## 4 Simulation Results
### Simulation Results  
Figure 13 shows the transmit power at each node in Schemes 2 and 3 and the packet train method. The following eight scenarios in Table 3 are simulated. The former four provide a similar data rate to each other as so do the latter four. Among the eight scenarios, scenarios 1, 5 and 7 do not satisfy BER below 1% criterion even without noise. Their worst BER values are 1.1%, 5.3% and 3.0%, respectively. Therefore, five curves are drawn in Figure 13. Scheme 2 with six repetitions is compared with Scheme 3 with four repetitions and the packet train method with two repetitions. Another comparison is between Scheme 2 with three repetitions and the packet train method without repetition. The power in Figure 13 presents an instantaneous value during a transmission frame. The abnormally high value at the 10 th node of Scheme 2 with three repetitions is attributed to high interference from the surrounding active nodes. The  
Figure 12: Transmission/reception timing of the packet train method. \begin{table}
\begin{tabular}{c c} \hline
**Scenario** & **Scheme** \\ \hline
1 & Scheme 1 with 6 repetitions \\
2 & Scheme 2 with 6 repetitions \\
3 & Scheme 3 with 4 repetitions \\
4 & Packet train method with 2 repetitions \\
5 & Scheme 1 with 3 repetitions \\
6 & Scheme 2 with 3 repetitions \\
7 & Scheme 3 with 2 repetitions \\
8 & Packet train method without repetitions \\ \hline \end{tabular}
\end{table} 
Table 3: Simulation scenarios. 10 th node suffers from a BER of 0.91% without noise. Therefore, the node is vulnerable to noise and the margin for noise is only 0.09%. Because the noise level is given, a high transmit power is required for the node. In addition, owing to the short inter-node distance of 100 m and achievable signal-to-noise ratio (SNR) for 1% BER, the transmit power level appears somewhat small between 0.4 mW to 300 mW. Overall, Scheme 2 outperforms the packet train method and Scheme 3 in terms of the instantaneous power, which could be a critical issue in cases where the maximum power is limited. In Scheme 2, the case of six repetitions performs better in terms of power consumption than that of three repetitions but the data rate becomes a half. One of the reasons for this is that the energy is doubly accumulated at the receiver because one information bit is doubly repeated at the transmitter. In other words, the case of six repetitions provides the same bit energy with only a half power. Therefore, 3 dB gain is achieved in Figure 13. The additional gain is from interference averaging. When repetitions of one information bit are summed at the receiver, the interference values are random because scrambling has been applied. Therefore, the summed interference values are not serious in most cases, and this phenomenon is similar to interference averaging in code division multiple access. For the above two reasons, the difference between Schemes 2 with six repetitions and with three repetitions is greater than 3 dB, as shown in Figure 13. The difference becomes greater for the packet train method. On the other hand, in terms of battery life, the time average of the transmit power over the entire time period, including both the active and inactive states, is more important. Figure 14 shows the time average of each node's transmit power. In case of Scheme 2 with six repetitions, compared to the packet train method with two repetitions, the power saving is clearer at the lower indexed nodes, even though performance degradation may occur at higher indexed nodes where data accumulation becomes heavier. In the case of Scheme 2 with six repetitions, compared to Scheme 3 with four repetitions, they show similar performance but Scheme 2 is slightly better. Because Scheme 3 cannot support two repetitions, Scheme 3, which is a variant of Scheme 2, is a good candidate in low interference environments (with many repetitions) but it cannot work in high interference situations (with a few repetitions.)