The dual-carrier signal is generated using two dual polarization
16QAM transmitters (28 Gbaud). Pulse shaping is performed digitally
with square-root-raised-cosine FIR filters (0.15 roll-off). The
lasers are assumed to be stable so that the carrier spacing remains
32 GHz during the simulated time window. Different input powers
are considered in order to optimize the system performance. Each
span consists of 80 km of SSMF and of an EDFA (4 dB NF) compensating
exactly for the span loss. The signal is stored after each span
(see ).
The signal is received using a polarization-diversity coherent
receiver including four ADCs working at 84 Gs (50 GHz bandwidth)
followed by a DSP unit
().
The signal processing includes the following steps:
Digital resampling (4 samples per symbol)
Joint frequency domain CD equalization
Carrier separation (performed by down- or upconverting the signal)
Low-pass filtering (downsampling to 2 samples per symbol)
Carrier frequency recovery
Adaptive filtering (MMA MIMO) to optimize the receiver response and reduce crosstalk between carriers
Carrier-phase estimation (blind phase search algorithm)
The system performance (BER) vs. input power is reported in
for 400 and 800 km transmission length.
