function y = R_d_func(t, range, velocity)
    global c f_n T_r;

    ti = t - (2 / c) * r(t, range, velocity);
    p = rem(ti, T_r); % p = t - nT_r
    n = round((ti - p) / T_r);

    if (n < 0)
        n = 0;
    end

    y = R_x_func(t, range, velocity) * exp(1j * -2 * pi * f_n(n + 1) * (t - n * T_r));
end