w2z_link.m

function [rxBits, rxSyms, txSyms] = w2z_link(txBits, SNR, isChannelIdea)
    % This function simulates WiFi-to-ZigBee transmission of WEBee.

    persistent freqDiff;
    persistent OSR;
    persistent FFTLength;
    persistent numSampPerWiFiSym;

    if isempty(freqDiff)
        subcrWidth = 0.3125; % Bandwith of a WiFi subcarrier (MHz).
        numShiftSubc = 0; % 16; % # of shift subcarriers. Set to zero for now.
        freqDiff = (subcrWidth * numShiftSubc) / 20; % MHz
    end

    if isempty(OSR)
        OSR = 20;
    end

    if isempty(FFTLength)
        FFTLength = 64;
    end

    %% WiFi sender link
    % Waveform configuration
    cfgNonHT = wlanNonHTConfig;
    cfgNonHT.Modulation = 'OFDM';
    cfgNonHT.ChannelBandwidth = 'CBW20'; % Channel bandwidth
    cfgNonHT.NumTransmitAntennas = 1; % Number of transmit antennas
    cfgNonHT.MCS = 7; % Modulation and coding scheme
    cfgNonHT.PSDULength = 3885;
    cfgNonHT.BandwidthOperation = 'Static';

    %% Channel
    % Channel settings
    Channel = wlanTGnChannel;
    Channel.DelayProfile = 'Model-B';
    Channel.CarrierFrequency = 2.44e9;
    Channel.NumTransmitAntennas = cfgNonHT.NumTransmitAntennas;
    Channel.NumReceiveAntennas = 1;
    Channel.TransmitReceiveDistance = 5;
    Channel.LargeScaleFadingEffect = 'None'; % 'Pathloss'?
    fsWiFi = wlanSampleRate(cfgNonHT);
    Channel.SampleRate = fsWiFi;

    % Idea ZigBee signals that WiFi aimts to generate.
    [objZigBeeSignal, txSyms, numPkt] = zigbee_txer(txBits, OSR, true);

    % Shift the ideal ZigBee signal in the frequency domain so that it can be processed
    % from WiFi's perspective.
    freqShiftOptZ2W = repmat(exp(1i * 2 * pi * -freqDiff * (0:size(objZigBeeSignal, 1) - 1))', 1, size(objZigBeeSignal, 2));
    objZigBeeSignal = objZigBeeSignal .* freqShiftOptZ2W;

    % Generate waveform of WiFi packet payload.
    txSignal = zeros(size(objZigBeeSignal, 1) - OSR / 2, size(objZigBeeSignal, 2));
    field = 'NonHT-Data';

    if isempty(numSampPerWiFiSym)
        numSampPerWiFiSym = 80;
    end

    numWiFisym = ceil(size(txSignal, 1) ./ numSampPerWiFiSym);

    for ithPkt = 1:1:size(objZigBeeSignal, 2)
        focusObjZigBeeSignal = reshape(objZigBeeSignal(1:1:end - (OSR / 2), ithPkt), numSampPerWiFiSym, numWiFisym);
        focusObjZigBeeSignal(1:1:16, :) = []; % Remove the CP part.
        rxSignalWiFi = fftshift(fft(focusObjZigBeeSignal, [], 1), 1);
        info = wlanNonHTOFDMInfo(field, cfgNonHT);
        % Demodulation based on MATLAB WLAN toolbox.
        rxSignalWiFi = rxSignalWiFi(info.ActiveFFTIndices, :);
        rxSignalWiFi = rxSignalWiFi .* sqrt(FFTLength);
        rxSignalWiFi = rxSignalWiFi(info.DataIndices, :);
        rxSignalWiFi(1:1:(size(info.DataIndices, 1) / 2 - 5), :) = 0;
        rxSignalWiFi((end - size(info.DataIndices, 1) / 2 + 6):1:end, :) = 0;

        % WEBee part.
        [txBitsW, scramInit] = webeeBitGenerator(rxSignalWiFi, cfgNonHT);

        % Send out emulate signals.
        cfgNonHT.PSDULength = length(txBitsW) / 8; % Payload length in bytes
        txSignal(:, ithPkt) = wlanNonHTData(txBitsW, cfgNonHT, scramInit);
    end

    % Normalization so that the power of each signal is 1
    for pkt_ith = 1:1:size(txSignal, 2)
        tmp = txSignal(:, pkt_ith);
        txSignal(:, pkt_ith) = tmp / sqrt(sum(abs(tmp.^2), 1)/size(tmp, 1));
    end

    if isChannelIdea == true
        rxSignal = txSignal;
    else
%         % Create an instance of the AWGN channel per SNR point simulated
%         awgnChannel = comm.AWGNChannel;
%         awgnChannel.NoiseMethod = 'Signal to noise ratio (SNR)';
%         awgnChannel.SignalPower = 1 / Channel.NumReceiveAntennas;
%         % Account for noise energy in nulls so the SNR is defined per
%         % active subcarrier
%         awgnChannel.SNR = SNR - 10 * log10(FFTLength);
%         reset(Channel); % Reset channel for different realization
%         txSignal = [txSignal; zeros(50, cfgNonHT.NumTransmitAntennas)];
%         rxSignal = Channel(txSignal);
%         rxSignal = awgnChannel(rxSignal);
%         rxSignal = rxSignal(1:end-50, 1);

%         rxSignal = awgn(txSignal(1:end-50, 1), SNR, 'measured');
        rxSignal = awgn(txSignal, SNR, 'measured');
    end

    % Shift emulated ZigBee signal in the frequency domain so that it can be processed
    % from ZigBee's perspective.
    freqShiftOptW2Z = repmat(exp(1i * 2 * pi * freqDiff * (0:size(rxSignal, 1) - 1))', 1, size(rxSignal, 2));
    rxSignal = rxSignal .* freqShiftOptW2Z;
    rxSignal = lowpass(rxSignal, 1.1e6, fsWiFi, 'Steepness', 0.99);

    % ZigBee receiver link
    rxSignal = rxSignal(1:5:end, :);
    [rxBits, rxSyms] = zigbee_rxer(rxSignal, OSR/5, true, numPkt);
    rxBits = reshape(rxBits, 1, size(rxBits, 1) * size(rxBits, 2));
    rxSyms = reshape(rxSyms, 1, size(rxSyms, 1) * size(rxSyms, 2));
    txSyms = reshape(txSyms, 1, size(txSyms, 1) * size(txSyms, 2));