Breakthrough the communication bottleneck between sky and underwater Open Access

The undersea domain imposes well-known limits on communication due to the high dielectric loss factor of sea water.¹ In particular, it hinders the development of two-way wireless communication between sky and underwater. While acoustic waves have been widely used for underwater communication, they attenuated significantly in air.² Thus, there was an insurmountable gap between sky and underwater communication for a long time. Fortunately, Wang et al. realized one-way wireless signal transmission from sky to underwater based on the thermoacoustic effect (TE),^3,4 which converts electromagnetic energy into thermal energy and further produces ultrasound waves through the thermal expansion.^5–9 After that, Tonolini and Adib presented a new communication technology, called translational acoustic-radiofrequency communication (TARF), which achieved one-way wireless signal transmission from underwater to sky based on microwave vibration measurement (MVM).¹⁰ In this study, we experimentally demonstrate for the first time that combined TE with MVM can realize two-way communication between sky and underwater.

The homemade experimental system used in this study is schematically shown in Fig. 1. Wireless signal transmission from sky to underwater is based on the thermoacoustic induced acoustic waves generated in the water–air interface with pulsed microwaves. The pulsed microwaves were emitted from a homemade microwave generator (center frequency: 3.0 GHz, peak power: 60 kW, and pulse duration: 550 ns) and then coupled to the dipole antenna via a semi-rigid coaxial cable (3.0 m long with 2.2 dB insertion loss), as previously described.¹¹ For effective thermoacoustic signal (TAS) evoking, the antenna is 1 cm away from the water interface, and at the same time, saline water is used. The microwave generator (MG) can be externally triggered for signal coding. A 2.25 MHz transducer (V323-SU, Olympus) was immersed at 10 cm underwater to collect the TAS. The received TAS was first amplified by a homemade pre-amplifier (bandwidth: 0.2 MHz–2.0 MHz and gain: 55 dB) and then digitally acquired (EPH1026, Chengdu EnPHT Technology, China) at a sampling rate of 10 MHz.

On the other hand, the wireless signal transmission from underwater to sky is based on detection of the displacement information in the water–air interface induced by modulated ultrasound signals.¹² In this study, the underwater system sent the modulated ultrasound signal via a 3.5 MHz transducer (SU-102, Sonic concepts) placed at 10 cm underwater, where the signal comes from a function generator (33220A, Agilent). The function generator (FG) is triggered by a micro-controller unit (Arduino UNO R3). In order to obtain effective vibration on the water–air interface, the FG was set to work in burst mode for enhancing the energy of ultrasonic waves through multiple cycles. At the same time, a radiofrequency (RF) amplifier (325LA, ENI) was utilized to further increase the output power of the transducer. With the help of microwave vibration measurement (MVM) technology,^13,14 the transmitting antenna transmits continuous waves, while the receiving antenna records the amplitude of the reflected microwaves that carry information with different displacements on the air–water interface. In this paper, when effective displacement appeared, MVM can be performed through a pair of Vivaldi transceiver antennas¹⁵ to capture the information carried by the water–air interface vibration. More precisely, the MVM system transmits a continuous microwave through one of the Vivaldi antennas. When the microwave encounters a water–air interface with tiny vibration, another Vivaldi antenna records the reflected microwave signals that carry the vibration information simultaneously. Among them, the generation and detection of microwave signals are completed by a vector network analyzer (E5071C, Agilent).

For two-way communication between sky and underwater, a binary code was compiled by a PC according to the information that needs to be transformed and a burst with this information was emitted by the FG, which was modulated by a Micro-Controller Unit (MCU) according to the binary code. After that, the burst signal was sent to the MG and RF amplifier for signal transmission, respectively.

Figures 2(a) and 2(b) show the picture of the two-way communication system. As indicated in Fig. 2(c), transmitting signals from sky to underwater, TAS was successfully excited by the MG and acquired by the underwater transducer. While transmitting signals from underwater to sky, an un-modulated continuous sine signal is generated by the FG in burst mode, where the pulse frequency is 3.0 MHz with a duty cycle of 50% and the number of pulses is 50 000 cycles per 0.2 s and the peak to peak voltage (Vp-p) is 3.0 V. In the signal-receiving part, the antenna is placed at a height of 30 cm from the water–air interface. Once the transducer sends signals from underwater, a continuous microwave is transmitted from the transmitting antenna, where the amplitude of the reflected microwave is synchronously recorded by the receiving antenna. Figure 2(d) shows two seconds of recorded vibration signals through the receiving antenna. By analyzing the obtained data in Fig. 2(d), the transmitted information from water to sky can be demodulated. Therefore, a communication protocol that carries different information by setting different repetition frequencies will be applied for underwater to sky communication and vice versa.

To further evaluate the feasibility of the proposed two-way communication method, five letters (UESTC) are compiled into five eight-bit binary codes through the UTF-8 protocol, which are “01 110 101,” “01 100 101,” “01 110 011,” “01 110 100,” and “01 100 011,” respectively. According to the conclusion from Fig. 2(c), based on TE, the underwater transducer can detect the pulsed microwave induced TAS in the water–air interface, which is displayed as a pulsed ultrasound signal. According to this phenomenon, the information carried by the pulsed microwave signals, such as repetition frequency, can be obtained based on the detected TAS. In this study, “0” is defined as the repetition frequency of 5 Hz and “1” is defined as the repetition frequency of 25 Hz. This protocol is written in the MCU. When the PC sends an eight-bit binary code (such as “01 110 101”) to the MCU, the MCU will output a burst signal according to this protocol to trigger the MG through FG. And then, the pulsed microwave signal carries the information to the water–air interface modulated via the repetition frequency and further generates the corresponding TAS. After the TAS was captured by the underwater transducer, the signal shown in Fig. 3(a) was collected through a data acquisition card. Among them, Figs. 3(b) and 3(c) shows the real TAS when it receives “1” and “0,” respectively, and the binary code can be obtained according to the protocol as mentioned above. Finally, the information conveyed by the sky system can be realized by decoding.

Similar to the method of sky-to-water transmitting, according to the conclusion from Fig. 2(d), “0” is defined as the repetition frequency of 0.5 Hz and “1” is defined as the repetition frequency of 2.5 Hz. The other settings of the FG were un-changed. Here, the MCU is used to trigger the FG to realize the repetition frequency modulation under different information. Once the transducer generates the ultrasound signal, the transmitting antenna in the reception part starts emitting a continuous microwave with a frequency of 8.0 GHz and the other antenna continuously records echo signals synchronously. Figure 4 shows a part of the recorded microwave’s amplitude reflected from the water–air interface when vibration appeared. The binary code can be recovered by processing the recorded data through the Fourier transform and digital filter. Finally, the corresponding string information from underwater can be decoded by the sky system.

In conclusion, we have successfully realized two-way communication between sky and underwater by combined TE with MVM. However, in the face of a complex marine environment, there are still improvements needed. First, the frequency, the pulse duration, the peak power of the MG, and the polarization, gain, and directivity of the antenna needs to be optimized for effective TAS generation. Furthermore, the relationship between the underwater emitted ultrasound power and induced vibration amplitude on the water–air interface must be calculated. Finally, due to the influence of sea surface fluctuation, it is necessary to investigate the received signal demodulation algorithm of the received MVM signal. To achieve a high sensitivity of the MVM, a smaller wavelength in transmitting and phase detection in receiving is currently under investigation in our lab. Nonetheless, this study has demonstrated the potential that TE combined with MVM has the potential to be used for two-way wireless communication between sky and underwater.