function txspectrum(scenario, bw, fs, fshift, L)
if nargin == 0
    fprintf('Use this function to plot your Transmit spectrum\n');
    fprintf('\n');
    fprintf('First argument is to run 5 example scenarios.  \n');
    fprintf('txpsectrum(1) or txspectrum(2)...to plot and study various scenarios\n');
    fprintf('\n');
    fprintf('For custom numbers function can be called as follows\n');
    fprintf('txspectrum([], data bandwidth, sampling frequency,shift or carrier frequency, x-axis plotting limit(optional arugment)\n');
elseif (nargin == 1)
    %% System Parameters
    sys          = [];
    sys.scenario = scenario;
    switch sys.scenario
        case 1
            % Use Nyquist Zone 1 to tx signal at center frequency = 1 GHz with Fs = 4
            % GHz.
            sys.fs     = 4e9;
            sys.bw     = 400e6;
            sys.fshift = 1e9;
            L          = 7;
            sys.xlim   = [-L L];
            sys.xticks = -L : L;
        case 2
            % Use Nyquist Zone 2 to tx signal at center frequency = 3 GHz with Fs = 4
            % GHz.
            sys.fs     = 4e9;
            sys.bw     = 400e6;
            sys.fshift = -1e9;
            L          = 7;
            sys.xlim   = [-L L];
            sys.xticks = -L : L;
        case 3
            % This is a difficult scenario. We want to tx the signal at center frequency
            % = 1.8 GHz. But with Fs = 4 GHz, there is also a copy of the spectrum
            % centered at 2.2 GHz touching the spectrum at 1.8 GHz. Unless we have a
            % ideal brick-wall analog lowpass filter, we cannot remove the spectrum at
            % 2.2 GHz.
            %
            sys.fs     = 4e9;
            sys.bw     = 400e6;
            sys.fshift = 1.8e9;
            L          = 7;
            sys.xlim   = [-L L];
            sys.xticks = sort([-L : -3, -1 : 1, 3:L, 1.8, -1.8, 2.2, -2.2]);
        case 4
            % This scenario is a solution to scenario 3. There are two key ideas:
            %
            %   (1) When converting complex baseband signal to real bandpass signal, use
            %       a value of fshift that is large enough to avoid overlap between the
            %       spectra at +ve frequency and -ve frequency. In our example, the
            %       complex baseband signal has frequency content from -200 MHz to 200
            %       MHz, so we can pick fshift = -400 MHz and the spectrum at -400 MHz
            %       does not overlap with the spectrum at 400 MHz.
            %
            %   (2) Digital signal's spectrum repeats every Fs. Use this fact to create
            %       spectrum at higher frequency. We have the desired spectrum at -0.4
            %       GHz and so use Fs = 2.2 GHz to create the spectrum at -0.4 GHz + 2.2
            %       GHz = 1.8 GHz.
            %
            % In this scenario, we use a lower sampling rate of 2.2 GHz and use Nyquist
            % zone 2 to transmit our signal at 1.8 GHz. Now there is a resonable gap
            % between the spectrum at 1.8 GHz and the spectrum at 2.6 GHz and we can
            % rely on a bandpass filter to filter everything except the the spectrum at
            % 1.8 GHz.
            %
            sys.fs     = 2.2e9;
            sys.bw     = 400e6;
            sys.fshift = -0.4e9;
            L          = 5;
            sys.xlim   = [-L L];
            sys.xticks = sort([-L : L, 1.8, -1.8, 2.2, -2.2, 0.4, -0.4, 2.6, -2.6]);
        case 5
            % Better solution than scenario 4.
            sys.fs     = 2.6e9;
            sys.bw     = 400e6;
            sys.fshift = -0.8e9;
            L          = 5;
            sys.xlim   = [-L L];
            sys.xticks = sort([-L : L, 1.8, -1.8, 2.2, -2.2, 0.4, -0.4, 2.6, -2.6]);
        otherwise
            error('Invalid sys.scenario');
    end
elseif (nargin == 4)   %if(nargin == 1)

    %% System Parameters
    sys          = [];
    sys.scenario = scenario;
    sys.bw = bw;
    sys.fs = fs;
    sys.fshift = fshift;
    L = 7;
    sys.xlim   = [-L L];
    sys.xticks = -L : L;
elseif (nargin == 5)
    %% System Parameters
    sys          = [];
    sys.scenario = scenario;
    sys.bw = bw;
    sys.fs = fs;
    sys.fshift = fshift;
    sys.xlim   = [-L L];
    sys.xticks = -L : L;
end
sys.xunit = 'GHz';


%%
if nargin > 0
    if isempty(findobj('Number', sys.scenario))
        figure(sys.scenario)
        set(gcf, 'Position', [227         140        1231         617])
    else
        figure(sys.scenario)
        clf
    end


    %%
    subplot(311)
    xlim(sys.xlim);
    xticks(sys.xticks);
    set(gca, 'Position', [0.0463    0.7521    0.9277    0.18])
    f1 = 0 + (-4:4)*sys.fs;
    plotspectrum('asym', f1, sys.bw, 'r', sys.xunit);
    s = sprintf('Complex Baseband (Fs = %.2f GHz. BW = %d MHz)', ...
        sys.fs/1e9, sys.bw/1e6);
    plotannotations(sys.fs, s)
    addtext3(-4.5, 1.15, '|Z(f)|')


    %%
    subplot(312)
    xlim(sys.xlim);
    xticks(sys.xticks);
    set(gca, 'Position', [0.0463    0.4221    0.9277    0.18])
    f1 = f1 + sys.fshift;
    plotspectrum('asym', f1, sys.bw, 'r', sys.xunit);
    s = sprintf('Complex Bandpass (Fs = %.2f GHz. ', sys.fs/1e9);
    if abs(sys.fshift) >= 1e9
        s = [s, sprintf('fshift = %.2f GHz)', sys.fshift/1e9)];
    else
        s = [s, sprintf('fshift = %d MHz)', sys.fshift/1e6)];
    end
    plotannotations(sys.fs, s)
    addtext3(-4.5, 1.15, '|U(f)|')


    %%
    subplot(313)
    xlim(sys.xlim);
    xticks(sys.xticks);
    set(gca, 'Position', [0.0463    0.0921    0.9277    0.18])
    f2 = -f1;
    plotspectrum('asym',     f1, sys.bw, 'r', sys.xunit);
    plotspectrum('asymflip', f2, sys.bw, 'r', sys.xunit);
    s = sprintf('Real Bandpass (Fs = %.2f GHz)', sys.fs/1e9);
    plotannotations(sys.fs, s)
    addtext3(-4.5, 1.15, '|S(f)| = |Y(f)|')


end
end


% ------------------------------------------------------------------------------
%                           Helper Functions
% ------------------------------------------------------------------------------


function plotannotations(fs, titlestr)

%% Plot DC.
xline(0, 'LineWidth', 3, 'Color', 'g', 'LineStyle', '--')
y = 0.9;
addtext1(0, y, '0')

%% Plot fs/2.
xline(fs/2/1e9, 'LineWidth', 3, 'Color', 'g', 'LineStyle', '--')
xline(-fs/2/1e9, 'LineWidth', 3, 'Color', 'g', 'LineStyle', '--')
addtext1(0 + fs/2/1e9, y, 'fs/2')
addtext1(0 - fs/2/1e9, y, '-fs/2')

%% Plot fs.
xline(fs/1e9, 'LineWidth', 3, 'Color', 'g', 'LineStyle', '--')
xline(-fs/1e9, 'LineWidth', 3, 'Color', 'g', 'LineStyle', '--')
addtext1(0 + fs/1e9, y, 'fs')
addtext1(0 - fs/1e9, y, '-fs')

%% Plot 3*fs/2.
xline(fs*1.5/1e9, 'LineWidth', 3, 'Color', 'g', 'LineStyle', '--')
xline(-fs*1.5/1e9, 'LineWidth', 3, 'Color', 'g', 'LineStyle', '--')
addtext1(0 + 1.5*fs/1e9, y, '3*fs/2')
addtext1(0 - 1.5*fs/1e9, y, '-3*fs/2')

%% Plot ...
ntext4(0 + 1.6*fs/1e9, 0.6, '...', 'FontSize', 30, 'FontWeight', 'bold');
ntext4(0 - 1.7*fs/1e9, 0.6, '...', 'FontSize', 30, 'FontWeight', 'bold');

%%
addtext2(fs/4/1e9,            0.7, {'Nyquist', 'Zone 1'})
addtext2(fs/4/1e9 + fs/2/1e9, 0.7, {'Nyquist', 'Zone 2'})
% addtext2(fs/4/1e9 + fs/1e9,   0.7, {'Nyquist', 'Zone 3'})

%% Title.
title(titlestr, 'FontSize', 12);

%%
h = gca;
h.XAxis.FontSize = 11;

end


function addtext1(x, y, t)

h                     = ntext4(x, y, t);
h.FontSize            = 15;
h.FontWeight          = 'bold';
h.HorizontalAlignment = 'center';

end


function addtext2(x, y, t)

h                     = ntext4(x, y, t);
h.FontSize            = 15;
h.FontWeight          = 'bold';
h.Color               = [0    0.4470    0.7410];
h.HorizontalAlignment = 'center';

end


function addtext3(x, y, t)

h                     = ntext4(x, y, t);
h.FontSize            = 20;
h.FontWeight          = 'bold';
h.Color               = 'r';
h.HorizontalAlignment = 'center';

end