Experimental Investigation of Information Bit Scrambling for Physical-Layer Security in Coherent Fiber-Optic Systems

In fiber-optic transmission systems, security against eavesdroppers is of increasing importance. Resource-efficient methods of physical-layer security (PLS) are particularly attractive, as they allow for secure optical transmission without requiring secret cryptographic keys known to both communication parties. Combining conventional forward-error correction (FEC) coding with information bit scrambling is a way to achieve PLS. For security, a linear feed-forward shift register is employed at the transmitter side as information bit scrambler, which necessitates the use of a corresponding (self-synchronized) linear feedback shift register to recover the data at the receiver side. Therefore, residual post-FEC errors in the eavesdropped signal create an error avalanche in the eavesdropper’s descrambler. We experimentally assess the achievable security gaps by carrying out back-to-back system experiments using a polarization-division multiplexing (PDM) digital-coherent optical system setup operated at a symbol rate of 64 GBd with offline processing. The security gaps of the scrambling method are compared for quadrature phase shift keying (PDM-04QAM), 16ary quadrature amplitude modulation (PDM-16QAM) and PDM-64QAM using conventional error-correcting low-density parity-check (LDPC) codes. This results in up to 640 Gb/s net secure data rates. Compared to our previous experimental investigations of special punctured LDPC codes (targeting both, reliability and security), the combination of conventional LDPC codes with information bit scrambling yields significantly improved (reduced) security gaps.