Analyzing Physical Layer Security of Antenna Subset Modulation as Block Encryption Ciphers

In this paper, a novel framework for analyzing Physical Layer Security (PLS) of Directional Modulation (DM) techniques has been introduced. The proposed framework maps the concepts of PLS techniques to cryptographic techniques, enabling the analysis of DM techniques as block encryption ciphers. The relevance of the proposed framework has been shown by applying it on Antenna Subset Modulation (ASM). After appropriate physical layer mappings, the encryption strength of ASM is analyzed using National Institute of Standards and Technology's Statistical Test Suite (NIST's STS). The performance of ASM is benchmarked against strong block encryption cipher of Advanced Encryption Standard (AES) using data types of image, text, and audio. A new metric, namely Physical Layer Randomness (PLR), has been introduced for direct comparison of encryption strength of PLS techniques to that of cryptographic techniques. The analysis shows that Optimized Antenna Subset Selection (OASS) that reduced average Side-Lobe Levels (SLLs) and improved Symbol Error Rate (SER), has rather adverse effects on encryption strength of ASM. Furthermore, it has been found that scrambling the selection of antenna subsets imparts negligible improvement in PLR.