Sea Technology

MAY 2018

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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18 ST | May 2018 T he significant growth in the use of UUVs and other underwater networks of nodes in the dynamic ocean environment requires communication systems with far greater capability than is currently available. Since the beginning of acoustic communications, the state-of-the- art technology has been limited to half-duplex signals: transmit with the receiver off and then turn the transmit- ter off and receiver on and wait to receive, because the direct transmission at the source saturates the receiver electronics if they are both on at the same time. Half du- plex is simply accepted as "the way it is." There are three typical outcomes of half-duplex ap- proaches. One is that time domain multiple access (TDMA) is the only practical approach for multiple nodes to communicate in a network. The "time slice" method of TDMA is a workaround to the limitation of using half du- plex. The TDMA approach requires precise knowledge of the distance between communicating nodes so that the data will arrive at the node at the correct time slice when the node is ready to receive. TDMA networks containing moving nodes, like UUVs, tend to fail because the mov- ing nodes can't time their transmission correctly to arrive at the receiving node. The dynamic nature of the ocean makes TDMA networks fragile, even when the distance is well known and the nodes are stationary, and they are virtually useless for moving nodes. Another typical outcome is that the communications channel capacity utilization is dramatically reduced due to the need to stop transmitting to receive data. This is ex- acerbated as the bit error rate (BER) increases and as the number of nodes increases. The additional retransmis- sions required to account for the BER reduce the chan- nel capacity utilization, resulting in lower data exchange rates or throughput. A third outcome is that adding and removing nodes in a network is problematic and disruptive due to the need to establish a new time slice slot for each node. This makes reliable self-forming mesh networks extreme- ly difficult, if not impossible, to create in acoustic envi- ronments using half-duplex approaches. Full-Duplex Comms Simultaneous Transmit, Receive for Static and Mobile Nodes By Justin Manley • Michael Murphree • Greg Folts Simplified DOLPHIN Comms duplex design.

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