We show experimentally and numerically that reducing the bandwidth of the sequence spectrum does not necessarily improve the propagation on a dispersive fibre. For instance an ideal duobinary coding-based transmission is strongly affected after only 65 km, while the conventional NRZ binary signal experiences a quite smaller penalty. However, adding only some small differences to this ideal case will not broaden the spectrum but may induce a dramatic improvement in the performance of the transmission. Therefore the spectrum bandwidth is not the relevant parameter to assess the transmission performance over a dispersive fibre.
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