# FD-LTDA-MAC: Full-Duplex Unsynchronised Scheduling in Linear Underwater Acoustic Chain Networks
## 1 Introduction
The LTDA-MAC protocol provides better network performance and improved efficiency by using optimised packet scheduling in linear UASN-based pipeline monitoring systems without clock synchronisation at the sensor nodes [11; 18] for short pipeline half-duplex linear underwater networks. However, this protocol performs poorly for long pipeline scenarios as packet schedules become extremely long, which reduces the frequency at which sensor data can be delivered to the required destination. With the recent advances in self-interference cancellation for in-band full-duplex communication (a phenomenon whereby network nodes can transmit and receive data packets simultaneously within the same frequency bandwidth), new opportunities are on the horizon for improving spectral use and throughput in acoustic communication systems [19; 20] are available. Interestingly, this can solve some of the MAC layer problems by potentially improving network performance in terms of providing higher throughput, lower latency, and by providing an opportunity for a node to simultaneously sense the channel while receiving a packet [21; 22; 23]. This has motivated the design of a new LTDA-MAC protocol for full-duplex based underwater chain network scenarios. Figure 1: A typical linear UASN subsea asset monitoring scenario (Taken with permission from [11]. Copyright 2019 IEEE Networking Letters.). The LTDA-MAC protocol is designed to generate efficient collision-free packet schedules with significantly shorter frame duration. It can leverage full-duplex communications. This can significantly enhance spatial spectrum reuse, especially in the long range pipeline scenarios. We explored the benefit of full-duplex in [24], which investigated the potential performance gains that can be achieved in full-duplex network scenarios by switching on full-duplex capabilities without having to change the LTDA-MAC protocol. Although simultaneous packet scheduling in the full-duplex nodes achieved collision-free packet schedules with up to 39% and 34% throughput improvement for simple (short pipeline) and challenging (long pipeline) cases, respectively, compared to the half-duplex case, it was observed that spatial re-use could be improved especially for longer pipelines by designing a new protocol capable of fully exploiting the full-duplex capabilities of nodes. This paper proposes the FD-LTDA-MAC protocol which is designed to achieve further performance improvement by re-developing the traditional LTDA-MAC protocol to fully exploit full-duplex capabilities and enhance spatial reuse for full-duplex underwater multi-hop chain networks. Consequently, this protocol provides much more efficient packet scheduling to achieve higher monitoring rates over long range underwater pipelines using low cost, mid range, low rate, and low power acoustic modems, such as those presented in [25]. This study is based on numerical simulation and a BELLHOP [26] based underwater channel model. It builds on prior work, in particular, related to the LTDA-MAC protocol. Hence, this paper presents a new protocol designed for full-duplex communication in linear networks. The remainder of this work is organised as follows. Section 2 provides a description of the FD-LTDA-MAC protocol, while, the simulation scenarios are presented in Section 3. Section 4 presents the numerical results and discussion, and conclusions are provided in Section 5.