## 2 Related Work  
The rapid technological progress in development of small size wearable devices dedicated for WBANs does not eliminate the key problem of ensuring a stable radio link. In these types of networks one has to deal with high system losses, i.e., introduced by the human body and multipath propagation in the environment, especially for applications with dynamic users. The indoor propagation issues, concerning WBANs, are widely analyzed by researchers. The majority of currently designed WBANs are used in indoor environments in which radio wave propagation is significantly disturbed compared to open space, mainly due to the stronger multipath effect. In the current state of the art, many indoor off-body channel models were proposed, but still there is a need to perform additional measurements and analytical studies for other types of environments. Therefore, it was decided to carry out measurements in a passenger ferryboat, which is an atypical and very harsh environment in terms of a radio wave propagation. Due to its metal structure, it cannot be treated as a typical indoor environment.  
The obtained measurements allowed for development of an off-body channel model, which will facilitate the design of WBANs. It is very important, both from the perspective of multimedia service providers, as well as the security point of view. The practical example of WBAN operating in the ferryboat is a system for remote tracking and monitoring of people, developed at the Department of Radio Communication Systems and Networks, Gdansk University of Technology [11]. This system was developed for the Polish Maritime Regional Unit of Border Guard, and its goal was to increase the security of the officers on duty. It consists of off-body personal identification modules, which are mounted directly on the human body, while reference nodes are attached to the ferryboat's construction elements. The telemetry data about the officers are sent to the operating center.  
Several previous research studies in this environment analyzed ultra-wide band (UWB) channels operating in the 6-9 GHz band and narrowband (NB) channels operating in the 2.4 GHz band. In [12], measurements of NB 2.4 GHz radio channel in a similar ferrryboat environment were carried out Despite the environments' similarities, the main goal of the analysis was to evaluate a simple model for stationary surveillance network. In [13], measurements were carried out for an on-body UWB channel. The proposed model consists of a normally distributed random variable, which represents the received signal variations due to fading. The work in [14] is focused on obtaining a statistical model for the UWB propagation channel in a ferryboat passenger cabin. Moreover, the measurements were focused on determining and further modeling of the channel impulse response, not the system loss. In [15], the developed path loss model was evaluated for UWB off-body communication, where the fading is modeled by a Gaussian variable, but the model was not investigated in the ferryboat environment. The most complex model for WBANs in a ferryboat is proposed in [16]. It accounts for the mean system loss (MSL), and attenuation due to fading. The measurement scenarios were dynamic, but only the NB channel at 2.45 GHz is analyzed.  
Considering the limited number of publications related to channel modeling in the passenger ferryboat and the growing interest in the use of WBANs in such an environment, the conducted research and proposed novel system loss model is unique. It should also be noted that the authors have used custom measurement equipment, which uses radio distance measurements (RDMs) to dynamically estimate the distance between the moving person and the static reference node (RN). This approach gives accurate data despite the different user motion velocities during the measurements.