function a_tag = gen_tag(m_transmitter, N_point_FFT);

%this function is to generate a 1*MG line vecter a_tag

index = 1 : m_transmitter * N_point_FFT;
for index_m = 1: m_transmitter
    for index_g = 1: N_point_FFT
        a_tag(1,(index_m - 1) * N_point_FFT + index_g) = ...
            exp(2j * pi *(index_m - 1 + (index_g - 1) * m_transmitter) / (m_transmitter * N_point_FFT));
    end;
end;