Figure 8

Illustration of a typical QPSK data modulation with 16-QAM QEPS encryption using displacement operator \(\hat{d}(\alpha )\) for a back-to-back scheme without fibres. Fig. (a) is a constellation with \(\hat{d}(\alpha )\) encryption and then direction coherent detection without decryption using \(\hat{d}(-\alpha )\), and Fig. (b) denotes the constellation after \(\hat{d}(-\alpha )\) decryption from Fig. (a). Fig. (c) is a constellation diagram applying DSP directly to Fig. (a). Fig. (d) is a constellation diagram applying DSP directly to Fig. (b). Figs. (A), (B), (C), (D) are the corresponding constellation diagrams at 80 km, respectively