ULTIMATE CAPACITY OF A LASER-DIODE IN TRANSPORTING MULTICHANNEL M-QAMSIGNALS

In this paper, we investigate the ultimate M-ary quadrature amplitude modulated (M-QAM) channel capacity of a laser diode which is limited b y the laser clipping induced nonlinear distortions, Our study includes a spectral analysis, a complete system simulation, and an experiment which used up to 70 channels of vector arbitrary waveform synthesizer generated quadrature phase shift keyed (QPSK) or 16-QAM signals to mod ulate an isolated/cooled distributed-feedback (DFB) laser and two unis olated/uncooled Fabry-Perot (FP) lasers, respectively, Our analytical results show that for an upstream laser diode, over 1000 QPSK channels or 170 16-QAM channels can be delivered, even in the presence of a hi gh relative intensity noise (RIN) of -115 dB/Hz, However, these high c apacities are reduced significantly when we consider the effect of col lision-based medium access control (MAC) protocols, We found that, in the worst case condition (collisions occur in all but one channels), t he ultimate QPSK channel capacity of an upstream laser diode is dramat ically reduced from over 1000 to 125 for eight collisions/channel. The se results have important implications to systems transporting frequen cy-stacked return-path bands with or without collision-based MAC chann els, As regard to the ultimate capacity of a down-stream laser diode w ith a RIN level of -135 dB/Hz, we found that as high as 600 and 128 ch annels of 64-QAM and 256-QAM signals (equivalent to 3600 and 1152 chan nels of MPEG-II live video signals) can be transported, respectively.