Covert NOMA in Uplink: A Two-User Framework

This paper pioneers the exploitation of power domain superposition to embed covert messages within overt ones in a two-user uplink Non-Orthogonal Multiple Access (NOMA) system. This eliminates the need for a dedicated covert trans-mitter, enhancing practicality. Our method thwarts conventional radiometer detection at Willie, necessitating Likelihood Ratio Testing (LRT), which we evaluate under optimized overt transmission rates and imperfect Successive Interference Cancellation (SIC) scenarios at Willie. We then minimize Willie's detection error probability by optimizing his detection thresholds and subsequently maximize his minimum DEP by exploiting the user's transmit power and overt-covert power allocation. We show that covert communication is safe if the warden, Willie, doesn't decode any overt message and converges to 1 under high transmission power at users.