## 2 Related Work
### Parallel Transmission MAC Protocols for Underwater Acoustic Networks  
Parallel transmission is one of the effective ways to improve the performance of a communication network when the transmission rates of the network links are limited. Many MAC protocols based on this technique have been proposed for underwater acoustic networks in recent years. In contention-free MAC protocols, the researches on parallel transmission were mostly focused on TDMA. Many TDMA protocols used the time domain parallel transmission to optimize their time slot allocations [6; 7; 8; 9; 10; 11; 12]. Some of them also made use of the network topology to reuse some time slots [8; 9; 10; 11; 12], which can be regarded as passive space domain parallel transmission. Some information such as propagation delay, link attenuation or node location needed to be known prior for calculating the communication resource in these protocols. The time and space domain parallelism help to form a more compact time slot schedule so that the channel can be utilized more efficiently in TDMA networks. Because no control information is needed in TDMA, the design of parallel transmission is simplified to the problem of static communication resource allocation. This problem can be solved by many mathematical tools and methods. Therefore, TDMA protocols with parallel transmission are easy to implement and have attracted many attentions in the previous studies. In contention-based MAC protocols, all communication resources are allocated dynamically, so designing a parallel transmission contention-based protocol should consider not only how to optimize the allocation of communication resources, but also the exchange process of control information. The contention-based MAC protocols with parallel transmission can be further divided into single channel and multi-channel protocols. In single channel contention-based MAC protocols, all control and data packets are transmitted through a common channel. Therefore, most of the parallel transmission opportunities in the single channel underwater acoustic networks come from the time and/or space domains. Time domain parallel transmission was usually implemented by scheduling the transmission time of the source  
Figure 2: An example of creating a space domain parallel transmission opportunity by power control. (**a**) transmit with normal power; (**b**) transmit with reduced power. nodes [13; 14; 15; 16; 17; 18], while space domain parallel transmission was achieved by controlling the source node's transmission power [19; 20; 21]. Optimization technique for parallel transmission in TDMA can be used to implement the communication resource allocation in the single channel contention-based MAC protocols, except that the allocation of the time or space communication resources needs to be performed every transmission cycle in the latter. The exchange process of control information is another important question in designing the parallel transmission protocol. Usually multiple successful handshakes are required before the data transmission. Some multiple handshake mechanisms have been proposed in the previous researches. One common technique is to use a sender- or receiver-initiated handshake process, where when two nodes shake hands, other nodes that have data to send to the source or destination node send their handshake packets to this source or destination along with the original handshake packets. However, in this method, additional concurrent transmissions only occur when there are nodes have data to send to the current source or destination node, which will sometimes reduce the concurrent transmission opportunities. Another method is to send the RTS and CTS packets at a time randomly distributed in the RTS and CTS phases. This method imposes less constraint on the source and destination nodes but will cause CTS packet collisions, which will lead to miscalculating the transmission time of the source node.