  Next-generation high-speed passive optical network (HS-PON) transceivers
supporting 25, 50 and 100 Gb/s are under the early stage of their
standardization process. One key aspect of this process is the choice of the
best modulation format. To this end, performance comparisons among several
modulation formats against different physical constraints have been presented
in literature and are still being carried out. In our contribution, we
performed an exhaustive analysis on the impact of transceivers electrical
frequency response shape on the performance of 2-levels pulse amplitude
modulation (PAM-2), 4-levels PAM (PAM-4), electrical and optical duobinary
modulation formats with adaptive equalizer at the receiver side. We show by
means of numerical simulations that the specification of the typically used
-3dB bandwidth is not sufficient, since also out-of-band electrical frequency
response specifications (such as the -20dB bandwidth) has a huge impact on the
performance of the analyzed modulation formats. The normalized graphs given at
the end of the paper in terms of -3dB and -20 dB bandwidths can thus be useful
for the design of the next generation of HS-PON transceivers.
