Characterization of the confidentiality of a green time reversal communication system: Experimental measurement of the spy BER sink

Time Reversal (TR) is a multiple-input single-output (MISO) pre-filtering technique that allows a signal to be sent towards a target antenna by focusing the wave at the receiver both in the time and in the space domains. Recently, TR has been advertised for both green communications and confidential communications. This paper proposes an experimental characterization of the confidentiality of an indoor MISO-TR communication. The transmit system is made of 16 transmit antennas distributed within a room and the data stream is transmitted with a 256-Quadratic Amplitude Modulation (256QAM). The received signal is demodulated by the target user but also by a spy or eavesdropper antenna, both using a Single Tap Receiver. The Bit Error Rate (BER) achieved by the spy is then evaluated for variable positions of the spy nearby the target. The "spy BER" is plotted as a function of the spy-to-target distance. This experimental measurement clearly shows the presence of a spy BER "sink" centered on the target.