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1. Secure Intelligent Reflecting Surface-Aided Integrated Sensing and Communication

   https://doi.org/10.1109/TWC.2023.3280179  

   Hua, Meng; Wu, Qingqing; Chen, Wen; Dobre, Octavia A; Swindlehurst, A Lee (January 2024, IEEE Transactions on Wireless Communications)

   In this paper, an intelligent reflecting surface (IRS) is leveraged to enhance the physical layer security of an integrated sensing and communication (ISAC) system in which the IRS is deployed to not only assist the downlink communication for multiple users, but also create a virtual line-of-sight (LoS) link for target sensing. In particular, we consider a challenging scenario where the target may be a suspicious eavesdropper that potentially intercepts the communication-user information transmitted by the base station (BS). To ensure the sensing quality while preventing the eavesdropping, dedicated sensing signals are transmitted by the BS. We investigate the joint design of the phase shifts at the IRS and the communication as well as radar beamformers at the BS to maximize the sensing beampattern gain towards the target, subject to the maximum information leakage to the eavesdropping target and the minimum signal-to-interference-plus-noise ratio (SINR) required by users. Based on the availability of perfect channel state information (CSI) of all involved user links and the potential target location of interest at the BS, two scenarios are considered and two different optimization algorithms are proposed. For the ideal scenario where the CSI of the user links and the potential target location are perfectly known at the BS, a penalty-based algorithm is proposed to obtain a high-quality solution. In particular, the beamformers are obtained with a semi-closed-form solution using Lagrange duality and the IRS phase shifts are solved for in closed form by applying the majorization-minimization (MM) method. On the other hand, for the more practical scenario where the CSI is imperfect and the potential target location is uncertain in a region of interest, a robust algorithm based on the $$\cal S$$ -procedure and sign-definiteness approaches is proposed. Simulation results demonstrate the effectiveness of the proposed scheme in achieving a trade-off between the communication quality and the sensing quality, and also show the tremendous potential of IRS for use in sensing and improving the security of ISAC systems. 

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2. Online Millimeter Wave Phased Array Calibration Based on Channel Estimation

   https://doi.org/10.1109/VTS.2019.8758627  

   Moon, Thomas; Gaun, Junfeng; Hassanieh, Haitham (April 2019, 2019 IEEE 37th VLSI Test Symposium (VTS))

   This paper proposes a new over-the-air (OTA) calibration method for millimeter wave phased arrays. Our method leverages the channel estimation process which is a fundamental part of any wireless communication system. By performing the channel estimation while changing the phase of an antenna element, the phase response of the element can be estimated. The relative phase of the phased array can also be obtained by collecting all the estimated phase responses with a shared reference state. Hence, the phase mismatches of the phased array can be resolved. Unlike prior work, our calibration method embraces all the array components such as power-divider, phase shifter, amplifier and antenna and thus, spans the full chain. By overriding channel estimation, our proposed technique does not require any additional circuits for calibration. Furthermore, the calibration can be performed online without the need to pause the communication. We tested our method on an eight element phased array at 24GHz which we designed and fabricated in PCB for verification. The measured beam patterns prove the viability of our proposed method. 

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3. Retroreflective optical ISAC using OFDM: channel modeling and performance analysis

   https://doi.org/10.1364/OL.528029  

   Cui, Yuanxin; Chen, Chen; Cai, Yueping; Zeng, Zhihong; Liu, Min; Ye, Jia; Shao, Sihua; Haas, Harald (July 2024, Optics Letters)

   In this Letter, we propose and investigate a retroreflective optical integrated sensing and communication (RO-ISAC) system using orthogonal frequency division multiplexing (OFDM) and corner cube reflector (CCR). To accurately model the reflected sensing channel of the RO-ISAC system, both a point source model and an area source model are proposed according to the two main types of light sources that are widely used. Detailed theoretical and experimental results are presented to verify the accuracy of the proposed channel models and evaluate the communication and sensing performance of the considered RO-ISAC system. 

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4. Design of a Compact 24 GHz Antenna Array for Unmanned Aerial Vehicle-to-Vehicle (V2V) Communication

   https://doi.org/10.1109/AP-S/USNC-URSI47032.2022.9886200  

   Kakaraparty, Karthik; Roy, Sunanda; Mahbub, Ifana (September 2022, 2022 IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting (AP-S/URSI))

   This paper presents the design of a compact 4 × 4 antenna array suitable for unmanned aerial vehicle-to-vehicle (V2V) communication applications. The proposed antenna array can offer a narrow beamwidth, high gain, wide beam steering capability and is highly compact. The substrate material used is Rogers 5880 with a thickness of 0.2 mm, and copper is used for the patch and ground material with 0.14 mm thickness. The di-electric constant and the tangent loss of the Rogers substrate are 2.2 and 0.0004, respectively. 45° phase shifters are incorporated in the feeding paths to facilitate the beamsteering. The dimensions of the proposed antenna array are 32 × 32 × 0.48 mm 3 . The designed antenna array has the resonating frequency at 24 GHz and has a bandwidth of 0.83 GHz (3.5% fractional bandwidth). The measured far field gain of the designed antenna array is 16.7 dBi. The beamwidth derived from the array’s far-field radiation pattern is 14.6°, and the maximum beam steering range of the array is 102° along the θ axis. 

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5. Stabilizing Terahertz MIMO Channel Capacity with Controlled Diffuse Reflections

   https://doi.org/10.1109/ICC45041.2023.10279474  

   Singh, Suresh; Tran, Ha; Le, Thanh (May 2023, IEEE International Conference on Communications)

   Communication at the terahertz band is increasingly seen as vital for future short-range very high datarate channels. However, these channels suffer from significant environmental impairments and, as a result, providing coverage in indoor settings requires the use of directional line of sight (LoS) paths to visible users and reflected paths, using smooth metal reflectors, for users in the shadow of an obstruction. Previous work has shown that these types of reflected paths display similar characteristics to LoS paths and we call them R-LoS (reflected LoS). MIMO for LoS and R-LoS channels is feasible at terahertz frequencies and delivers very high capacity at some distances. Unfortunately, channel capacity varies greatly with small changes in distance (the channel matrix fluctuates between full rank and rank 1) which is undesirable for communication systems. In this paper, we utilize diffusive reflectors to create multiple reflections such that the MIMO channel capacity for R-LoS is better behaved. We conduct experiments at 410 GHz for reflections from different artificially created diffuse surfaces. We use measurements to estimate channel capacity for 2×2 MIMO when the only path is the diffuse reflected one. We show that by creating multiple controlled reflections, it is possible to achieve relatively stable capacity up to 13 - 16 bits/sec/Hz at varying distances. We also analyze the phase of the received signals and the beam profile in detail. Overall, our results indicate that by utilizing artificially created reflections, we can maintain a stable MIMO channel at high capacity. 

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