Covert Communications Over Non-Orthogonal Multiple Overt Channels

The paper exploits overt information of non-orthogonal multiple access (NOMA) systems for camouflage to hide covert information. The optimum strategy of hiding information at the transmitter, accounting for the probability of the warden's detection error and the resulting spectral efficiency of the covert information under the covertness requirement, is determined. The results show that the optimum strategy of hiding the covert information is to superimpose it onto the overt NOMA information which is hardest-decoded, increasing to 1 the probability of detection error at the warden when the number of NOMA users accretes. This indicates that the multiplicity of NOMA users can be leveraged to hide the covert information in NOMA systems. In addition, various results prove the increase in the covert spectral efficiency with increasing the number of NOMA users while guaranteeing the probability of the warden's detection error close to 1.