+3*n_id_1;

% Channel estimation
ce_tfg=NaN(n_ofdm,72,4);
[ce_tfg(:,:,1) np0]=chan_est(peak,tfg,0);
[ce_tfg(:,:,2) np1]=chan_est(peak,tfg,1);
[ce_tfg(:,:,3) np2]=chan_est(peak,tfg,2);
[ce_tfg(:,:,4) np3]=chan_est(peak,tfg,3);

% Try various numbers of ports and various frame timing offsets
%disp('Check code here!!!');
for frame_timing_guess=0:3
%for frame_timing_guess=3
  %frame_timing_guess
  % Extract the 4 frames that will be used to find the PBCH
  %ofdm_sym_set=frame_timing_guess*10*2*n_symb_dl+1:10*2*n_symb_dl*(frame_timing_guess+4);
  ofdm_sym_set=frame_timing_guess*10*2*n_symb_dl+1;
  ofdm_sym_set=ofdm_sym_set:ofdm_sym_set+3*10*2*n_symb_dl+2*n_symb_dl-1;
  tfg_try=tfg(ofdm_sym_set,:);
  ce_try=ce_tfg(ofdm_sym_set,:,:);

  % Extract symbols and channel estimates for the PBCH
  [pbch_sym pbch_ce]=pbch_extract(peak,tfg_try,ce_try);
  %load override_pbch_ce.mat

  found=0;
  %disp('Check code here!!!');
  for n_ports=[1 2 4]
  %for n_ports=2
    %n_ports
    if (n_ports==1)
      np=np0;
      gain=conj(pbch_ce(1,:))./absx2(pbch_ce(1,:));
      syms=pbch_sym.*gain;
      np=np*absx2(gain);

    elseif (n_ports==2)
      np_mean=mean([np0 np1]);
      syms=NaN(1,length(pbch_sym));
      np=NaN(1,length(pbch_sym));
      for t=1:2:length(syms)
        % http://en.wikipedia.org/wiki/Space-time_block_coding_based_transmit_diversity
        h1=mean(pbch_ce(1,t:t+1));
        h2=mean(pbch_ce(2,t:t+1));
        x1=pbch_sym(t);
        x2=pbch_sym(t+1);
        scale=sum(absx2([h1 h2]));
        syms(t)=(conj(h1)*x1+h2*conj(x2))/scale;
        syms(t+1)=conj((-conj(h2)*x1+h1*conj(x2))/scale);
        np(t)=(abs(h1)/scale)^2*np_mean+(abs(h2)/scale)^2*np_mean;
        np(t+1)=np(t);
      end
      % 3dB factor comes from precoding for transmit diversity
      syms=syms*sqrt(2);

    elseif (n_ports==4)
      np_mean=mean([np0 np1 np2 np3]);
      syms=NaN(1,length(pbch_sym));
      np=NaN(1,length(pbch_sym));
      for t=1:2:length(syms)
        % http://en.wikipedia.org/wiki/Space-time_block_coding_based_transmit_diversity
        if (mod(t,4)==1)
          h1=mean(pbch_ce(1,t:t+1));
          h2=mean(pbch_ce(3,t:t+1));
        else
          h1=mean(pbch_ce(2,t:t+1));
          h2=mean(pbch_ce(4,t:t+1));
        end
        x1=pbch_sym(t);
        x2=pbch_sym(t+1);
        scale=sum(absx2([h1 h2]));
        syms(t)=(conj(h1)*x1+h2*conj(x2))/scale;
        syms(t+1)=conj((-conj(h2)*x1+h1*conj(x2))/scale);
        np(t)=(abs(h1)/scale)^2*np_mean+(abs(h2)/scale)^2*np_mean;
        np(t+1)=np(t);
      end
      % 3dB factor comes from precoding for transmit diversity
      syms=syms*sqrt(2);

    else
      error('Check code...');
    end

    % Extract the bits
    e_est=deqam(syms,np,'QAM','LTE');
    % Unscramble
    scr=lte_pn(n_id_cell,length(e_est));
    e_est(scr==1)=1-e_est(scr==1);
    % Undo ratematching
    d_est=lte_conv_deratematch(e_est,40);
    % If identical CE smoothing is used, C code matches perfectly until
    % here.
    % Viterbi decode
    c_est=lte_conv_decode(d_est);
    % Calculate the received CRC
    crc_est=lte_calc_crc(c_est(1:24),16);
    %c_est(25:end)
    % Apply CRC mask
    if (n_ports==2)
      crc_est=1-crc_est;
    elseif (n_ports==4)
      crc_est(2:2:end)=1-crc_est(2:2:end);
    end
    %crc_est
    % Success?
    if (all(crc_est==c_est(25:end)))
      found=1;
      break;
    end

  end
  if (found==1)
    break;
  end

end

% Record the extracted data if the MIB was properly decoded
if (found)
  disp('Found MIB!!!');
  %frame_timing_guess
  %n_ports
  c_est(1:24)

  % Record system BW
  bw_packed=c_est(1)*4+c_est(2)*2+c_est(3);
  switch (bw_packed)
    case 0
      n_rb_dl=6;
    case 1
      n_rb_dl=15;
    case 2
      n_rb_dl=25;
    case 3
      n_rb_dl=50;
    case 4
      n_rb_dl=75;
    case 5
      n_rb_dl=100;
    otherwise
      n_rb_dl=NaN;
  end
  peak_out.n_rb_dl=n_rb_dl;

  % PHICH duration
  if (c_est(4))
    peak_out.phich_dur='extended';
  else
    peak_out.phich_dur='normal';
  end

  % PHICH res
  phich_res=c_est(5)*2+c_est(6);
  phich_res_map=[1/6 1/2 1 2];
  peak_out.phich_res=phich_res_map(phich_res);

  % Calculate the SFN of the first frame in the tfg variable.
  sfn_bits=c_est(7:14);
  sfn=0;
  for t=1:length(sfn_bits)
    sfn=2*sfn+sfn_bits(t);
  end
  sfn=sfn*4-frame_timing_guess;
  sfn=mod(sfn,1024);
  peak_out.sfn=sfn;

end