The effects of fiber nonlinearity in Coherent Optical Orthogonal Frequency-Division Multiplexing (CO-OFDM)
transmission, such as self-phase modulation (SPM) and cross-phase modulation (XPM), are a major concern. In this
paper, we investigate the use of RF-Pilot (RFP) based nonlinearity compensation scheme in frequency domain to
compensate for fiber nonlinearity in a coherent OFDM optical system. It shows that the RFP-based compensation
scheme has superiority over a conventional pilot-based compensation scheme at FEC threshold.
Combination of quadrature amplitude modulation with coherent detection is attractive for optical transmission systems,
since it permits an increase of data rate without increasing the symbol rate or the required bandwidth. 16-point
Quadrature Amplitude Modulation (16-QAM) is most interesting in this context. In-phase (I) and quadrature (Q) signals
transmit 2 bit each. Together with polarization division multiplex this amounts to 8 bit/symbol.
2.5 Gbit/s synchronous coherent 16-QAM data is transmitted and received in a realtime intradyne setup with BER below
FEC (7% overhead) threshold. A phase noise tolerant feedforward carrier recovery concept with hardware-efficient
implementation was tested. Transmission was error-free in a back-to-back electrical test for various PRBS lengths. The
carrier recovery does not contain any feedback loop and is therefore highly tolerant against laser phase noise.