# Mutual-Information-Based Incremental Relay Communications for Wireless Biomedical Implant Systems
## 6 Performance Evaluation and Discussion  
The topology of the proposed protocol is shown in Figure 3. As shown in Figure 3a, wirelessly networked implanted sensors are placed inside the human body in predetermined locations and monitor physiological parameters such as glucose and insulin levels. The number of implanted sensors is limited to nine on the basis of Equation (26). Figure 3b,c illustrate the relay-based and direct communication, respectively. The coordinates of all implanted sensors (with IDs) are summarized in Figure 3d. The coordinator is fixed at the center of the human body with a coordinate (0.4, 0.85). The implanted sensors transmit the sensed data simultaneously on two data transmission paths via a cooperative scheme to reduce information packet loss. Once the mutual information value \(I\) becomes smaller than the predetermined threshold, a cooperative data transmission link is established. Relay nodes are used for cooperative transmission that promises implanted sensors multiple transmission links simultaneously. Moreover, it is worth noting that the data flow from \(S\) to \(R\) should be no more than the flow from \(R\) to \(D\). An initial energy value of 1 Joule (J) is offered equally to all implanted sensors. The number of relay nodes is limited to two with coordinates (1.65, 0.75) and (0.9, 1.65) in accordance with [19]. The packet length is set to 2000 bits, since this is the maximum payload regulated by the IEEE 802.15.6 standard [34]. Table 1 summarizes the simulation parameters. Simulations are conducted in MATLAB and compare the performance of the proposed protocol with our previously proposed WBAN routing protocols: two-relay-based, and incremental-relay-based routing protocols. Other simulation tools such as NS2, NS3, OMNET, and OPNET are also applicable; detailed information can be found in [35; 36]. It can be seen in Figure 4 that the stability period in the MI-based incremental relaying protocol is around 11,000 rounds, whereas it is 8700 rounds for the non-MI incremental relaying protocol and 4100 rounds for the two-relay-based routing technique. Also, the MI-based incremental relaying protocol achieves the most extended network lifetime at about 12,800 rounds while this is 11,800 rounds for the non-MI incremental relaying protocol and only 4250 rounds for two-relay-based routing. The reasons for the results are as follows: the incremental relaying technique promises longer network lifetime than the static two-relay-based routing scheme because the proposed relay node selection method is capable of balancing the implanted sensors' energy consumption in each round by considering both transmission distance and the residual energy status of each implanted sensor. Moreover, the MI-based routing protocol allows implanted sensors to not transfer redundant data, which results in extending the stability period and network lifetime. Figure 5 demonstrates the residual network energy status in each round of the three advanced communication techniques. The MI-based routing technique has the least average energy consumption at around 0.7 mJ per round in comparison to the two-relay-based protocol scheme at 2.1 mJ and the non-MI incremental relaying approach at 0.76 mJ. The MI-based protocol scheme is capable \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value (unit)** \\ \hline Frequency & 2.4 GHz \\ \(E_{Tx\_dec}\) & 16.7 mJ/bit \\ \(E_{Rx\_dec}\) & 36.1 mJ/bit \\ \(E_{amp}\) & 1.97 mJ/bit \\ \(E_{pro}\) & 0.3064 mJ/bit \\ \(E_{sen}\) & 0.12 \(\times\) 10\({}^{-9}\) mJ/bit \\ PL exponent & 3.6 \\ PL standard deviation & 2.93 \\ \(PL_{dB}\left(d_{ref}\right)\) & 23.49 dB \\ Payload & 2000 bits \\ Implant sensor initial power \(E_{0}\) & 1 J \\ Number of implanted sensors & 9 \\ \(I\left(D_{i};D_{j}\right)\) threshold & 0.5 \\ Predetermined BER & 10\({}^{-3}\) \\ Location of implanted sensors & shown in Figure 3b \\ Location of relays & (1.65, 0.75) and (0.9, 1.65) \\ \(P_{t}\) & 25 uW \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters.