摘要 :
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 th...
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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.
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In this paper, we first overview some traditional relaying technologies, and then present a Network Coding-Aware Cooperative Relaying (NC2R) scheme to improve the performance of downlink transmission for relay-aided cellular netwo...
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In this paper, we first overview some traditional relaying technologies, and then present a Network Coding-Aware Cooperative Relaying (NC2R) scheme to improve the performance of downlink transmission for relay-aided cellular networks. Moreover, systematic performance analysis and extensive simulations are performed for the proposed NC2R and traditional relaying and non-relaying schemes. The results show that NC2R outperforms conventional relaying and non-relaying schemes in terms of blocking probability and spectral efficiency, especially for cell-edge users. Additionally, the location selections for relays with NC2R are also discussed. These results will provide some insights for incorporating network coding into next-generation broadband cellular relay mobile systems.
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Network throughput and energy consumption are two important performance metrics for a multihop wireless network. Current state-of-the-art research is limited to either maximizing throughput under some energy constraint or minimizi...
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Network throughput and energy consumption are two important performance metrics for a multihop wireless network. Current state-of-the-art research is limited to either maximizing throughput under some energy constraint or minimizing energy consumption while satisfying some throughput requirement. Although many of these prior efforts were able to offer some optimal solutions, there is still a critical need to have a systematic study on how to optimize both objectives simultaneously. In this paper, we take a multicriteria optimization approach to offer a systematic study on the relationship between the two performance objectives. To focus on throughput and energy performance, we simplify link layer scheduling by employing orthogonal channels among the links. We show that the solution to the multicriteria optimization problem characterizes the envelope of the entire throughput-energy region, i.e., the so-called optimal throughput-energy curve. We prove some important properties of the optimal throughput-energy curve. For case study, we consider both linear and nonlinear throughput functions. For the linear case, we characterize the optimal throughput-energy curve precisely through parametric analysis, while for the nonlinear case, we use a piecewise linear approximation to approximate the optimal throughput-energy curve with arbitrary accuracy. Our results offer important insights on exploiting the tradeoff between the two performance metrics.
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This paper considers the application of compressed sensing (CS) to a wireless sensor network for data measurement communication and reconstruction, where N sensor nodes compete for medium access to a single receiver. Sparsity of t...
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This paper considers the application of compressed sensing (CS) to a wireless sensor network for data measurement communication and reconstruction, where N sensor nodes compete for medium access to a single receiver. Sparsity of the sensor data in three domains due to time correlation, space correlation and multiple access are being utilized. We first provide an in-depth analysis on the CS-based medium access control schemes from a physical layer perspective and reveal the impact of communication signal-to-noise ratio on the reconstruction performance. We show the process of the sensor data converted to the modulated symbols for physical layer transmission and how the modulated symbols being recovered via compressed sensing. This paper further identifies the decision problem of distinguishing between active and inactive transmitters after symbol recovery and shows a comprehensive performance comparison between carrier sense multiple access and the proposed CS-based scheme. Second, a network data recovery scheme that exploits both spatial and temporal correlations is proposed. Simulation results validate the effectiveness of the proposed method in terms of communication throughput and show that enhanced performance can be obtained by utilizing the sensed signal's temporal and spatial correlations.
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The article considers the variable process control scheme for two dependent process steps with incorrect adjustment. Incorrect adjustment of a process may result in shifts in process mean, ultimately affecting the quality of produ...
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The article considers the variable process control scheme for two dependent process steps with incorrect adjustment. Incorrect adjustment of a process may result in shifts in process mean, ultimately affecting the quality of products. We construct variable sampling interval (VSI) EWMA_(Z_x) and EWMA_(Z_e) control charts to effectively monitor the quality variable produced by the first process step with incorrect adjustment and the quality variable produced by the second process step with incorrect adjustment. The performance of the proposed VSI control charts is measured by the adjusted average time to signal (AATS) derived using a Markov chain approach. An example of the automobile braking system with incorrect adjustment shows the application and performance of the proposed VSI EWMA_(Z_x) and EWMA_(Z_e) control charts in detecting shifts in process mean. Furthermore, the performance of the proposed VSI EWMA_(Z_x) and EWMA_(Z_e) control charts and the fixed sampling interval (FSI) EWMA_(Z_x) and EWMA_(Z_e) control charts are compared by numerical analysis results. These demonstrate that the former is much faster in detecting small and median shifts in mean. The optimum VSI EWMA_(Z_x). and EWMA_(Z_e) control charts are also proposed using optimization technique to minimize AATS when quality engineers cannot specify the values of variable sampling intervals. It has been found that the optimum VSI EWMA_(Z_x) and EWMA_(Z_e) control charts always work better than the VSI EWMA_(Z_x) and EWMA_(Z_e) control charts.
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P2P platforms have been criticized because of the heavy strain that they can inflict on costly inter-domain links of network operators. It is therefore mandatory to develop network optimization schemes for controlling the load gen...
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P2P platforms have been criticized because of the heavy strain that they can inflict on costly inter-domain links of network operators. It is therefore mandatory to develop network optimization schemes for controlling the load generated by a P2P platform on an operator network. While many research efforts exist on centralized tracker-based systems, in recent years multiple DHT-based P2P platforms have been widely deployed and considered as commercial services due to their scalability and fault tolerance. Finding network optimization for DHT-based P2P applications has thereby potential large practical impacts. In this paper, we present THash, a simple scheme that implements a distributed and effective network optimization for DHT systems. THash uses standard DHT put/get semantics and utilizes a triple hash method to guide the DHT clients to choose their sharing peers in proper domains. We have implemented THash in a major commercial P2P system (PPLive), using the standard ALTO/P4P protocol as the network information source. We conducted experiments over this network in real operation and observed that compared with Native DHT, THash reduced respectively by 47.4% and 67.7% the inter-PID and inter-AS traffic, while reducing the average downloading time by 14.6% to 24.5%.
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Communication networks are an indispensable component in the smart grid power systems by providing the essential information exchange functions among the electrical devices that are located distributively in the grid. In particula...
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Communication networks are an indispensable component in the smart grid power systems by providing the essential information exchange functions among the electrical devices that are located distributively in the grid. In particular, wireless networks will be deployed widely in the smart grid for data collection and remote control purposes. In this paper, we model the smart grid wireless networks and present the communication delay analysis in typical wireless network deployment scenarios in the grid. As the time critical communications are coupled with the power system protections in the smart grid, it is important to understand the delay performance of the smart grid wireless networks. Our results provide the delay bounds that can help design satisfactory wireless networks to meet the demanding communication requirements in the smart grid.
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In our previous studies (Chu et al. in Heat Mass Transf 43(11): 1213-1224, 2007), a theoretical model of swirl atomizers was successfully established. From the analysis, the equations for the droplet size, velocity components, the...
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In our previous studies (Chu et al. in Heat Mass Transf 43(11): 1213-1224, 2007), a theoretical model of swirl atomizers was successfully established. From the analysis, the equations for the droplet size, velocity components, the boundary layer thickness and the spray cone angle were deduced based on the fundamental governing equations. The purpose of this study is to further compare the experimental result with the theoretical one already gained by a satisfactory embodiment of series of experiments. The aim is to corroborate the analytical results of the influence of atomizer construction and controlled pressure difference on typical swirl chambers. The results provide the droplet diameter as a function of pressure difference, swirl atomizer geometry, flow rate, spray cone angle. The experimental outputs also show a good confirmation of theoretical results and can also be used for further optimization on existing swirl chambers. Based on the results obtained, an optimization methodology on characteristics of swirl atomizers is proposed with the adjustment of individual design parameter and the matching flow number.
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Network traffic modeling is a fundamental problem in communication. A traffic model should be able to capture and reproduce various properties of a real trace. Despite the widespread success of most numerical models in various app...
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Network traffic modeling is a fundamental problem in communication. A traffic model should be able to capture and reproduce various properties of a real trace. Despite the widespread success of most numerical models in various applications, few actually focus on the oscillation behavior proven to be one of the basic properties in network traffic. In this paper, a new mathematical method is proposed to model and synthesize stationary and nonstationary oscillatory processes of network traffic. The proposed model is based on the structure of the hierarchical hidden Markov model, which includes two nested hidden Markov chains and one observable process. The first-layer hidden Markov chain with variable state-duration controls the time-varying oscillatory process. Conditional on the first-layer Markov chain, the local fluctuation process is modeled by the second-layer hidden Markov chain. Algorithms are derived for inference of model parameters and traffic synthesis. The proposed approach is compared with four classical models for performance evaluation. The selected performance criterion includes time structure, statistical properties, self-similarity, queuing behavior and multiscale properties. The flexibility and accuracy of the proposed model results in a close fit to the real traces.
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For two-tier networks consisting of macrocells and femtocells, the channel access mechanism can be configured to be open access, closed access, or hybrid access. Hybrid access arises as a compromise between open and closed access ...
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For two-tier networks consisting of macrocells and femtocells, the channel access mechanism can be configured to be open access, closed access, or hybrid access. Hybrid access arises as a compromise between open and closed access mechanisms, in which a fraction of available spectrum resource is shared to nonsubscribers while the remaining reserved for subscribers. This paper focuses on a hybrid access mechanism for multi-channel femtocells which employ orthogonal spectrum access schemes. Considering a randomized channel assignment strategy, we analyze the performance in the downlink. Using stochastic geometry as technical tools, we model the distribution of femtocells as Poisson point process or Neyman-Scott cluster process and derive the distributions of signal-to-interference-plus-noise ratios, and mean achievable rates, of both nonsubscribers and subscribers. The established expressions are amenable to numerical evaluation, and shed key insights into the performance tradeoff between subscribers and nonsubscribers. The analytical results are corroborated by numerical simulations.
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