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1. Impact of Blockers on User Equipment Angular Diversity in THz Microcellular Scenarios

   https://doi.org/10.1109/ICC45041.2023.10279799  

   Gomez-Ponce, Jorge; Abbasi, Naveed A.; Abu-Surra, Shadi; Xu, Gary; Zhang, Charlie; Molisch, Andreas F. (May 2023, IEEE International Conference on Communications)

   The availability of large bandwidths in the terahertz (THz) band will be a crucial enabler of high data rate applications in next-generation wireless communication systems. The urban microcellular scenario is an essential deployment scenario where the base station (BS) is significantly higher than the user equipment (UE). Under practical operating conditions, moving objects (i.e., blockers) can intermittently obstruct various parts of the BSUE link. Therefore, in the current paper, we analyze the effect of such blockers. We assume a blockage of the strongest beam pair and investigate the availability and extent of angular diversity, i.e., alternative beampairs that can sustain communication when the strongest is blocked. The analysis uses double-directional channel measurements in urban microcellular scenarios for 145- 146 GHz with BS-UE distances between 18 to 83 m. We relate the communication-system quantities of beam diversity and capacity to the wireless propagation conditions. We show that the SNR loss due to blockage depends on the blocked angular range and the specific location, and we find mean blockage loss to be on the order of 10-20 dB in line-of-sight (LOS) and 5-12 dB in NLOS (non-LOS). This analysis can contribute to the design of intelligent algorithms or devices (e.g., beamforming, intelligent reflective surfaces) to overcome the impact of the blockage. 

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2. Second-Best Beam-Alignment via Bayesian Multi-Armed Bandits

   https://doi.org/10.1109/GLOBECOM38437.2019.9013578  

   Hussain, Muddassar; Michelusi, Nicolo (December 2019, IEEE Global Communications Conference (GLOBECOM))

   Millimeter-wave (mm-wave) systems rely on narrow- beams to cope with the severe signal attenuation in the mm- wave frequency band. However, susceptibility to beam mis- alignment due to mobility or blockage requires the use of beam- alignment schemes, with huge cost in terms of overhead and use of system resources. In this paper, a beam-alignment scheme is proposed based on Bayesian multi-armed bandits, with the goal to maximize the alignment probability and the data-communication throughput. A Bayesian approach is proposed, by considering the state as a posterior distribution over angles of arrival (AoA) and of departure (AoD), given the history of feedback signaling and of beam pairs scanned by the base-station (BS) and the user- end (UE). A simplified sufficient statistic for optimal control is identified, in the form of preference of BS-UE beam pairs. By bounding a value function, the second-best preference policy is formulated, which strikes an optimal balance between exploration and exploitation by selecting the beam pair with the current second-best preference. Through Monte-Carlo simulation with analog beamforming, the superior performance of the second- best preference policy is demonstrated in comparison to existing schemes based on first-best preference, linear Thompson sampling, and upper confidence bounds, with up to 7%, 10% and 30% improvements in alignment probability, respectively. 

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3. mm Wave Antenna Gain Switching to Mitigate Indoor Blockage

   https://doi.org/10.1109/APUSNCURSINRSM.2018.8608384  

   Bshara, Oday; Liu, Yuqiao; Tekin, Ibrahim; Taskin, Baris; Dandekar, Kapil R. (July 2018, 2018 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting)

   Indoor blockage in a millimeter wave (mmWave) wireless communication link introduces significant signal attenuation. Solving the indoor blockage problem is critical to effectively using the unlicensed 60 GHz band spectrum. This work used various V-band horn antennas to collect signal measurements in an indoor lab environment. As an object blocks the Tx- Rx line of sight (LOS) path, the signal fades deeply. Experimental results showed that switching to a wider beam with lower gain has the potential to partially restore or maintain a communicating link. Effective beam switching and coordinated beam steering can shorten deep fades which is crucial for mm Wave communication systems that are very sensitive to the spatial characteristics of the environment. The experimental results in this paper thus motivate the design of future indoor mm Wave antennas capable of beam switching and facilitate fast beam search. 

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4. THz Band Channel Measurements and Statistical Modeling for Urban Microcellular Environments

   https://doi.org/10.1109/TWC.2023.3328236  

   Abbasi, Naveed A; Gomez-Ponce, Jorge; Kondaveti, Revanth; Kumar, Ashish; Bhagat, Eshan; Rao, Rakesh_N S; Abu-Surra, Shadi; Xu, Gary; Zhang, Charlie; Molisch, Andreas F (July 2024, IEEE Transactions on Wireless Communications)

   The THz band has attracted considerable attention for next-generation wireless communications due to the large amount of available bandwidth that may be key to meet the rapidly increasing data rate requirements. Before deploying a system in this band, a detailed wireless channel analysis is required as the basis for proper design and testing of system implementations. One of the most important deployment scenarios of this band is the outdoor microcellular environment, where the Transmitter (Tx) and the Receiver (Rx) have a significant height difference (typically ≥10 m). In this paper, we present double-directional (i.e., directionally resolved at both link ends) channel measurements in such a microcellular scenario encompassing street canyons and an open square. Measurements are done for a 1 GHz bandwidth between 145–146 GHz and an antenna beamwidth of 13 degree; distances between Tx and Rx are up to 85 m and the Tx is at a height of 11.5 m from the ground. The measurements are analyzed to estimate path loss, shadowing, delay spread, angular spread, and multipath component (MPC) power distribution. These results allow the development of more realistic and detailed THz system performance assessment. 

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5. 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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