## 1 Introduction
developed a method for PPM-based communication; however, this method should be applied for communication based on the special multi-wire cables [54; 55; 56]. [PERSON] et al. proposed an approach to data transmission along a cylindrical pipe [57]. In their work, a simple modulation scheme chirp-on-off keying (Chirp-OOK) is applied, where the bit '1' is represented by a chirp signal with selected frequencies and the bit '0' is represented by a null signal [57; 58]. With the implementation of a low-power hardware system, data transmission was successfully achieved. However, due to the effect of channel frequency selectivity [58], the available frequencies for chirp-signals in Chirp-OOK varies with the channels. Additionally, [PERSON] et al. investigated the potential of phase-modulated signals conveying information in a planar waveguide with dispersion compensation based on the guided-wave modes group velocity [59]. A field experiment conducted on a buried water-filled pipe also verified the feasibility of data transmission through an actual pipe [60]. According to the characteristics of a stress wave propagating along a cylindrical pipe, the signal attenuation can undergo steep change with a slight frequency deviation. Thus, PPM is an appropriate modulation scheme for stress wave communication along pipeline structures, since there is no need to consider the available and suitable signal frequency ranges and the waveform refocusing method is verified to be useful to combat the dispersion. PPM modulation has already been widely applied to the stress wave communication successfully in many different structures, such as pipeline structures, special multi-wire cables, among others. There is still no an easy-to-use system for the PPM modulation stress wave communication, to the authors' best knowledge. To address this challenge, a new easy-to-use stress wave PPM communication system, including the supporting circuit with a built-in amplifier, a touch type of piezoceramic transducer, and easy-to-use supporting software, is developed in this research in order to achieve data transmission along the pipeline structures. The contributions of our new stress wave PPM communication system are summarized in the following three aspects:  
1. To realize stress wave communication, the system integrates PPM modulation circuit and an amplifier to a stress wave generator that can produce high-voltage, modulated, information-containing waveforms. 2. To eliminate the need of an installing device, a new piezoceramic transducer can transmit stress waves through a simple contact with a pipe surface. 3. For the convenience of the end users, an easy-to-use user interface has been designed and implemented. End users can control the data to be transmitted through this easy-to-use interface. To implement this system, a PPM signal generator, a piezoceramic transducer, and a communication protocol, are designed and implemented. In our design, the piezoceramic generator is utilized to deliver modulated stress wave signals based on the communication protocol. The transducer can be adequately coupled with the transmission medium by a simple contact or touching, thus allowing for flexible usage. The motivation for developing such a piezoceramic transducer is for future robotics applications and the actuator design can be incorporated on a hand of a robot to enable stress wave communication upon \"touch.\" The rest of this paper is organized as follows. Section 2 illustrates the principle of the stress wave communication system design. Section 3 presents the design process of three key components. Section 4 describes the experiment set up and discusses experimental results. Concluding remarks and future works are summarized in Section 5.