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ABSTRACT: Csiszár and Körner's characterization of the rate-equivocation region of a general wiretap channel involves two auxiliary random variables: U, which represents rate splitting and V, which represents channel prefixing. For some channels, one or both of these auxiliary random variables are unnecessary, simplifying the expression and evaluation of the rate-equivocation region. In this paper, we provide new conditions under which channel prefixing or rate splitting does not improve the rate-equivocation region. In particular, we show that when the main channel is more capable than the eavesdropping channel, channel prefixing is unnecessary; the entire rate-equivocation region can be achieved by rate splitting alone. Conversely, we show under a mild assumption that if the main receiver is not more capable, then channel prefixing is strictly necessary. Moreover, we show that if the main channel is more capable but not less noisy, then rate splitting is strictly necessary. Next, we focus on the set of cyclic shift symmetric channels. We prove that for these channels, if in addition I(X; Y) - I(X;Z) is maximized at the uniform distribution, then rate splitting is unnecessary. Our results apply to BSC-BEC and BEC-BSC wiretap channels. We identify the conditions on the parameters of the BSC and BEC under which channel prefixing and/or rate splitting are unnecessary.
Information Theory Proceedings (ISIT), 2011 IEEE International Symposium on; 09/2011
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ABSTRACT: In this paper, we investigate the transmission completion time minimization problem in a two-user additive white Gaussian noise (AWGN) broadcast channel, where the transmitter is able to harvest energy from the nature. The harvested energy is modeled to arrive at the transmitters randomly. In this paper, under a deterministic system setting, we assume that the energy harvesting times and harvested energy amounts are known before the transmission starts. The transmitter has a fixed number of packets to be delivered to each receiver. Our goal is to minimize the time by which all of the packets for both users are delivered to their respective destinations. To this end, we optimize the transmit powers and transmission rates intended for both users. We first analyze the structural properties of the optimal transmission policy. We prove that the optimal total transmit power has the same structure as the optimal single-user transmit power. We also prove that there exists a cut-off power level for the stronger user. If the optimal total transmit power is lower than this level, all transmit power is allocated to the stronger user, and when the optimal total transmit power is larger than this level, all transmit power above this level is allocated to the weaker user. Based on these structural properties of the optimal policy, we propose an algorithm that yields the globally optimal off-line scheduling policy.
Communications (ICC), 2011 IEEE International Conference on; 07/2011
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ABSTRACT: We consider the minimization of the transmission completion time with a battery limited energy harvesting transmitter in a two-user AWGN broadcast channel. The transmitter has fixed number of packets for each receiver and energy is modeled to arrive (be harvested) at the transmitter at random instants. The battery at the transmitter has a finite storage capacity, hence energy may overflow without being utilized for data transmission. We derive the optimal offline transmission policy that minimizes the time by which all of the data packets are delivered to their respective destinations. We analyze the structural properties of the optimal transmission policy using a dual problem. We find the optimal total transmit power sequence by a directional water-filling algorithm. We prove that there exists a cut-off power level such that if the allocated power is lower than this level, then only the stronger user is served in that epoch; otherwise, the power above this level is allocated to the weaker user. Based on these properties, we propose an algorithm that gives the globally optimal offline policy. The proposed algorithm uses directional water-filling repetitively.
Modeling and Optimization in Mobile, Ad Hoc and Wireless Networks (WiOpt), 2011 International Symposium on; 06/2011
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ABSTRACT: Wireless systems comprised of rechargeable nodes have a significantly prolonged lifetime and are sustainable. A distinct characteristic of these systems is the fact that the nodes can harvest energy throughout the duration in which communication takes place. As such, transmission policies of the nodes need to adapt to these harvested energy arrivals. In this paper, we consider optimization of the transmission policy of an energy harvesting transmitter which has a limited battery capacity, communicating in a wireless fading channel. In particular, we identify the optimal offline transmission policies that maximize the number of bits delivered by a deadline, and minimize the transmission completion time of the communication session. We introduce a directional water-filling algorithm which provides a simple and concise interpretation of the necessary optimality conditions as well as energy storage capacity and causality. We solve the throughput maximization problem for the fading channel using the directional water-filling algorithm, which simultaneously adapts to the energy harvested as well as the channel variations in time. We then solve the transmission completion time minimization problem by utilizing its equivalence to its throughput maximization counterpart.
INFOCOM, 2011 Proceedings IEEE; 05/2011
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ABSTRACT: In this paper, we consider a single-user communication system, where an energy harvesting transmitter communicates with a receiver over a fading wireless channel. We design adaptive transmission policies that adapt to the random energy arrivals at the transmitter and random fluctuations in the channel, in order to maximize the average number of bits transmitted by a finite deadline T. We solve for the optimum transmission scheme using stochastic dynamic programming. This optimal solution does not admit a closed form expression and is computationally expensive. We then propose several suboptimal event based adaptive transmission policies that react to the changes in energy arrivals and fading states. We provide extensive simulation results that compare the performances of the optimal and proposed simpler solutions.
Information Sciences and Systems (CISS), 2011 45th Annual Conference on; 04/2011
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ABSTRACT: In energy harvesting communication systems, an exogenous recharge process supplies energy for the data transmission and arriving energy can be buffered in a battery before consumption. Transmission is interrupted if there is not sufficient energy. We address communication with such random energy arrivals in an information-theoretic setting. Based on the classical additive white Gaussian noise (AWGN) channel model, we study the coding problem with random energy arrivals at the transmitter. We show that the capacity of the AWGN channel with stochastic energy arrivals is equal to the capacity with an average power constraint equal to the average recharge rate. We provide two different capacity achieving schemes: save-and-transmit and best-effort-transmit. Next, we consider the case where energy arrivals have time-varying average in a larger time scale. We derive the optimal offline power allocation for maximum average throughput and provide an algorithm that finds the optimal power allocation.
Personal, Indoor and Mobile Radio Communications Workshops (PIMRC Workshops), 2010 IEEE 21st International Symposium on; 10/2010
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ABSTRACT: In this work we consider the allocation of buffer space to data streams sharing a common high-speed wireless transmitter. As an example, we focus on an OFDMA-based downlink system scenario. Scheduling for maximum throughput has been extensively studied in the literature. However, the practically interesting case of a finite buffer has not been sufficiently addressed before. Especially in the case of overloaded packet queues, the choice of buffer management policy substantially affects the throughput performance. We consider a physicallayer scheduling scheme that allocates users to subcarriers based on channel state, in order to make the most use of multiuser diversity. We then consider optimal buffer partitioning to accommodate the resulting rates. We study the system throughput by simulations. As a benchmark, we also simulate MaxWeight, a well-known cross-layer channel and queue-aware scheduling policy that is throughput-optimal in the absence of a finite buffer constraint. We observe that a suitable buffer management policy with a simple channel-aware queuing policy achieves cross-layer scheduling performance, and can exceed it.
Personal Indoor and Mobile Radio Communications (PIMRC), 2010 IEEE 21st International Symposium on; 10/2010
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ABSTRACT: We consider a finite buffer shared by multiple packet queues. Throughput can be considerably improved by partitioning the buffer space among the queues judiciously, especially under a high load regime. We formulate optimal buffer partitioning as a resource allocation problem, the solution of which is found through a greedy incremental algorithm in polynomial time. The rest of the work is devoted to applying the optimal buffer allocation strategy in different scenarios modeling a wireless downlink. First, the strategy is applied in a general parallel M/M/1/m<sub>i</sub> system and a numerical study verifies that the strategy may boost the throughput considerably. Then, a multichannel extension of this system is considered when the users have different arrival rates and channels have different outage probabilities. Jointly optimal buffer space allocation and channel assignment problems in this scenario are shown to be separable. Lastly, buffer allocation is considered in a system where users need to be multiplexed and scheduled based on channel state. It is shown that this system can be modeled as a set of parallel M/G/1/m<sub>i</sub> queues to which the optimum buffer allocation strategy is again applicable. The improvement brought by optimal buffer allocation to scheduling based solely on channel-state is explored. It is observed that buffer optimization can result in remarkable throughput increase on top of channel-based user selection.
Information Theory and Applications Workshop (ITA), 2010; 03/2010
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ABSTRACT: When rate expectations of users in a wireless network cannot all be satisfied, one choice is to discard some users from the system, in a mechanism called admission control. However, in a data network, users have a certain tolerance to occasional rate outages. In this paper we argue that it may be preferable for users to reduce their rate objectives smoothly, by considering an outage probability tolerance, rather than not be provided any service at all. We propose a distributed utility based algorithm for doing this. The smoothness of reactions is obtained by making transmit power reaction curves absolutely subhomogeneous functions of interference. This is done using an ¿objective reduction factor¿, in addition to a linear price. We first provide conditions for a unique Nash equilibrium. Assuming that distributed nodes use gradient based optimization, convergence and error sensitivity of gradient based iterative algorithms are analyzed. Lastly, the continuous-time counterpart of the problem is considered and a stability condition is established for the system.
Communication, Control, and Computing, 2009. Allerton 2009. 47th Annual Allerton Conference on; 11/2009
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ABSTRACT: In this paper, we study distributed power control in an interference network. In particular, distributed power control mechanisms are devised by exploiting a one-shot non-cooperative game based on a suitably chosen utility function. The utility is a function of quality of service (QoS) objectives defined in terms of fading-induced outage probabilities. Equilibrium analysis of the resulting power control game is made, and its relationship with admission control is discussed. The main contribution of the paper is a mechanism for obtaining smooth non-monotonic reaction curves, in contrast to sharp cut-offs with increasing interference that are characteristic of admission control. This is done via the introduction of a factor f<sub>d</sub>(.) into the utility function, allowing users to smoothly decrease their objectives in response to interference. The resulting algorithm is called non-monotonic power control (NMPC). We provide sufficient conditions for a unique Nash equilibrium (NE) under NMPC. The equilibria are studied in numerical examples, which exhibit that NMPC increases the number of users who achieve their objectives, without removing any user, as compared to previous utility-based power control algorithms with harsher reaction curves. Considerable energy efficiency is gained by a transfer of resources from the disadvantaged user to the advantaged: users whose SIR objectives are infeasible under current channel gains reduce their own transmit power thus helping on others. We view this solution as an attractive alternative to pricing in wireless networks formed by cooperative nodes (such as sensor networks) where an economic model is not natural.
Game Theory for Networks, 2009. GameNets '09. International Conference on; 06/2009
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ABSTRACT: In this paper, the application of game theory to the purpose of obtaining energy-efficient distributed power control algorithms in interference-limited wireless networks is considered. Utility for users has been defined as a function of rate and power that encourages energy-efficient behavior, and several games have been constructed based on this utility function. Nash equilibria in wireless interference network models with single and multiple access-points are characterized. Later, the utility function is modified to have a variable degree of energy efficiency and the tradeoff between energy and spectral efficiency is verified. Implications of the results toward the design of utility functions are discussed.
Signal Processing and Communications Applications Conference, 2009. SIU 2009. IEEE 17th; 05/2009
