Until now underwater communications (UComms) have been mainly focused on point to point links in open sea between an underwater vehicle or seabed sensors and the support ship. Theoretically radio waves are usable in water; however they are of little use for this purpose because of their great attenuation at short distances. Optical solutions such as green-blue laser suffers from scattering and needs a high precision in pointing. The acoustic communication appears as the preferable solution; however the channel is far from ideal, especially in the very shallow water conditions of a port. It has a very limited bandwidth, and causes severe signal dispersion both in time and frequency domain.
The SHOAL project will develop an Underwater Mobile Ad-hoc Network (UMANet) in harsh environmental conditions.
Another important key point is the impact of such communications on the existing environmental constituents and especially real fishes which use the port as their life area. Special care will be taken to protect this unstable balance in selecting appropriate frequency range, reducing acoustic level and minimising data exchange.
Regarding the communication system, the main challenges this project will have to solve will be at least:
- The reduced available bandwidth,
- The time delay due to acoustic propagation,
- The high level of bit error rate,
- The power consumption of modems,
- The integration of modem on a very small platform.
A secondary feature will be the localisation of fish within the operational area. Acoustic underwater positioning systems already exist as independent functions. Such systems have generally been single role, application specific. The target would be to integrate this function within the communications. The hub will be used as positioning reference for the swarm.
In a classical navigation system the mobile has to send recurrent signal to be tracked. Applied to a swarm manoeuvring in a reduced area, this solution will generate a permanent acoustic noise which could affect other systems and disturb the environment.
In the frame of this project, the positioning methodology will take advantage of navigation intelligence of the robot. The positioning system will stay silent until a fish makes a request through the communication link:
- The fish will send a message including its diving depth and ID,
- The positioning system will generate a coded acoustic pulse,
- The fish will answer,
- The positioning system will compute bearing and distance.
- The hub will broadcast a navigation message including 3D position of dedicated fish.
With this methodology the fish positioning could be reached with one base.
Nevertheless depending on the port topology, two or three bases will be necessary to extend the coverage. The communication protocol will take this constraint into account even if, in the frame of sea trials only one base will be used.
The communication system will have to transmit to fish relevant information for:
- Network management,
- Direct way back for each fish to their hub for recharging.
In progressing beyond the state of the art this could be done using different multiplexing schemes:
- TDMA protocol with time slot allocation for each fish and the hub. The different subscribers will use a common time reference which could be the GPS time sustained by an internal stable clock. This principle which has already been patented for ground vehicle positioning has never been used on underwater fleet. Especially due to the very low available data rate in AComms, classical protocols, generating a lot of additional data, cannot be used. Dedicated protocol will have to be developed to satisfy the positioning accuracy required by the SHOAL objective within the data rate capability of low size, low power communication modem. Validated solutions could then be proposed to standardisation committees for other underwater applications.
- FDMA protocol where different frequency channels are used to transmit messages simultaneously. This could have better promise than TDMA, in term of delay, however it could be difficult for the network manager (hub) to administer the frequency allocations as well as the modem to be dynamically tuned to different bands.
- CDMA where dedicated codes are used to multiplex information onto the same frequency band simultaneously by multiple modems. This scheme is based on special DSSS (Direct Sequence Spread Spectrum) modulation where each signal transmitted by a modem looks like noise to the other one until it decodes appropriate code. Using this scheme, data transfer can be carried out efficiently, on only one frequency channel.
SHOAL will address underwater communications in the port environmental conditions which could be seen as one of the most difficult in terms of acoustic propagation, and also in UUV fleet management. This will benefit not only to pollution and chemical analysis but also all new projects where many vehicles or divers will have to cooperate in a constrained area merging together their collected data for a global and real time situation analysis.
SHOAL will also investigate how to exchange data in restricted area without disturbing environmental balance.