## 1 Introduction
One common technique is to use a sender- or receiver-initiated handshaking method, where when two nodes shake hands, other nodes that have data to send to the source or destination send their handshake packets to this source or destination along with the original handshake packets. However, in this method, concurrent transmissions only occur when other nodes have data to send to the current source or destination, which will sometimes reduce the chance of concurrent transmission. In [18], we proposed a handshaking method where multiple nodes sent their request to send (RTS) and clear to send (CTS) packets at a time randomly distributed in the RTS and CTS stages. This method imposes less constraint on the source and destination nodes so that can increase the opportunity of concurrent transmission in a transmission cycle and provide better performance. The major drawback of this method is that a node will fail to receive CTS packets due to collision and miscalculate its transmission time. This phenomenon becomes more severe when the number of nodes that participate in the handshaking process increases. In this paper, we propose a new collision-free hybrid MAC protocol based on pipeline parallel transmission for distributed multi-channel underwater acoustic networks. The contributions of our work include:* A new collision-free hybrid MAC framework is proposed, where the handshaking and data transmission are implemented as a pipeline on multiple acoustic channels. Handshaking is implemented using TDMA in a dedicated control channel and the data packets are transmitted in one or multiple data channels. This framework has two advantages. The first is that the TDMA based handshaking can support multiple successful handshakes in a transmission cycle while avoiding the control packet loss due to collision. The second is that the pipeline transmission of control and data packets can provide high throughput for the network by making use of the multi-channel transmission parallelism. * The algorithms for optimizing the time slots of the TDMA based handshaking and the schedules of data packet transmission are proposed. These algorithms can achieve collision-free concurrent transmission by making use of the time domain parallel transmission opportunities in the underwater environment, which will let the transmission schedule more compact and improve the transmission efficiency effectively. * The replication computation technique is employed in the data packet scheduling to reduce the control overhead in distributed environments. Replication computation, also known as redundant computing, is a technique that uses unnecessary and redundant computation to reduce the communication and/or execution time in parallel computing. By using replication computation, the data packet scheduling can be implemented via a two-way handshake, which is the simplest form of the handshaking protocol and has the least overhead. The rest of this paper is organized as follows: in Section 2, the works related to parallel transmission in underwater acoustic networks are reviewed. In Section 3, the new collision-free hybrid MAC protocol is proposed. Sections 4 and 5 present and discuss the simulation results, respectively. Finally, Section 6 concludes the paper.