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In-vehicle signal transmission requires dedicated communication cables such as twisted metal wires and optical fiber cables, which increase the weight and the volume of wiring harnesses in vehicles. Furthermore, the number of electronic control units (ECUs) and the other electronic devices in vehicles is increasing. As a result, in-vehicle networks are becoming more and more complex and unstable. In-vehicle power line communication (PLC) system has been proposed to carry out signal transmissions between ECUs via in-vehicle power lines. The in-vehicle PLC system simplifies in-vehicle networks and reduces the weight and the volume of wiring harnesses. To design communication systems, we analyze communication environments which are mainly characterized by propagation channels and additive noise. In this paper, the measurement and the analysis of frequency responses for in-vehicle power lines are presented to analyze the propagation channels. Thus we observe frequency-selective fading channels which have several notches in the frequency domain because of reflection of signals at impedance mismatching points. We also analyze the noise and the interference in order to design reliable communication systems. For this purpose, the measurement and analysis of the noise on in-vehicle power lines are presented. A lot of bursty impulsive noises are observed while actuators in vehicles are active. A time-frequency analysis is applied to the observed noise and we show time-varying frequency spectra of the observed noise. Finally, we discuss in-vehicle PLC schemes which are suitable for signal transmissions.