# A Cross-Layer Media Access Control Protocol for WBANs
## 5 Simulation Results  
To evaluate the proposed protocol, we conducted simulations using MATLAB in two classical WBAN scenarios, as depicted in Figure 5. The network consists of seven sensors and one coordinator. The sensors can select a transmission power ranging from \(-45\) dBm to \(-10\) dBm, whereas the receiver power is fixed at \(-30\) dBm. The values of \(P_{i}^{s}\) and \(\vartheta\) are set to 0.1 times \(P_{i}^{s}\). The values of \(\lambda_{1}\) and \(\lambda_{2}\) are set to 0.8 and 0.2, respectively. We compared our protocol with IEEE 802.15.6 [33]in terms of total throughput, network lifetime, and packet reception ratio. We considered two variants of our protocol: one with only power adjustment (PACR-PA) and another that incorporates both power adjustment and cooperative transmission (PACR-CP). The length of the superframe is set to 1 s, and each sensor generates an event-triggered packet or emergency data on average every ten minutes. The lengths of the generated packets range from 20 bytes to 100 bytes, with a step value of 20 bytes. Firstly, we evaluated the impact of our cross-layer protocol on network lifetime by using the adaptive power adjustment mechanism. Secondly, we assessed the additional improvement in network lifetime achieved by employing cooperative transmission. Figure 6 demonstrates the lifetimes of the two considered WBANs. When larger packets are generated, sensors need to exhaust more energy to upload data per superframe, resulting in a decrease in the network lifetime with increasing packet size. It is obvious that our proposed protocol outperforms the typical IEEE 802.15.6 since sensors can choose smaller transmission power based on the wireless channel conditions. If no power adjustment scheme is adopted, all the sensors must choose a high transmission power to ensure that the sensor located far from the coordinator can upload packets to the coordinator successfully. Consequently, a sensor located near the coordinator would unnecessarily consume  
Figure 5: Illustration of the simulation scenarios. (**a**) Network topology in the first scenario; (**b**) Network topology in the second scenario. high energy, as its packets could be received by the coordinator using a relatively lower transmission power. Therefore, it is necessary for sensors in resource-constrained WBANs to adaptively adjust the transmission power to conserve energy. As shown in Figure 6, the network lifetime of the PACR-PA scheme is 3.5 to 2.9 times and 3.7 to 3.1 times that of the IEEE 802.15.6 protocol under different packet sizes in the two considered networks. Therefore, our proposed protocol can effectively improve network lifetime through adaptive adjustment of node power. After adopting the transmission power adjustment scheme, sensors experience different energy consumption rates, leading to an imbalance in the energy distribution within the network over time. Therefore, enabling cooperation among the sensors is necessary to further prolong the network lifetime. As depicted in Figure 5, for the first network, sensors \(n_{1}\) and \(n_{7}\), located relatively far from the coordinator, can potentially benefit from relays such as nodes \(n_{2}\), \(n_{3}\), \(n_{5}\), and \(n_{6}\). The network lifetime can be improved by 17% to 20% for the PACR-CP scheme compared to the PACR-PA scheme. In the second network, only \(n_{7}\) is located relatively far from the coordinator, and four potential relays can cooperate with \(n_{7}\). The network lifetime is improved by 38% to 44% for the PACR-CP scheme compared to the PACR-PA scheme. The simulation results indicate that our cross-layer protocol can further improve network lifetime through the provided mechanism of collaborative transmission among nodes. Although our proposed protocol can improve network lifetime through power adjustment, reducing the power can affect packet reception rates. Therefore, it is necessary to evaluate the packet reception rate of the network.