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With the emergence of cloud computing, users are increasingly using storage services of cloud providers to satisfy their storage needs. However, these cloud providers charge at different rates and are not 100% reliable, resulting in occasional service outages. Thus the costs on end users are two folds: (1) cost of storage service, and (2) cost of information loss in cloud service outages. End users require a solution that minimizes the overall cost to balance the storage cost and the probability of successfully retrieving stored objects in cases of provider failures. In this paper, based on the concept of federated cloud agent proposed in our previous work [1], we study the cloud storage management strategy which allows end users to simultaneously deploy storage objects to multiple cloud service providers, based on their unit storage costs and failure behaviors. We propose to use random network coding and a decision model to solve this storage management problem. The random network coding over information objects to be stored in clouds helps simplify the cloud storage management and the decision model forms the foundation of solving the problem in a dynamic environment. We solve the decision model analytically and also propose two practical solutions based on efficient local search algorithms which can achieve near-optimal performance and lower computational complexity. In the experiments, we evaluate the proposed cloud storage strategies against a few reference schemes and find our schemes can achieve a good balance between the information success recovery rate and storage cost.

With the emergence of cloud computing, users are increasingly using storage services of cloud providers to satisfy their storage needs. However, these cloud providers are not 100% reliable, resulting in periodic service outages. End users require a solution that minimizes the impact of cloud service outages and optimizes the chances of successfully retrieving stored objects in cases of provider failures. In this paper, we propose to build a federated cloud agent, enabling end users to simultaneously deploy storage objects to multiple cloud service providers, based on their unit storage costs and failure probabilities. We study two service provider failure models: the first case assumes that at most one service provider fails at any given time, and the second case assumes that cloud service providers may fail independently with certain probabilities. We cast both scenarios as constrained optimization problems to minimize the overall storage cost while enhancing the chances of recovering all the storage objects in case of service outages. In the experiments, we evaluated the proposed optimal storage schemes against a fixed server deployment strategy and find our schemes can achieve good recovery ratios, while incurring minimal storage costs.

Efficient transmission control (e.g., power/rate control) in the physical layer, link scheduling in the link layer and routing in the network layer are critical design issues of wireless ad hoc networks. By considering both the quality-of-service and the utilization of resources under the temporally correlated uncertainties of the network conditions, a sequential decision framework is proposed for protocol design and layering. By observing that there are usually different performance concerns in the network layer versus lower layers, two correlated sequential decision models for the operations at different layers are proposed. With the decision model in the link and physical layers, scheduling and transmission control are jointly optimized. By adding a decision model in the network layer, the benefit of cross-layer information is quantified and the optimal routing/forwarding strategy is characterized. Practical algorithms based on the proposed decision models are also developed to achieve optimal/near-optimal performance.

In this paper, we investigate the low-complexity synchronization design for multi-band orthogonal-frequency-division-multiplexing (MB-OFDM) ultra-wideband (UWB) systems. We propose a unified synchronizer design based on auto-correlation-function. The key component in the proposed synchronizer is a parallel signal detector structure in which multiple auto-correlation units are instantiated and their outputs are shared by other functional units in the synchronizer, including time-frequency pattern detection, symbol timing, carrier frequency offset estimation and correction and frame synchronization. We show that, via analysis and simulations, such a design achieves not only a low computation cost which makes it attractive in implementation, but also equal or better performance compared to the cross-correlation based designs.

We consider the scenario of distributed data aggregation in wireless sensor networks, where each sensor can obtain and estimate the information of the whole sensing field through local data exchange and aggregation. The intrinsic trade-off between energy and delay in aggregation operations imposes a crucial question on nodes to decide optimal instants for forwarding their samples. The samples could be composed of the information from their own sensor readings or an aggregation of information with other samples forwarded from neighboring nodes. By considering the randomness of the sample arrival instants and the uncertainty of the availability of the multiaccess communication channel due to the asynchronous nature of information exchange among neighboring nodes, we propose a decision process model to analyze this problem and determine the optimal decision policies at nodes with local information. We show that, once the statistics of the sample arrival and the availability of the channel satisfy certain conditions, there exist optimal control-limit type policies which are easy to implement in practice. In the case that the required conditions are not satisfied, we provide two learning algorithms to solve a finite-state approximation model of the decision problem. Simulations on a practical distributed data aggregation scenario demonstrate the effectiveness of the developed policies, which can also achieve a desired energy-delay tradeoff.

In mobile ad hoc networks, relaying is one of the most fundamental functions. In this paper, we analyze the stability of several relaying strategies. By using the notion of node lifetime instead of the commonly used link lifetime, our analysis offers a complementary and unique finding not available in the literature previously. Our results show that a route of parallel relays have a significantly longer lifetime than a route of serial relays although the latter of the two has previously predominated the networking research.

In order to adapt to the time-varying nature of wireless channels, various channel-adaptive schemes have been proposed to exploit inherent spatial diversity in wireless ad hoc networks where there are usually alternate forwarding nodes available at a given forwarding node. However, existing schemes along this line are designed based on heuristics, implying room for performance enhancement. Thereby, to seek a theoretical foundation for improving spatial diversity gain, we formulate the selection of the next-hop relay as a sequential decision problem and derive a general "Optimal Stopping Relaying (OSR)" framework for designing such spatial-diversity schemes. As a particular example, assuming Rayleigh fading channels, we implement an OSR strategy to optimize information efficiency (IE) in a protocol stack consisting of Greedy Perimeter Stateless Routing (GPSR) and IEEE 802.11 MAC protocols. We present an analysis of the algorithm for a single node. In addition, we perform extensive simulations (using QualNet) to evaluate the end-to-end performance of the proposed forwarding strategy. The results demonstrate the superiority of OSR over other existing schemes.

For several decades, research activities in mobile ad hoc networking have predominantly relied on the notion of serial relays where a data packet hops from one node to another in a serial fashion. Recent research activities in signal processing and information theory suggest that parallel relays have a greater potential than serial relays. In this paper, a significant diversity gain of using parallel relays over serial relays is highlighted, the lifetimes of a single-path route of parallel relays and multi-path routes of serial relays are compared, and the routing overheads for each type of routes are also investigated. This study strongly supports that parallel relays have great advantages over serial relays in dense and mobile ad hoc networks.

Ultra-wideband (UWB) systems spread the transmitted signal power over an extremely large frequency band, and the power spectral density of the signal is very low. Due to the wide bandwidth of the transmitted signal, UWB signal energy spreads over the frequency bands allocated to other radio systems. Evaluating the interference issues of these narrowband systems to UWB systems in its overlay band is an important issue for understanding and designing efficient UWB systems. An analysis of the power spectral density (PSD) of generic narrowband signals is presented. The theoretical and simulation results reveal that the bit-error-rate (BER) performance of a direct sequence UWB (DS-UWB) system with a minimum mean square error (MMSE) receiver is robust in the presence of narrowband interference caused by existing applications.

Ultra-wideband (UWB) technology is one of the promising solutions for short-range indoor wireless communication applications. UWB systems spread the transmitted signal power over an extremely large frequency band, and the power spectral density of the signal is very low. Due to the wide bandwidth of the transmitted signal, UWB signal energy will spread over the frequency bands allocated to other radio systems. In order to evaluate the effects of UWB system to narrowband systems (such as IEEE802.11a wireless LAN) in its overlay band, it is important to understand and quantify the power spectral density (PSD) of UWB signal. In this paper, a detailed analysis on the PSD of direct sequence coded UWB (DS-UWB) signal is presented. The theoretical and simulation results reveal that the interfering DS-UWB power can be reduced by regulating UWB chip rate, pulse shape, pulse width and scrambling codes in the frequencies of victim narrowband systems.