# UCMAC: A Cooperative MAC Protocol for Underwater Wireless Sensor Networks
## 5 Performance Evaluation
### Simulation Results  
First of all, we define performance metrics as follows:  
* System throughput: The average number of DATA bits successfully received by the intended destinations per second (measured in bps)
* Latency: The average time interval between generation and successful delivery of DATA packets at the intended destinations (measured in s)
* Single-hop PDR: The ratio of the number of DATA packets successfully delivered at the intended destinations to the total number of DATA packets generated (measured in %)
* Energy efficiency: The average number of DATA bits successfully received by the intended destinations per Joule (measured in bits/J)  
Figure 9 demonstrates the existence of an appropriate \(N_{\text{coop}}^{\text{max}}\) in terms of the system throughput for various DATA sizes. It is generally accepted to assume that high cooperative diversity gains lead to a high system throughput. If \(N_{\text{coop}}^{\text{max}}\) is small, the cooperative diversity gains may not be maximally \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Grid size & 1 km \(\times\) 1 km \\ Propagation speed & 1500 m/s \\ Transmission range & 2500 m \\ Data rate & 2400 bps \\ Tx Power & 20 W \\ Rx power & 756 mW \\ Maximum number of RTS transmission & 5 \\ Control packet size 1  & 120 bits \\ \hline \hline \end{tabular}
\end{table} 
Table 2: System parameters for simulation. obtained even though only a small amount of overheads is produced by additional cooperation process at the same time. On the other hand, a high \(N_{\text{coop}}^{\text{max}}\) makes excessive overheads exceed the cooperative diversity gains. Consequently, an appropriate level of \(N_{\text{coop}}^{\text{max}}\) exists depending on network circumstances. For example, when the DATA size is 3 kbits, we expect that \(N_{\text{coop}}^{\text{max}}\) of 3 results in the highest system throughput. Meanwhile, the optimal \(N_{\text{coop}}^{\text{max}}\) also varies with the data size. As the data packet size becomes larger, the susceptibility to packet error increases. Therefore, if the data size is too large, the data packets retransmitted by cooperators are also likely to be erroneous. In this situation, increasing \(N_{\text{coop}}^{\text{max}}\) just brings about increase in overheads. On the contrary, a small data packet is more likely to be successfully received, and therefore, a relatively higher \(N_{\text{coop}}^{\text{max}}\) is allowed at the expense of additional overheads. In Figure 10, we compare UCMAC with MACA-U and CD-MACA by setting \(N_{\text{coop}}^{\text{max}}\) and DATA size at 2 and 1 kbits, respectively. Overall, UCMAC outperforms the other schemes as a result of its well-coordinated cooperation process. MACA-U, which is non-cooperative, shows relatively low performances compared to cooperative protocols. First, our analysis shows that UCMAC offers much better system throughput than other schemes (Figure 10a). This is because retransmitted DATAs arrive at the destinations more quickly with the aid of the well-coordinated cooperators. Although CD-MACA also benefits from cooperation, such cooperative gains are smaller than those of UCMAC mainly due to the lack of coordination between the cooperators. CD-MACA performs slightly better than MACA-U, allowing neighboring nodes or cooperators to retransmit DATA opportunistically. In MACA-U's case, when packet errors happen, the destinations do not request any cooperation and just rely on source retransmissions. If packet errors persist, each source is more likely to take a longer backoff time, which will result in larger decreases in the system throughput. The result of latency can be understood in the same context (Figure 10b). In CD-MACA, packet collisions may occur between DATAs retransmitted by the cooperators; UCMAC escapes such collisions by making the cooperators retransmit DATA one at a time. This increases the likelihood that the intended destinations successfully receive DATAs after a smaller number of retransmission, thus reducing the latency.