Realtime implementation of square 16-QAM transmission system

摘要

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