@article{li_stacked_2025,

title = {Stacked Intelligent Metasurface-Based Transceiver Design for Near-Field Wideband Systems},

author = {Qingchao Li and Mohammed El-Hajjar and Chao Xu and Jiancheng An and Chau Yuen and Lajos Hanzo},

url = {https://ieeexplore.ieee.org/document/10900449},

doi = {10.1109/TCOMM.2025.3544929},

issn = {1558-0857},

year  = {2025},

date = {2025-09-01},

urldate = {2025-10-08},

journal = {IEEE Transactions on Communications},

volume = {73},

number = {9},

pages = {8125–8139},

abstract = {Intelligent metasurfaces may be harnessed for realizing efficient holographic multiple-input and multiple-output (MIMO) systems, at a low hardware-cost and high energy-efficiency. As part of this family, we propose a hybrid beamforming design for stacked intelligent metasurfaces (SIM) aided wideband wireless systems relying on the near-field channel model. Specifically, the holographic beamformer is designed based on configuring the phase shifts in each layer of the SIM for maximizing the sum of the baseband eigen-channel gains of all users. To optimize the SIM phase shifts, we propose a layer-by-layer iterative algorithm for optimizing the phase shifts in each layer alternately. Then, the minimum mean square error (MMSE) transmit precoding method is employed for the digital beamformer to support multi-user access. Furthermore, the mitigation of the SIM phase tuning error is also taken into account in the digital beamformer by exploiting its statistics. The power sharing ratio of each user is designed based on the iterative waterfilling power allocation algorithm. Additionally, our analytical results indicate that the spectral efficiency attained saturates in the high signal-to-noise ratio (SNR) region due to the phase tuning error resulting from the imperfect SIM hardware quality. The simulation results show that the SIM-aided holographic MIMO outperforms the state-of-the-art (SoA) single-layer holographic MIMO in terms of its achievable rate. We further demonstrate that the near-field channel model allows the SIM-based transceiver design to support multiple users, since the spatial resources represented both by the angle domain and the distance domain can be exploited.},

keywords = {Array signal processing, Channel models, Hardware, holographic beamforming architecture, Low earth orbit satellites, Metamaterials, Metasurfaces, near-field channel model, phase tuning error, Stacked intelligent metasurface, Transceivers, Tuning, Vectors, Wideband, wideband system},

pubstate = {published},

tppubtype = {article}

}

@article{liu_near-field_2025,

title = {Near-field Communications: What Will Be Different?},

author = {Yuanwei Liu and Jiaqi Xu and Zhaolin Wang and Xidong Mu and Lajos Hanzo},

url = {https://ieeexplore.ieee.org/document/10944643},

doi = {10.1109/MWC.001.2300588},

issn = {1558-0687},

year  = {2025},

date = {2025-04-01},

urldate = {2025-10-08},

journal = {IEEE Wireless Communications},

volume = {32},

number = {2},

pages = {262–270},

abstract = {The design dilemma of “What will be different between near-field communications (NFC) and far-field communications (FFC)?” is addressed from four perspectives. First, from the channel modelling perspective, the differences between near-field and far-field channel models are discussed. A novel Green's function-based channel model is proposed for continuous-aperture antennas, which is contrasted to conventional channel models tailored for spatially-discrete antennas. Second, from the performance analysis per-spective, analytical results for characterizing the degrees of freedom and the power scaling laws in the near-field region are provided for both spatially-discrete and continuous-aperture antennas. Third, from the beamforming perspective, far-field beamforming is analogous to a “flashlight” that enables beamsteering, while near-field beamforming can be likened to a “spotlight” that facilitates beamfocusing. As a further advance, a couple of new beamforming structures are proposed for exploiting the new characteristics of NFC. Fourth, from the application perspective, new designs are discussed in the context of promising next-generation technologies in NFC, where our preliminary numerical results demonstrate that distance-aware target sensing and enhanced physical layer security can be realized in NFC. Finally, several future research directions of NFC are discussed.},

keywords = {Antennas, Array signal processing, Channel models, Green's function methods, Next generation networking, Performance analysis, Physical layer security, Sensors},

pubstate = {published},

tppubtype = {article}

}

@article{zhang_space-air-ground_2025,

title = {Space-Air-Ground Integrated Networks: Their Channel Model and Performance Analysis},

author = {Chao Zhang and Qingchao Li and Chao Xu and Lie-Liang Yang and Lajos Hanzo},

url = {https://ieeexplore.ieee.org/document/11018358},

doi = {10.1109/OJVT.2025.3575360},

issn = {2644-1330},

year  = {2025},

date = {2025-01-01},

urldate = {2025-10-08},

journal = {IEEE Open Journal of Vehicular Technology},

volume = {6},

pages = {1501–1523},

abstract = {Given their extensive geographic coverage, low Earth orbit (LEO) satellites are envisioned to find their way into next-generation (6G) wireless communications. This paper explores space-air-ground integrated networks (SAGINs) leveraging LEOs to support terrestrial and non-terrestrial users. We first propose a practical satellite-ground channel model that incorporates five key aspects: 1) the small-scale fading characterized by the Shadowed-Rician distribution in terms of the Rician factor K, 2) the path loss effect of bending rays due to atmospheric refraction, 3) the molecular absorption modelled by the Beer-Lambert law, 4) the Doppler effects including the Earth's rotation, and 5) the impact of weather conditions according to the International Telecommunication Union Recommendations (ITU-R). Harnessing the proposed model, we analyze the long-term performance of the SAGIN considered. Explicitly, the closed-form expressions of both the outage probability and of the ergodic rates are derived. Additionally, the upper bounds of bit-error rates and of the Goodput are investigated. The numerical results yield the following insights: 1) The shadowing effect and the ratio between the line-of-sight and scattering components can be conveniently modelled by the factors of K and m in the proposed Shadowed-Rician small-scale fading model. 2) The atmospheric refraction has a modest effect on the path loss. 3) When calculating the transmission distance of waves, Earth's curvature and its geometric relationship with the satellites must be considered, particularly at small elevation angles. 3) High-frequency carriers suffer from substantial path loss, and 4) the Goodput metric is eminently suitable for characterizing the performance of different coding as well as modulation methods and of the estimation error of the Doppler effects.},

keywords = {Absorption, Atmospheric modeling, Attenuation, Bending, Channel model, Channel models, Doppler effect, Earth, Fading channels, goodput, Meteorology, Performance analysis, Rician channels, Space-air-ground integrated networks},

pubstate = {published},

tppubtype = {article}

}