# Concurrent Transmission Based on Distributed Scheduling for Underwater Acoustic Networks
## 4 Simulations and Results
### Performance Under Different Network Loads  
The performance of UWALOHA, UW-CSMA/CA, SFAMA, and ROPA protocols, and the proposed method under different network loads are shown in Figure 3, where \"DS\" and \"DS without SO\" denote the proposed distributed scheduling method with and without sending sequence optimization, respectively. Figure 2(a) shows the normalized throughputs of these protocols under different network loads. Figure 2(a) shows that the proposed method reaches its maximum normalized throughput around 0.85 when the network load is higher than 2.8. The normalized throughputs of DS and DS without SO are both higher than the four conventional MAC protocols. DS produces higher normalized throughput than DS without SO, which shows the effectiveness of the proposed joint optimization method. UWALOHA provides the highest normalized throughput when network load is 0.3-0.6, but decreases rapidly as the network load increases due to the massive conflicts caused by uncontrolled transmission. UW-CSMA/CA and SFAMA reach their maximum normalized throughputs around 0.35 and 0.56 under the network loads of 1.5 and 2.3, respectively. The maximum normalized throughputs of UW-CSMA/CA and SFAMA are much lower than those of the proposed method because, in UW-CSMA/CA and SFAMA, only one pair of nodes can communicate in a transmission cycle. This means that a node needs to compete for the transmission opportunity using the RTS/CTS handshake mechanism for each data packet, which significantly reduces the channel use in underwater environments. When only one node needs to transmit data, the proposed method provides the same performance as UW-CSMA/CA and SFAMA. However, when there is more than one node that has data to send, the proposed method can shorten the data transmission time by optimizing the transmission schedules of each source node, and the concurrent handshaking also reduces the overhead. Therefore, the proposed method can support heavier network loads and achieve higher maximum normalized throughput than UW-CSMA/CA and SFAMA. ROPA has similar normalized throughputs as the proposed method when the network load is around 0.3-1.3 but becomes lower as the network load increases because in ROPA, additional concurrent transmissions only occur when other nodes have data to send to the source node, which will sometimes reduce the concurrent transmission opportunity. The proposed method imposes no constraints on the source and destination nodes and can support multiple transmissions with optimized sequences and time in a transmission cycle, so that can provide better performance than ROPA. Figure 3b shows the delivery rates of the four conventional protocols and that of the proposed method. The delivery rates of all methods drop as the network load increases. UWALOHA provides the lowest delivery rate and tends to be zero when the network load is higher than 2.8. The delivery rates of UW-CSMA/CA, SFAMA, and ROPA are higher than UWALOHA because they use handshaking to avoid conflicts. The proposed method provides the highest delivery rate in the simulation because collisions only occur when a node loses the CTS packets and the collision rate of the CTS packets is usually low due to their small size. Concurrent transmission can also improve the total number of transmitted data packets in a fixed time interval, which produces a higher delivery rate. DS had a higher delivery rate than DS without SO because the former can transmit more data packets than the latter in the same time interval under the same collision rates of the CTS packets. Figure 3c shows the average end-to-end delays of the four conventional protocols and that of the proposed method. UWALOHA produced the lowest average delay when the network load was below 0.6 but worsened in high network load environments due to the collisions and retransmission. UW-CSMA/CA and SFAMA produced high average delays because handshaking is needed for each data packet transmission.