## 2 Related Works  
Currently, signal transmission technologies based on radio frequency (RF) from the 900 MHz to 2.4 GHz and are widely used in pipe inspection. However, the signal is not stable and the most important feature is the limited signal transmission distance due to the huge attenuation in the small diameter pipe. Apart from this, some other wireless solutions based on laser techniques have been investigated, as the laser beam possesses a special feature which can be described as point-to-point transmission. For stable communication in a pipe, the high directivity of the laser transmitter and receiver is important, however, it is difficult to realize with a mobile robot system. Moreover, the size of the laser generator and the heat sink is another critical factor which limits application in small diameter pipes. Infrared radiation (IR) is also commonly used as a communication approach for pipe inspection robots, but it cannot avoid drawbacks such as a low transmission rate, low anti-interference capability, etc. [11]. As one type of 5G communication technique, visible light communication (VLC), has been regarded as an alternative communication method to challenge existing wireless communication, especially in complex environments such as underground, underwater, and air. The utilization of the visible light band can drastically increase the available bandwidth for wireless communication. The other advantages such as high transmission rate, high energy efficiency, and strong anti-inference capability will also make this technology more competitive than the common wireless transmission strategies. In previous research, a robot chain system (RCS) based on VLRC and illuminated assessment for pipe inspection has been proposed. The robot system can not only achieve high-quality and low-loss communication with the external controller through the \"light signal relay\", but also can detect the adjacent distance by the illuminance-based evaluation approach [12]. However, the transmission of essential information such as image through the visible light has not been realized. There are many related studies on image transmission based on visible light. In 2013, [PERSON] et al. developed an integrated audio, video, and data transceiver system based on VLC. The real-time analog closed circuit television (CCTV) video signals and pulse-position modulated stereo audio signals could be transmitted by a white light emitting diode (LED) between a tablet and an android smartphone. Through VLC, the mobile equipment which installed visible light transceiver could access to the internet very easily [13]. In 2013, [PERSON] et al. developed a high-bandwidth wireless optical communication device called AquaOptical II. This system could transmit digital \(1288\times 964\) pixels images reliably through a unidirectional underwater channel of 50 m length. Besides, the excellent performance of real-time image acquisition and high-quality displaying under 200 m depth of water made it possible to have a wide range of applications such as underwater robot tele-operation and interactive remote seabed monitoring [14]. In 2016, [PERSON] et al. demonstrated their research results using on-board camera video transmission through a vehicle lamp. With the support of direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) technique, [PERSON] et al. finally realized the line of sight (LOS) inter-vehicular video transmission based on VLC with the data rate of 6.42 Mbps and lower latency than 32 ms in the experiments [15]. In 2018, [PERSON] et al. proposed and implemented an optical-acoustic hybrid network. In this network, optics provided good quality real-time video streaming while acoustic maintains a channel for the network topology and transmission control. The acoustic channel was also used for frame video delivery when the optical channel fails. [PERSON] et al.