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Journal of Lightwave Technology

Quantum Noise-Assisted Coherent Radio-Over-Fiber Cipher System for Secure Optical Fronthaul and Microwave Wireless Links

Ken Tanizawa, Fumio Futami

Published Apr 20, 2020• 20 citations
Publisher

Abstract

This article reports on a symmetric-key direct-data encryption technique that directly protects the interception of optical and microwave signals from physical layer in a coherent analog radio-over-fiber (RoF) system. Secrecy is realized by signal masking by quantum (shot) noise. The quantum noise masking for the encryption is achieved by converting data into extremely high-order signals at an optical frequency. After signal transmission over an optical fronthaul link, the frequency of the encrypted signal is shifted to a microwave frequency via an optical heterodyne process using a local oscillator for wireless transmission. The effect of the quantum noise masking is naturally and seamlessly kept in the heterodyne process. We experimentally demonstrate 12 Gbit/s coherent RoF cipher systems utilizing the quantum noise masking for a 30 GHz wireless band. Adequate signal quality and high security against interception with sufficient quantum noise masking are simultaneously achieved in the optical fronthaul and microwave wireless links.

DOI
10.1109/jlt.2020.2987213

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This article reports on a symmetric-key direct-data encryption technique that directly protects the interception of optical and microwave signals from physical layer in a coherent analog radio-over-fiber (RoF) system. Secrecy is realized by signal masking by quantum (shot) noise. The quantum noise masking for the encryption is achieved by converting data into extremely high-order signals at an optical frequency. After signal transmission over an optical fronthaul link, the frequency of the encrypted signal is shifted to a microwave frequency via an optical heterodyne process using a local oscillator for wireless transmission. The effect of the quantum noise masking is naturally and seamlessly kept in the heterodyne process. We experimentally demonstrate 12 Gbit/s coherent RoF cipher systems utilizing the quantum noise masking for a 30 GHz wireless band. Adequate signal quality and high security against interception with sufficient quantum noise masking are simultaneously achieved in the optical fronthaul and microwave wireless links.
Security enhancement is a major challenge in future 6G wireless networks. In this study, we have introduced a symmetric-key encryption system, which utilizes multilevel signaling, for the security of microwave signals against wireless interception. The system achieves quantum-noise signal masking at microwave frequencies via optical-tomicrowave frequency conversion based on the homodyne process, thus providing high signal security. We show experimental demonstrations of quantum-noise randomized phase-shift keying modulation/quadrature amplitude modulation cipher generation at a frequency of ~GHz via the intensity modulation/direct detection analog intermediate frequency over fiber transmission. The proposed system simultaneously achieves microwave signal delivery over a fiber link and signal encryption based on sufficient quantum-noise signal masking for wireless communication systems.
This study proposes an intensity modulation/direct detection intermediate frequency-over-fiber (IFoF) system that generates a microwave orthogonal frequency-division multiplexing (OFDM) quantum-noise randomized phase-shift-keying (PSK) cipher. The system directly prevents wireless interception by an eavesdropper without a private key. We theoretically analyze the system and derive a fundamental tradeoff between the security and signal quality at an intermediate frequency (IF). The system achieves balanced security and signal quality. We experimentally demonstrate 4.09-Gbit/s PSK cipher generation at an IF of 3.6 GHz. The cipher has 216phase levels, and more than 100 adjacent signals are masked by quantum noise. The quantum-noise signal masking imposes irreducible uncertainty on illegitimate reception of the cipher, and a symbol error ratio (SER) that an eavesdropper can reach approaches 1. A lower bound of the SER is promised because truly random signal masking by quantum noise cannot be reduced. The error vector magnitude of the IF signals after decryption is maintained at a low value for wireless transmission. The cipher directly and effectively protects the wireless transmission of personal data from interception.
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.