## 1 Introduction  
Signal transmission based on the human body, also termed intra-body communication (IBC), is a technology using the human body as the transmission medium for electrical signals [1]. Compared with short distance wireless communication technologies, such as Bluetooth and Zigbee, _etc._, this technology has several novel characteristics, which can be summarized as follows: (1) due to the fact that the signal mainly transmits within the human body and little radiation leaks out, it avoids the disturbance of environmental electromagnetic noise and can achieve comparatively higher data rates; (2) as a special type of cable communication using the human body as transmission medium, it needs comparatively lower energy consumption [2]; (3) using this technology, communication can be started or stopped by touching, standing and sitting down of the human body [3]. Due to the advantages mentioned above, it is believed that the signal transmission technology based on the human body medium will offer significant advantages in BSNs used for healthcare [1,4] and other related fields. Sensors used for signal detection are very important for achieving reliable signal transmission based on the human body. Firstly, to guarantee the safety of the human body, signals injected into the human body should be low [1,5]. Moreover, the impedance of human body also results in signal attenuation [6], therefore, sensors used for detecting the signal transmission within the human body should have high sensitivity. Secondly, with the influence from the floating ground of wearable electronical devices, signal transmission within the human body may suffer from great distortion, therefore, anti-interference properties are another important characteristic required by the sensor. Recently, two kinds of sensors have been chosen, the electrical sensor and electro-optical (EO) sensor. However, due to the fact that the electrical sensor has comparatively low input impedance and is easy to be interfered with by electromagnetic noise, the typical signal transmission distance based on this kind of sensor is only approximately 30 cm and the corresponding signal transmission rate is limited to 40 kbps [3]. As for the EO sensor, due to its extremely high input impedance, the influence of the electrical noise can be greatly decreased. Moreover, the ground electrode of the EO sensor is electrically isolated from the electronic circuits, which eliminates the influence of the floating ground potential [7]. As a result, both the noise and the distortion of the receiving signal can be greatly decreased, thereby high signal transmission rate can be achieved [3]. Therefore, EO sensors are believed to be suitable for detecting signal transmissions within the human body. On the other hand, recent works with respect to the EO sensor used for signal transmission based on the human body medium mainly focus on sensors based on bulk electro-optical crystals [3,7,8]. This kind of EO sensor has an additional phase delay caused by natural binary refraction, which is very sensitive to the temperature that thereby influences the signal transmission quality. Moreover, the phase delay of this kind of EO sensor depends on the aspect ratio of EO crystals, which results in the comparatively big size of the EO sensor and will limit its application in BSNs. In our previous works, we proposed a novel signal transmission method based on the human body medium by using a Mach-Zehnder EO sensor, which will help to achieve signal transmission based on the human medium with the characteristics of good temperature dependence properties, small size and low power consumption [9]. In this paper, we present a signal transmission system based on the proposed method, and the frequency response properties as well as the parameters of the proposed system have been discussed. Firstly, we described the proposed signal transmission system, which consists of a transmitter, a Mach-Zehnder EO sensor and a corresponding receiving circuit.