Concurrent data transmission through PSTN by CDMA

This paper proposes a novel method to perform concurrent data transmission through the public switched telephone network (PSTN). More specifically, a hidden channel is created by removing the voice components below the perceptual masking threshold. The method of code division multiple access (CDMA) is then used to transmit data through the hidden channel. At the receiver, since digital data is imperceptibly embedded into the hidden channel, conventional telephone sets are able to access the voice signal without additional equipments. Meanwhile, a customized telephone set can extract the hidden data and provide concurrent services. Experimental results show that a data rate at hundreds bits per second can be achieved with a bit error rate (BER) of 10^-3