Design of Practical Scrambling Schemes for Physical-Layer Security

Abstract

In physical-layer security, coding and modulation are employed to prevent successful eavesdropping without the need for secret keys. Most promising coding approaches apply channel coding in combination with information-bit scrambling, which leads to error multiplication at the descrambling process. In this letter, the strategies of shift-register based scrambling and block-wise scrambling (using matrix multiplications) are assessed for a Gaussian wiretap channel scenario w.r.t. the security gap metric. We show that the former leads to an unequal protection of communicated message bits. This becomes apparent only upon the evaluation of individual bit positions’ error rates and security gaps. In contrast, the more general block scrambling enables schemes that provide equal protection. An optimized scrambling matrix design is presented, which yields the best error-rate performance. In addition to the theoretical examination of error propagation in the descrambling process, numerical results that combine low-density parity-check codes and scrambling support those findings.