## 1 Introduction  
Human body communication (HBC) is a wireless communication method that uses the human body as part of the signal transmission medium, and it is expected to be among the technologies used in wireless body area networks [1]. In the IEEE (Institute of Electrical and Electronics Engineers) 802.15.6 standard, HBC was adopted as one of the three physical layers for the body area network. The other two layers are narrowband and ultra-wide band [2]. In comparison with other wireless communication technologies such as Bluetooth and Zigbee, HBC is advantageous in terms of its communication secrecy, low power consumption, and intuitive tactile operation. Because of these advantages, HBC is expected to be utilized in wearable devices for health care systems [3]. There are two transmission mechanisms of HBC: galvanic and capacitive coupling [4]. In galvanic HBC, the signal frequency is between 100 kHz and 10 MHz and the current flowing through the human body between the transmitter and the receiver contributes to the transmission [5; 6]. In capacitive coupling, the signal frequency is between 100 kHz and 100 MHz and the current through the human body and the electric field around the human body contribute to the transmission [7; 8; 9; 10]. Detailed studies on the electric field distribution and its contribution to capacitive coupling have been carried out using calculation methods such as the finite element method (FEM) and the finite-difference time-domain method (FDTD) [11; 12]. Although the conditions of the contact between the electrodes and the human body greatly influence the transmission characteristic of HBC, the electrode contact condition has not yet been standardized. The shape of HBC devices, and the electrode dimensions and arrangements, vary from study to study. In most HBC studies, researchers have adopted a one-electrode contact for both the transmitter and the receiver; the signal electrode is in contact with the human body while the ground electrode is left floating [13; 14]. In recent years, some studies have used a two-electrode transmitter in which the signal and ground electrodes of the transmitter are both in contact with the human body. Some studies have compared the transmission characteristics and electric field distributions of one- and two-electrode contact transmitters [15; 16; 17; 18; 19; 20]. [PERSON] et al. conducted numerical simulation and investigated the difference between the received voltages and electric field distributions of one- and two-electrode transmitter models. They found that the received voltage and electric field were both enhanced by attaching the ground electrode to the human body [17]. [PERSON] et al. investigated the effect of the ground electrode on the current distribution through numerical simulation. They found that attaching the ground electrode to the body increased the current from the signal electrode and altered the distribution of electric current in the human body [19]. [PERSON] et al. numerically and experimentally analyzed the received signal levels using a one-electrode transmitter, a two-electrode transmitter, and a one-electrode receiver. A biological tissue-equivalent solid phantom was used in their experiments. In both the simulations and the experiments, the received signal was larger when the two-electrode transmitter was used [20]. In these studies, the simulation models included only the human arm, and the influence of the torso and the environment surrounding the human body was not considered. In addition, only the strength of the electric field was investigated for the two types of electrode contact conditions. [PERSON] et al. reported a simulation-based study on the electrode configuration, transmitter/receiver placement on various human body postures, and susceptibility to physiological variations [21].