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1. Clutter Suppression for Target Detection Using Hybrid Reconfigurable Intelligent Surfaces

   https://doi.org/10.1109/RadarConf2351548.2023.10149583  

   Wang, Fangzhou ; Li, Hongbin ; Swindlehurst, A. Lee ( May 2023 , 2023 IEEE Radar Conference (RadarConf23))

   Reconfigurable intelligent surface (RIS) technology is a promising approach being considered for future wireless communications due to its ability to control signal propagation with low-cost elements. This paper explores the use of an RIS for clutter mitigation and target detection in radar systems. Unlike conventional reflect-only RIS, which can only adjust the phase of the reflected signal, or active RIS, which can also amplify the reflected signal at the cost of significantly higher complexity, noise, and power consumption, we exploit hybrid RIS that can configure both the phase and modulus of the impinging signal by absorbing part of the signal energy. Such RIS can be considered as a compromise solution between conventional reflect-only and active RIS in terms of complexity, power consumption, and degrees of freedoms (DoFs). We consider two clutter suppression scenarios: with and without knowledge of the target range cell. The RIS design is formulated by minimizing the received clutter echo energy when there is no information regarding the potential target range cell. This turns out to be a convex problem and can be efficiently solved. On the other hand, when target range cell information is available, we maximize the received signal-to-noise-plus-interference ratio (SINR). The resulting non-convex optimization problem is solved through fractional programming algorithms. Numerical results are presented to demonstrate the performance of the proposed hybrid RIS in comparison with conventional RIS in clutter suppression for target detection. 

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2. Tractable Planning for Coordinated Story Capture: Sequential Stochastic Decoupling

   Diptanil Chaudhuri, Hazhar Rahmani ( January 2021 , International Symposium on Distributed Autonomous Robotic Systems)

   We consider the problem of deploying robots to observe the evolution of a stochastic process in order to output a sequence of observations that fit some given specification. This problem often arises in contexts such as event reporting, situation depiction, and automated narrative generation. The paper extends our prior work by formulating and examining the multi-robot case: a team of robots move about, each recording what they observe, and, if they manage to capture some event, communicating that fact with the group. In the end, all events from all the robots are collated to provide a cumulative output. A plan is used to decide what each robot will attempt to capture next, based on the state of the world and the events that have been captured (collectively) so far. This paper focuses on the question of how to compute effective multi-robot plans. A monolithic treatment, involving the optimal selection of joint choices, i.e., choosing the next elements to attempt to capture by all robots, is formulated where costs are minimized in an expected sense. Since such plans are prohibitive to compute, variants based on an approximation scheme based on solving a sequence of individual planning problems are then introduced. This scheme sacrifices some solution quality but requires far less computational expense; we show this permits one to scale to greater numbers of robots. 

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3. Hybrid Ris-Assisted Interference Mitigation for Spectrum Sharing

   https://doi.org/10.1109/ICASSP49357.2023.10096639  

   Wang, Fangzhou ; Swindlehurst, A. Lee ( June 2023 , ICASSP 2023 - 2023 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP))

   This paper explores reconfigurable intelligent surfaces (RIS) for mitigating cross-system interference in spectrum sharing applications. Unlike conventional reflect-only RIS that can only adjust the phase of the incoming signal, a hybrid RIS is considered that can configure the phase and modulus of the impinging signal by absorbing part of the signal energy. We investigate two spectrum sharing scenarios: (1) Spectral coexistence of radar and communication systems, where a convex optimization problem is formulated to minimize the Frobenius norm of the channel matrix from the communication base station to the radar receiver, and (2) Spectrum sharing in device-to-device (D2D) communications, where a max-min scheme that optimizes the worst-case signal-to-interference-plus-noise ratio (SINR) among the D2D links is formulated, and then solved through fractional programming. Numerical results show that with a sufficient number of elements, the hybrid RIS can in many cases completely eliminate the interference, unlike a conventional non-absorptive RIS. 

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4. Power control and beamforming design for SWIPT in AF two-way relay networks

   https://doi.org/10.1109/ICCS.2016.7833632  

   Wang, Wei ; Wang, Rui ; Mehrpouyan, Hani ; Zhang, Guoan ( December 2016 , IEEE International Conference on Communication Systems (ICCS), 2016)

   In this paper, we study the problem of joint power control and beamforming design for simultaneous wireless information and power transfer (SWIPT) in an amplify-and-forward (AF) based two-way relaying (TWR) network. The considered system model consists of two source nodes and a relay node. Two single-antenna source nodes receive information and energy simultaneously via power splitting (PS) from the signals sent by a multi-antenna relay node. Our objective is to maximize the weighted sum energy at the two source nodes subject to quality of service (QoS) constraints and the transmit power constraints. However, the joint optimization of the relay beamforming matrix, the source transmit power and PS ratio is intractable. To find a closed-form solution of the formulated problem, we decouple the primal problem into two subproblems. In the first problem, we intend to optimize the beamforming vectors for given transmit powers and PS ratio. In the second subproblem, we optimize the remaining parameters with obtained beamformers. It is worth noting that although the corresponding subproblem are nonconvex, the optimal solution of each subproblem can be found by using certain techniques. The iterative optimization algorithm finally converges. Simulation results verify the effectiveness of the proposed joint design. 

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5. RIS-Assisted Interference Mitigation for Uplink NOMA

   https://doi.org/10.1109/WCNC55385.2023.10119107  

   Tabeshnezhad, Azadeh ; Swindlehurst, A. Lee ; Svensson, Tommy ( March 2023 , 2023 IEEE Wireless Communications and Networking Conference (WCNC))

   Non-orthogonal multiple access (NOMA) has become a promising technology for next-generation wireless communications systems due to its capability to provide access for multiple users on the same resource. In this paper, we consider an uplink power-domain NOMA system aided by a reconfigurable intelligent surface (RIS) in the presence of a jammer that aims to maximize its interference on the base station (BS) uplink receiver. We consider two kinds of RISs, a regular RIS whose elements can only change the phase of the incoming wave, and an RIS whose elements can also attenuate the incoming wave. Our aim is to minimize the total power transmitted by the user terminals under quality-of-service constraints by controlling both the propagation from the users and the jammer to the BS with help of the RIS. The resulting objective function and constraints are both non-linear and non-convex, so we address this problem using numerical optimization. Our numerical results show that the RIS can help to dramatically reduce the per user required transmit power in an interference-limited scenario. 

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