Aiming at the constraints in cross-medium wireless communication and according to the characteristics of atmospheric and oceanic communication, the laser-induced acoustic communication adopting time slot-hopping pulse position modulation (PPM) is proposed to achieve cross-medium signal transmission. The mechanism of optoacoustic effect is studied. Meanwhile, the time- and frequency-domain characteristics of the laser-induced sound in the underwater conditions are analyzed. Combining atmospheric optical wireless communication with underwater acoustic communication and reasonably selecting time slot combination for hopping, the reliability and security for information transmission can be effectively improved. Based on the pulsed laser-induced sound, a time slot-hopping PPM communication link from air to underwater is established, where field programmable gate array (FPGA) is adopted to achieve coding and decoding. Tests for information transmission by frames are accomplished. Note that each frame has one-byte data. Using time slot-hopping technology can effectively reduce the interception probability of the transmitted signal.
The interaction of laser pulse with gas, liquid and solid can induce acoustic waves. Firstly, according to the basic theory of laser-induced acoustic waves in liquid, including the effects of laser characteristics, liquid characteristics and incidence angle on optoacoustic energy conversion efficiency as well as excitation mechanism, a technical methodology for cross-medium communication from air to water is introduced in this paper, where the pulsating acoustic source radiates radially outward in the form of spherical wave (Fig. 1). Subsquently, the PPM format with variable time slots is used. Framing with two pilot pulses and one-byte data, three frames with different time slots are presented (Fig. 3). Finally, the coding and decoding of time slot-hopping PPM is achieved by FPGA, and then the experimental test platform based on laser-induced acoustic waves is established for data transmission and analysis (Fig.7).
By the experimental analysis of the time- and frequency-domain characteristics of laser-induced acoustic waves (Fig. 6), it has been shown that the time-domain waveform has many peaks, and the sound pulse gradually decays to disappear. By Fourier transform, the spectral characteristic is obtained, wherein the peak frequency is about 30 kHz. The components with frequencies greater than 100 kHz decay rapidly, and the energy is concentrated at the components with lower frequencies. The noise is regarded as an additive white Gaussian noise (AWGN), whose amplitude is much lower than that of the laser-induced sound pulse. The technologies of time slot-hopping PPM and framing with one-byte data are adopted. A combination of 256PPM, 16PPM and 4PPM is considered and the received waveform is obtained for the case that the optical energy arriving at the water surface is greater than 15 mJ, as shown in Fig. 8, wherein the top one represents the waveform detected and amplified by a hydrophone and the bottom one is a shaped signal as the input of the FPGA for decoding. The relationship between the data rate and frame length is discussed by comparisons among the three modulation orders (i.e., 8, 4 and 2, respectively). The data rate depends much on the modulation order and the time slot duration of PPM. Assuming that the laser source meets the requirements of pulse repetition frequency and response time, a data rate of 5 kbit/s could be achieved in the laser-induced acoustic communication with a time slot duration of 10 μs. In experiments, the time slot duration is set as 10 ms. Experimental results show that the symbol error rate could be lower than 10-5 if the optical energy arriving at the water surface is greater than 25 mJ. Combining more PPM formats with different orders is beneficial to reduce the probability of interception.
In the cross-medium communication from air to water based on the laser-induced sound, the optoacoustic integration is used to give full play to the advantages of atmospheric laser communication and underwater acoustic communication. Due to the controllable position of the induced acoustic pulse by airborne laser remote excitation, the flexibility and initiative of communication operations could be greatly improved. The characteristics of laser-induced acoustic source can meet the requirement of the frequency range in underwater acoustic communication. Using time slot-hopping technology and reasonably selecting time slots and time slot combination can effectively enhance the anti-interception ability, which could provide reference for the air-to-water communication.
