Sea Technology

MAY 2018

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20 ST | May 2018 www.sea-technology.com and/or moving nodes (i.e., swarms of UUVs) can be im- plemented with full-duplex communications capabili- ties, much like radio-based mesh networks. Nodes can receive multiple incoming transmissions while they are transmitting. Mobile nodes can be fully accommodated in self-forming networks. Channel capacity utilization remains high and is only reduced slightly as the BER in- creases. DOLPHIN full-duplex technology is signal-ag- nostic. Multiple-access modulation methods such as CDMA (code-di- vision multiple access), OFDMA (orthogonal frequency-division multiple access) and FHSS (fre- quency-hopping spread spectrum) can be chosen for their optimality for specific user requirements (high signal-to-noise ratio, high bit rate, many users, etc.). Accurate posi- tion, navigation and timing (PNT) are continuously available from all nodes but not required for effective communication to take place in the network. In practice, the impact of DOLPHIN full-duplex com- ms versus standard half-duplex comms can be viewed in terms of the change in data throughput (speed of trans- mission of meaningful information). This can be generi- cally viewed in terms of data rate versus range. The posi- tive benefits are particularly clear as the number of nodes in a half-duplex (mobile or static) network increases. While increasing nodes degrades the overall throughput of conventional networks, modeling and testing show that DOLPHIN Comms networks will maintain consis- tent performance. The receive rate is independent of the number of nodes in the network or whether the nodes are static or moving. If you start adding nodes to the network, the receive rate is decreased even more for half-duplex static networks, while the DOLPHIN Comms full-duplex receive rate is basically unchanged. This demonstrates the significant importance of the DOLPHIN Comms technology in enabling networks of moving nodes, like swarms of UUVs. A more concrete example of the benefits can be seen by considering the case of two UUVs attempting to com- municate using assured data communications (ADT). Traditional half-duplex approaches, with multiple cycles of transmission of packets that require a receive acknowl- edgment from the receiving node, can require up to 20 sec. to deliver a typical data message between UUVs. But using a DOLPHIN full-duplex approach can deliver about a 400 percent improvement of throughput. To be clear, this improvement is entirely due to the signal pro- cessing approach, but the overall limitations of acoustic transmission remain. To put it casually, while the laws of physics have not changed, DOLPHIN Comms provides better lawyers. Lab, Field Testing A DOLPHIN Comms and Sonar proof-of-concept (POC) demonstration was successfully completed in separation are typically applied in today's systems, but do not provide enough dynamic range to enable full-duplex communications. DOLPHIN Comms technology pro- vides the analog cancellation of the transmission source before the LNA in order to reduce the dynamic range of the combined signal to within the limits of the analog to digital (A/D) converter. Using analog cancellation to reduce the transmission signal by 80 dB, the overall dy- namic range of the combined signal can pass through the dynamic limitations of the A/D conversion (typically 100 dB for a 16-bit A/D) without loss of the lower order terms from the receiver. The surviving digital signal after the A/D converter can be further improved with digital cancellation but only if the receiver content has not been suppressed by the A/D converter. DOLPHIN Comms en- sures the receiver contents are preserved for the entirety of the bandwidth of the signal. What Full Duplex Offers While ocean physics are fixed, the development of DOLPHIN Communications technology enables true full-duplex acoustic telemetry. This new technology re- solves many of the issues of half-duplex acoustic com- munications. DOLPHIN technology cancels the transmit signal at the receiver in real time. This eliminates the receiver saturation caused by self-signal and enables si- multaneous transmitting and receiving on the same fre- quency, with collocated source and hydrophones. The DOLPHIN technology is frequency and range indepen- dent—thus yielding new approaches to underwater com- munications. There are several immediate benefits of the DOLPHIN technology. Because all the nodes in a network can transmit and receive simultaneously, TDMA is no longer required to segregate the transmission of data among nodes, which are now all free to transmit data at will. Data can be sent and verification acknowledgments can be received simultaneously using the same frequency until the data transfer is completed. Robust assured data transfer (ADT) between nodes is feasible. Over a 400 percent increase in ADT throughput speeds has been ob- served using DOLPHIN Comms. It is no longer required to know the distance between each node. DOLPHIN-en- abled communications can transmit to any node and to multiple nodes within range at any time necessary. Ro- bust self-forming acoustic networks of randomly placed "TDMA networks containing moving nodes, like UUVs, tend to fail because the moving nodes can't time their transmission correctly to arrive at the receiving node."

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