| With the proposed national strategic targets such as 'Ocean Power',”Smart Ocean' and 'Digital Ocean',research on the next generation of marine equipment has gradually gained more and more attention of industry,academia and military.Underwater transmission technology is the core technology of marine equipment.Underwater transmission technology can provide reliable communication capability for marine equipment.It could also make the marine equipment become informatization,intelligence and autonomous.Underwater cable communication,underwater acoustic communications,underwater optical communication,underwater electromagnetic wave communication and underwater quantum communication are the most common underwater communication technologies used in underwater environments.Underwater acoustic communication technology is currently the most popular underwater communication technology.It has been widely studied and applied in underwater environment for its transmission distance,transmission reliability and flexibility.Underwater acoustic channel is considered as one of the most complex communication channels in nature for its limited bandwidth,multi-path effect,Doppler effect,impulse noise and large propagation delay.The transmission rate,transmission capability and transmission range of underwater acoustic communications is restricted by the characteristics of underwater acoustic channel.We need to study the physical layer and network layer of underwater acoustic communications to reduce or eliminate the influence of underwater acoustic channel for a better communication performance.At present,many research institutes have proposed a variety of physical layer and network layer designs for underwater acoustic communications.These designs are usually designed for specific underwater environment and application scenario,and cannot be applied to real underwater environment.Therefore,we need to redesign physical layer and network layer with anti-interference and adaptive ability.In this paper,we study the the characteristics of the real underwater acoustic channel,and propose new designs of physical layer and network layer of underwater acoustic communications that can be applied to the real complex and variable underwater environment.In the design of physical layer of acoustic communication,in order to solve the problem of poor adaptive ability of the physical layer in traditional underwater acoustic communications,we adopt adaptive modulation technology to improve the adaptive ability of underwater acoustic communications.We propose three adaptive modulation schemes: adaptive modulation scheme based on pre-training,adaptive modulation scheme based on LFM signal feedback and adaptive modulation scheme based on environment sensing.The adaptive modulation scheme based on pre-training realizes adaptive modulation by measuring the applicable transmission power of each modulation mode.It is suitable for a static underwater acoustic channel.The adaptive modulation scheme based on LFM signal feedback is implemented by a feedback LFM signal.It improves the reliability of the feedback channel and can be used in a dynamic underwater application with stable acoustic channel.The adaptive modulation scheme realized based on the environment sensing technology and adaptive modulation scheme with LFM signal feedback.It adopt the joint estimation of Doppler error and impulse noise,and can be applied to the dynamic underwater application with complex acoustic channel.We demonstrate the feasibility and effectiveness of three adaptive modulation schemes through simulation with different acoustic channel conditions.In the design of network layer of acoustic communication,we focus on the MAC layer protocol design in underwater acoustic communications.We proposed TDMA-based MAC protocol to eliminate influence of the long transmission delay,impulse noise,and transmission shadow area of underwater acoustic channel.It improves the anti-interference ability of underwater acoustic communications.The protocol we proposed is based on the minimum power transmission interference model and underwater multi-hop grid network.According to the acoustic channel and structure of underwater acoustic network,we optimize the scheduling scheme in TDMA protocol.The scheduling scheme could guarantee collision-free transmission in the network.Meanwhile,it optimize the throughput and energy consumption of the underwater acoustic network.We change the transmission order in the underwater acoustic network to increase the space diversity of the source information.By this way,we improve the anti-interference ability of the underwater acoustic network.Simulation results show that the proposed protocol has good transmission performance and anti-interference ability.Finally,we develop the acoustic communication platform and the I-SEA sea-air integrated cross-media communication platform for practical underwater environment experiments and tests.The underwater acoustic communication platform is a software defined underwater acoustic communication platform.We construct four modulation modes and the adaptive modulation scheme based on pre-training in the platform,which can be used in most underwater environment scenarios.The platform has a good software refactoring,and users can customize any algorithms and functions through software modification.The I-SEA sea-air integrated cross-media communication platform contains the underwater vehicles,unmanned aerial vehicles and relay buoys.It can achieve two-way sea and air cross-media communication,and improve the communication ability and coverage effectively.We introduce the design ideas and implementation schemes of the I-SEA underwater acoustic communication platform and sea-air integrated cross-medium communication platform from the hardware structure and software structure respectively.We use the two platforms to conduct a lake experiment to verify the performance and functions of the two platforms respectively.The experimental results prove the performance and the functions of the two platforms. |
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