# Interference-Robust Transmission in Wireless Sensor Networks
## 5 Performance Evaluation  
We evaluate the performance of the proposed scheme by computer simulation using a lab-developed WSN simulator written in C++. Figure 3 depicts the simulator structure, which considers data transmission from the network coordinator to its child nodes in the presence of IEEE 802.11g WLAN interference signals [38; 39] in a Ricean fading channel with a maximum Doppler frequency of \(f_{D}\)[36]. For the performance evaluation, we use Monte Carlo simulation of 300 iterations, each of which runs \(1.5\times 10^{5}\) simulation time slots. The simulation environment is summarized in Table 1, which is mainly based on the specification of the IEEE 802.15.4 PHY layer. For comparison, we also consider the performance of seven schemes; an IEEE 802.15.4 baseline scheme at a transmission rate of 250 Kbps with a fixed payload size, DRACER, which adjusts a transmission rate with a fixed payload size [22], DPLC, which adjusts a payload size at a fixed transmission rate of 250 Kbps [23], DRACER with DPLC that adjusts the transmission rate and the payload size by using DRACER and DPLC, respectively, a streaming data link layer scheme, which is a static PPR scheme referred to as Seda [25], a hybrid frame fragmentation scheme, which is a dynamic PPR scheme referred to as HiFrag [27], and a green frame fragmentation scheme, which is a combination of HiFrag and transmit power adaptation, referred to as GreenFrag [28]. The proposed scheme and DRACER use one of four transmission rates, 250, 500, 1000 and 2000 Kbps, by adjusting the spreading factor with an appropriate coding set, while using the same spectrum bandwidth as conventional IEEE 802.15.4 [22; 41; 42]. The transmission rate can be informed to the receiver using a start frame delimiter (SFD) without additional signaling overhead [22]. Considering application areas of WSNs, we assume that the maximum payload size is 1024 bytes, which is larger than that of the conventional IEEE 802.15.4 PHY layer (i.e., 127 bytes). Note that IEEE 802.15.4g, a recent amendment of IEEE 802.15.4, supports a maximum payload size of up to 2047 bytes with using almost the same PHY layer techniques as IEEE 802.15.4 [43]. the transmitter can employ all of the transmission rates. It can be seen that when \(\rho=0\), the data throughput increases indifferently from the transmission rate as the payload size increases, which is mainly due to the decrease of the header signaling overhead. When \(\rho=0.2\), however, there exists a payload size maximizing the data throughput at each transmission rate. It is mainly due to the fact that the use of a larger payload size may become more susceptible to the collision. It can also be seen that the analytical results agree very well with the simulation results. Figure 5 depicts the energy consumption (in uJoule/bit) of the WSN transmitter and receiver according to the data payload size when \(\rho=0\) and \(0.2\), which is measured from the power consumption for the transmission and reception of all packets and the power consumption during idle listening (i.e., waiting a packet), as well. It can be seen that when \(\rho=0\), the energy consumption somewhat decreases as the payload size increases mainly due to the increase of the throughput. Note that the energy consumption \(E\left(=P_{avg}/S\right)\) may increase as the payload size increases, where \(P_{avg}\) denotes the average power consumption. When \(\rho=0.2\), however, there exists a payload size that minimizes the  
Figure 4: Throughput according to the payload size. (**a**) When \(\rho=0\); (**b**) When \(\rho=0.2\). energy consumption, which is slightly different from the one that maximizes the throughput. This is mainly due to the fact that \(P_{avg}\) varies with the payload size. It can also be seen that the use of a higher transmission rate considerably reduces the power consumption, implying that the transmission rate should be adjusted according to the channel condition. Figure 6 depicts the normalized throughput according to the transmission time.