A system comprising: an external transmitter configured to transmit a first far field radiative signal at a first frequency and a second far field radiative signal at a second frequency different from the first frequency; a first implantable medical device (106, 206, 306, 406, 506, 606, 706, 806) comprising a first receiver, a first charge storage device, and a first therapy unit, wherein the first receiver is configured to receive the first far field radiative signal from the external transmitter and to charge the first charge storage device using the first far field radiative signal, the first therapy delivery unit powered by the first charge storage device and configured to deliver a first therapy to a first target tissue of a patient, wherein the first therapy includes delivery of a first electrical stimulation signal to the first target tissue; and a second implantable medical device (106, 206, 306, 406, 506, 606, 706, 806) comprising a second receiver, a second charge storage device, and a second therapy unit, wherein the second receiver is configured to receive the second far field radiative signal at the second frequency different from the first frequency from the external transmitter and to charge the second charge storage device using the second far field radiative signal, the second therapy delivery unit powered by the second charge storage device and configured to deliver a second therapy to a second target tissue of the patient, wherein the second therapy includes delivery of a second electrical stimulation signal to the second target tissue, wherein the external transmitter is configured to transmit the first far field radiative signal and the second far field radiative signal according to a treatment protocol, characterised in that the treatment protocol determines when to transmit the first far field charging signal and the second far field charging signal. an external transmitter configured to transmit a first far field radiative signal at a first frequency and a second far field radiative signal at a second frequency different from the first frequency; a first implantable medical device (106, 206, 306, 406, 506, 606, 706, 806) comprising a first receiver, a first charge storage device, and a first therapy unit, wherein the first receiver is configured to receive the first far field radiative signal from the external transmitter and to charge the first charge storage device using the first far field radiative signal, the first therapy delivery unit powered by the first charge storage device and configured to deliver a first therapy to a first target tissue of a patient, wherein the first therapy includes delivery of a first electrical stimulation signal to the first target tissue; and a second implantable medical device (106, 206, 306, 406, 506, 606, 706, 806) comprising a second receiver, a second charge storage device, and a second therapy unit, wherein the second receiver is configured to receive the second far field radiative signal at the second frequency different from the first frequency from the external transmitter and to charge the second charge storage device using the second far field radiative signal, the second therapy delivery unit powered by the second charge storage device and configured to deliver a second therapy to a second target tissue of the patient, wherein the second therapy includes delivery of a second electrical stimulation signal to the second target tissue, wherein the external transmitter is configured to transmit the first far field radiative signal and the second far field radiative signal according to a treatment protocol, characterised in that the treatment protocol determines when to transmit the first far field charging signal and the second far field charging signal.