Decentralized Beamforming Design for Intelligent Reflecting Surface-enhanced Cell-free Networks

Cell-free networks are considered as a promising distributed network architecture to satisfy the increasing number of users and high rate expectations in beyond-5G systems. However, to further enhance network capacity, an increasing number of high-cost base stations (BSs) are required. To address this problem and inspired by the cost-effective intelligent reflecting surface (IRS) technique, we propose a fully decentralized design framework for cooperative beamforming in IRS-aided cell-free networks. We first transform the centralized weighted sum-rate maximization problem into a tractable consensus optimization problem, and then an incremental alternating direction method of multipliers (ADMM) algorithm is proposed to locally update the beamformer. The complexity and convergence of the proposed method are analyzed, and these results show that the performance of the new scheme can asymptotically approach that of the centralized one as the number of iterations increases. Results also show that IRSs can significantly increase the system sum-rate of cell-free networks and the proposed method outperforms existing decentralized methods.Comment: 5 pages, 6 figure

Relations of blood lead levels to echocardiographic left ventricular structure and function in preschool children

Lead (Pb) has been proved to exert adverse effect on human cardiovascular system. However, the cardiotoxicity of Pb on children is still unclear. The aim of this study was to evaluate left ventricular (LV) structure and function, by using echocardiographic indices, in order to elucidate the effect of Pb on low-grade inflammation related to left ventricle in healthy preschool children. We recruited a total of 486 preschool children, 310 from Guiyu (e-waste-exposed area) and 176 from Haojiang (reference area). Blood Pb levels, complete blood counts, and LV parameters were evaluated. Associations between blood Pb levels and LV parameters and peripheral leukocyte counts were analyzed using linear regression models. The median blood level of Pb and the counts of white blood cells (WBCs), monocytes, and neutrophils were higher in exposed group. In addition, the exposed group showed smaller left ventricle (including interventricular septum, LV posterior wall, and LV mass index) and impaired LV systolic function (including LV fractional shortening and LV ejection fraction) regardless gender. After adjustment for confounding factors, elevated blood Pb levels were significantly associated with higher counts of WBCs and neutrophils, and lower levels of LV parameters. Furthermore, counts of WBCs, monocytes, and neutrophils were negatively correlated with LV parameters. Taken together, smaller left ventricle and impaired systolic function were found in e-waste-exposed children and associated with chronic low-grade inflammation and elevated blood Pb levels. It indicates that the heart health of e-waste-exposed children is at risk due to the long-term environmental chemical insults. (C) 2020 Elsevier Ltd. All rights reserved

Toward understanding of differences in current cloud retrievals of ARM ground-based measurements

Accurate observations of cloud microphysical properties are needed for evaluating and improving the representation of cloud processes in climate models and better estimate of the Earth radiative budget. However, large differences are found in current cloud products retrieved from ground-based remote sensing measurements using various retrieval algorithms. Understanding the differences is an important step to address uncertainties in the cloud retrievals. In this study, an in-depth analysis of nine existing ground-based cloud retrievals using ARM remote sensing measurements is carried out. We place emphasis on boundary layer overcast clouds and high level ice clouds, which are the focus of many current retrieval development efforts due to their radiative importance and relatively simple structure. Large systematic discrepancies in cloud microphysical properties are found in these two types of clouds among the nine cloud retrieval products, particularly for the cloud liquid and ice particle effective radius. Note that the differences among some retrieval products are even larger than the prescribed uncertainties reported by the retrieval algorithm developers. It is shown that most of these large differences have their roots in the retrieval theoretical bases, assumptions, as well as input and constraint parameters. This study suggests the need to further validate current retrieval theories and assumptions and even the development of new retrieval algorithms with more observations under different cloud regimes

Rate, Reliability and Secrecy Performance Analysis and Optimization for Millimeter WaveCommunications

With the fast development of electronic devices and computer science, various emerging applications (e.g., virtual and augmented reality, ultra-high-definition three-dimensional video, autonomous driving, big data analysis, etc.) have created an explosive growth of mobile data traffic and caused growing demands for higher communication rates, more reliable and secure connectivity in the future wireless communications, e.g., the fifth-generation (5G) and beyond mobile communications. In recent years, millimeter wave (mmWave) communication, as a promising candidate to meet the aforementioned demands, has attracted extensive research attention, and is regarded as one of the key enablers for the 5G and beyond mobile communications. The main features of mmWave communications include:  abundant spectral resources, high penetration loss, severe pathloss, and narrow antenna beams, and these particular features make the potential challenges and solutions with mmWave significantly different from those in the conventional sub-6 GHz systems.   It is known that beamforming is a crucial stage of mmWave communication to support high antenna gains and suppress inter-channel interference. However, the related research on beamforming design is fairly recent and insufficient. Motivated by the urgent needs for further development, in this thesis, we investigate beamforming optimization and the reliability and secrecy performance for mmWave communications. Our main research regarding beamforming design and secrecy performance can be categorized into the following three aspects:   1)      Hybrid beamforming (HBF) for mmWave systems with learning machines: We propose two low-complexity and robust learning schemes to design HBF for mmWave full-duplex systems and multi-user multi-input and multi-output (MU-MIMO) systems i.e., extreme learning machine based HBF and convolutional neural networks based HBF. To provide accurate labels for off-line training, effective HBF algorithms are proposed to achieve joint self-interference cancellation and HBF optimization for mmWave full-duplex systems and to achieve joint transmitting and receiving HBF optimization for mmWave MU-MIMO systems. The convergence of the proposed algorithms is proven and the computation complexity is analyzed. Results show that the proposed learning based methods can achieve higher spectral efficiency, lower complexity, and more robust HBF performance than conventional optimization based methods.   2)      Decentralized beamforming for intelligent reflecting surface (IRS)-enhanced cell-free networks: To avoid the centralized computation and inspired by the cost-effective IRS technique, we propose a fully decentralized design framework for cooperative beamforming in IRS-aided cell-free networks, in which transmitting digital beamformers and IRS-based analog beamformers are jointly optimized. We first derive a closed-form expression of each updating variable and then propose a fully decentralized beamforming scheme based on the alternating direction method of multipliers to incrementally and locally update the beamformers. Results reveal that the new scheme outperforms existing decentralized methods and IRSs can significantly improve the system sum-rate.   3)      Physical layer security of mmWave non-orthogonal multiple access (NOMA) networks: Considering the limited scattering characteristics of mmWave channels and imperfect successive interference cancellation at receivers, we develop an analytic framework on the secrecy outage probability for mmWave NOMA networks. Based on the directional transmission property of mmWave signals, we propose a minimal angle-difference user pairing scheme to reduce the secrecy outage probability of users. Considering the spatial correlation between the selected user pair and eavesdroppers, we develop two maximum ratio transmission beamforming schemes to further enhance the secrecy performance of mmWave NOMA networks. Closed-form secrecy outage probability for the paired users with different eavesdropper detection capacities is derived   4)  Achievable rates and reliability analysis of mmWave channels: We leverage random coding error exponent to investigate the achievable rate of mmWave channels under reliability and packet duration (finite blocklength) constraints. Under the assumption of perfect and imperfect channel state information at the receiver (CSIR), exact and approximate analytical expressions of achievable rates are derived to capture the relationship of rate, latency, and reliability. Furthermore, we show that the achievable rate always increases as the bandwidth increases with perfect CSIR. However, there exists a critical bandwidth that maximizes the achievable rate for non-line-of-sight mmWave signals with imperfect CSIR, beyond which the achievable rate will decrease with increasing bandwidth. For imperfect CSIR, the optimal training symbol length and power allocation factor at the training phase are investigated and closed-form expressions for special cases are derived. QC 20201106</p

Rate, Reliability and Secrecy Performance Analysis and Optimization for Millimeter WaveCommunications

With the fast development of electronic devices and computer science, various emerging applications (e.g., virtual and augmented reality, ultra-high-definition three-dimensional video, autonomous driving, big data analysis, etc.) have created an explosive growth of mobile data traffic and caused growing demands for higher communication rates, more reliable and secure connectivity in the future wireless communications, e.g., the fifth-generation (5G) and beyond mobile communications. In recent years, millimeter wave (mmWave) communication, as a promising candidate to meet the aforementioned demands, has attracted extensive research attention, and is regarded as one of the key enablers for the 5G and beyond mobile communications. The main features of mmWave communications include:  abundant spectral resources, high penetration loss, severe pathloss, and narrow antenna beams, and these particular features make the potential challenges and solutions with mmWave significantly different from those in the conventional sub-6 GHz systems.   It is known that beamforming is a crucial stage of mmWave communication to support high antenna gains and suppress inter-channel interference. However, the related research on beamforming design is fairly recent and insufficient. Motivated by the urgent needs for further development, in this thesis, we investigate beamforming optimization and the reliability and secrecy performance for mmWave communications. Our main research regarding beamforming design and secrecy performance can be categorized into the following three aspects:   1)      Hybrid beamforming (HBF) for mmWave systems with learning machines: We propose two low-complexity and robust learning schemes to design HBF for mmWave full-duplex systems and multi-user multi-input and multi-output (MU-MIMO) systems i.e., extreme learning machine based HBF and convolutional neural networks based HBF. To provide accurate labels for off-line training, effective HBF algorithms are proposed to achieve joint self-interference cancellation and HBF optimization for mmWave full-duplex systems and to achieve joint transmitting and receiving HBF optimization for mmWave MU-MIMO systems. The convergence of the proposed algorithms is proven and the computation complexity is analyzed. Results show that the proposed learning based methods can achieve higher spectral efficiency, lower complexity, and more robust HBF performance than conventional optimization based methods.   2)      Decentralized beamforming for intelligent reflecting surface (IRS)-enhanced cell-free networks: To avoid the centralized computation and inspired by the cost-effective IRS technique, we propose a fully decentralized design framework for cooperative beamforming in IRS-aided cell-free networks, in which transmitting digital beamformers and IRS-based analog beamformers are jointly optimized. We first derive a closed-form expression of each updating variable and then propose a fully decentralized beamforming scheme based on the alternating direction method of multipliers to incrementally and locally update the beamformers. Results reveal that the new scheme outperforms existing decentralized methods and IRSs can significantly improve the system sum-rate.   3)      Physical layer security of mmWave non-orthogonal multiple access (NOMA) networks: Considering the limited scattering characteristics of mmWave channels and imperfect successive interference cancellation at receivers, we develop an analytic framework on the secrecy outage probability for mmWave NOMA networks. Based on the directional transmission property of mmWave signals, we propose a minimal angle-difference user pairing scheme to reduce the secrecy outage probability of users. Considering the spatial correlation between the selected user pair and eavesdroppers, we develop two maximum ratio transmission beamforming schemes to further enhance the secrecy performance of mmWave NOMA networks. Closed-form secrecy outage probability for the paired users with different eavesdropper detection capacities is derived   4)  Achievable rates and reliability analysis of mmWave channels: We leverage random coding error exponent to investigate the achievable rate of mmWave channels under reliability and packet duration (finite blocklength) constraints. Under the assumption of perfect and imperfect channel state information at the receiver (CSIR), exact and approximate analytical expressions of achievable rates are derived to capture the relationship of rate, latency, and reliability. Furthermore, we show that the achievable rate always increases as the bandwidth increases with perfect CSIR. However, there exists a critical bandwidth that maximizes the achievable rate for non-line-of-sight mmWave signals with imperfect CSIR, beyond which the achievable rate will decrease with increasing bandwidth. For imperfect CSIR, the optimal training symbol length and power allocation factor at the training phase are investigated and closed-form expressions for special cases are derived. QC 20201106</p

Achievable Rate Analysis of Millimeter Wave Channels with Random Coding Error Exponent

Millimeter Wave (mmWave) communication has attracted massive attention, since the abundant available bandwidth can potentially provide reliable communication with orders of magnitude capacity improvements relative to microwave. However, the achievable rate of mmWave channels under latency and reliability constraints is still not quite clear. We investigate the achievable rates of mmWave channels by random coding error exponent (RCEE) with finite blocklength. With imperfect channel state information at the receiver, the exact and approximate analytical expressions of the training based maximum achievable rate are derived to capture the relationship among rate-latency-reliability. Additionally, the relationship between the training based maximum achievable rate and bandwidth is investigated. We show that there exists critical bandwidth to maximize the training based maximum achievable rate for the non-line-of-sight (NLoS) propagation. Numerical results show that the approximate expression of the training based maximum achievable rate are tight and can capture the tendency at low SNRs. In addition, results show that for a given rate, one can reduce both packet duration and decoding error probability by increasing bandwidth. Results also suggest that in some mmWave bands, e.g. 57-64 GHz band, the performance, i.e., Gallager function, is significantly affected by frequency selective power absorption.QC 20201106Not duplicate with Diva 1349489 </p