Abstract
The finite electrical bandwidth of digital coherent transceivers together
with limited wavelength division multiplexing (WDM) channel spacing constrains
the maximum forward error correction (FEC) overhead that can be advantageously
applied to fiber optic transmission systems. We investigate through simulations
of a 100 GbE system employing coherent polarization division multiplexed quaternary
phase shift keying, the impact of these bandwidth constraints on the optimal
code rate thereby minimizing the required optical signal-to-noise ratio (OSNR)
at the receiver. The optimum FEC overhead is found to increase linearly with
available bandwidth and the required OSNR is found to reduce for lower order
transponder filters. Fabrication of transmitters with a broader spectral response
than their receiver counterpart is found to be advantageous. The optimum electrical
bandwidth is found to be approximately half the WDM channel spacing for nonreturn
to zero (NRZ) transmission and slightly less for return to zero (RZ). There
is shown to be no performance advantage in using RZ over NRZ modulation formats
when the optimum FEC overhead and filter bandwidth is employed in each case.
Nonlinear simulations demonstrate that transpacific transmission is attainable
with a margin that increases as the spectral efficiency is relaxed.
© 2011 IEEE
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