## 1 Introduction
RF can cover longer operating distances and enable interactive implanted sensors and devices to communicate with on-body devices wirelessly [1; 4; 5]. Furthermore, due to the technical constraints of batteries, improving the energy efficiency of RF modules is a key factor in the I2O body channel and one that can be realised by using small-sized implantable antennas [10; 11; 12; 13]. Figure 1: The proposed typical structure of the health IoT system. [PERSON] et al. proposed the first in-body path loss (PL) model for homogeneous human muscle tissue at 2.45 GHz in 2009 [14]. Later they reported an extended PL model, which includes the conductivity and permittivity of the human tissues in [15]. However, those PL models and other studies are antenna-specific and only applicable to the virtual family models provided by either of the SEMCAD X or FEKO software packages [4; 5; 14; 15]. In this paper, the PL model is obtained by using software from Computer Simulation Technology (CST) [16] and the recently proposed 3D heterogeneous human body model developed by [PERSON] et al. [7]. The PL result is believed to be more accurate because the CST software takes the loss tangent parameters of human tissues and organs into account in the simulations and the deviation value is smaller than that seen in [11; 12; 13; 14; 15; 17]. Furthermore, it is vital to choose suitable digital modulation techniques to overcome the strong power attenuation over the shadow fading channel caused by the intra-body environment [18]. The relationship between a large range of data rates from 250 kbps to 30 Mbps and the operating communication distance for safe transmission powers is discussed and investigated when using the selected modulation schemes. Routing protocols have been reported to discover and analyze the most energy efficient route [19; 20]. Relay based protocol solutions have also been studied that minimize the energy consumption of the in-body sensor nodes by reducing the length of the transmission distance [21]. A two relay based WBAN routing protocol has been investigated by [PERSON] and [PERSON] [22] and the results show that the proposed routing scheme outperforms direct communication and single relay methods in terms of transmission reliability and energy efficiency. Cooperative communication techniques have gained significant attention as an effective strategy to improve energy efficiency and spatial diversity in wireless fading channels [21]. However, the incremental relaying routing protocol strategy has not been investigated in I2O WBANs. In this paper, an accurate statistical I2O body path loss model that describes the signal propagation between the transmitter (Tx) and receiver (Rx) antennas is obtained by using CST electromagnetic solvers, based on a heterogeneous and innovative 3D virtual human body model at 2.45 GHz. The SAR is assessed to ensure adherence to the authorities' safety requirements. The BER performance for the I2O shadow fading channel employing the binary phase shift keying (BPSK), quadrature PSK (QPSK), 16 quadrature amplitude modulation (16 QAM) and 16 PSK modulation schemes is obtained. The threshold signal-to-noise ratio (SNR) of the four modulation methods is derived when using an acceptable BER performance of \(10^{-3}\). Here, our proposed incremental relaying routing protocol is analyzed and compared with the existing two-relay WBAN routing scheme. A series of QoS metrics such as network lifetime, throughput, average energy consumption, residual energy and propagation delay are also investigated.