Collaborative Research: Distributed Electro-Mechanical Transmitters for Adaptive and Power-Efficient Wireless Communications in RF-Denied Environments

在某些情况下，射频(RF)电磁波的传播变得不可行，从而导致“射频被拒绝”的环境。例如地下和深水设施(地雷、掩体、储存区、隧道、潜水艇、海底电缆等)。最近发生的事件突显了与这种环境进行无线通信的重要性。一个突出的例子是2018年7月从泰国Tham Luang Non Caves营救一支足球队，他们被困在地下两周多。这种射频干扰环境从根本上说是由于电磁波在地球或海水等导电介质中的“皮肤深度”较短，从而导致了高衰减。幸运的是，皮肤深度随着频率的降低而增加，因此千赫范围内的极低频(ELF)无线电波可以在名义上拒绝射频的环境中穿透很长距离。例如，海水中的皮肤深度为7.1米，频率为1千赫，如果能够有效地将极低频无线电波耦合到介质中，这将允许海底通信达到约30米的深度和合理的发射功率水平。然而，传统的天线非常大，不可能在这个频率范围内部署，而电短天线的功率效率非常低。这个项目试图通过采用一种全新的方法来解决这个根本问题，这种方法是基于永久磁铁的机械运动来实现的。这项拟议的研究将对射频拒绝环境中双向无线通信的可用性产生广泛影响。具体地说，它将使用便携、强大、低功耗和低成本的设备建立低数据速率无线链路。这种链路有望在水下或地下环境中的传感和联网、近地表地球物理、大气科学、搜救行动、采矿和石油和天然气勘探等领域有广泛的应用。便携式低功率ELF收发机的可用将通过实现与地面的双向低数据速率无线链路，立即实现射频拒绝环境中的通信。虽然有小型化和高灵敏度的ELF接收器可用，但ELF发射器(通常是偶极或环状天线)是实现这种链路的关键障碍，因为它们物理上很大，而且需要耗电。因此，该项目的重点是小型化和高能效的ELF发射器，它能够在导电介质中的中短程(最长约1公里)无线链路上实现双向通信。特别是，拟议的研究将探索一种全新的、全机械的极低频发射机设计方法，有可能使这一区域的电磁频谱得到有效利用。这个项目的主要智力贡献集中在这一总体方法的不同方面。它们包括：i)提出了基于有线或无线网络上的同步的分布式全机械发射机的概念，并展示了它如何克服现有ELF发射机体系结构的关键限制；ii)为使用具有显著机械惯性的系统来设计功率高效的无线通信创建了理论基础，从而将衰落信道上的信息传输的数学与机械设备的物理联系起来；和iii)为网络高速机器的精确控制奠定了第一个理论基础，通过无缝建模复杂的3-D(维度)机器参数变化及其与高速机器振动和发射器网络内的能量/功率波动的紧密耦合，有可能极大地提高当前的技术水平。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Propagation of radio frequency (RF) electromagnetic waves becomes infeasible in certain situations, thus resulting in "RF-denied" environments. Examples include underground and deep-water facilities (mines, shelters, storage areas, tunnels, submarines, undersea cables, etc.). Recent events have highlighted the importance of wireless communications with such environments. A prominent example is the July 2018 rescue of a soccer team from the Tham Luang Nang Non cave in Thailand after they had been trapped underground for over two weeks. Such RF-denied environments are fundamentally produced by the short "skin depth" of electromagnetic waves within conductive media such as earth or seawater, which results in high attenuation. Fortunately, the skin depth increases as the frequency decreases, so extremely low frequency (ELF) radio waves in the kHz range can penetrate long distances in nominally RF-denied environments. For example, the skin depth in sea water is 7.1 m at 1 kHz, which would allow undersea communications to depths of about 30 m with reasonable transmit power levels if one could effectively couple ELF radio waves into the medium. However, conventional antennas are extremely large and impossible to deploy in this frequency range, while electrically-short antennas have very poor power efficiency. This project seeks to solve this fundamental problem by adopting a radically new approach to ELF antennas that is based on the mechanical motion of permanent magnets. The proposed research will have a broad impact on the availability of bidirectional wireless communications in RF-denied environments. Specifically, it will enable low-data-rate wireless links to be established using portable, robust, low-power, and low-cost devices. Such links are expected to have a multitude of applications in fields such as sensing and networking in underwater or underground environments, near-surface geophysics, atmospheric science, search and rescue operations, mining, and oil and gas exploration.The availability of portable low-power ELF transceivers would immediately enable communications within RF-denied environments by enabling bidirectional low-data-rate wireless links with the surface. While miniaturized and highly-sensitive ELF receivers are available, ELF transmitters (typically dipole or loop antennas) are the key obstacles for realizing such links since they are physically large and power-hungry. Thus, this project focuses on miniaturized and power-efficient ELF transmitters that enable bidirectional communications over short- and medium-range (up to about 1 km) wireless links in conductive media. In particular, the proposed research will explore a fundamentally new all-mechanical approach to ELF transmitter design that has the potential to enable efficient use of this region of the EM spectrum. The major intellectual contributions of this project focus on different aspects of this overall approach. They include: i) Proposing the concept of distributed all-mechanical transmitters based on synchronization over either wired or wireless networks and showing how it overcomes the key limitations of existing ELF transmitter architectures; ii) Creating a theoretical basis for the design of power-efficient wireless communications using systems that have significant mechanical inertia, thus linking the mathematics of information transfer over fading channels to the physics of the mechanical devices; and iii) Laying the first theoretical groundwork for the precise control of networked high-speed machines, which has the potential to dramatically advance the current state of the art by seamlessly modeling complex 3-D (dimensional) machine parameter variations and their tight coupling with high-speed machine vibrations and energy/power fluctuations within the transmitter network.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.