In this paper, we characterize the fundamental limits of a communication system with three users (i.e., three transmitters) and a single receiver where communication from two covert users must remain undetectable to an external warden. Our results show a tradeoff between the highest rates that are simultaneously achievable for the three users. They further show that the presence of a non-covert user in the system can enhance the capacities of the covert users under stringent secret-key constraints. To derive our fundamental limits, we provide an information-theoretic converse proof and present a coding scheme that achieves the performance of our converse result. Our coding scheme is based on multiplexing different code phases, which seems to be essential to exhaust the entire tradeoff region between the rates at the covert and the two non-covert users. This property is reminiscent of the setup with multiple non-covert users, where multiplexing is also required to exhaust the entire rate-region.

翻译：本文刻画了一个包含三用户（即三个发射机）与单个接收机的通信系统的基本极限，其中两个隐蔽用户与外部监视者之间的通信必须保持不可检测性。研究结果表明，三用户同时可达的最高速率之间存在权衡关系，并进一步揭示系统中非隐蔽用户的存在能够在严格密钥约束下提升隐蔽用户的信道容量。为推导这一基本极限，我们给出了信息论层面的逆定理证明，并设计了一种能够实现该逆性能的编码方案。该编码方案基于多码相复用技术，这一方法对于完全遍历隐蔽用户与两个非隐蔽用户之间的完整速率折衷区域至关重要。该特性与需通过复用实现全速率区域覆盖的多非隐蔽用户场景具有相似性。