The data encryption is accomplished after source coding and before channel coding. After channel coding the data id is digitally modulated. So, there is no reason for not encrypting the data before transmission for any digital communication system.
This is from the conceptual point of view. From the realization point of view one can use also the SIMULINK to implement the AES algorithm.
For implementing AES using SIMULINK please refer to the link:http://textlab.io/doc/118108/matlab-simulink-implementation-of-aes-algorithm-for-image...
Encryption is indepaendant of the transmission media. You can sendAES encrypted data through PLC, twisted pair, athmosphere or otical fiber. PLC is on teh Physical layer and encryption is on the presentation layer in OSI model.