Physical Layer Anonymous Communications in Trustworthy 6G: Fundamentals, Recent Advances, and Future Trends

Along with performance enhancement, sixth-generation (6G) technology must address various threats associated with the explosion of accessible user information. When exchanging confidential personal data in emerging applications, user privacy becomes a fundamental attribute of 6G trustworthiness. As a critical design principle in the field of privacy, in this article, we discuss anonymous communication designs at the physical (PHY) layer, a complementary defense line to higher-layer anonymizers. We first discuss the recent progress on sender detection where the users' PHY unique characteristics, such as channel and radio frequency signatures, are used to break the users' anonymity. Then, we review the design of anonymous precoding, which is used to mask the sender's multiple characteristics during communication to preserve anonymity while also paying attention to the tradeoff among anonymity, multiplexing, and diversity performance. Finally, we discuss some open technical issues in the area of PHY anonymous communications. Applicability to standard transceiver architecture and compatibility with classical optimization algorithms make anonymous precoding a promising PHY privacy enabler in trustworthy 6G.