1,474 publications from this institution
A slot‐type ground radiation antenna and a high‐speed digital circuit are modeled within a Bluetooth headset to demonstrate a useful method to reduce high‐speed digital noise. The method of noise reduction is based on the theory of characteristic modes. Modifying the structure of the antenna and thereby changing the characteristic mode excited by the antenna leads to decoupling between the antenna and the circuit, achieving the high‐sensitivity property of the antenna. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:2104–2108, 2015
Opportunistic spectrum access (OSA) is a key technique enabling the secondary users (SUs) in a cognitive radio (CR) network to transmit over the "spectrum holes" unoccupied by the primary users (PUs). In this paper, we focus on the OSA design in the presence of reactive PUs, where PU's access probability in a given channel is related to SU's past access decisions. We model the channel occupancy of the reactive PU as a 4-state discrete-time Markov chain. We formulate the optimal OSA design for SU throughput maximization as a constrained finite-horizon partially observable Markov decision process (POMDP) problem. We solve this problem by first considering the conventional short-term conditional collision probability (SCCP) constraint. We then adopt a long-term PU throughput (LPUT) constraint to effectively protect the reactive PU transmission. We derive the structure of the optimal OSA policy under the LPUT constraint and propose a suboptimal policy with lower complexity. Numerical results are provided to validate the proposed studies, which reveal some interesting new tradeoffs between SU throughput maximization and PU transmission protection in a practical interaction scenario.
The line-of-sight (LoS) air-to-ground channel brings both opportunities and challenges in cellular-connected unmanned aerial vehicle (UAV) communications. On one hand, the LoS channels make more cellular base stations (BSs) visible to a UAV as compared to the ground users, which leads to a higher macro-diversity gain for UAV-BS communications. On the other hand, they also render the UAV to impose/suffer more severe uplink/downlink interference to/from the BSs, thus requiring more sophisticated inter-cell interference coordination (ICIC) techniques with more BSs involved. In this paper, we consider the uplink transmission from a UAV to cellular BSs, under spectrum sharing with the existing ground users. To investigate the optimal ICIC design and air-ground performance trade-off, we maximize the weighted sum-rate of the UAV and existing ground users by jointly optimizing the UAV's uplink cell associations and power allocations over multiple resource blocks. However, this problem is non-convex and difficult to be solved optimally. We first propose a centralized ICIC design to obtain a locally optimal solution based on the successive convex approximation (SCA) method. As the centralized ICIC requires global information of the network and substantial information exchange among an excessively large number of BSs, we further propose a decentralized ICIC scheme of significantly lower complexity and signaling overhead for implementation, by dividing the cellular BSs into small-size clusters and exploiting the LoS macro-diversity for exchanging information between the UAV and cluster-head BSs only. Numerical results show that the proposed centralized and decentralized ICIC schemes both achieve a near-optimal performance, and draw important design insights based on practical system setups.
With the fast development of Radio Frequency Identification(RFID) and tag usage,security has become the key issue for RFID related applications.In this paper,a new RFID secure protocol is proposed,which uses a synchronized random number created in the backend system,for tag authentication.After tag successful authentication each time,both tag and random number update synchronously.Meanwhile,the performance and security of the protocol are analyzed theoretically.Also the BAN logic is used to prove its security in further.The result show that this protocol can provide authentication between tag and reader,moreover,it can resist attacks such as replay,location tracking,traffic analysis,counterfeiting,denial of service,and illegal access,etc.From the implementation point,it just uses Hash and exclusive XOR operations,which reduces the computing complexity and makes it fitting for low price tags applications.
Intelligent reflecting surface (IRS) is a new and promising paradigm to substantially improve the spectral and energy efficiency of wireless networks, by constructing favorable communication channels via tuning massive low-cost passive reflecting elements. Despite recent advances in the link-level performance optimization for various IRS-aided wireless systems, it still remains an open problem whether the large-scale deployment of IRSs in wireless networks can be a cost-effective solution to achieve their sustainable capacity growth in the future. To address this problem, we study in this paper a new hybrid wireless network comprising both active base stations (BSs) and passive IRSs, and characterize its achievable spatial throughput in the downlink as well as other pertinent key performance metrics averaged over both channel fading and random locations of the deployed BSs/IRSs therein based on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">stochastic geometry</i> . Compared to prior works on characterizing the performance of wireless networks with active BSs only, our analysis needs to derive the power distributions of both the signal and interference reflected by distributed IRSs in the network under spatially correlated channels, which exhibit channel hardening effects when the number of IRS elements becomes large. Extensive numerical results are presented to validate our analysis and demonstrate the effectiveness of deploying distributed IRSs in enhancing the hybrid network throughput against the conventional network without IRS, which <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">significantly boosts the signal power</i> but results in only <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">marginally increased interference</i> in the network. Moreover, it is unveiled that there exists an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">optimal IRS/BS density ratio</i> that maximizes the hybrid network throughput subject to a total deployment cost given their individual costs, while the conventional network without IRS (i.e., zero IRS/BS density ratio) is generally suboptimal in terms of throughput per unit cost.
This paper considers spectrum sharing for wireless communication between a cognitive radio (CR) and a primary radio (PR). An effective means known in the literature for the CR to protect the PR is by applying the so-called interference-temperature constraint, under which the CR is allowed to transmit regardless of the PR's on/off status provided that the resultant interference power level at the PR receiver is kept below some predefined threshold. For the fading PR and CR channels, the interference-power constraint at the PR receiver is usually one of the following two types: One is to regulate the average interference power (AIP) over all the fading states, while the other is to limit the peak interference power (PIP) at each fading state. From the CR's perspective, given the same average and peak power-constraint threshold, the AIP constraint is more favorable than the PIP counterpart because of its more flexibility for dynamically allocating the CR's transmit powers over the fading states. On the contrary, from the perspective of protecting the PR, the more restrictive PIP constraint appears at a first glance to be a better option. Some surprisingly, this paper proves that in terms of the achievable ergodic capacity of the PR fading channel, the AIP constraint is indeed superior over the PIP. This proof is based upon an interesting interference diversity phenomenon: Randomized interference powers over the fading states in the AIP case are more advantageous over deterministic ones in the PIP case for minimizing the resultant PR capacity loss. Therefore, the AIP constraint results in larger ergodic capacities than the PIP for both the CR and PR.
This paper considers spectrum sharing for wireless communication between a cognitive radio (CR) link and a primary radio (PR) link. It is assumed that the CR protects the PR transmission by applying the so-called ldquointerference-temperaturerdquo constraint, whereby the CR is allowed to transmit regardless of the PR's on/off status provided that the resultant interference power level at the PR receiver is kept below some predefined threshold. For the fading PR and CR channels, the interference-power constraint at the PR receiver is usually one of the following two types: one is to regulate the average interference power (AIP) over all different fading states, while the other is to limit the peak interference power (PIP) at each fading state. From the CR's perspective, given the same average and peak power threshold, the AIP constraint is more favorable than the PIP counterpart because of its more flexibility for dynamically allocating transmit powers over different fading states. On the contrary, from the perspective of protecting the PR, the more restrictive PIP constraint appears at a first glance to be a better option than the AIP. Some surprisingly, this paper shows that in terms of various forms of capacity limits achievable for the PR fading channel, e.g., the ergodic and outage capacities, the AIP constraint is also superior over the PIP. This result is based upon an interesting interference diversity phenomenon, where randomized interference powers over the fading states in the AIP case are more advantageous over deterministic ones in the PIP case for minimizing the resultant PR capacity losses. Therefore, the AIP constraint results in larger fading channel capacities than the PIP for both the CR and PR transmissions.
A composite generation and transmission system is such a power system which consists of generators, transformers, buses, circuit breakers, protective relaying, auto reclosure and transmission lines. In this paper the judgment of system failure mode and the definitions of risk indices, the mathematical model of sub systems and the implementation of algorithms are discussed. The results of example calculation show that this method is correct and feasible.
No abstract is provided for this article.
In wide band wireless communication systems, apropos scheduling and link adaptation are significant to meet the high spectrum efficiency and high rate data transmission demand. Channel quality indication (CQI) feedback is crucial for the system to obtain accurate information used in scheduling. But with the increasing of user number and bandwidth, the CQI feedback will bring intolerable overhead. To solve this problem, we propose a new CQI feedback method which utilizes the sparse attribute of wide band wireless channel. Compared with traditional CQI feedback method via analysis and simulation, the proposed scheme provides high throughput performance while keeping the feedback overhead at a low level.
This paper studies spatial capacity in a stochastic wireless ad hoc network. We propose a novel signal-to-interference-ratio (SIR) threshold based scheduling scheme with multi-stage probing and data transmission, where each transmitter iteratively decides to further probe or stay idle, depending on whether the estimated SIR in the proceeding probing is no smaller than a predefined threshold. Though the locations of the initial transmitters can be modeled as a homogeneous Poisson Point Process (PPP), the SIR based scheduling makes the PPP model no longer applicable in the subsequent probing and data transmission phases. We first focus on single-stage probing and find that when the SIR threshold is set sufficiently small to assure an acceptable network interference level, the proposed scheme can greatly outperform the reference scheme without any transmission scheduling in terms of spatial capacity. We clearly characterize the spatial capacity with exact/approximate closed-form expressions, by proposing a new approximate approach to deal with the correlated SIR distributions over non-PPPs. Then, we successfully extend to multi-stage probing, by properly designing the multiple SIR thresholds to assure gradual improvement of the spatial capacity. Furthermore, we analyze the impact of multi-stage probing overhead and present a probing-capacity tradeoff in scheduling design. Finally, extensive numerical results are presented to demonstrate the scheduling performance.
No abstract is provided for this article.
Powered by unreliable renewable energy sources, microgrids with limited storage capacities need to purchase supplementary energy from the main grid to support local consumers' random demand. The high variability of renewable energy increases a microgrid's energy purchasing fluctuation over time and makes the main grid's energy generation and supply more unscheduled and risky. This paper proposes a dynamic contract mechanism to regulate microgrids' energy purchasing over time, while the mismatch between renewable supply and local demand in each microgrid is efficiently resolved. The contract sets time-specific purchasing commitments for a microgrid to fulfill, yet still provides the microgrid with flexibility to change future commitments. Under the contract, a stochastic dynamic program is formulated for each microgrid to update commitments according to current storage status and future load prediction. As the microgrid's optimal purchase planning is complex to analyze, we further propose a heuristic algorithm to develop a suboptimal purchasing policy which is threshold-based and easy to implement. Extensive simulations show that the proposed contract can significantly reduce microgrids' purchasing fluctuation even though a large flexibility is allowed to change commitments. The contract smoothes out more purchasing fluctuations when fewer microgrids connect to the main grid or the net load variation in each microgrid increases.
In this paper, to minimize the on-grid energy cost in a large-scale green cellular network, we jointly design the optimal base station (BS) on/off operation policy and the on-grid energy purchase policy from a network-level perspective. Due to the fluctuations of the on-grid energy prices, the harvested renewable energy, and the network traffic loads over time, as well as the BS coordination to hand over the traffic offloaded from the inactive BSs to the active BSs, it is generally NP-hard to find a network-level optimal adaptation policy that can minimize the on-grid energy cost over a long-term and yet assures the downlink transmission quality at the same time. Aiming at the network-level dynamic system design, we jointly apply stochastic geometry (Geo) for large-scale green cellular network analysis and dynamic programming (DP) for adaptive BS on/off operation design and on-grid energy purchase design, and thus propose a new Geo-DP design approach. By this approach, we obtain the optimal BS on/off policy, which shows that the optimal BSs' active operation probability in each horizon is just sufficient to assure the required downlink transmission quality with time-varying load in the large-scale cellular network. We also propose a suboptimal on-grid energy purchase policy with low-complexity, where the low-price on-grid energy is over-purchased in the current horizon only when the current storage level and the future renewable energy level are both low. We compare the proposed policy with the existing schemes and show that our proposed policy can more efficiently save the on-grid energy cost over time.
A new form of multiuser diversity, named multiuser interference diversity, is investigated for opportunistic communications in cognitive radio (CR) networks by exploiting the mutual interference between the CR and the existing primary radio (PR) links. The multiuser diversity gain and ergodic throughput are analyzed for different types of CR networks and compared against those in the conventional networks without the PR link.
