Abstract
Quadrature amplitude modulation (QAM) allows increase of transmission capacity while keeping the symbol rate [1]. But QAM signals show a high sensitivity to amplitude and phase noise. In particular, amplitude noise does not only enlarge the overlap between the signal states, but due to the Gordon-Mollenauer effect in the transmission line [2], it is also converted into nonlinear phase noise which is the major limiting factor for phase-shift-keying (PSK) transmission [3]. Phase-preserving amplitude regeneration was considered to reduce amplitude fluctuations and, therefore, the origin of nonlinear phase noise for binary [4] and quadrature PSK [5]. It has already been demonstrated that the periodic behaviour of the power transfer characteristic of a nonlinear amplifying loop mirror (NALM) can create multiple flat, so-called plateau regions, which can be used also for QAM signal regeneration [6].
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