The papers in this special issue aim to report on the latest advances in signal processing for reconfigurable intelligent surface (RIS)-aided 6G networks with focus on theoretical development, algorithmic design, and future applications.RIS or intelligent reflecting surface (IRS) technology has been widely regarded as one of the most promising techniques to deal with the blockage issue in millimeter-wave (mmWave) communications. An RIS is a planar surface consisting of an array of nearlypassive reflecting elements, each of which can independently induce an controllable phase shift on the incident signal. By installing RIS on walls or ceilings, a virtual line-of-sight (LoS) link between a mmWave base station (BS) and the users can be established so as to bypass any blockage between them. In addition, RIS is a promising technology for applications in conventional sub-6 GHz communications. Specifically, by judiciously adjusting the phase shifts of the reflecting elements, the reflected signals can be constructively superimposed (even with the direct path if it is available) to enhance the desired signal power or can be cancelled out to mitigate the impact of co-channel interference or signal leakage to eavesdroppers. Since the elements of an RIS reflect the incoming signals without any signal processing operations that require radio-frequency (RF) chains, an RIS has a lower implementation cost than conventional active receivers and transmitters. An RIS can be fabricated with light weight and small-thickness layers, and thus can be readily integrated into the environment.
With the development of space optical remote sensing imaging technology, Space remote sensing systems have reached a new stage which is changing towards higher spatial resolution, higher spectral resolution, higher radiometric resolution, and higher temporal resolution.In recent years, applying TDI CCD to earth observation from space has become a growing trend, and it can make integrals to the same moving target by using multilevel sensitive units, which has improved the S/N and the dynamic range of optical system effectively.This paper proposes a semi-physical simulation system designed for space optical remote sensing based on 6-DOF motion platform.The system ignores the atmospheric radiation and it can obtain high quality images by simulating the whole physical process of the space optical remote sensing imaging chain which includes the sensor of TDI CCD, the motion platform of satellite, the ground scene, and the access to final remote sensing.
Nowdays HVDC technology is developing very fast. As the core component of HVDC technology, high-voltage DC circuit breaker has got much attention. High-voltage DC circuit breaker consists of series of semiconductors. In this paper, Researches of transient voltage imbalance mechanism of connected MOSFETS is done The effects of parasitic parameters on the switching characteristics of the MOSFET is analyzed. The dynamic and static voltage imbalance mechanism from two aspects is analyzed. The theoretical basis for the design of the high-voltage DC circuit breaker and the design of equalizing circuits is proposed. A high frequency carrier isolation driver strategy is proposed and a pulse control circuit, drive circuit is designed which implements to provides a controlled drive signal to connected-MOSFETS. In the end, a DC circuit breaker is designed, and the feasibility of the design is verified by the analysis of the experimental waveforms.
Energy harvesting is a promising solution to prolong the operation of energy-constrained wireless networks. In particular, scavenging energy from ambient radio signals, namely wireless energy harvesting (WEH), has recently drawn significant attention. In this paper, we consider a point-to-point wireless link over the narrowband flat-fading channel subject to time-varying co-channel interference. It is assumed that the receiver has no fixed power supplies and thus needs to replenish energy opportunistically via WEH from the unintended interference and/or the intended signal sent by the transmitter. We further assume a single-antenna receiver that can only decode information or harvest energy at any time due to the practical circuit limitation. Therefore, it is important to investigate when the receiver should switch between the two modes of information decoding (ID) and energy harvesting (EH), based on the instantaneous channel and interference condition. In this paper, we derive the optimal mode switching rule at the receiver to achieve various trade-offs between wireless information transfer and energy harvesting. Specifically, we determine the minimum transmission outage probability for delay-limited information transfer and the maximum ergodic capacity for no-delay-limited information transfer versus the maximum average energy harvested at the receiver, which are characterized by the boundary of so-called "outage-energy" region and "rate-energy" region, respectively. Moreover, for the case when the channel state information (CSI) is known at the transmitter, we investigate the joint optimization of transmit power control, information and energy transfer scheduling, and the receiver's mode switching. The effects of circuit energy consumption at the receiver on the achievable rate-energy trade-offs are also characterized. Our results provide useful guidelines for the efficient design of emerging wireless communication systems powered by opportunistic WEH.
As an effective fluid control technology, the groove flow control technique is used to improve axial-flow pump energy performance. Determining the values of groove parameters reasonably is essential to maximize the energy performance of axial-flow pumps. In order to solve the limitation of traditional groove design in groove flow control technology, this paper presents a multi-objective groove optimization method developed by integrating OD (orthogonal design), RSM (response surface methodology), and PSO (particle swarm optimization). A case study of an axial-flow pump is conducted. By using the OD method, the sample space has been designed. The performance prediction model of axial flow pump is established by RSM. In addition, the linear weighted method is employed to construct a comprehensive evaluation function as the fitness value of PSO. Finally, an optimal combination of the groove flow control technique parameters is determined using the PSO algorithm. The result shows that this method proposed in this paper is helpful for solving multi-objective optimization of the groove flow control technique and could effectively improve the hydraulic performance of the axial-flow pump under three typical stall conditions. The optimization effectively suppresses the formation of a saddle zone. In addition, the optimization increases the pump head by 0.30, 4.41 and 9.70 m under the critical stall condition, moderate stall condition and deep stall condition, respectively. Moreover, the optimization leads to better inflow conditions for the impeller and enhances its performance under the stall conditions. This study provides a basis for the early-stage design of the groove flow control technique for axial-flow pump and provides a valuable reference for the optimal design of groove for other fluid machineries.
In this paper, we propose a novel semi-passive elements-aided channel estimation framework for intelligent reflecting surface (IRS), where a small portion of IRS reflecting elements are able to process the received signal for facilitating the channel estimation. Specifically, the BS-IRS channel is estimated by applying the estimation of signal parameters via rotational invariance technique (ESPRIT), while the user-IRS channels are estimated by combining the use of total least square (TLS) ESPRIT and multiple signal classification (MUSIC) methods. The required training time of the proposed channel estimation scheme is irrelevant to the number of IRS reflecting elements, thus substantially reducing the training overhead. Simulation results show the great advantages of our proposed scheme over both the conventional compressed sensing (CS)-based channel estimation and cascaded channel estimation schemes.
Unmanned aerial vehicles (UAVs) are expected to be an important new class of users in the fifth generation (5G) and beyond 5G cellular networks. In particular, there are emerging UAV applications such as aerial photograph and data relaying that require high-speed communications between the UAVs and the ground base stations (GBSs). Due to the high UAV altitude, the strong line-of-sight (LoS) links generally dominate the channels between the UAVs and GBSs, which brings both opportunities and challenges in the design of future wireless networks supporting both terrestrial and aerial users. Although each UAV can associate with more GBSs for communication as compared to terrestrial users thanks to the LoS-dominant channels, it also causes/suffers more severe interference to/from the terrestrial communications in the uplink/downlink. This paper studies the uplink communication from a multi-antenna UAV to a set of GBSs within its signal coverage by considering a practical yet challenging scenario when the number of antennas at the UAV is smaller than that of co-channel GBSs. To achieve high-rate transmission yet avoid interfering with any of the existing terrestrial communications at the co-channel GBSs, we propose a novel multi-beam transmission strategy by exploiting the non-orthogonal multiple access (NOMA) technique. Specifically, the UAV sends each data stream to a selected subset of the GBSs, which can decode the UAV's signals and then cancel them before decoding the messages of their served terrestrial users, and in the meanwhile nulls its interference at the other GBSs via zero-forcing (ZF) beamforming. To draw essential insight, we first characterize in closed-form the degrees-of-freedom (DoF) under the proposed strategy. Then, we propose an efficient algorithm to maximize the UAV's transmit rate subject to the interference avoidance constraints for protecting the terrestrial users.
Intelligent reflecting surface (IRS) can be densely deployed in wireless networks to significantly enhance the communication channels. In this letter, we consider the downlink transmission from a multi-antenna base station (BS) to a single-antenna user, by exploiting the cooperative passive beamforming (CPB) and line-of-sight (LoS) path diversity gains of multi-IRS signal reflection. Unlike existing works where only one single multi-IRS reflection path from the BS to user is selected, we propose a new and more general multi-path beam routing scheme. Specifically, the BS sends the user's information signal via multiple orthogonal active beams (termed as active beam splitting), which point towards different IRSs. Then, these beamed signals are subsequently reflected by selected IRSs via their CPB in different paths, and finally coherently combined at the user's receiver (thus named {\it \textbf{passive beam combining}}). For this scheme, we formulate a new multi-path beam routing design problem to jointly optimize the number of IRS reflection paths, the selected IRSs for each of the reflection paths, the active/passive beamforming at the BS/each selected IRS, as well as the BS's power allocation over different active beams, so as to maximize the received signal power at the user. To solve this challenging problem, we first derive the optimal BS/IRS beamforming and BS power allocation for a given set of reflection paths. The clique-based approach in graph theory is then applied to solve the remaining multi-path selection problem efficiently. Simulation results show that our proposed multi-path beam routing scheme significantly outperforms its conventional single-path beam routing special case.
Because of hundreds of megahertz bandwidth is a necessary condition for high speed data rates, the millimeter wave band will be used for 5G communication systems. Propagation channel measurements at mm-wave band were conducted in a conference room with an advanced MIMO channel sounder. An omnidirectional antenna at the transmitter and a horn antenna at the receiver were used. The transmitter antenna was fixed in one location while the receiver antenna was rotated 360 ° in azimuth in the measurement. The receiver antenna was conducted in 7.5° increments to capture measured data with eleven locations. The results of path loss and shadow fading factors are presented here using reference distance path loss model based on measured data.
Wireless charging is a promising way to power wireless nodes' transmissions. This paper considers new dual-function access points (APs) which are able to support the energy/information transmission to/from wireless nodes. We focus on a large-scale wireless powered communication network (WPCN), and use stochastic geometry to analyze the wireless nodes' performance tradeoff between energy harvesting and information transmission. We study two cases with battery-free and battery-deployed wireless nodes. For both cases, we consider a harvest-then-transmit protocol by partitioning each time frame into a downlink (DL) phase for energy transfer, and an uplink (UL) phase for information transfer. By jointly optimizing frame partition between the two phases and the wireless nodes' transmit power, we maximize the wireless nodes' spatial throughput subject to a successful information transmission probability constraint. For the battery-free case, we show that the wireless nodes prefer to choose small transmit power to obtain large transmission opportunity. For the battery-deployed case, we first study an ideal infinite-capacity battery scenario for wireless nodes, and show that the optimal charging design is not unique, due to the sufficient energy stored in the battery. We then extend to the practical finite-capacity battery scenario. Although the exact performance is difficult to be obtained analytically, it is shown to be upper and lower bounded by those in the infinite-capacity battery scenario and the battery-free case, respectively. Finally, we provide numerical results to corroborate our study.
This paper studies a new and challenging wireless surveillance problem where a legitimate monitor (e.g., the National Security Agency in the USA) attempts to eavesdrop more than one suspicious communication links simultaneously to maximally protect public security. To facilitate concurrent eavesdropping, our multi-antenna monitor employs a proactive eavesdropping via jamming approach, by purposely jamming suspicious receivers to lower the transmission rates of the target links. In particular, we are interested in characterizing the monitor's achievable eavesdropping signal-to-interference-plus-noise ratio (SINR) region of two suspicious links, by optimizing the legitimate monitor's transmit covariance matrix for jamming the two suspicious receivers. As the monitor cannot hear more than what suspicious links transmit, the achievable eavesdropping SINR region is essentially the intersection of the achievable region for the two suspicious links and that for the two eavesdropping links, and the former region can be purposely altered by the monitor's jamming transmit covariance matrix subject to its power budget. As both suspicious links' SINRs are affected by monitor's jamming covariance matrix, we first analyze the achievable region bounds for both suspicious links and then characterize the achievable eavesdropping SINR region. Finally, numerical results are provided to corroborate our analysis.
