Sub Project OptoCOMM

The project OPTOCOMM has developed and demonstrated a high-speed Optical Underwater Wireless Communication (OUWC) link as new technology integrated in the Littoral Ocean Observatory Network (LOON) test-bed of the SUNRISE infrastructures. OPTOCOMM system is based on visible light communication, with Light Emitting Diodes (LEDs) as basic components. The project has been conducted by the “end users” SUNRISE partners UNIGE-ISME, the Italian Interuniversity Research Ctr. on Integrated Systems for the Marine Environment, (legally established at the University of Genova, and participating with the University of Pisa and Polytechnic University of Marche ISME units), and the Scuola Superiore Sant’Anna of Pisa (SSSA), in collaboration with the core partners CMRE and URM for the integration in the LOON and SUNRISE GATE framework.

Being oriented to littoral observatories, the optical systems developed in OPTOCOMM had the ambitious goal of providing robust, high bandwidth optical communication in very shallow water and in daylight. Both these operating conditions are extremely challenging, because sunlight introduces wide-band noise in any visible light communication system, while sediment suspensions, always largely present in littoral waters, increase the turbidity and eventually the scattering of transmitted light, greatly reducing the available receiving power. Existing LED-based underwater optical communication systems work mainly in deep water far from the Ocean boundaries, i.e., in dark, clear water conditions.

Overall, the OptoCOMM goals have been achieved in terms of

•  transmission performance (10Mb/s @ 10 m range in harbour waters during sunlight);
• integration with the LOON test-bed and with the SUNRISE interface;
• demonstration of the transmission between a fixed and a mobile node.

It is underlined that the transmission performance reached was the target transmission performance from the start, and that to our knowledge no other visible light communication system has yet reached these performances in similar conditions.

While the feasibility of the result was part of the background work of the same group (ISME and SSSA), the realization of small dimension optical modems ready to use in the underwater environment required a critical and by all means non trivial tuning of the optical filters, of the electronic realization, and of the steering software protocols and drives.

The final optical modems are illustrated in Fig. 1, where two modems are shown in the set-up of the final experimentation. In order to measure the communication performance, the two modems have been clamped to two poles anchored to the harbour pier, so that the pole distance could be measured on the pier. The modems were submerged so that the communication layer depth was at 1 m below the water surface. The overall water depth at the experimental site (CMRE harbour, La Spezia) is 4 m. In order to test file transfer, we repeatedly used two types files, one of 3.5 MB size (photo) and one of 14.6 MB size (video). Each file was segmented in smaller packets of 1024 B, and the protocol includes re-transmission mechanisms for lost packets. During the experiments, the turbidity level of water was constantly monitored with a Turbidimeter probe, and the sunlight disturbance in the optical modem band expressed in terms of background noise level.

 Fig 1. The two optical modems ready for immersion in the final experimentation. The upper part of each modem, inside a transparent case, contains the optical physical communication layer, while the lower part, in the aluminum case, contains the electronics and the power source of the modem. The modems in the figure are clamped to two poles for experimental purposes.

Furthermore, one optical modem was installed on the micro-ROV VideoRay Pro4 (Figure 2), to test the possibility of fixed vs. mobile node communication, in particular to assess the robustness in presence of mismatch between modem alignement. The ROV was completely steered in manual mode, with no auto-depth or auto-heading aids, in order to increase the perturbation in the communication process. File transmission at 10 MB/S from the ROV to the fixed node to a maximum range of 5 m was successfully achieved in full daylight and high turbidity water (Figure 3).

Fig 2. The optical modem installed on the VideoRay ROV.

Fig 3. Still image taken during optical transmission between the moving modem on VideoRay and the still modem connected to the LOON station. Blue LED transmission from the modem is barely recognizable at the top of the image.