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1. Programming Wireless Security Through Learning‐Aided Spatiotemporal Digital Coding Metamaterial Antenna

   https://doi.org/10.1002/aisy.202300341  

   Nooraiepour, Alireza; Vosoughitabar, Shaghayegh; Wu, Chung-Tse_Michael; Bajwa, Waheed_U; Mandayam, Narayan_B (August 2023, Advanced Intelligent Systems)

   The advancement of future large‐scale wireless networks necessitates the development of cost‐effective and scalable security solutions. Specifically, physical layer (PHY) security has been put forth as a cost‐effective alternative to cryptographic mechanisms that can circumvent the need for explicit key exchange between communication devices. Herein, a space–time‐modulated digitally‐coded metamaterial (MTM) leaky wave antenna (LWA) is proposed that can enable PHY security by achieving the functionalities of directional modulation (DM) using a machine learning‐aided branch‐and‐bound (B&B) optimized coding sequence. Theoretically, it is first shown that the proposed space–time MTM antenna can achieve DM through both the spatial and spectral manipulation of the orthogonal frequency division multiplexing signal. Simulation results are then provided as proof‐of‐principle, demonstrating the applicability of the approach for achieving DM in various communication settings. Furthermore, a prototype of the proposed architecture controlled by a field‐programmable gate array is realized, which achieves DM via an optimized coding sequence carried out by the learning‐aided B&B algorithm corresponding to the states of the MTM LWA's unit cells. Experimental results confirm the theory behind the space–time‐modulated MTM LWA in achieving DM, which is observed via both the spectral harmonic patterns and bit error rate measurements. 

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2. Frequency selective wave beaming in nonreciprocal acoustic phased arrays

   https://doi.org/10.1038/s41598-020-77489-x  

   Adlakha, Revant; Moghaddaszadeh, Mohammadreza; Attarzadeh, Mohammad A.; Aref, Amjad; Nouh, Mostafa (December 2020, Scientific Reports)

   Abstract Acoustic phased arrays are capable of steering and focusing a beam of sound via selective coordination of the spatial distribution of phase angles between multiple sound emitters. Constrained by the principle of reciprocity, conventional phased arrays exhibit identical transmission and reception patterns which limit the scope of their operation. This work presents a controllable space–time acoustic phased array which breaks time-reversal symmetry, and enables phononic transition in both momentum and energy spaces. By leveraging a dynamic phase modulation, the proposed linear phased array is no longer bound by the acoustic reciprocity, and supports asymmetric transmission and reception patterns that can be tuned independently at multiple channels. A foundational framework is developed to characterize and interpret the emergent nonreciprocal phenomena and is later validated against benchmark numerical experiments. The new phased array selectively alters the directional and frequency content of the incident signal and imparts a frequency conversion between different wave fields, which is further analyzed as a function of the imposed modulation. The space–time acoustic phased array enables unprecedented control over sound waves in a variety of applications ranging from ultrasonic imaging to non-destructive testing and underwater SONAR telecommunication. 

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3. Time-Varying Metamaterial-Enabled Directional Modulation Schemes for Physical Layer Security in Wireless Communication Links

   https://doi.org/10.1145/3513088  

   Nooraiepour, Alireza; Vosoughitabar, Shaghayegh; Wu, Chung-Tse Michael; Bajwa, Waheed U.; Mandayam, Narayan B. (January 2022, ACM Journal on Emerging Technologies in Computing Systems)

   Novel transmission schemes, enabled by recent advances in the fields of metamaterial (MTM), leaky-wave antenna (LWA) and directional modulation, are proposed for enhancing the physical layer (PHY) security. MTM-LWAs, which offer compact, integrated, and cost-effective alternatives to the classic phased-array architectures, are particularly of interest for emerging wireless communication systems including Internet-of-Things (IoT). The proposed secure schemes are devised to accomplish the functionalities of directional modulation (DM) transmitters for orthogonal frequency-division multiplexing (OFDM) and non-contiguous (NC) OFDM transmissions, while enjoying the implementation benefits of MTM-LWAs. Specifically, transmitter architectures based on the idea of time-modulated MTM-LWA have been put forth as a promising solution for PHY security for the first time. The PHY security for the proposed schemes are investigated from the point of view of both passive and active attacks where an adversary aims to decode secret information and feed spurious data to the legitimate receiver, respectively. Numerical simulations reveal that even when the adversary employs sophisticated state-of-the-art deep learning based attacks, the proposed transmission schemes are resistant to these attacks and reliably guarantee system security. 

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4. Demo: Experimentation with Wideband Real-Time Adaptive Full-Duplex Radios

   https://doi.org/10.1145/3603269.3610853  

   Levin, Alon Simon; Kadota, Igor; Garikapati, Sasank; Zhang, Bo; Jolly, Aditya; Kohli, Manav; Seok, Mingoo; Krishnaswamy, Harish; Zussman, Gil (September 2023, ACM SIGCOMM'23)

   We present a set of experiments utilizing wideband real-time adaptive full-duplex (FD) radios, demonstrating simultaneous transmission and reception on the same frequency channel. Each FD radio consists of a circulator-based antenna interface, a switched-capacitor delay-line-based configurable Radio-Frequency Integrated Circuit (RFIC) that implements Self-Interference Cancellation (SIC), an FPGA that optimizes the RFIC configuration in under 1.1 sec and can adapt to environmental changes in under 0.3 sec, and a Software-Defined Radio (SDR) transmitting OFDM-like packets. We demonstrate a real-time adaptive FD radio that achieves the SIC necessary to reach the noise floor across a wide bandwidth of 50 MHz. Then, we use two FD radios to create a wireless link and showcase the superior FD throughput. 

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5. Nanoscale nonreciprocity via photon-spin-polarized stimulated Raman scattering

   https://doi.org/10.1038/s41467-019-11175-z  

   Lawrence, Mark; Dionne, Jennifer A. (July 2019, Nature Communications)

   Abstract Time reversal symmetry stands as a fundamental restriction on the vast majority of optical systems and devices. The reciprocal nature of Maxwell’s equations in linear, time-invariant media adds complexity and scale to photonic diodes, isolators, circulators and also sets fundamental efficiency limits on optical energy conversion. Though many theoretical proposals and low frequency demonstrations of nonreciprocity exist, Faraday rotation remains the only known nonreciprocal mechanism that persists down to the atomic scale. Here, we present photon-spin-polarized stimulated Raman scattering as a new nonreciprocal optical phenomenon which has, in principle, no lower size limit. Exploiting this process, we numerically demonstrate nanoscale nonreciprocal transmission of free-space beams at near-infrared frequencies with a 250 nm thick silicon metasurface as well as a fully-subwavelength plasmonic gap nanoantenna. In revealing all-optical spin-splitting, our results provide a foundation for compact nonreciprocal communication and computing technologies, from nanoscale optical isolators and full-duplex nanoantennas to topologically-protected networks. 

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