Toward Secure and High-Speed OFDM-PONs Leveraging Variable Multi-Band Chaotic Non-Orthogonal Matrices

The broadcasting nature of passive optical networks (PONs) makes the downstream data susceptible to illegal eavesdropping. Therefore, the physical-layer encryption in PONs has been gaining attention recently. This work proposes a novel variable multi-band chaotic non-orthogonal matrix (CNOM) encryption scheme for secure orthogonal frequency division multiplexing (OFDM) PONs. In the proposed variable scheme, the number and the size of sub-bands for each OFDM symbol are relaxed to be variable and entirely determined by the chaotic sequences to further increase the transmission and security performance, compared with the previous fixed scheme with only fixed sub-band number and size. Besides, to estimate the security performance of this type of variable encryption scheme, this work derives a new algorithm based on the generation function method. Thanks to the encryption exploiting an independent CNOM in each sub-band, a new dimension of non-orthogonality is explored, further enhancing the physical-layer security. In a proof-of-concept experiment with 10.6 Gb/s 4-quadrature amplitude modulation (QAM) transmission over a 20-km standard single-mode fiber, the impacts of sub-bands' variable number and size on the transmission and security performance were first investigated. Results show that (i) the transmission performance can be improved through the variable scheme compared to the fixed scheme when the number of sub-bands exceeds a critical value, and (ii) the upper bound of the security performance presented in the conventional fixed scheme can be easily broken by also enabling a variable number/size of sub-bands without degrading the transmission performance. Finally, the results of the proposed scheme show that there is similar to 3.3-dB sensitivity improvement compared with the conventional OFDM signals at the hard-decision-forward error correction (HD-FEC) threshold, in addition to the considerable enhancement of the security performance.