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1. Cross Evaluation of Multiple Access Schemes Using Post-Experimental Field Data for Underwater Acoustic Communications

   Xiangzhao Qin, Y. Rosa (September 2019, MTS/IEEE Oceans Conf)

   This paper proposes a post-experimental field data reuse method to test the single carrier modulation (SCM) and orthogonal frequency division multiplexing (OFDM) signals interchangeably for multiple access underwater acoustic (UWA) communications. We call this approach the cross evaluation that transforms a set of SCM or OFDM post-experimental field data to their corresponding OFDM or SCM scheme under test (SUT) via linear matrix operation such as fast Fourier transform (FFT) and its inverse (IFFT). At the receiver side, we derived a general framework of turbo equalization (TEQ) that alters the two physical layer schemes but keeps the passband transmitted and received data unchanged. Inherently, some efficient techniques such as pre-cursor and post-cursor interference cancellation (IC), and overlap adding (OLA) operations enhance the equivalence of input and output (I/O) system model between the SCM and OFDM. The proposed approach will bring the gap between the SCM and OFDM, and evaluate the two physical layer schemes under similar or tougher test conditions. The experimental results of the undersea 2008 Surface Processes and Acoustic Communications Experiment (SPACE08) have verified the feasibility of the cross evaluation approach in terms of the BER benchmark. 

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2. Physical Layer Encryption for Wireless OFDM Communication Systems

   https://doi.org/10.1007/s41635-020-00097-8  

   Jacovic, Marko; Juretus, Kyle; Kandasamy, Nagarajan; Savidis, Ioannis; Dandekar, Kapil R. (July 2020, Journal of Hardware and Systems Security)

   Our everyday lives are impacted by the widespread adoption of wireless communication systems integral to residential, industrial, and commercial settings. Devices must be secure and reliable to support the emergence of large scale heterogeneous networks. Higher layer encryption techniques such as Wi-Fi Protected Access (WPA/WPA2) are vulnerable to threats, including even the latest WPA3 release. Physical layer security leverages existing components of the physical or PHY layer to provide a low-complexity solution appropriate for wireless devices. This work presents a PHY layer encryption technique based on frequency induction for Orthogonal Frequency Division Multiplexing (OFDM) signals to increase security against eavesdroppers. The secure transceiver consists of a key to frequency shift mapper, encryption module, and modified synchronizer for decryption. The system has been implemented on a Virtex-7 FPGA. The additional hardware overhead incurred on the Virtex-7 for both the transmitter and the receiver is low. Both simulation and hardware evaluation results demonstrate that the proposed system is capable of providing secure communication from an eavesdropper with no decrease in performance as compared with the baseline case of a standard OFDM transceiver. The techniques developed in this paper provide greater security to OFDM-based wireless communication systems. 

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3. Securing Wireless Communication via Hardware-Based Packet Obfuscation

   https://doi.org/10.1007/s41635-019-00070-0  

   Chacko, James; Juretus, Kyle; Jacovic, Marko; Sahin, Cem; Kandasamy, Nagarajan; Savidis, Ioannis; Dandekar, Kapil R. (May 2019, Journal of Hardware and Systems Security)

   Obfuscation of the orthogonal frequency-division multiplexing (OFDM) physical layer is described in this paper as a means to enhance the security of wireless communication. The standardization of the communication channel between two trusted parties results in a variety of security threats, including vulnerabilities in WPA/WPA2 protocols that allow for the extraction of the software layer encryption key. Obfuscating the physical layer of the OFDM pipeline provides an additional layer of security in the event that the software layer key is compromised and allows for rolling updates of the physical layer key without altering the software layer key. The interleaver stage of the OFDM pipeline is redesigned to utilize a physical layer key, which is termed Phy-Leave. The Phy-Leave interleaver is evaluated through both MATLAB simulation and hardware prototyping on the Software Defined Communication (SDC) testbed using a Virtex6 FPGA. The implemented rolling physical layer key policy and Phy-Leave system resulted in a less than 1% increase in the area of a Virtex6 FPGA, demonstrating physical layer obfuscation as a means to increase the security of wireless communication without a significant cost in hardware. 

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4. Collaborative Hybrid ARQ for CDMA-based Reliable Underwater Acoustic Communications

   https://doi.org/10.1109/UComms.2018.8493231  

   Rahmati, Mehdi; Pompili, Dario; Petroccia, Roberto (August 2018, 2018 Fourth Underwater Communications and Networking Conference (UComms))

   Achieving high throughput and reliability in underwater acoustic networks is a challenging task due to the bandwidth-limited and unpredictable nature of the channel. In a multi-node structure, such as in the Internet of Underwater Things (IoUT), the efficiency of links varies dynamically because of the channel variations. When the channel is not in good condition, e.g., when in deep fade, channel-coding techniques fail to deliver the required information even with multiple rounds of retransmissions. An efficient and agile collaborative strategy among the nodes is required to assign appropriate resources to each link based on their status and capability. Hence, a cross-layer collaborative strategy is introduced to increase the throughput of the network by allocating unequal share of system resources to different nodes/links. The proposed solution adjusts the physical- and link-layer parameters in a collaborative manner for a Code Division Multiple Access (CDMA)-based underwater network. An adaptive Hybrid Automatic Repeat Request (HARQ) solution is employed to guarantee reliable communications against errors in poor communication links. Results are being validated using data collected from the LOON underwater testbed, which is hosted by the NATO STO Centre for Maritime Research and Experimentation (CMRE) in La Spezia, Italy. 

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5. The Underwater Backscatter Channel: Theory, Link Budget, and Experimental Validation

   https://doi.org/10.1145/3570361.3613265  

   Akbar, Waleed; Allam, Ahmed; Adib, Fadel (October 2023, ACM)

   Underwater backscatter is a recent networking technology that enables net-zero-power communication and sensing in underwater environments. Existing research on underwater backscatter has focused on designing and demonstrating early systems with impressive capabilities; however, what remains critically missing is an end-to-end analysis of the underwater backscatter communication channel, which is necessary to understand the potential of this technology to scale to real-world applications and practical deployments. This paper presents the first comprehensive theoretical and empirical analysis of the underwater backscatter channel, including the downlink and uplink of end-to-end backscatter. We introduce a closed-form analytical model that encompasses the physical properties of piezoelectric materials, electromechanical coupling, electrical impedance, and the underwater acoustic channel. We verify the correctness of this theoretical analysis through both finite-element-model physical simulations and real-world experimental validation in a river, demonstrating that the analytical model matches our real-world experiments with a median deviation of only 0.76 dB. Using this model, we then simulate the theoretical limits of underwater backscatter as a function of different design parameters and identify pathways for pushing underwater backscatter toward its theoretical limits. 

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