NB-IoT Cell Search and MIB Recovery

% Initialize NBIoTDownlinkWaveformGenerator object
%
% Use the 'rc' values of 'R.NB.5' and 'R.NB.6' for MIB decoding. Note that
% RMCs 'R.NB.5-1', 'R.NB.6-1' and 'R.NB.7' correspond to Non-Anchor type
% carriers which do not contain the required NPSS, NSSS and NPBCH.
rc = 'R.NB.5';
ngen = NBIoTDownlinkWaveformGenerator(rc);
ngen.Config.NNCellID = 120;
ngen.Config.NBRefP = 1;
ngen.Config.NFrame = 640;
ngen.Config.TotSubframes = 50;
ngen.Config.NPBCH.DataSource = 'MIB';

% Generate time domain NB-IoT waveform
[eNodeBOutput,grid,ofdmInfo] = generateWaveform(ngen);
sr = ofdmInfo.SamplingRate;

% Introduce a delay to simulate the unknown timing alignment associated
% with an arbitrary signal capture. This is compensated at the receiver
% while synchronizing the received signal.
delay = 50;
waveform = circshift(eNodeBOutput, delay);

% Initialize plots
if (~exist('channelFigure','var') || ~isvalid(channelFigure))
    channelFigure = figure('Visible','off');
end
[spectrumScope,synchCorrPlot,pdcchConstDiagram] = ...
    hSIB1RecoveryExamplePlots(channelFigure,sr);

% Display received signal spectrum
fprintf('\nPlotting received signal spectrum...\n');
spectrumScope(awgn(waveform, 100.0));

% Cell search
fprintf('\nPerforming cell search...\n');
alg.SSSDetection = 'PostFFT';
alg.MaxCellCount = 1;
[NNCellID, offset, peak] = hNBCellSearch(waveform, alg);
nbenb = struct;
nbenb.NNCellID = NNCellID;

[~,corr] = lteNBDLFrameOffset(setfield(nbenb,'OperationMode','Standalone'),waveform); %#ok<SFLD>

% As NB-IoT has similar waveform structure as LTE waveform, use functions
% pertaining to LTE to work with NB-IoT. For this purpose, create a
% cell-wide settings structure enb.
enb = nbenb;

% Plot NPSS/NSSS correlation and threshold
synchCorrPlot.YLimits = [0 max(corr(:))*1.1];
synchCorrPlot(corr);

% Perform timing synchronization
fprintf('Timing offset to frame start: %d samples\n',offset);
timesynced = waveform(1+offset:end,:);

% Show cell-wide settings
fprintf('Cell-wide settings after cell search:\n');
disp(nbenb);

% Frequency offset estimation
fprintf('\nPerforming frequency offset estimation...\n');
enb.NULRB = 6; % For frequency offset computation
enb.DuplexMode = 'FDD';
foffset = lteFrequencyOffset(enb,timesynced,0);
fprintf('Frequency offset: %0.3fHz\n',foffset);

% Compensating for frequency offset
freqsynced = lteFrequencyCorrect(enb,timesynced,foffset);

% Channel estimator configuration
cec.PilotAverage = 'UserDefined';     % Type of pilot averaging
cec.FreqWindow = 13;                  % Frequency window size
cec.TimeWindow = 9;                   % Time window size
cec.InterpType = 'Cubic';             % 2D interpolation type
cec.InterpWindow = 'Centered';        % Interpolation window type
cec.InterpWinSize = 1;                % Interpolation window size
cec.Reference = 'NRS';                % NB-IoT downlink channel estimation

% Assume 4 cell specific and 2 narrowband reference signals for initial
% decoding attempt. This ensures channel estimates are available for all
% reference signals
enb.CellRefP = 4;
enb.NBRefP = 2;
enb.NTxAnts = 2;
nbenb.NBRefP = 2;

fprintf('Performing OFDM demodulation...\n\n');
griddims = lteResourceGridSize(enb); % Resource grid dimensions
L = griddims(2);                     % Number of OFDM symbols in a subframe

enb.NBULSubcarrierSpacing = '15kHz'; % Required for NB-IoT downlink OFDM demodulation
rxgrid = lteSCFDMADemodulate(enb,freqsynced); % For OFDM demodulation in NB-IoT
if (isempty(rxgrid))
    fprintf('After timing synchronization, signal is shorter than one subframe so no further demodulation will be performed.\n');
    return;
end

% Perform channel estimation
enb.NSubframe = 0; % Initialize NSubframe for channel estimation
[hest, nest] = lteDLChannelEstimate(enb, cec, rxgrid(:,1:L,:));

% Decode the MIB
% Extract resource elements (REs) corresponding to the NPBCH from the first
% subframe across all receive antennas and channel estimates
separator = repmat('-',1,50);
fprintf('%s\n',separator);
fprintf('Performing MIB decoding...\n');
fprintf('%s\n\n',separator);
npbchIndices = lteNPBCHIndices(nbenb);
[npbchRx, npbchHest] = lteExtractResources(npbchIndices, ...
                              rxgrid(1:12,1:L,:), hest(:,1:L,:,:));

% Decode NPBCH
dstate = [];
[bchBits, dstate,npbchSymbols,nfmod64,mib,nbenb.NBRefP] = lteNPBCHDecode( ...
    nbenb, npbchRx, npbchHest, nest, dstate);

% Parse MIB bits
nbenb = hNBMIB(mib,nbenb);

if (nbenb.NBRefP == 0)
    fprintf('MIB decoding failed (nbenb.NBRefP=0).\n\n');
    return;
end

% Incorporate the nfmod64 value from the function lteNPBCHDecode, as
% the NFrame value established from the MIB is the SFN modulo 64 (it is
% stored in the MIB as floor(SFN/64)).
if nbenb.NBRefP % If NPBCH decoding is passed then set the NFrame value
    nbenb.NFrame = nbenb.NFrame+nfmod64;
end

if strcmpi(nbenb.OperationMode, 'Inband-SamePCI')
    nbenb.NCellID = NNCellID;
end

% Display cell-wide settings after MIB decoding
fprintf('Cell-wide settings after MIB decoding:\n');
disp(nbenb);