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Julian Cheng

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Intelligent Reflecting Surface Assisted Multi-Cluster AirComp via Dynamic Beamforming

Jun 11, 2023
Yapeng Zhao, Qingqing Wu, Wen Chen, Celimuge Wu, Julian Cheng, Octavia A. Dobre

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This paper studies an multi-cluster over-the-air computation (AirComp) system, where an intelligent reflecting surface (IRS) assists the signal transmission from devices to an access point (AP). The clusters are activated to compute heterogeneous functions in a time-division manner. Specifically, two types of IRS beamforming (BF) schemes are proposed to reveal the performancecost tradeoff. One is the cluster-adaptive BF scheme, where each BF pattern is dedicated to one cluster, and the other is the dynamic BF scheme, which is applied to any number of IRS BF patterns. By deeply exploiting their inherent properties, both generic and lowcomplexity algorithms are proposed in which the IRS BF patterns, time and power resource allocation are jointly optimized. Numerical results show that IRS can significantly enhance the function computation performance, and demonstrate that the dynamic IRS BF scheme with half of the total IRS BF patterns can achieve near-optimal performance which can be deemed as a cost-efficient approach for IRS-aided multi-cluster AirComp systems.

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Performance Analysis of Cell-Free Massive MIMO Systems with Asynchronous Reception

Oct 17, 2022
Jiakang Zheng, Zhuoyi Zhao, Jiayi Zhang, Julian Cheng, Victor C. M. Leung

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Cell-free (CF) massive multiple-input multiple-output (MIMO) is considered as a promising technology for achieving the ultimate performance limit. However, due to its distributed architecture and low-cost access points (APs), the signals received at user equipments (UEs) are most likely asynchronous. In this paper, we investigate the performance of CF massive MIMO systems with asynchronous reception, including both effects of delay and oscillator phases. Taking into account the imperfect channel state information caused by phase asynchronization and pilot contamination, we obtain novel and closed-form downlink spectral efficiency (SE) expressions with coherent and non-coherent data transmission schemes, respectively. Simulation results show that asynchronous reception destroys the orthogonality of pilots and coherent transmission of data, and thus results in poor system performance. In addition, getting a highly accurate delay phase is substantial for CF massive MIMO systems to achieve coherent transmission gain. Moreover, the oscillator phase of UEs has a larger effect on SE than that of the APs, because the latter can be significantly reduced by increasing the number of antennas.

* Accepted in IEEE GLOBECOM Workshops 2022 
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Federated Deep Reinforcement Learning for RIS-Assisted Indoor Multi-Robot Communication Systems

Jul 17, 2022
Ruyu Luo, Wanli Ni, Hui Tian, Julian Cheng

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Indoor multi-robot communications face two key challenges: one is the severe signal strength degradation caused by blockages (e.g., walls) and the other is the dynamic environment caused by robot mobility. To address these issues, we consider the reconfigurable intelligent surface (RIS) to overcome the signal blockage and assist the trajectory design among multiple robots. Meanwhile, the non-orthogonal multiple access (NOMA) is adopted to cope with the scarcity of spectrum and enhance the connectivity of robots. Considering the limited battery capacity of robots, we aim to maximize the energy efficiency by jointly optimizing the transmit power of the access point (AP), the phase shifts of the RIS, and the trajectory of robots. A novel federated deep reinforcement learning (F-DRL) approach is developed to solve this challenging problem with one dynamic long-term objective. Through each robot planning its path and downlink power, the AP only needs to determine the phase shifts of the RIS, which can significantly save the computation overhead due to the reduced training dimension. Simulation results reveal the following findings: I) the proposed F-DRL can reduce at least 86% convergence time compared to the centralized DRL; II) the designed algorithm can adapt to the increasing number of robots; III) compared to traditional OMA-based benchmarks, NOMA-enhanced schemes can achieve higher energy efficiency.

* IEEE Transactions on Vehicular Technology, 2022, early access  
* The paper has been accepted for publication in the IEEE Transactions on Vehicular Technology 
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Ultrasonic Backscatter Communication for Implantable Medical Devices

Feb 14, 2022
Qianqian Wang, Quansheng Guan, Julian Cheng, Yuankun Tang

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This paper proposes an ultrasonic backscatter communication (UsBC) system for passive implantable medical devices (IMDs) that can operate without batteries, enabling versatile revolutionary applications for future healthcare. The proposed UsBC system consists of a reader and a tag. The reader sends interrogation pulses to the tag. The tag backscatters the pulses based on the piezoelectric effect of a piezo transducer. We present several basic modulation schemes for UsBC by impedance matching of the piezo transducer. To mitigate the interference of other scatters in the human body, the tag transmits information bits by codeword mapping, and the reader performs codeword matching before energy detection in the reader. We further derive the theoretical bit-error rate (BER) expression. Monte Carlo simulations verify the theoretical analysis and show that passive UsBC can achieve low BER and low complexity, which is desirable for size- and energy-constrained IMDs.

* 5 pages and 5 figures 
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On the Ergodic Mutual Information of Keyhole MIMO Channels with Finite Inputs

Dec 29, 2021
Chongjun Ouyang, Ralf R. Müller, Yuanwei Liu, Julian Cheng, Hongwen Yang

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This letter studies the ergodic mutual information (EMI) of keyhole multiple-input multiple-output (MIMO) channels having finite input signals. At first, the EMI of single-stream transmission is investigated depending on whether the channel state information at the transmitter (CSIT) is available or not. Then, the derived results are extended to the case of multi-stream transmission. For the sake of providing more system insights, asymptotic analyses are performed in the regime of high signal-to-noise ratio (SNR), which suggests that the high-SNR EMI converges to some constant with its rate of convergence (ROC) determined by the diversity order. All the results are validated by numerical simulations and are in excellent agreement.

* 5 pages 
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On the Average Mutual Information of MIMO Keyhole Channels with Finite Inputs

Dec 08, 2021
Chongjun Ouyang, Yuanwei Liu, Julian Cheng, Ralf R. Müller, Hongwen Yang

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This letter studies the average mutual information (AMI) of keyhole multiple-input multiple-output (MIMO) systems having finite input signals. At first, the AMI of single-stream transmission is investigated under two cases where the state information at the transmitter (CSIT) is available or not. Then, the derived results are further extended to the case of multi-stream transmission. For the sake of providing more system insights, asymptotic analyses are performed in the regime of high signal-to-noise ratio (SNR), which suggests that the high-SNR AMI converges to some constant with its rate of convergence determined by the diversity order. All the results are validated by numerical simulations and are in excellent agreement.

* 5 pages 
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Asymptotic Average Mutual Information Over Finite Input Mixture Gamma Distributed Channels

Nov 24, 2021
Chongjun Ouyang, Sheng Wu, Chunxiao Jiang, Yuanwei Liu, Julian Cheng, Hongwen Yang

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This letter establishes a unified analytical framework to study the asymptotic average mutual information (AMI) of mixture gamma (MG) distributed fading channels driven by finite input signals in the high signal-to-noise ratio (SNR) regime. It is found that the AMI converges to some constant as the average SNR increases and its rate of convergence (ROC) is determined by the coding gain and diversity order. Moreover, the derived results are used to investigate the asymptotic optimal power allocation policy of a bank of parallel fading channels having finite inputs. It is suggested that in the high SNR region, the sub-channel with a lower coding gain or diversity order should be allocated with more power. Finally, numerical results are provided to collaborate the theoretical analyses.

* 5 pages 
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Multi-Pair Two-Way Massive MIMO DF Relaying Over Rician Fading Channels Under Imperfect CSI

Oct 28, 2021
Zhangjie Peng, Shuxian Wang, Cunhua Pan, Xianzhe Chen, Julian Cheng, Lajos Hanzo

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We investigate a multi-pair two-way decode-andforward relaying aided massive multiple-input multiple-output antenna system under Rician fading channels, in which multiple pairs of users exchange information through a relay station having multiple antennas. Imperfect channel state information is considered in the context of maximum-ratio processing. Closedform expressions are derived for approximating the sum spectral efficiency (SE) of the system. Moreover, we obtain the powerscaling laws at the users and the relay station to satisfy a certain SE requirement in three typical scenarios. Finally, simulations validate the accuracy of the derived results.

* Accepted by IEEE Wireless Communications Letters 
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Beam Tracking for UAV-Assisted FSO Links With a Four-Quadrant Detector

Sep 16, 2021
Hossein Safi, Akbar Dargahi, Julian Cheng

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A ground-to-air free-space optical link is studied for a hovering unmanned aerial vehicle (UAV) having multiple rotors. For this UAV, a four-quadrant array of photodetectors is used at the optical receiver to alleviate the adverse effect of hovering fluctuations by enlarging the receiver field-of-view. Extensive mathematical analysis is conducted to evaluate the beam tracking performance under the random effects of hovering fluctuations. The accuracy of the derived analytical expressions is corroborated by performing Monte-Carlo simulations. It is shown that the performance of such links depends heavily on the random fluctuations of hovering UAV, and, for each level of instability there is an optimal size for the array that minimizes the tracking error probability

* 12 pages, 5 figures, accepted for publication in the IEEE Communications Letters 
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