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Journal of Lightwave Technology

All-Digital Device-Compatible Real-Time Physical Layer Secure Transmission Based on Delta-Sigma Modulation

Linsheng Zhong, Ruiyan Zhao, Tian Qiu, Yuanxiang Wang, Hanwen Luo, Xiaoxiao Dai, Mengfan Cheng, Lei Deng, Deming Liu, Shixiang Wang, Kailin Gao, Peng Yang, Bin Zhang, Zhiwen Fan, Du Ping, Liang Mei, Junbo Xu, Yaqin Wang, Qi Yang

Published Oct 14, 2024• 5 citations
Publisher

Abstract

A novel physical layer digital encryption optical transmission method based on delta-sigma modulation is proposed and implemented using field programmable gate arrays (FPGA) for real-time transceivers. This approach converts ultra-high-order ciphertext, encrypted by a first-layer pseudorandom matrix, into 2-level signals via delta-sigma modulator (DSM), enabling all-digital transceiver operation with commercial optical transmission network. By employing a dynamically perturbed parameter matrix, the quantization noise introduced by the DSM is randomized, achieving a secondary masking of the ultra-high-order ciphertext. This method shares conceptual security principles with quantum noise stream cipher (QNSC) but focuses on enhancing device compatibility for physical layer secure optical transmission systems and removes the necessity for additional clock links often required by QNSC and most digital domain secure transmission technologies. In the experiment, the proposed system demonstrated bidirectional quantization noise masking physical layer secure transmission on a coarse wavelength division multiplexing (CWDM) grid, achieving a plaintext capacity of 8 × 10 Gb/s. Subsequently, real-time long-haul transmission was achieved using commercial coherent optical terminal unit (OTU4E) modules. In a 1000 km link, a 96-grid system covering the entire C-band was employed, with 9 grids measured at equal intervals across the band. Each measured grid occupied 50 GHz frequency range and achieved a plaintext transmission rate of 24.88 Gbps. This demonstrates that a potential capacity-distance product of 2.4 Pb/s·km can be achieved.

DOI
10.1109/jlt.2024.3480703

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A novel physical layer digital encryption optical transmission method based on delta-sigma modulation is proposed and implemented using field programmable gate arrays (FPGA) for real-time transceivers. This approach converts ultra-high-order ciphertext, encrypted by a first-layer pseudorandom matrix, into 2-level signals via delta-sigma modulator (DSM), enabling all-digital transceiver operation with commercial optical transmission network. By employing a dynamically perturbed parameter matrix, the quantization noise introduced by the DSM is randomized, achieving a secondary masking of the ultra-high-order ciphertext. This method shares conceptual security principles with quantum noise stream cipher (QNSC) but focuses on enhancing device compatibility for physical layer secure optical transmission systems and removes the necessity for additional clock links often required by QNSC and most digital domain secure transmission technologies. In the experiment, the proposed system demonstrated bidirectional quantization noise masking physical layer secure transmission on a coarse wavelength division multiplexing (CWDM) grid, achieving a plaintext capacity of 8 × 10 Gb/s. Subsequently, real-time long-haul transmission was achieved using commercial coherent optical terminal unit (OTU4E) modules. In a 1000 km link, a 96-grid system covering the entire C-band was employed, with 9 grids measured at equal intervals across the band. Each measured grid occupied 50 GHz frequency range and achieved a plaintext transmission rate of 24.88 Gbps. This demonstrates that a potential capacity-distance product of 2.4 Pb/s·km can be achieved.
We propose and experimentally study a novel, to the best of our knowledge, quantum noise stream cipher (QNSC) secure transmission scheme based on the delta-sigma modulation (DSM) technique. The cooperation of the QNSC and DSM mechanisms makes it possible to transmit an ultrahigh-order encrypted signal in the non-return-to-zero (NRZ) on-off keying (OOK) format. The delivery of the NRZ OOK waveform over the fiber link allows us to send and receive signals using digital ports, instead of high-speed and high-resolution digital-to-analog converters (DACs) and analog-to-digital converters (ADCs) in conventional QNSC systems. Meanwhile, clock synchronization can be achieved by using a simple clock data recovery algorithm. The extra clock signal transmission link in conventional QNSC systems is no longer needed. The proposed scheme is also compatible with wavelength division multiplexing (WDM) systems. In this work, 4 × 12.9 Gbit/s plaintext is encrypted to a 65,536-level QNSC signal and then transmitted over a 10-km standard single-mode fiber. The transmitter and receiver are established by commercial 100G QSFP28 optical modules with clock data recovery. This proposed scheme can be easily deployed in commercial systems due to its minimalist implementation architecture and relatively low hardware cost.
Abstract Data security is a key feature of future communications networks. Physical layer introduces rich physical mechanisms to increase the complexity of deciphering and provides extensive protection, but faces challenges in compatibility with commercial systems. Quantum noise stream cipher (QNSC) has been proposed as a promising solution to overcome this problem by fusing the stream cryptography regime with the quantum noise masking physical mechanism. However, it has limitations in terms of digital to analog conversion and clock data synchronization of ultra-high-order ciphertext as well as flexible control of masking noise. Here we report a 147.9-Gbps device-compatible quadrature amplitude modulation (QAM) QNSC secure scheme over 75-km fiber. Thanks to delta-sigma modulator, the transmission of 220 × 220-order QAM-QNSC signal are established through the low-order digital signal. We develop a theoretical model for flexibly regulating the transmission rate and security performance. Broadband optical chaos introduces true randomness and acts on the masking noise.
In this paper, we propose a novel physical layer security optical transmission scheme utilizing randomized quantization noise. The proposed approach encrypts low-order plaintext signals into ultra-high-order ciphertext using principles similar to quantum noise stream cipher (QNSC). While, the ultra-dense quadrature amplitude modulation (QAM) ciphertext waveforms are masked by intrinsic quantization noise. The combination of digital delta-sigma quantization and analog chaotic random scrambling not only produces randomized quantization noise, but also naturally supports one-time processing of masking ciphertext and generating keystream. Whereby, the in-band quantization noise (IBN) conceals nearby ciphertext levels bolstering security, and the out-of-band quantization noise (OOBN) is digitized to generate keystream using the Toeplitz hashing extractor. Experimental results show that 256-QAM plaintext signals were securely transmitted over standard single-mode fiber: 163-Gbps signals over 400 km, and 79.7-Gbps signals over 1800 km. To evaluate system performance, theoretical models for signal-to-noise ratio (SNR), bit error rate (BER), and number of masked signals (NMS) are derived as functions of the oversampling ratio (OSR). Our findings reveal a trade-off between transmission performance and security performance. Our results confirm that this scrambling effectively eliminates the correlation between the masking noise components. Toeplitz hashing extractor can effectively reduce the complexity of keystream generation and obtain a source-independent random keystream.
We describe in detail our recent demonstration of a 10 Tbit/s secure physical layer transmission that we achieved by using digital coherent QAM quantum noise stream cipher (QNSC) and injection-locked WDM techniques. We used an FPGA-based transmitter and receiver to demonstrate a 165 channel polarization-multiplexed WDM 5 Gbaud 128 QAM/QNSC (70 Gbit/s) on-line transmission over 160 km with a spectral efficiency of 6 bit/s/Hz. In the present system, the original 128 QAM data were encrypted in a 1024 × 1024 QAM format using basis information. The encrypted signal was then masked by a large ASE noise, which reduced the detection “success” probability for an eavesdropper to 0.13% for each symbol. Furthermore, the multiplicity of the original QAM data and the seed keys used to generate the basis information were arbitrarily changed with time, which makes the decryption much more difficult.
We present a symmetric-key direct data encryption technique utilizing signal masking by quantum noise, called Y-00 quantum stream cipher. Physical encryption of phase-shift-keying (PSK) Y-00 quantum stream cipher is achieved by our proposed extremely high-order phase modulation. Two cascaded phase modulators are driven with two synchronized digital-to-analog converters for coarse-to-fine modulations, resulting in a record 217 phase levels for the quantum noise masking. Decryption of the cipher is implemented in digital signal processing after intra-dyne coherent detection with a free-running local oscillator. We experimentally demonstrate transmission of 10-Gbaud digital-coherent PSK Y-00 cipher with 217 levels over a 240-km field-installed single-mode fiber.