This paper studies the newly emerging wireless powered communication network in which one hybrid access point (H-AP) with a constant power supply coordinates the wireless energy/information transmissions to/from a set of distributed users that do not have fixed 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 allocated to the DL wireless power transfer and the UL information transmissions given a total time constraint based on the users' DL and UL channels as well as their average harvested energy values. Our result 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 the nearer users in the DL, has to transmit with more power in the UL for reliable communication. As a result, the sum-throughput maximization solution allocates substantially more time to the near users than the far users, thus resulting in unfair rate allocation among users. To overcome this problem, we furthermore propose to maximize a new metric so-called common-throughput with an additional constraint that all users should be allocated with equal rates regardless of their distances to the H-AP. Simulation results demonstrate the effectiveness of the common-throughput approach for solving the uniquely new doubly near-far problem in wireless powered communication networks.
This paper investigates an orthogonal frequency division multiplexing (OFDM)-based wireless powered communication system, where one user harvests energy from an energy access point (EAP) to power its information transmission to a data access point (DAP). The channels from the EAP to the user, i.e., the wireless energy transfer (WET) link, and from the user to the DAP, i.e., the wireless information transfer (WIT) link, vary over both time slots and sub-channels (SCs) in general. To avoid interference at DAP, WET and WIT are scheduled over orthogonal SCs at any slot. Our objective is to maximize the achievable rate at the DAP by jointly optimizing the SC allocation over time and the power allocation over time and SCs for both WET and WIT links. Assuming availability of full channel state information (CSI), the structural results for the optimal SC/power allocation are obtained and an offline algorithm is proposed to solve the problem. Furthermore, we propose a low-complexity online algorithm when causal CSI is available.
This paper studies bandwidth-efficient transmission protocol for a two-hop communication network with one source, one destination, and two assisting relays. The two-path successive relaying protocol is considered where the source/destination transmits/receives continuously in time, and two half-duplex relays both operating in a time-division-duplex (TDD) mode successively in turn forward messages from source to destination. By mimicking a full-duplex relay with two alternating half-duplex relays, this protocol ideally achieves the same bandwidth efficiency. However, a major issue for this protocol to be effectively implemented is to tackle the inter-relay interference (IRI), which occurs when one relay listens to the source, it is interfered with by the other relay that transmits to the destination. This paper characterizes achievable rates for the protocol of interest for a simplified symmetric two-path relay channel when both relays employ the decode-and-forward (DF) operation or digital relaying. First, the achievable rate of digital relaying using the conventional repetition coding is derived, and is compared to that of a full-duplex relay. It is shown that owing to the IRI, this method leads to severe rate loss at high signal-to-noise- ratio (SNR) regime. Then, this paper presents a new scheme that can be easily incorporated into the considered protocol to avoid completely any rate loss caused by the IRI. The proposed scheme utilizes the dirty-paper-coding (DPC) -based interference pre-subtraction at the source transmitter, and is shown to be capacity-optimal.
In order to realize the high precision indoor positioning system, an embedded gateway is designed for data transmission and forwarding, and the server is developed on Windows platform for storing data, obtaining and showing location information. The indoor positioning system architecture is given, focusing on the design and implementation of gateway and server. ARM11 processor is used to design a gateway. Gateway architecture and software development are introduced. A server solution is described. The results show that the designed gateway and server meet the requirements of data transmission and good positioning accuracy.
HVDC circuit breakers are the core equipment for the reliable operation of VSC-HVDC. The controllable resonant DC circuit breaker amplifies the oscillating current through excitation capacitor oscillation and boost, quickly generating a current zero point, and achieving the transmission of fault current and circuit breaking. In this paper, controlled resonant DC circuit breaker with a half-bridge topology structure is first simulated. But this topology structure cannot generate a current crossing point when the oscillation frequency decreases, resulting in the mechanical switch cannot be broken. So, a DC circuit breaker with a full-bridge topology structure is proposed, and the simulation results show that the topology structure can make the oscillation current rises quickly. Compared with the half-bridge topology circuit, its current crossing zero point can be created when the oscillation frequency reduces. Finally, in order to verify the safety and reliability of the full-bridge topology, stress analysis is conducted on the full control device and diodes, which provides a basis for the safe and reliable operation and electrical structure design of the high-voltage DC circuit breaker.
Wireless communication with unmanned aerial vehicles (UAVs) is a promising technology for future communication systems. In this paper, assuming that the UAV flies horizontally with a fixed altitude, we study energy-efficient UAV communication with a ground terminal via optimizing the UAV's trajectory, a new design paradigm that jointly considers both the communication throughput and the UAV's energy consumption. To this end, we first derive a theoretical model on the propulsion energy consumption of fixed-wing UAVs as a function of the UAV's flying speed, direction, and acceleration. Based on the derived model and by ignoring the radiation and signal processing energy consumption, the energy efficiency of UAV communication is defined as the total information bits communicated normalized by the UAV propulsion energy consumed for a finite time horizon. For the case of unconstrained trajectory optimization, we show that both the rate-maximization and energy-minimization designs lead to vanishing energy efficiency and thus are energy-inefficient in general. Next, we introduce a simple circular UAV trajectory, under which the UAV's flight radius and speed are jointly optimized to maximize the energy efficiency. Furthermore, an efficient design is proposed for maximizing the UAV's energy efficiency with general constraints on the trajectory, including its initial/final locations and velocities, as well as minimum/maximum speed and acceleration. Numerical results show that the proposed designs achieve significantly higher energy efficiency for UAV communication as compared with other benchmark schemes.
The implementing way of transmitting binary Direct Series Spreading System by BPSK and its anti jamming function were analyzed. Anti jamming emulational pattern of DS SS was put forward. The emulational result showed greatly the anti jamming function of DS SS was stronger against single frequency single interfering signals.
This paper studies cooperative schemes for the inter-cell interference control in orthogonal-frequency-divisionmultiple- access (OFDMA) cellular systems. The downlink transmission in a simplified two-cell system is examined, where both cells simultaneously access the same frequency band using OFDMA. The joint power and subcarrier allocation over the two cells is investigated for maximizing their sum throughput with both centralized and decentralized implementations. Particularly, the decentralized allocation is achieved via a new cooperative interference control approach, whereby the two cells independently implement resource allocation to maximize individual throughput in an iterative manner, subject to a set of mutual interference power constraints. Simulation results show that the proposed decentralized resource allocation schemes achieve the system throughput close to that by the centralized scheme, and provide substantial throughput gains over existing schemes.
The misuse of wireless communications by malicious users imposes new threats on the public security, and authorized parities 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 the case when Alice employs the widely- used binary phase-shift keying (BPSK) modulation, we obtain the optimal solution to the average SSER minimization problem. Numerical results show that the symbol-level spoofing scheme with optimized transmission achieves a much lower average SSER, as compared to other benchmark schemes.
In this paper, we propose several spectrum sensing methods designed using the generalized likelihood ratio test (GLRT) paradigm, for application in a cognitive radio network. The proposed techniques utilize the eigenvalues of the sample covariance matrix of the received signal vector, taking advantage of the fact that in practice, the primary signal in a cognitive radio environment will either occupy a subspace of dimension strictly smaller than the dimension of the observation space, or have a spectrum that is non-white. We show that by making various assumptions on the availability of side information such as noise variance and signal space dimension, several feasible algorithms result which all outperform the standard energy detector.
This paper studies the energy management in the coordinated multi-point (CoMP) systems powered by smart grids, where each base station (BS) with local renewable energy generation is allowed to implement the two-way energy trading with the grid. Due to the unevenly generated renewable energy over distributed BSs and the difference in the prices for buying/selling energy from/to the gird, it is in general beneficial for the cooperative BSs to jointly manage their energy trading with the grid and energy consumption in CoMP based communication for reducing the total energy cost. We consider the downlink transmission in one CoMP cluster by jointly optimizing the BSs' purchased/sold energy units from/to the grid and their cooperative transmit beamforming, so as to minimize the total energy cost subject to the given quality of service (QoS) constraints for the users. By applying techniques from convex optimization and uplink-downlink duality, we propose an efficient algorithm to solve this problem optimally. Through simulations, we show the performance gain achieved by our proposed joint energy trading and communication cooperation scheme in terms of cost reduction, as compared to a baseline scheme with the separate designs of communication cooperation and energy trading.
This paper considers the two-hop relay broadcast channel (BC) in a wireless cellular network where the downlink transmission from the base station (BS) to mobile users is assisted by a fixed relay station (RS). Assuming both the BS and RS equipped with multi-antennas, we consider joint optimization of beamforming weights and power allocations at the BS and RS so as to minimize their weighted sum-power consumption under the minimum signal-to-interference-noise-ratio (SINR) constraints at user terminals. We extend the two well-known criteria for design of linear precoding in the conventional MIMO-BC without relay, namely, the SINR balancing and the channel-inversion, to the MIMO relay BC. First, an iterative SINR-balancing algorithm is proposed that in each iteration optimizes transmit parameters at one station (BS or RS) with those at the other fixed. Secondly, a joint BS and RS channel-inversion algorithm is proposed together with a novel technique termed "eigenmode switching" at the RS. Simulation results show that optimized joint beamforming and power control are able to achieve substantial power savings for the MIMO relay BC.
Compton-thick active galactic nuclei (CT-AGNs), which are defined by column density $\mathrm{N_H} \geqslant 1.5 \times 10^{24} \ \mathrm{cm}^{-2}$, emit feeble X-ray radiation, even undetectable by X-ray instruments. Despite this, the X-ray emissions from CT-AGNs are believed to be a substantial contributor to the cosmic X-ray background (CXB). According to synthesis models of AGNs, CT-AGNs are expected to make up a significant fraction of the AGN population, likely around 30% or more. However, only $\sim$11% of AGNs have been identified as CT-AGNs in the Chandra Deep Field-South (CDFS). To identify hitherto unknown CT-AGNs in the field, we used a Random Forest algorithm for identifying them. First, we build a secure classified subset of 210 AGNs to train and evaluate our algorithm. Our algorithm achieved an accuracy rate of 90% on the test set after training. Then, we applied our algorithm to an additional subset of 254 AGNs, successfully identifying 67 CT-AGNs within this group. This result significantly increased the fraction of CT-AGNs in the CDFS, which is closer to the theoretical predictions of the CXB. Finally, we compared the properties of host galaxies between CT-AGNs and non-CT-AGNs and found that the host galaxies of CT-AGNs exhibit higher levels of star formation activity.
Intelligent reflecting surface (IRS) is a promising technology to improve the performance of backscatter communication systems by smartly reconfiguring the multi-reflection channel. To fully exploit the passive beamforming gain of IRS in backscatter communication, channel state information (CSI) is indispensable but more practically challenging to acquire than conventional IRS-assisted systems, since IRS passively reflects signals over both the forward and backward (backscattering) links between the reader and tag. To address this issue, we propose in this letter a new and efficient channel estimation scheme for the IRS-assisted backscatter communication system. To minimize the mean-square error (MSE) of channel estimation, we formulate and solve an optimization problem by designing the IRS training reflection matrix for channel estimation under the constraints of unit-modulus elements and full rank. Simulation results verify the effectiveness of the proposed channel estimation scheme as compared to other baseline schemes.
Block diagonalization (BD) is a practical linear precoding technique that eliminates the inter-user interference in downlink multiuser multiple-input multiple-output (MIMO) systems. In this paper, we apply BD to the downlink transmission in a cooperative multi-cell MIMO system, where the signals from different base stations (BSs) to all the mobile stations (MSs) are jointly designed with the perfect knowledge of the downlink channels and transmit messages. Specifically, we study the optimal BD precoder design to maximize the weighted sum-rate of all the MSs subject to a set of per-BS power constraints. This design problem is formulated in an auxiliary MIMO broadcast channel (BC) with a set of transmit power constraints corresponding to those for individual BSs in the multi-cell system. By applying convex optimization techniques, this paper develops an efficient algorithm to solve this problem, and derives the closed-form expression for the optimal BD precoding matrix. It is revealed that the optimal BD precoding vectors for each MS in the per-BS power constraint case are in general non-orthogonal, which differs from the conventional orthogonal BD precoder design for the MIMO-BC under one single sum-power constraint. Moreover, for the special case of single-antenna BSs and MSs, the proposed solution reduces to the optimal zero-forcing beamforming (ZF-BF) precoder design for the weighted sum-rate maximization in the multiple-input single-output (MISO) BC with per-antenna power constraints. Suboptimal and low-complexity BD/ZF-BF precoding schemes are also presented, and their achievable rates are compared against those with the optimal schemes.
