Research

Beyond Diagonal Reconfigurable Intelligent Surface (BD-RIS)

Reconfigurable intelligent surface (RIS) is a promising technology for the 6G communication. It can intelligently adjust the phase shift of each element to reconfigure the wireless environment to enhance the wireless system. Conventional RIS, also known as single-connected RIS, does not connect RIS element with each other and is characterized by a diagonal scattering matrix, which has limited performance.

To breakthrough the limitation, we have proposed beyond diagonal reconfigurable intelligent surface (BD-RIS). BD-RIS connects RIS elements through tunable impedance component, so that it is characterized by a beyond diagonal scattering matrix. Specifically, we have proposed different achitectures including fully/group-connected BD-RIS and different modes including hybrid transmitting and reflecting BD-RIS and multi-sector BD-RIS, which provide a full space coverage and high channel gain, and developed the corresponding algorithms for BD-RIS optimization. We have also prototyped and experimented BD-RIS.

Related publications:

1. B. Peng, K. L. Besser, S. Shen, F. S. Poschmann, R. Raghunath, D. Mittleman, V. Jamali, and E. A. Jorswieck, "RIS-assisted NOMA with partial CSI and mutual coupling: a machine learning approach," GLOBECOM 2025 - 2025 IEEE Global Communications Conference, Taipei, Taiwan, 2025.

2. B. Peng, K. L. Besser, S. Shen, F. S. Poschmann, R. Raghunath, D. Mittleman, V. Jamali, and E. A. Jorswieck, "RISnet: a domain-knowledge driven neural network architecture for RIS optimization with mutual coupling and partial CSI," in IEEE Transactions on Wireless Communications, vol. 24, no. 5, pp. 4469-4482, May 2025. [Link]

3. H. Li, M. Nerini, S. Shen, and B. Clerckx, "Beyond diagonal reconfigurable intelligent surfaces in wideband OFDM communications: circuit-based modeling and optimizations," in IEEE Transactions on Wireless Communications, vol. 24, no. 4, pp. 3623-3636, April 2025. [Link]

4. B. Wang, H. Li, S. Shen, Z. Cheng, and B. Clerckx, "A dual-function radar-communication system empowered by beyond diagonal reconfigurable intelligent surface," in IEEE Transactions on Communications, vol. 73, no. 3, pp. 1501-1516, March 2025. [Link]

5. H. Li, M. Nerini, S. Shen, and B. Clerckx, "Wideband modeling and beamforming for beyond diagonal reconfigurable intelligent surfaces," 2024 IEEE 25th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), Lucca, Italy, 2024, pp. 926-930. [Link]

6. H. Li, S. Shen, Y. Zhang, and B. Clerckx, "Channel estimation and beamforming for beyond diagonal reconfigurable intelligent surfaces," in IEEE Transactions on Signal Processing, vol. 72, pp. 3318-3332, 2024. [Link]

7. Z. Li, A. Dubey, S. Shen*, N. K. Kundu, J. Rao, and R. Murch, "Radio tomographic imaging with reconfigurable intelligent surfaces," in IEEE Transactions on Wireless Communications, vol. 23, no. 11, pp. 15784-15797, Nov. 2024. [Link]

8. M. Nerini, S. Shen, H. Li, M.D. Renzo, and B. Clerckx, "A universal framework for multiport network analysis of reconfigurable intelligent surfaces," in IEEE Transactions on Wireless Communications, vol. 23, no. 10, pp. 14575-14590, Oct. 2024. [Link]

9. M. Nerini, S. Shen*, H. Li, and B. Clerckx, "Beyond diagonal reconfigurable intelligent surfaces utilizing graph theory: modeling, architecture design, and optimization," in IEEE Transactions on Wireless Communications, vol. 23, no. 8, pp. 9972-9985, Aug. 2024. [Link]

10. H. Li, S. Shen*, and B. Clerckx, "Synergizing beyond diagonal reconfigurable intelligent surface and rate-splitting multiple access," in IEEE Transactions on Wireless Communications, vol. 23, no. 8, pp. 8717-8729, Aug. 2024. [Link]

11. Y. Zhou, Y. Liu, H. Li, Q. Wu, S. Shen, and B. Clerckx, "Optimizing power consumption, energy efficiency and sum-rate using beyond diagonal RIS - a unified approach," in IEEE Transactions on Wireless Communications, vol. 23, no. 7, pp. 7423-7438, July 2024. [Link]

12. M. Nerini, S. Shen, and B. Clerckx, “Static grouping strategy design for beyond diagonal reconfigurable intelligent surfaces," in IEEE Communications Letters, vol. 28, no. 7, pp. 1708-1712, July 2024. [Link]

13. Z. Liu, Y. Liu, S. Shen, Q. Wu, and Q. Shi, "Enhancing ISAC network throughput using beyond diagonal RIS," in IEEE Wireless Communications Letters, vol. 13, no. 6, pp. 1670-1674, June 2024. [Link]

14. H. Li, S. Shen, M. Nerini, M.D. Renzo, and B. Clerckx, "Beyond diagonal reconfigurable intelligent surfaces with mutual coupling: modeling and optimization," in IEEE Communications Letters, vol. 28, no. 4, pp. 937-941, April 2024. [Link]

15. H. Li, S. Shen*, M. Nerini, and B. Clerckx, "Reconfigurable intelligent surfaces 2.0: beyond diagonal phase shift matrices," in IEEE Communications Magazine, vol. 62, no. 3, pp. 102-108, March 2024. [Link] (ESI Highly Cited Paper)

16. M. Nerini, S. Shen*, and B. Clerckx, "Closed-form global optimization of beyond diagonal reconfigurable intelligent surfaces," in IEEE Transactions on Wireless Communications, vol. 23, no. 2, pp. 1037-1051, Feb. 2024. [Link] (ESI Highly Cited Paper)

17. M. Nerini, S. Shen*, and B. Clerckx, "Discrete-value group and fully connected architectures for beyond diagonal reconfigurable intelligent surfaces," in IEEE Transactions on Vehicular Technology, vol. 72, no. 12, pp. 16354-16368, Dec. 2023. [Link]

18. H. Li, S. Shen*, and B. Clerckx, "A dynamic grouping strategy for beyond diagonal reconfigurable intelligent surfaces with hybrid transmitting and reflecting mode," in IEEE Transactions on Vehicular Technology, vol. 72, no. 12, pp. 16748-16753, Dec. 2023. [Link]

19. H. Li, S. Shen*, and B. Clerckx, "Beyond diagonal reconfigurable intelligent surfaces: a multi-sector mode enabling highly directional full-space wireless coverage," in IEEE Journal on Selected Areas in Communications, vol. 41, no. 8, pp. 2446-2460, Aug. 2023. [Link]

20. H. Li, S. Shen*, and B. Clerckx, "Beyond diagonal reconfigurable intelligent surfaces: from transmitting and reflecting modes to single-, group-, and fully-connected architectures," in IEEE Transactions on Wireless Communications, vol. 22, no. 4, pp. 2311-2324, April 2023. [Link] (ESI Highly Cited Paper)

21. Z. Li, N. K. Kundu, J. Rao, S. Shen*, M. R. Mckay, R. Murch, "Performance analysis of RIS-assisted communications with element grouping and spatial correlation," in IEEE Wireless Communications Letters, vol. 12, no. 4, pp. 630-634, April 2023. [Link]

22. T. Fang, Y. Mao, S. Shen, Z. Zhu, and B. Clerckx, "Fully connected reconfigurable intelligent surface aided rate-splitting multiple access for multi-user multi-antenna transmission," 2022 IEEE International Conference on Communications Workshops (ICC Workshops), Seoul, Korea, Republic of, 2022, pp. 675-680. [Link]

23. J. Rao, Y. Zhang, S. Tang, Z. Li, S. Shen, C. Y. Chiu, and R. Murch, "A novel reconfigurable intelligent surface for wide-angle passive beamforming," in IEEE Transactions on Microwave Theory and Techniques, vol. 70, no. 12, pp. 5427-5439, Dec. 2022. [Link]

24. N. K. Kundu, Z. Li, J. Rao, S. Shen*, M. R. Mckay, and R. Murch, "Optimal grouping strategy for reconfigurable intelligent surfaces assisted wireless communications," in IEEE Wireless Communications Letters, vol. 11, no. 5, pp. 1082-1086, May 2022. [Link]

25. S. Shen*, B. Clerckx, and R. Murch, "Modeling and architecture design of reconfigurable intelligent surfaces using scattering parameter network analysis," in IEEE Transactions on Wireless Communications, vol. 21, no. 2, pp. 1229-1243, Feb. 2022. [Link] (ESI Highly Cited Paper)

Toward Electromagnetic Information Theory: Exploring Novel Antenna Approaches

Electromagnetic Information Theory (EIT) is an interdisciplinary framework to design next-generation wireless communications that amalgamates electromagnetic theory, signal processing theory, and information theory. EIT is aimed at conceiving and designing physically consistent communication models, algorithms, and protocols for the transmission and processing of information in communication networks. In simple terms, EIT can be defined as physics-aware information theory and signal processing for communications. Under the framework of EIT, we have actively explored novel antenna technologies to enhance the next-generation wireless systems, including electronically steerable parasitic array radiator (ESPAR) antenna and pixel antennas.

ESPAR antennas consist of an active antenna and multiple parasitic antennas loaded by tunable reactance and the radiation pattern of ESPAR antennas can be adjusted by tuning the loaded reactance, so that transmit symbol can be modulated in beamspace. Based on this, we have developed two methods based on characteristic mode analysis and pattern correlation decomposition to find the orthogonal beamspace basis for beamspace channel representation, spatial modulation in beamspace to achieve multiplexing gain, bandwidth efficient pulse shaping technique, broadband OFDM transmission, and beamforming for compact massive MIMO systems. We have shown that using ESPAR can greatly enhance the spectral and energy efficiencies of wireless systems with compact size, lower complexity, and low cost.

On the other hand, pixel antenna is based on discretizing a continuous radiating structure into a grid of small metals, named as pixels, which are connected through RF switches, so that the radiation pattern can be flexibly adjusted by controlling the switch states. Based on this, we have proposed antenna coding technique, which is to control antenna configuration and characteristics through binary codes representing switches for enhancing wireless systems, and developed the antenna coding optimization in different scenarios including SISO, MIMO, multi-user MIMO systems. We have shown that antenna coding based on pixel antennas is a promising and emerging technology to enable the next-generation wireless systems.

Related publications:

1. C. Zhang, S. Shen*, H. Li, D. Ma, Z. Han, J. Qian, B. Clerckx, and R. Murch, "Compact millimeter wave massive MIMO system utilizing ESPAR," in IEEE Transactions on Communications (Early Access), 2025. [Link]

2. H. Li and S. Shen, "Antenna coding design based on pixel antennas for multi-user MISO systems," 2025 IEEE 26th International Workshop on Signal Processing and Artificial Intelligence for Wireless Communications (SPAWC), Surrey, UK, 2025, pp. 1-5.

3. T. Qiao, S. Shen, Z. Han, C. Y. Chiu, Q. S. Cheng, and R. Murch, "A wideband compact 20-port antenna design developed using electromagnetic information theory for MIMO-OFDM systems," 2025 IEEE International Symposium on Antennas and Propagation and ITNC-USNC-URSI Radio Science Meeting, Ottawa, Canada, 2025.

4. T. Qiao, Z. Han, S. Shen, C. Y. Chiu, Q. S. Cheng, and R. Murch, "Performance bounds on the partial gain to Q-factor quotient for loaded N-port antenna systems," 2024 IEEE International Symposium on Antennas and Propagation and INC/USNC-URSI Radio Science Meeting (AP-S/INC-USNC-URSI), Firenze, Italy, 2024, pp. 2003-2004. [Link]

5. J. Qian, S. Shen*, and R. Murch, "Including antenna effects into capacity formulations of line-of-sight MIMO channels," in IEEE Wireless Communications Letters, vol. 13, no. 9, pp. 2477-2481, Sept. 2024. [Link]

6. Z. Han, Y. Zhang, S. Shen, J. Rao, T. Qiao, J. Jin, Q. Wang, and Ross Murch, "Characteristic modes for orthogonal beamsteering using ESPAR," 2024 IEEE International Symposium on Antennas and Propagation and INC/USNC-URSI Radio Science Meeting (AP-S/INC-USNC-URSI), Firenze, Italy, 2024, pp. 585-586. [Link]

7. Z. Han, S. Wang, J. Jin, Q. Wang, S. Shen and R. Murch, “Loaded N-port structure for reconfigurable compact MIMO antenna design,” 2023 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications (IMWS-AMP), Chengdu, China, 2023, pp. 1-3. [Link]

8. Z. Han, S. Shen*, Y. Zhang, S. Tang, C. Y. Chiu, and R. Murch, "Using loaded N-port structures to achieve the continuous-space electromagnetic channel capacity bound," in IEEE Transactions on Wireless Communications, vol. 22, no. 11, pp. 7592-7605, Nov. 2023. [Link]

9. Z. Han, S. Shen*, Y. Zhang, S. Tang, C. Y. Chiu, and R. Murch, "Spectrally efficient pulse shaping for beamspace space shift keying in single-RF ESPAR systems," in IEEE Transactions on Vehicular Technology, vol. 72, no. 8, pp. 10548-10560, Aug. 2023. [Link]

10. C. Zhang, S. Shen*, Z. Han, and R. Murch, "Analog beamforming using ESPAR for single-RF precoding systems," in IEEE Transactions on Wireless Communications, vol. 22, no. 7, pp. 4387-4400, July 2023. [Link]

11. Z. Han, S. Shen*, Y. Zhang, S. Tang, C. Y. Chiu, and R. Murch, "Single-RF MIMO-OFDM using ESPAR," in IEEE Transactions on Vehicular Technology, vol. 72, no. 5, pp. 6080-6089, May 2023. [Link]

12. Z. Han, S. Shen*, Y. Zhang, C. Y. Chiu, and R. Murch, "A pattern correlation decomposition method for analysis of ESPAR in single-RF MIMO systems," in IEEE Transactions on Wireless Communications, vol. 21, no. 7, pp. 4654-4668, July 2022. [Link]

13. Z. Han, Y. Zhang, S. Shen*, Y. Li, C. Y. Chiu, and R. Murch, "Characteristic mode analysis of ESPAR for single-RF MIMO systems," in IEEE Transactions on Wireless Communications, vol. 20, no. 4, pp. 2353-2367, April 2021. [Link]

Integrated Sensing and Communications Utilizing Frequency Diverse Antennas

Integrated sensing and communications plays an important role in the 6G communication. However, conventional sensing technologies based on synthetic aperture radar or phased array methods generally rely on mechanical or electrical scanning to collect the sensing data, which results in low sensing speed and high complexity. Therefore, a sensing method with low complexity while maintaining high resolution is desired.

To achieve this goal, we have developed an efficient sensing method based on MIMO frequency diverse antennas. Frequency diverse antennas provide diverse radiation patterns at different frequencies, e.g. the beam can be steered by sweeping frequency. Hence, using frequency diverse antennas can collect sensing data simply by sweeping frequency without mechanical moving or active circuit switching. In addition, combining frequency diverse antennas with MIMO technique can provides additional degrees of freedom and high spatial resolution to further enhance sensing. We have shown that using MIMO frequency diverse antennas provides a higher sensing capacity and enhanced sensing quality.

Related publications:

1. D. Ma, S. Shen*, H. Zhou, C. Zhang, Q. Zhang and R. Murch, "Integrated sensing, identification, and backscatter Communication system utilizing inverse scattering approach," in IEEE Transactions on Antennas and Propagation, vol. 73, no. 8, pp. 5877-5889, Aug. 2025. [Link]

2. D. Ma, A. Dubey, Z. Xu, S. Shen, Q. Zhang, and R. Murch, "Reducing the number of measurement nodes in RF imaging using antenna pattern diversity with an extended Rytov approximation," in IEEE Transactions on Antennas and Propagation, vol. 71, no. 11, pp. 8881-8893, Nov. 2023. [Link]

3. D. Ma, Y. Zhang, A. Dubey, S. Deshmukh, S. Shen*, Q. Zhang*, and R. Murch, "Millimeter-wave 3D imaging using leaky-wave antennas and an extended Rytov approximation in a frequency-diverse MIMO system," in IEEE Transactions on Microwave Theory and Techniques, vol. 71, no. 4, pp. 1809-1825, April 2023. [Link]

4. D. Ma, J. Zhong, S. Shen, A. Dubey, C. Zhang, Q. Zhang, and R. Murch, "Single-shot frequency-diverse near-field imaging using high-scanning-rate leaky-wave antenna," in IEEE Transactions on Microwave Theory and Techniques, vol. 69, no. 7, pp. 3399-3412, July 2021. [Link]

MIMO Ambient Backscatter Communication System

Ambient backscatter communication (AmBC) is a novel technology for implementing battery-free wireless communications. It enables backscatter devices to transmit information to near-by receivers using ambient RF signals. Thus, AmBC has the benefit that it does not need any dedicated RF source and frequency allocation. However, the key issue in AmBC is the low power level and the uncertainty of the ambient RF signal, making signal detection difficult.

To solve this issue, we have proposed multi-antenna Tag and multi-antenna Reader AmBC systems, i.e. MIMO AmBC systems, and develop efficient signal detection algorithms based on average power mechanism and ratio detector to overcome the weakness and uncertainty of the ambient RF signal. Based on the proposed MIMO AmBC systems, we have also applied coherent and non-coherent orthogonal space-time codes to enhance the signal detection performance, and we have shown that the bit error rate can be effectively decreased by using MIMO AmBC system with more Tag or Reader antennas, making AmBC promising for battery-free wireless communications.

Related publications:

1. W. Liu, S. Shen*, D. H. K. Tsang, R. K. Mallik, and R. Murch, "An efficient ratio detector for ambient backscatter communication," in IEEE Transactions on Wireless Communications, vol. 23, no. 6, pp. 5908-5921, June 2024. [Link]

2. W. Liu, S. Shen*, D. H. K. Tsang, and R. Murch, "MIMO ambient backscatter communication: capacity maximization and beamforming optimization," in IEEE Transactions on Vehicular Technology, vol. 72, no. 12, pp. 15829-15843, Dec. 2023. [Link]

3. W. Liu, S. Shen, C. Zhang, D. H. K. Tsang, and R. Murch, "Optimal antenna selection and time sharing in RF-powered cognitive networks with ambient backscatter communication," 2023 IEEE 97th Vehicular Technology Conference (VTC2023-Spring), Florence, Italy, 2023, pp. 1-6. [Link]

4. W. Liu, S. Shen*, D. H. K. Tsang, and R. Murch, "Enhancing ambient backscatter communication utilizing coherent and non-coherent space-time codes," in IEEE Transactions on Wireless Communications, vol. 20, no. 10, pp. 6884-6897, Oct. 2021. [Link]

Nonlinear Multi-Sine MIMO Wireless Power Transfer System

Wireless power transfer (WPT) uses rectenna to harvest RF energy from a dedicated source, instead of ambient source. WPT is related to ambient RF energy harvesting, but it is more reliable, controllable, user-friendly, and cost-effective to energize massive devices of Internet-of-Things. In addition, it supports simultaneous information and power transfer to enhance both spectrum and energy efficiency. However, the key challenge of WPT is to increase the power transfer efficiency.

To overcome this challenge, we have proposed the nonlinear multi-sine MIMO WPT systems to enhance the transfer efficiency and developed joint waveform, beamforming, and DC/RF combining optimization algorithm. We have shown that the nonlinear multi-sine MIMO WPT with optimized waveform and beamforming can greatly improve the transfer efficiency. In addition, we have prototyped a WPT system with distributed antennas using antenna and frequency selections and a limited feedback WPT system using adaptive waveform and beamforming, to experimentally demonstrate the enhanced transfer efficiency and the capability and promise of WPT.

Related publications:

1. M. Kumar, C. Song, Y. Huang, J. Zhou, S. Shen, P. Mitcheson, Y. Ding, G. Goussetis, and M. Wagih, "From MHz to mmWaves: a review of application-driven UK-based wireless power research," in IEEE Microwave Magazine, vol. 26, no. 7, pp. 78-92, July 2025. [Link]

2. T. Zhan, S. Shen and D. H. K. Tsang, "Machine learning based accurate modeling of rectenna nonlinear behavior," 2025 IEEE Wireless Power Technology Conference and Expo (WPTCE), Rome, Italy, 2025, pp. 1-6. [Link]

3. S. Shen*, J. Kim, and B. Clerckx, "Closed-loop wireless power transfer with adaptive waveform and beamforming: design, prototype, and experiment," in IEEE Journal of Microwaves, vol. 3, no. 1, pp. 29-42, Jan. 2023. [Link]

4. S. Shen*, J. Kim, C. Song, and B. Clerckx, "Wireless power transfer with distributed antennas: system design, prototype, and experiments," in IEEE Transactions on Industrial Electronics, vol. 68, no. 11, pp. 10868-10878, Nov. 2021. [Link]

5. S. Shen* and B. Clerckx, "Joint waveform and beamforming optimization for MIMO wireless power transfer," in IEEE Transactions on Communications, vol. 69, no. 8, pp. 5441-5455, Aug. 2021. [Link]

6. S. Shen* and B. Clerckx, "Beamforming optimization for MIMO wireless power transfer with nonlinear energy harvesting: RF combining versus DC combining," in IEEE Transactions on Wireless Communications, vol. 20, no. 1, pp. 199-213, Jan. 2021. [Link]

Pixel Antenna Design and Optimization

Pixel antennas are a flexible antenna design approach. The concept of pixel antenna is to discretize a continuous radiating structure with arbitrary shape and size into a grid of small metals, named as pixels. Adjacent pixels can be connected through either hardwires or RF switches. Given a pixelized structure, different antenna characteristics, including operating frequency, radiation pattern, and polarizations, can be obtained by changing the connection states between them. Therefore, pixel antennas can be used to either implement highly reconfigurable antennas (by using RF switches) or flexibly design and optimize antenna (by using hardwire).

Inspired by this, we have proposed the pattern-reconfigurable pixel antennas, which can reconfigure the radiation pattern to implement full-space single- or multibeam steering. In addition to reconfigurable pixel antenna designs, we have designed MIMO antennas with pixelated surface, which provide low mutual coupling and spatial correlation to maximize the ergodic capacity, and defected ground structures with pixelated grid, which suppress cross-polarization and achieve circular polarization. Furthermore, we have investigated the pixel antenna optimization, which is essentially a binary optimization problem, and proposed different algorithms, including successive Exhaustive Boolean optimization, mixed integer linear programming, N-port characteristic mode analysis, perturbation sensitivity analysis, and the adjoint method combined with the method of moving asymptote, to efficiently optimize the pixel antenna.

Related publications:

1. Z. Ming, S. Shen, and R. Murch, "A 2-bit phase reconfigurable aperture coupled pixel antenna," 2024 IEEE International Symposium on Antennas and Propagation and INC/USNC-URSI Radio Science Meeting (AP-S/INC-USNC-URSI), Firenze, Italy, 2024, pp. 143-144. [Link]

2. T. Qiao, F. Jiang, S. Shen*, Z. Zhang, M. Li, C. Y. Chiu, Q. S. Cheng*, and R. Murch, "Pixel antenna optimization using the adjoint method and the method of moving asymptotes," in IEEE Transactions on Antennas and Propagation, vol. 71, no. 3, pp. 2873-2878, March 2023. [Link]

3. F. Jiang, Z. Zhang, M. Li, S. Shen, C. Y. Chiu, Y. Zhang, Q. S. Cheng, and R. Murch, "Multi-port pixel antenna optimization using characteristic mode analysis and sequential feeding port search," in IEEE Transactions on Antennas and Propagation, vol. 70, no. 10, pp. 9160-9174, Oct. 2022. [Link]

4. Y. Zhang, Z. Han, S. Tang, J. Rao, S. Shen, M. Li, C. Y. Chiu, and R. Murch, "A low-profile microstrip vertically polarized endfire antenna with 360\(^{\circ}\) beam-scanning and high beam-shaping capability," in IEEE Transactions on Antennas and Propagation, vol. 70, no. 9, pp. 7691-7702, Sept. 2022. [Link]

5. Y. Zhang, Z. Han, S. Tang, S. Shen, C. Y. Chiu, and R. Murch, "A highly pattern-reconfigurable planar antenna with 360\(^{\circ}\) single- and multi-beam steering," in IEEE Transactions on Antennas and Propagation, vol. 70, no. 8, pp. 6490-6504, Aug. 2022. [Link]

6. F. Jiang, S. Shen, C. Y. Chiu, Z. Zhang, Y. Zhang, Q. S. Cheng, and R. Murch, "Pixel antenna optimization based on perturbation sensitivity analysis," in IEEE Transactions on Antennas and Propagation, vol. 70, no. 1, pp. 472-486, Jan. 2022. [Link]

7. Y. Zhang, S. Shen*, Z. Han, C. Y. Chiu, and R. Murch, "Compact MIMO systems utilizing a pixelated surface: capacity maximization," in IEEE Transactions on Vehicular Technology, vol. 70, no. 9, pp. 8453-8467, Sept. 2021. [Link]

8. Y. Zhang, Z. Han, S. Shen*, C. Y. Chiu, and R. Murch, "Polarization enhancement of microstrip antennas by asymmetric and symmetric grid defected ground structures," in IEEE Open Journal of Antennas and Propagation, vol. 1, pp. 215-223, 2020. [Link]

9. F. Jiang, C. Y. Chiu, S. Shen, Q. S. Cheng, and R. Murch, "Pixel antenna optimization using N-port characteristic mode analysis," in IEEE Transactions on Antennas and Propagation, vol. 68, no. 5, pp. 3336-3347, May 2020. [Link]

10. S. Shen, C. Y. Chiu, and R. D. Murch, "Optimization of 2.45-GHz pixel rectenna for wireless power transmission using mixed integer linear programming," 2017 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, San Diego, CA, USA, 2017, pp. 351-352. [Link]

11. S. Shen*, Y. Sun, S. Song, D. P. Palomar, and R. D. Murch, "Successive Boolean optimization of planar pixel antennas," in IEEE Transactions on Antennas and Propagation, vol. 65, no. 2, pp. 920-925, Feb. 2017. [Link]

Liquid Antenna Design

The concept of liquid antennas stems from the pursuit of highly reconfigurable and versatile wireless systems. Traditional solid-metal antennas are fixed in their shape and properties once manufactured. which limits their ability to adapt to changing conditions, such as different frequencies or radiation patterns. Liquid antennas address this by using conductive fluids (e.g., saltwater, liquid metals like Galinstan, or mercury) as the radiating element. The key idea is that the physical properties of a liquid, its shape, length, and position, can be easily manipulated using various methods (e.g., pumps, electric fields, magnetic fields, or mechanical actuators), leading to dynamic control over the antenna's performance. Liquid antennas are also the key technology to enable the fluid antenna system which is an emerging wireless system.

Inspired by this, we have proposed and designed gravitational liquid dielectric resonator antennas to achieve passive beamforming which is invariant to the change of position and rotation of the antenna. Furthermore, we have proposed using liquid crystal to implement a flexible reconfigurable antenna to tune the operating frequency. These liquid antennas enable the promise of reconfigurable and versatile wireless systems.

Related publications:

1. X. Yang, X. Yu, M. Liu, K. Kurskiy, S. Shen, and Y. Huang, "A passive 3D beam-steering gravitational liquid dielectric resonator antenna with polarization diversity," in IEEE Transactions on Antennas and Propagation, vol. 73, no. 7, pp. 4192-4207, July 2025. [Link]

2. X. Yang, S. Shen, and Y. Huang, "A passive 3D beam-steering gravitational liquid dielectric resonator antenna," 2024 IEEE 12th Asia-Pacific Conference on Antennas and Propagation (APCAP), Nanjing, China, 2024, pp. 1-2. [Link]

3. L. Yang, Y. Zhang, C. Y. Chiu, S. Shen, and R. Murch, "Investigation of a flexible reconfigurable antenna using liquid crystal," 2019 8th Asia-Pacific Conference on Antennas and Propagation (APCAP), Incheon, Korea (South), 2019, pp. 430-431. [Link]

MIMO Antenna Design

MIMO communications, utilizing multiple antennas at transmitter and receiver, are the dominant technology is modern wireless communications. However, the mutual coupling between multiple antennas will degrade the MIMO system performance. Thus, the key challenge in designing MIMO antennas is to ensure that the multiple antennas have low mutual coupling and high isolation.

To overcome this challenge, we have proposed using rotation symmetry among multiple antennas to exploit the phase and polarization diversity to reduce the mutual coupling. We have also leveraged theory of characteristic modes to analyze, design, and prototype massive MIMO antennas with low mutual coupling. In addition, we have utilized the space mapping technique to efficiently design and optimize the decoupling network for MIMO antenna. We have shown by measurement that these techniques can effectively suppress the mutual coupling to achieve good isolations.

Related publications:

1. F. Jiang, Z. Zhang, Q. S. Cheng, S. Shen, C. Y. Chiu, and R. Murch, "An efficient optimization scheme for MIMO antenna decoupling networks using space mapping techniques," in IEEE Journal on Multiscale and Multiphysics Computational Techniques, vol. 6, pp. 56-61, 2021. [Link]

2. C. Y. Chiu, S. Shen*, B. K. Lau, and R. Murch, "The design of a trimodal broadside antenna element for compact massive MIMO arrays: utilizing the theory of characteristic modes," in IEEE Antennas and Propagation Magazine, vol. 62, no. 6, pp. 46-61, Dec. 2020. [Link]

3. C. Y. Chiu, F. Xu, S. Shen, and R. D. Murch, "Mutual coupling reduction of rotationally symmetric multi-port antennas," in IEEE Transactions on Antennas and Propagation, vol. 66, no. 10, pp. 5013-5021, Oct. 2018. [Link]

4. C. Y. Chiu, S. Shen, and R. D. Murch, "Dual-band antenna pair for MIMO WiFi compact mobile terminals," 2016 IEEE International Symposium on Antennas and Propagation (APSURSI), Fajardo, PR, USA, 2016, pp. 65-66. [Link]

Transparent Antenna Design

Transparent antenna design is based on constructing antennas by optically transparent materials without obstructing the view. Transparent antennas can be used in display electronics, smart windows/glass, and solar energy harvesting. However, the key challenge is to find the optimal material and pattern that provides sufficient conductivity for efficient radiation while maintaining the required level of transparency.

To that end, we have proposed using conductive copper meshes to construct transparent antennas. Conductive copper meshes are very thin metal grids or patterns, where the gaps between the lines are large enough to allow light to pass through, so that are invisible to the naked eye from a distance. Using conductive copper meshed, we have proposed dual-band/broadband/multiport transparent antennas and also integrated with the solar cell for hybrid RF and solar energy harvesting. By measurement, we have shown that using conductive copper meshes can achieve good radiation efficiency and transparency simultaneously.

Related publications:

1. Y. Zhang, S. Shen, C. Y. Chiu, and R. Murch, "A broadband transparent antenna integrated with an indoor solar cell for WLAN applications," 2020 IEEE International Symposium on Antennas and Propagation and North American Radio Science Meeting, Montreal, QC, Canada, 2020, pp. 437-438. [Link]

2. Y. Zhang, S. Shen*, C. Y. Chiu, and R. Murch, "Hybrid RF-solar energy harvesting systems utilizing transparent multiport micro-meshed antenna," in IEEE Transactions on Microwave Theory and Techniques, vol. 67, no. 11, pp. 4534-4546, Nov. 2019. [Link]

3. Y. Zhang, S. Shen, C. Y. Chiu, and R. D. Murch, "A dual-band transparent coplanar patch antenna for WLAN systems," 2018 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, Boston, MA, USA, 2018, pp. 465-466. [Link]

4. C. Y. Chiu, Y. Zhang, S. Shen, and R. D. Murch, "Co-design of antenna and illumination systems," 2018 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, Boston, MA, USA, 2018, pp. 1675-1676. [Link]

5. C. Y. Chiu, S. Shen, and R. D. Murch, "Transparent dual-band antenna for smart watch applications," 2017 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, San Diego, CA, USA, 2017, pp. 191-192. [Link]

Multiport Rectenna Design for Ambient RF Energy Harvesting

Ambient RF energy harvesting uses rectenna to harvest RF energy in ambient environment to energize massive devices of Internet-of-Things. However, the key challenge is that the low RF power density of the ambient RF energy limits the power available for harvesting.

To address this limitation, we have proposed and developed multiport rectenna to enhance harvested RF energy. We have performed analysis, design, and prototyping of various kinds of multiport rectennas and shown by measurement that using multiport rectenna can linearly increase the harvested RF energy compared to traditional single-port rectenna of similar size. Furthermore, we have proposed to combine multiport rectenna with multi-band/broadband/dual-polarized rectenna to jointly leverage the spatial, frequency, and polarization domains to maximize the harvested ambient RF energy.

Related publications:

1. M. T. Chiu, C. Y. Chiu, C. Ng, L.O. Wong, S. Shen, and R. Murch, "An ambient RF powered wireless sensor system," in IEEE Open Journal of Antennas and Propagation, vol. 3, pp. 1382-1393, 2022. [Link]

2. C. Song, P. Lu, and S. Shen, "Highly efficient omnidirectional integrated multiband wireless energy harvesters for compact sensor nodes of internet-of-things," in IEEE Transactions on Industrial Electronics, vol. 68, no. 9, pp. 8128-8140, Sept. 2021. [Link]

3. S. Shen*, Y. Zhang, C. Y. Chiu and R. Murch, "Directional multiport ambient RF energy-harvesting system for the internet of things," in IEEE Internet of Things Journal, vol. 8, no. 7, pp. 5850-5865, April 2021. [Link]

4. S. Shen*, Y. Zhang, C. Y. Chiu, and R. Murch, "A triple-band high-gain multibeam ambient RF energy harvesting system utilizing hybrid combining," in IEEE Transactions on Industrial Electronics, vol. 67, no. 11, pp. 9215-9226, Nov. 2020. [Link]

5. R. Murch, C. Y. Chiu, and S. Shen, "Harnessing ambient RF waves for novel applications," 2020 IEEE Asia-Pacific Microwave Conference (APMC), Hong Kong, Hong Kong, 2020, pp. 42-44. [Link]

6. Z. Zeng, S. Shen, B. Wang, J. E. Lopez, R. Murch, and E. S. Sinencio, "An ultra-low-power management circuit with output bootstrapping and reverse leakage reduction function for RF energy harvesting," 2020 IEEE/MTT-S International Microwave Symposium (IMS), Los Angeles, CA, USA, 2020, pp. 1059-1062. [Link]

7. Z. Zeng, S. Shen, X. Zhong, X. Li, C. Y. Tsui, A. Bermak, R. Murch, and E. S. Sinencio, "Design of sub-GHz reconfigurable RF energy harvester from -22 to 4 dBm with 99.8% peak MPPT efficiency," in IEEE Journal of Solid-State Circuits, vol. 54, no. 9, pp. 2601-2613, Sept. 2019. [Link]

8. S. Shen*, Y. Zhang, C. Y. Chiu, and R. Murch, "An ambient RF energy harvesting system where the number of antenna ports is dependent on frequency," in IEEE Transactions on Microwave Theory and Techniques, vol. 67, no. 9, pp. 3821-3832, Sept. 2019. [Link]

9. S. Shen, Y. Zhang, C. Y. Chiu, and R. Murch, "Multiport rectennas for ambient RF energy harvesting," 2019 IEEE MTT-S International Wireless Symposium (IWS), Guangzhou, China, 2019, pp. 1-3. [Link]

10. S. Shen, Y. Zhang, C. Y. Chiu, and R. D. Murch, "A compact quad-port dual-polarized dipole rectenna for ambient RF energy harvesting," 12th European Conference on Antennas and Propagation (EuCAP 2018), London, UK, 2018, pp. 1-4. [Link]

11. S. Shen*, C. Y. Chiu and R. D. Murch, "Multiport pixel rectenna for ambient RF energy harvesting," in IEEE Transactions on Antennas and Propagation, vol. 66, no. 2, pp. 644-656, Feb. 2018. [Link]

12. S. Shen*, C. Y. Chiu, and R. D. Murch, "A dual-port triple-band L-probe microstrip patch rectenna for ambient RF energy harvesting," in IEEE Antennas Wireless Propagation Letters, vol. 16, pp. 3071-3074, 2017. [Link]

13. S. Shen, C. Y. Chiu, and R. D. Murch, "A broadband L-probe microstrip patch rectenna for ambient RF energy harvesting," 2017 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, San Diego, CA, USA, 2017, pp. 2037-2038. [Link]

14. S. Shen* and R. D. Murch, "Impedance matching for compact multiple antenna systems in random RF fields," in IEEE Transactions on Antennas and Propagation, vol. 64, no. 2, pp. 820-825, Feb. 2016. [Link]

15. S. Shen, C. Y. Chiu, and R. D. Murch, "Multi-port ladder impedance matching for compact MIMO communication systems," 2016 IEEE International Symposium on Antennas and Propagation (APSURSI), Fajardo, PR, USA, 2016, pp. 197-198. [Link]

16. S. Shen and R. D. Murch, "Designing dual-port pixel antenna for ambient RF energy harvesting using genetic algorithm," 2015 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, Vancouver, BC, Canada, 2015, pp. 1286-1287. [Link]