@article{wu_hughes_2023, title={A Principal-Component Approach to Antenna Impedance Estimation at MISO Receivers}, volume={27}, ISSN={["1558-2558"]}, url={https://doi.org/10.1109/LCOMM.2022.3215305}, DOI={10.1109/LCOMM.2022.3215305}, abstractNote={Impedance matching between receive antenna and front-end significantly impacts channel capacity in wireless channels. To implement the optimal matching, the receiver must know the antenna impedance. This impedance varies with the antenna’s loading conditions. Techniques are derived to estimate antenna impedance, but the optimality of these techniques remains unclear. In this letter, we study antenna impedance estimation at MISO receivers over Rayleigh fading channels and derive the optimal ML estimator in closed-form. Numerical results suggest a computationally efficient, principal-components approach that estimates antenna impedance in real-time and shows sizable improvement against a reference estimator at low SNR.}, number={1}, journal={IEEE COMMUNICATIONS LETTERS}, author={Wu, Shaohan and Hughes, Brian L.}, year={2023}, month={Jan}, pages={288–292} }
 @article{wu_2022, title={Impedance Variation Detection at MISO Receivers}, url={https://doi.org/10.1109/LCOMM.2022.3208875}, DOI={10.1109/LCOMM.2022.3208875}, abstractNote={Techniques have been proposed to estimate unknown antenna impedance due to time-varying near-field loading conditions at multiple-input single-output (MISO) receivers. However, it remains unclear when a change occurs and impedance estimation becomes necessary. In this letter, we address this problem by formulating it as a hypothesis test. Our contributions include deriving a generalized likelihood-ratio test (GLRT) detector to decide if the antenna impedance has changed over two groups of packets. This GLRT formulation leads to a novel optimization problem, but we propose a binary search based algorithm to solve it efficiently. Our derived GLRT detector enjoys a better detection and false alarm trade-off when compared with a well-known, reference detector in simulations. As one result, more transmit diversity significantly improves detection accuracy at a given false alarm rate, especially in slow fading channels.}, journal={IEEE Communications Letters}, author={Wu, Shaohan}, year={2022} }
 @inproceedings{wu_2021, title={A Hybrid Approach to Joint Estimation of MIMO Channel and Antenna Impedance Matrices}, url={http://dx.doi.org/10.1109/ciss50987.2021.9400229}, DOI={10.1109/ciss50987.2021.9400229}, abstractNote={Antenna impedance matching significantly affects the channel capacity of compact MIMO receivers. When antenna impedance is known to the receiver, channel capacity can be optimized. However, channel capacity may diminish, when antenna impedance varies due to time-varying near-field loading. This motivates impedance estimation in real-time. In this paper, we derive joint MAP/ML estimators for channel and impedance matrices in closed-form. As one result, we develop a design principle leveraging a trade-off between channel and impedance estimation, which depends on transmit diversity.}, booktitle={2021 55th Annual Conference on Information Sciences and Systems (CISS)}, publisher={IEEE}, author={Wu, Shaohan}, year={2021}, month={Mar} }
 @article{wu_2019, title={Moments of Complex Gaussian Ratios}, volume={23}, url={https://doi.org/10.1109/LCOMM.2018.2883309}, DOI={10.1109/LCOMM.2018.2883309}, abstractNote={We study the moments of general complex Gaussian ratios (CGRs), whose numerator and denominator are correlated and have arbitrary mean. In particular, we calculate the mean of these ratios in a closed form and prove that the mean-square and higher order absolute moments are unbounded in general. Then, we show that the earlier results generalize existing results in the literature and apply the mean of general CGR in a novel problem of antenna impedance estimation at single-antenna receivers.}, number={1}, journal={IEEE Communications Letters}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Wu, Shaohan}, year={2019}, month={Jan}, pages={88–91} }
 @inproceedings{wu_hughes_2018, place={Pacific Grove, CA, USA}, title={A Hybrid Approach to Joint Estimation of Channel and Antenna impedance}, url={http://dx.doi.org/10.1109/acssc.2018.8645068}, DOI={10.1109/acssc.2018.8645068}, abstractNote={This paper considers a hybrid approach to joint estimation of channel information and antenna impedance, for single-input, single-output channels. Based on observation of training sequences via synchronously switched load at the receiver, we derive joint maximum a posteriori and maximum-likelihood (MAP/ML) estimators for channel and impedance over multiple packets. We investigate important properties of these estimators, e.g., bias and efficiency. We also explore the performance of these estimators through numerical examples.}, booktitle={2018 52nd Asilomar Conference on Signals, Systems, and Computers}, author={Wu, S. and Hughes, B.L.}, year={2018}, month={Oct}, pages={1789–1794} }
 @article{wu_hughes_2018, place={Princeton, NJ}, title={Training-Based Joint Channel and Antenna Impedance Estimation}, url={http://dx.doi.org/10.1109/ciss.2018.8362280}, DOI={10.1109/ciss.2018.8362280}, abstractNote={We consider joint channel and antenna impedance estimation based on training data for single-input, single-output channels. The aim is to leverage the resources available in wireless systems for channel estimation to estimate antenna impedance as well. We assume the transmitter sends a known training sequence, during which the receiver varies its impedance in a known way. Based on this observation, we derive maximum-likelihood estimators for the channel and impedance. We show these estimators generally exhibit heavy tails, and explore estimator performance through numerical examples. Our results suggest that antenna impedance can be accurately estimated, in exchange for a small, controlled increase in channel estimation error.}, journal={2018 52ND ANNUAL CONFERENCE ON INFORMATION SCIENCES AND SYSTEMS (CISS)}, author={Wu, Shaohan and Hughes, Brian L.}, year={2018} }