# Double-Scale Adaptive Transmission in Time-Varying Channel for Underwater Acoustic Sensor Networks
## 7 Performance Evaluation
### Data Transmission Performance Comparison  
Four modulation and coding modes are used, which is indexed in order of increasing rate, as shown in Table 2. Mode 0 refers to no transmission. The maximal amount of bits that can be carried during one slot is computed based on the transmission mode with the highest data rate, namely Mode 4, as \(1000\times 1/2\times log_{2}16\), where 1000 is the number of symbols per block. The double-scale adaptive transmission mechanism is compared with the contrast schemes in terms of communication performance. Contrast schemes include combination adaptive transmission (AT), channel-based adaptive transmission, and buffer-based adaptive transmission, as shown in Table 3. Each adaptive transmission scheme consists  
Figure 19: Predicted small-scale channel state of Data 2 by the decomposition-based prediction model and AR prediction. Figure 18: Predicted small-scale channel state of Data 1 by the decomposition-based prediction model and auto-regressive (AR) prediction. When the packet arrival rate is 2 kb/slot, the predicted large-scale channel states and the scheduled future modulation and coding modes of Data 1 and Data 2 are shown in Figures 21 and 22, respectively. When the predicted channel state is good, the higher modulation and coding mode is scheduled; otherwise, the lower modulation and coding mode is adopted. The energy overhead of transmitting a certain amount of data in a good channel state is smaller than that of sending when the channel state is bad. Therefore, in order to improve energy efficiency, the transmitter should send more when the channel is good, and send less or stop sending when the channel is bad. In Figures 23 and 24, the energy cost per kb of all schemes are compared with data arrival rate from 0.2 kb/slot to 1.0 kb/slot. Overall, compared with the comparison algorithm, the energy cost of double-scale adaptive transmission is the lowest. In other words, double-scale adaptive transmission achieves higher energy efficiency than contrast \begin{table}
\begin{tabular}{c c c} \hline \hline
**Condition** & **Buffer \(\leq\) 10 kb** & **10 kb \textless{} Buffer \textless{} 30 kb** & **Buffer \textgreater{} 30 kb** \\ \hline \(h>h_{min}+2\Delta h/3\) & Mode 2 & Mode 3 & Mode 4 \\ \(h_{min}+\Delta h/3<h\leq h_{min}+2\Delta h/3\) & Mode 1 & Mode 2 & Mode 3 \\ \(h\leq h_{min}+\Delta h/3\) & stop & Mode 1 & Mode 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Combination AMC. Figure 21: Scheduled transmission mode according to predicted large-scale channel state for Data 1. \begin{table}
\begin{tabular}{c c} \hline \hline
**Buffer State** & **Mode** \\ \hline Buffer = 0 kb & stop \\
0 kb \textless{} Buffer \(\leq\) 5 kb & Mode 1 \\
5 kb \textless{} Buffer \(\leq\) 15 kb & Mode 2 \\
15 kb \textless{} Buffer \(\leq\) 25 kb & Mode 3 \\ Buffer \textgreater{} 25 kb & Mode 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Buffer-based AMC. schemes. The good energy efficiency results from the reasonable schedule of modulation and coding mode, as shown in Figure 21.  
Figure 23: Average energy cost per kb of for comparative strategies (Data 1). Figure 22: Scheduled transmission mode according to predicted large-scale channel state for Data 2. In Figures 25 and 26, the average buffer length of each scheme is shown with data arrival rate from 0.2 kb/slot to 1.0 kb/slot. The average buffer length of DSAT is longer than that of other schemes. This reflects that the transmitter often waits until the channel is good. With the increase of the data arrival rate, the average buffer length of each scheme increases. Figure 24: Average energy cost per kb for comparative strategies (Data 2). Figure 25: Average buffer length for comparative strategies (Data 1). In Figures 27 and 28, the average transmission delay of each scheme is shown with data arrival rate from 0.2 kb/slot to 1.0 kb/slot. The average transmission delay of DSAT is longer than that of other schemes.