FFT IFFT CP D/A LPF Re Im D/A LPF MZM MZM I Q 90 D Channel Channel 90 D A/D LPF Re A/D LPF Im FFT De CP FFT IFFT CP D/A LPF Re Im D/A LPF MZM MZM I Q 90 D Channel Channel 90 D A/D LPF Re A/D LPF Im FFT De CP Linear MMSE receiver ˆ$_{b}$ X M M M M M M Noise Noise Electrical transmitter Electrical-to-optical upconverter Fiber link  Optical-to-electrical downconverter Electrical receiver M M ˆ$_{a}$ X a X b X IFFT IFFT Figure 1: Block diagram for an optical DFT-spread OFDM system with PDM and coherent receiver. 0 1 2 3 4 5 6 7 8 9 10 −5 10 −4 10 −3 10 −2 10 −1 10 0 PAPR(dB) CCDF QPSK 16−QAM 16−QAM QPSK DFT−S−OFDM Clipped OFDM Unclipped OFDM QPSK 16−QAM Figure 2: Comparison of the complementary cumulative distribution function (CCDF) of the PAPR. 13