Open Access Publications

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Open access publications by faculty, postdocs, and graduate students in the Department of Electrical and Computer Engineering


Recent Submissions

Now showing 1 - 5 of 51
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    Structured Hybrid Message Passing Based Channel Estimation for Massive MIMO-OFDM Systems
    (IEEE Transactions on Vehicular Technology, 2023-01-26) Liu, Xiaofeng; Wang, Wenjin; Gong, Xinrui; Fu, Xiao; Gao, Xiqi; Xia, Xiang-Gen
    This paper investigates uplink channel estimation for massive multi-input multi-output orthogonal frequency division multiplexing (MIMO-OFDM) systems with uniform planar array (UPA) antennas at the base station (BS). We first establish a triple beam-based channel model using sampled steering vectors. Based on the presented channel model, we further develop a three-dimensional (3D) Markov random field (MRF) probability model to capture the structured channel sparsity. Then constrained Bethe free energy (BFE) minimization is introduced to provide a systematic theoretical framework for message passing. Under this framework, we derive a structured hybrid message passing (SHMP) algorithm to address the channel estimation problem. The proposed algorithm can significantly improve the estimation accuracy by exploiting the clustered sparse structure of channels with low complexity. Moreover, aiming at the fine factors of the triple beam-based channel model and the coupling parameter of the 3D-MRF sparsity model, we analyze the effect of their different settings in the numerical simulation. Finally, extensive simulation results verify the superiority of the proposed SHMP algorithm.
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    Near Space Communications: A New Regime in Space-Air-Ground Integrated Networks
    (IEEE Wireless Communications, 2022-12-29) Xiao, Zhenyu; Mao, Tianqi; Han, Zhu; Xia, Xiang-Gen
    Precipitated by the technological innovations in near-space platform stations (NSPSs), the near-space communication (NS-COM) network has emerged as an indispensable part of the next-generation space-air-ground integrated network (SAGIN) that facilitates ubiquitous coverage and broadband data transfer. This article aims to provide a comprehensive overview of NS-COM. First, we investigate the differences between NS-COM and the existing terrestrial cellular networks as well as satellite-based and unmanned-aerial-vehicle (UAV)-based communication networks, which is followed by a review of NS-COM development. Then we explore the unique characteristics of NS-COM regarding the platforms and the propagation environment of near space. The main issues of NS-COM are identified, resulting from the extremely long transmission distance, limitations of the communication payloads on NSPS, and the complex atmospheric constitution of near space. Then various application scenarios of NS-COM are discussed, where the special technical requirements are also revealed, from physical-layer aspects like transceiver design to upper-layer aspects like computational offloading and NSPS placement. Furthermore, we investigate the coexistence of NS-COM and ground networks by treating each other as interferers or collaborators. Finally, we list several potential technologies for NS-COM from the perspective of spectrum usage, and highlight their technical challenges for future research.
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    Cost-Efficient RIS-Assisted Transmitter Design With Discrete Phase Shifts for Wireless Communication
    (IEEE Wireless Communications Letters, 2022-12-30) Pi, Xiangyu; Yi, Pengfei; Xiao, Zhenyu; Zhang, Wei; Han, Zhu; Xia, Xiang-Gen
    In this letter, in order to achieve higher spectral and energy efficiency, we propose a novel cost-efficient transmitter conceptual design based on reconfigurable intelligent surface (RIS) with discrete phase shifts. The key idea is to directly utilize the digital signal to adjust the discrete reflection coefficients of RIS, resulting that the phases of the reflected carrier signal being modulated without the need for complex digital signal processing (DSP) hardware and costly radio frequency (RF) chains. Furthermore, a joint digital modulation and beamforming method is developed to enable information transmission as well as enhance signal strength. Based on the proposed transmitter, we derive the closed-form expressions of the signal-to-noise ratio (SNR) and bit error rate (BER) of the received signal and analyze the impact of hardware constraints on communication performance. Extensive simulation results validate that the novel design of RIS-assisted transmitter provides a cost-effective and power-efficient solution for wireless communications.
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    Revisiting Non-Coherent Detection of Differential PSK: A Factor Graph Perspective
    (IEEE Communications Letters, 2022-12-23) Shi, Long; Wang, Taotao; Mei, Zhen; Li, Jun; Xia, Xiang-Gen
    For non-coherent differential modulated communications, the multiple-symbol differential detection (MSDD) may approach the coherent detection performance by jointly detecting a block of symbols. However, MSDD cannot support large-size block detection due to its complexity exponentially increasing with the block size. Moreover, MSDD assumes that the channel phase is constant over the symbols in a block, which is not realistic. In this letter, we propose to formulate the signal model of non-coherent differential modulated communications by means of a factor graph. Based on the factor graph, we develop a sum-product message-passing algorithm to enable the maximum a posteriori probability detection. Compared with MSDD, the detection complexity of our proposed scheme is linearly proportional to the block size. Simulation results show that the proposed scheme can not only achieve the performance of MSDD over the constant-phase channel, but also outperform it over the random-walk phase channel.
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    Integrated photonic metasystem for image classifications at telecommunication wavelength
    (Nature Communications, 2022-04-19) Wang, Zi; Chang, Lorry; Wang, Feifan; Li, Tiantian; Gu, Tingyi
    Miniaturized image classifiers are potential for revolutionizing their applications in optical communication, autonomous vehicles, and healthcare. With subwavelength structure enabled directional diffraction and dispersion engineering, the light propagation through multi-layer metasurfaces achieves wavelength-selective image recognitions on a silicon photonic platform at telecommunication wavelength. The metasystems implement high-throughput vector-by-matrix multiplications, enabled by near 103 nanoscale phase shifters as weight elements within 0.135 mm2 footprints. The diffraction manifested computing capability incorporates the fabrication and measurement related phase fluctuations, and thus the pre-trained metasystem can handle uncertainties in inputs without post-tuning. Here we demonstrate three functional metasystems: a 15-pixel spatial pattern classifier that reaches near 90% accuracy with femtosecond inputs, a multi-channel wavelength demultiplexer, and a hyperspectral image classifier. The diffractive metasystem provides an alternative machine learning architecture for photonic integrated circuits, with densely integrated phase shifters, spatially multiplexed throughput, and data processing capabilities.
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