Under Ice Acoustic Communication - Results from a seatrial in the Bothnian Bay

As a part of the international collaboration USC (Undersea Surveillance and Communication) a sea trial was conducted in March of 2019 in the Bothnian Bay. USC is a collaboration between FOI and the following United States organizations; NIWCNaval Information Warfare Center, NRL-Naval Research Laboratory and NSWCNaval Surface Warfare Center, Carderock. The purpose of the experiment was further investigations of acoustic underwater communications under ice. In the experiment, specific signals for channel estimation and communication signals with a multitude of modulation formats were transmitted. Signals for both coherent and incoherent demodulation were used. It was intended that the experiment was to be carried out in "open waters", i.e. outside the archipelago. However, the ice situation at the time did not allow for this. Instead the trial area had to be moved closer to land. The receivers were fixed and closer to land, while the transmitter position was moved in steps as far as the ice would allow. The underwater acoustic channel is very stable under ice. There is no free water surface that introduces Doppler effects, and the transmitter and receiver are typically moored from the ice and hence introduce small variations in the received signals. In a first experiment a maximum distance of 16 km was reached, before the ice was too unstable to go any further. In a second experiment shorter ranges were the focus, and a maximum of 2 km was reached this time. The data rates were varied between 11 bits per second (bps) and about 11 kbps. At the lower centre frequency 10 kHz the results were good to at least 13 km range. At the higher frequency 24 kHz, the results were good out to 5 km. The bandspread signals with low bitrates (~11-117 bps) were correctly decoded at all distances and at both centre frequencies. The incoherent modulation formats MC-FSK and Janus, with bitrates between 40 and 141 bps, were received error free at the largest distance, 16 km, at the lower centre frequency 10 kHz. At 24 kHz both modulation formats were error free out to 5 km.