Data or control signals are communicated over a three phase power cable supplying power from a surface location to a motor/pump assembly located within a wellbore utilizing modulated radio frequency signals. The radio frequency signals may be impressed on the power cable through physical taps to the power cable conductors or by reactive coupling to the power cable. The transmission frequency is selected from a range of frequencies which propagate through the motor windings and up the power cable with sufficient amplitude to be received and processed. The modulated RF signal may be transmitted concurrently with the three phase power on the power cable, and simultaneous bidirectional communications between the surface and downhole locations may be supported utilizing, for example, discrete frequencies for transmission in different directions. A network of RF transceivers or nodes may be situated at various locations along the wellbore and the motor/pump assembly to gather information about conditions at different points (e.g., below the motor/pump assembly, above the motor/pump assembly, and at the wellhead of a subsea borehole), with transmission on the power cable shared among the nodes through a spread spectrum and/or multiple access protocol.