## 1 Introduction  
A wireless body area network (WBAN) consists of nodes that communicate wirelessly and are located in, on or outside the body of a person. To optimize the communication between the nodes placed within or outside the human body, a better understanding of propagation loss is required for the development of WBANs. This need arises, as the human body is a lossy medium, which considerably attenuates the electromagnetic waves traveling from the transmitter (Tx) to the receiver (Rx). The novelty of this research paper is the development of a simplified generic path loss (PL) model, which describes the signal attenuation between a Tx and an Rx antenna as a function of the propagation distance and other parameters (for example, conductivity), applicable for heterogeneous human models for in-to-out body communication at 2.45 GHz and which is independent of the antenna characteristics [1,2]. We select the 2.45-GHz frequency corresponding to the industrial, scientific and medical (ISM) band, as this is one way to reduce the antenna size and to make it available for implantation. Another advantage is that the larger bandwidth allows for higher bitrates. Various antennas are manufactured for the same reason [3, 4, 5]. The voting by the FCC to include the 2.36-2.4 GHz spectrum in medical device radiocommunications (MedRadio) for body area networks is also a motivation for carrying out this study [6]. In our previous work, we have presented the path loss models for insulated dipole antenna in a homogeneous lossy medium to understand the influence of the conductivity and permittivity of the medium on the path loss [7]; further, we extended the path loss model to include the range of conductivities and permittivities of the human body [2]. However, these path loss models were antenna specific (dipole antenna). Thus, in [8], we developed a method to find the antenna gain in the conducting medium and developed a path loss model by using the calculated gain, thereby making the model independent of the antenna-type. However, these models were specific for homogeneous medium only. Further, the path loss models were developed for a heterogeneous medium in [9,10], but are applicable only for the virtual family model provided by SEMCAD-X. Thus, in this paper, we provide a generic path loss model for a heterogeneous medium. A transmission channel includes both the antenna and channel properties that lie between the terminals of the two antennas. While designing communication systems for body-centric systems, PL models become more useful and widely applicable if the properties of the propagation channel can be extracted from the transmission channel [11]. Various PL models have been created for in-body and in-to-out body propagation by [1,2,7,12, 13, 14]. The main difference between these PL models and the simplified PL model developed in this paper is that all other PL models include antenna parameters, such as gain, and are only applicable for the considered antennas, thus limiting the general usability of the PL models. Antennas for in-body and on-body propagation have been designed [3,15] at 2.45 and 5.8 GHz, and a method to calculate the gain of these antennas in a conducting medium, as shown in [8], will help in developing PL models, which are independent of the antenna properties. Apart from developing a general useable and a simplified PL model, the aim is also to apply this PL model of a multilayered lossy structure to a heterogeneous human model, where the multilayer structure represents a cross-section of a heterogeneous human body. The PL calculated using the simplified model is compared with the simulated PL in a heterogeneous 3D adult human to verify the applicability in the heterogeneous medium. Thus, instead of using a complete human model for simulations, an equivalent layered structure can be utilized to carry out the PL calculations.