The transporter carrier wave was a subspace signal through which a transporter beam was transmitted. With both the Starfleet and Romulan military transporters operating on similar subspace frequencies, a Romulan engineer could with minor adjustments simulate a Federation carrier wave. This operation, though, was not perfect as it caused an increase in the matter to energy signal ratio. The increase was within operating parameters of the transporter.
In 2367, in his investigation into the death of Ambassador T'Pel, Data noted the increase. Upon delving further into the anomalous reading, he determined that the Romulans simultaneously transported T'Pel onto the IRW Devoras while transporting to the USS Enterprise-D a small amount of genetically similar material. This improbable feat was to cover up the fact that T'Pel was actually Sub-Commander Selok, who had been on an espionage mission within the Federation. (TNG: "Data's Day")