Short-Time Spot Communication Using COTS Stereo Speaker

Abstract

In this paper, we construct the spot communication method using a stereo speaker. In our proposed method, one speaker transmits differential binary phase-shift keying (DBPSK) symbols for communication, and the other speaker transmits sine waves whose frequency is close to that of the DBPSK symbols for jamming. Inside the spot, the jamming signal is canceled by the orthogonality of the communication symbols and jamming signals. Outside the spot, the jamming signal inverts the differential phase of the communication symbols, which prevents normal DBPSK demodulation rendering the communication symbols concealed in the area. This enables correct demodulation only inside the spot. In the conventional method using a stereo speaker, the effect of the jamming signal is weak; thus, transmitting at least 11 symbols is required. In our proposed method, the effective jamming using the interference between sine waves of close frequencies enables spot communication by transmitting only 3 symbols. In the evaluation experiments, we generate the spot from -10 to 20 degrees in a real environment and measure the signals at 9 positions inside and outside the spot, from -40 to 40 degrees at 10 degree intervals. The measurements were conducted 100 times at each position. Through these experiments, we confirmed that demodulation errors occur only outside the spot. In the measurement results, some systematic errors of the differential phase were observed inside the spot, where theoretically this error should not occur. To discuss this error, we conducted additional experiments and revealed the causes.