With bounded resources, traditional DBMS can't process and query the continuous and unbounded data stream in real time, and DBMS is researched widely as a new method to deal with this problem. The efficiency of the process and the query is improved with the new operator and the new query model introduced in DBMS. Firstly, the DSMS concept is introduced, and the principle of DSMS as well as the architecture of DSMS is discussed. Finally several typical DSMS instances are introduced and compared.
This paper studies the newly emerging wireless powered communication network in which one hybrid access point (H-AP) with constant power supply coordinates the wireless energy/information transmissions to/from a set of distributed users that do not have other energy sources. A "harvest-then-transmit" protocol is proposed where all users first harvest the wireless energy broadcast by the H-AP in the downlink (DL) and then send their independent information to the H-AP in the uplink (UL) by time-division-multiple-access (TDMA). First, we study the sum-throughput maximization of all users by jointly optimizing the time allocation for the DL wireless power transfer versus the users' UL information transmissions given a total time constraint based on the users' DL and UL channels as well as their average harvested energy values. By applying convex optimization techniques, we obtain the closed-form expressions for the optimal time allocations to maximize the sum-throughput. Our solution reveals an interesting "doubly near-far" phenomenon due to both the DL and UL distance-dependent signal attenuation, where a far user from the H-AP, which receives less wireless energy than a nearer user in the DL, has to transmit with more power in the UL for reliable information transmission. As a result, the maximum sum-throughput is shown to be achieved by allocating substantially more time to the near users than the far users, thus resulting in unfair rate allocation among different users. To overcome this problem, we furthermore propose a new performance metric so-called common-throughput with the additional constraint that all users should be allocated with an equal rate regardless of their distances to the H-AP. We present an efficient algorithm to solve the common-throughput maximization problem. Simulation results demonstrate the effectiveness of the common-throughput approach for solving the new doubly near-far problem in wireless powered communication networks.
본 논문에서는 모노폴 안테나의 각 모드별 전계 분포를 분석하여 전류가 상대적으로 약한 부분(전계가 강한 부분)에 스터브를 이용해 기본 공진 모드에는 영향을 주지 않으면서 안테나의 하모닉 성분인 3차 모드 공진 주파수를 효율적으로 제어하고, 스터브의 두께를 조절하여 임피던스 특성을 조절할 수 있는 방법을 제안한다. 이 방법을 이용하여 일반적인 USB Dongle 크기(<TEX>$20{\times}45mm$</TEX>)에 안테나를 설계하였으며, -10 dB 기준 2 GHz 대역에서 대역폭이 600 MHz(2.3~3 GHz), 5 GHz 대역의 대역폭은 1 GHz(4.9~5.9 GHz)로 WLAN 주파수 대역을 만족하는 성능과 50 % 효율이 달성되었다. This paper presents a new method for designing a dual-band WIFI antenna using the third-order harmonic mode of a monopole antenna whose first-order mode operates at the low frequency band of WIFI. As analysing the current distribution of the third-order mode of this monopole antenna, the strongest point of electric field can be found. Then by attaching a stub at this point, the resonant frequency of the stub radiator can be adjusted from the third-order mode of the monopole antenna into the high frequency band of WIFI and the input impedance at this resonant frequency can be controlled with the width of the branch, without affecting the low frequency band of WIFI (the first-order mode of the monopole antenna). The compact dual-band antenna is designed at the size of an USB(universal serial bus) dongle and the bandwidth covers 600 MHz(2.3~3 GHz) at 2 GHz and 1 GHz(4.9~5.9 GHz) at 5 GHz under -10 dB which is satisfied with WLAN frequency. Efficiency of proposed antenna achieves over 50 % at WLAN frequency.
A new universal algorithm of modulation and demodulation is presented to process most signals of digital phase-related modulation schemes such as M-DPSK, MSK and even some M-FSK with low modulation level. It can easily deal with modulation and demodulation of the signals with different modulation schemes and data rates by only setting a few input arguments of the versatile software modules. The computational complexity of the algorithm is far less than that of conventional methods. The average processing capacity of the algorithm is about 15 instructions per symbol when processing DQPSK signals. It can be applied to software radios.
Random beamforming (RBF) is a practically favorable transmission scheme for multiuser multi-antenna downlink systems. This paper studies the asymptotic rates achievable with RBF in a multi-cell system subject to the inter-cell interference, by assuming that the number of users in each cell scales in a given order with the signal-to-noise ratio (SNR). In particular, we investigate the achievable degrees of freedom (DoF) for the sum-rate in each cell when the SNR goes to infinity, and characterize the achievable DoF region for all the cells. Our results show that to achieve the DoF-optimal transmission in a multi-cell system with RBF, the numbers of transmit beams in all the cells need to be assigned in a collaborative manner based on the user densities.
With recent developments of wireless communication technologies, malicious users can use them to commit crimes or launch terror attacks, thus imposing new threats on the public security. To quickly respond to defend these attacks, authorized parities (e.g., the National Security Agency of the USA) need to intervene in the malicious communication links over the air. This paper investigates this emerging wireless communication intervention problem at the physical layer. Unlike prior studies using jamming to disrupt or disable the targeted wireless communications, we propose a new physical-layer spoofing approach to change their communicated information. Consider a fundamental three-node system over additive white Gaussian noise (AWGN) channels, in which an intermediary legitimate spoofer aims to spoof a malicious communication link from Alice to Bob, such that the received message at Bob is changed from Alice's originally sent message to the one desired by the spoofer. We propose a new symbol-level spoofing scheme, where the spoofer designs the spoofing signal via exploiting the symbol-level relationship between each original constellation point of Alice and the desirable one of the spoofer. In particular, the spoofer aims to minimize the average spoofing-symbol-error-rate (SSER), which is defined as the average probability that the symbols decoded by Bob fail to be changed or spoofed, by designing its spoofing signals over symbols subject to the average transmit power constraint. By considering two cases when Alice employs the widely-used binary phase-shift keying (BPSK) and quadrature phase-shift keying (QPSK) modulations, we obtain the respective optimal solutions to the two average SSER minimization problems. Numerical results show that the symbol-level spoofing scheme with optimized transmission achieves a much lower average SSER, as compared to other benchmark schemes.
Unmanned aerial vehicles (UAVs) have recently gained growing popularity in wireless communications owing to their many advantages such as swift and cost-effective deployment, line-of-sight (LoS) aerial-to-ground link, and controllable mobility in three-dimensional (3D) space. Although prior works have exploited the UAV's mobility to enhance the wireless communication performance under different setups, the fundamental capacity limits of UAV-enabled/aided multiuser communication systems have not yet been characterized. To fill this gap, we consider, in this paper, a UAV-enabled two-user broadcast channel (BC), where a UAV flying at a constant altitude is deployed to send independent information to two users at different fixed locations on the ground. We aim to characterize the capacity region of this new type of BC over a given UAV flight duration, by jointly optimizing the UAV's trajectory and transmit power/rate allocations over time, subject to the UAV's maximum speed and maximum transmit power constraints. First, to draw essential insights, we consider two special cases with asymptotically large/low UAV flight duration/speed, respectively. For the former case, it is shown that a simple hover-fly-hover (HFH) UAV trajectory with time division multiple access (TDMA)-based orthogonal multiuser transmission is capacity-achieving; while in the latter case, the UAV should hover at a fixed location that is nearer to the user with larger achievable rate and in general superposition coding (SC)-based non-orthogonal transmission with interference cancellation at the receiver of the nearer user is required. Next, we consider the general case with finite UAV speed and flight duration. We show that the optimal UAV trajectory should follow a general HFH structure, i.e., the UAV successively hovers at a pair of optimal initial and final locations above the line segment connecting the two users each with a certain amount of time and flies unidirectionally between them at the maximum speed, and SC is generally needed. Furthermore, when TDMA-based transmission is considered for low-complexity implementation, we show that the optimal UAV trajectory still follows an HFH structure, but the hovering locations can only be those above the two users. Extensive simulation results are provided to verify our analysis, which also reveal useful guidelines to the practical design of UAV trajectory and communication jointly.
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.
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.
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.
This paper studies the wireless two-way relay channel (TWRC), where two source nodes, S1 and S2, exchange information through an assisting relay node, R. It is assumed that R receives the sum signal from S1 and S2 in one time-slot, and then amplifies and forwards the received signal to both S1 and S2 in the next time-slot. By applying the principle of analogue network (ANC), each of S1 and S2 cancels the so-called "self-interference" in the received signal from R and then decodes the desired message. Assuming that S1 and S2 are each equipped with a single antenna and R with multi-antennas, this paper analyzes the capacity region of an ANC-based TWRC with linear processing (beamforming) at R. The capacity region contains all the achievable bidirectional rate-pairs of S1 and S2 under the given transmit power constraints at S1, S2, and R. We present the optimal relay beamforming structure as well as an efficient algorithm to compute the optimal beamforming matrix based on convex optimization techniques. Low-complexity suboptimal relay beamforming schemes are also presented, and their achievable rates are compared against the capacity with the optimal scheme.
