% FMin, FMax - bound of transmission channel
% F - carrier frequency
% B - baud rate
% N - samples per FMax
% bits - transmission data
function samples = modulation_qpsk(FMin, FMax, F, B, N, bits)
    % samples per baud
    k = ceil(FMax * N / B);

    % convers bits to QPSK symbols
    symbols = bits_to_symbols_qpsk(bits);

    % samples up - shift spectrum
    symbols = symbols_up(symbols, k);

    % shaping filter
    symbols = nyquist_filter(symbols, k, 4, 0.5);

    % show symbols spectrum
    if 0
        spectrum = abs(fft(symbols));
        spectrum = dB(spectrum, max(spectrum));
        spectrum = circshift(spectrum, floor(length(spectrum) / 2));
        hold on;
        plot(linspace(-pi(), pi(), length(spectrum)), spectrum)
        hold off;
    end

    % QAM modulation
    t = linspace(0, length(symbols), length(symbols));
    p = 2 * pi() * (t / (FMax * N)) * F;
    %samples = real(symbols.') .* cos(p) + imag(symbols.') .* sin(p);
    ñ = symbols.' .* exp(complex(0, -p));
    samples = real(ñ);
    hold on;

    if 1
        % show modulation samples
%        plot(real(symbols));
%        plot(imag(symbols));
        plot(samples);
        wavplay(samples, FMax * N);
    else
        %add MAX and MIN frequences
        %samples = samples + cos(2 * pi() * (t / (FMax * N)) * FMin);
        %samples = samples + cos(2 * pi() * (t / (FMax * N)) * FMax);

        % show modulation spectrum
        spectrum = abs(fft(samples));
        spectrum = dB(spectrum, max(spectrum));
        spectrum = circshift(spectrum.', floor(length(spectrum) / 2));
        plot(linspace(-pi(), pi(), length(spectrum)), spectrum);
    end
    hold off;