Sennur Ulukus

Loyola University Maryland, Baltimore, Maryland, United States

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Publications (209)195.47 Total impact

  • Pritam Mukherjee, Jianwei Xie, Sennur Ulukus
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    ABSTRACT: We consider three channel models: the wiretap channel with $M$ helpers, the $K$-user multiple access wiretap channel, and the $K$-user interference channel with an external eavesdropper, when no eavesdropper's channel state information (CSI) is available at the transmitters. In each case, we establish the optimal sum secure degrees of freedom (s.d.o.f.) by providing achievable schemes and matching converses. We show that the unavailability of the eavesdropper's CSIT does not reduce the s.d.o.f. of the wiretap channel with helpers. However, there is loss in s.d.o.f. for both the multiple access wiretap channel and the interference channel with an external eavesdropper. In particular, we show that in the absence of eavesdropper's CSIT, the $K$-user multiple access wiretap channel reduces to a wiretap channel with $(K-1)$ helpers from a sum s.d.o.f. perspective, and the optimal sum s.d.o.f. reduces from $\frac{K(K-1)}{K(K-1)+1}$ to $\frac{K-1}{K}$. For the interference channel with an external eavesdropper, the optimal sum s.d.o.f. decreases from $\frac{K(K-1)}{2K-1}$ to $\frac{K-1}{2}$ in the absence of the eavesdropper's CSIT. Our results show that the lack of eavesdropper's CSIT does not have a significant impact on the optimal s.d.o.f. for any of the three channel models, especially when the number of users is large. This implies that physical layer security can be made robust to the unavailability of eavesdropper CSIT at high signal to noise ratio (SNR) regimes by careful modification of the achievable schemes as demonstrated in this paper.
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    ABSTRACT: Wireless networks composed of energy harvesting devices will introduce several transformative changes in wireless networking as we know it: energy self-sufficient, energy self-sustaining, perpetual operation; reduced use of conventional energy and accompanying carbon footprint; untethered mobility; and an ability to deploy wireless networks in hard-to-reach places such as remote rural areas, within structures, and within the human body. Energy harvesting brings new dimensions to the wireless communication problem in the form of intermittency and randomness of available energy, which necessitates a fresh look at wireless communication protocols at the physical, medium access, and networking layers. Scheduling and optimization aspects of energy harvesting communications in the medium access and networking layers have been relatively wellunderstood and surveyed in the recent paper [1]. This branch of literature takes a physical layer rate-power relationship that is valid in energy harvesting conditions under large-enough batteries and long-enough durations between energy harvests so that information-theoretic asymptotes are achieved, and optimizes the transmit power over time in order to maximize the throughput. Another branch of recent literature aims to understand the fundamental capacity limits, i.e. information-theoretic capacities, of energy harvesting links under smaller scale dynamics, considering energy harvests at the channel use level. This branch necessitates a deeper look at the coding and transmission schemes in the physical layer, and ultimately aims to develop an information theory of energy harvesting communications, akin to Shannon's development of an information theory for average power constrained communications. In this introductory article, we survey recent results in this branch and point to open problems that could be of interest to a broad set of researchers in the fields of communication theory, information theory, signal processing, and netw- rking. In particular, we review capacities of energy harvesting links with infinite-sized, finitesized, and no batteries at the transmitter.
    IEEE Communications Magazine 04/2015; 53(4):126-132. DOI:10.1109/MCOM.2015.7081085 · 4.46 Impact Factor
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    ABSTRACT: The papers in this special issue presents cutting-edge research results in the emerging area of energy harvesting wireless communications and wireless energy transfer. This first issue starts with a review article coauthored by the guest editors that summarizes recent results in the broad area of energy harvesting communications, in particular, in information-theoretic, offline and online schedulingtheoretic, medium access, networking approaches to energy harvesting communications, as well as in energy cooperation and simultaneous wireless energy and information transfer.
    IEEE Journal on Selected Areas in Communications 03/2015; 33(3):357-359. DOI:10.1109/JSAC.2015.2406514 · 4.14 Impact Factor
  • Kaya Tutuncuoglu, Aylin Yener, Sennur Ulukus
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    ABSTRACT: We consider an energy harvesting network where the transmitter harvests energy from nature, and the harvested energy can be saved in an imperfect battery which suffers from charging/discharging inefficiency. In particular, when units of energy is to be stored in the battery, only units is saved and is lost due to charging/discharging inefficiency, where represents the storing efficiency. We determine the optimum offline transmit power schedule for such a system for single-user and broadcast channel models, for static and fading channels, with and without a finite battery size. We show that the optimum policy is a double-threshold policy: specifically, we store energy in the battery only when the harvested energy is above an upper threshold, and retrieve energy from the battery only when the harvested energy is below a lower threshold; when the harvested energy is in between these two thresholds, we use it in its entirety in the current slot. We show that the two thresholds remain constant unless the battery is depleted or full. We provide an algorithm to determine the sequence of optimum thresholds. For the case with fading, we develop a directional water-filling algorithm which has a double-threshold structure. Finally, we formulate the online problem using dynamic programming, and numerically observe that the online policy exhibits a double-threshold structure as well.
    IEEE Journal on Selected Areas in Communications 03/2015; 33(3):1-1. DOI:10.1109/JSAC.2015.2391511 · 4.14 Impact Factor
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    Pritam Mukherjee, Ravi Tandon, Sennur Ulukus
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    ABSTRACT: The two user multiple-input single-output (MISO) broadcast channel with confidential messages (BCCM) is studied in which the nature of channel state information at the transmitter (CSIT) from each user can be of the form $I_{i}$, $i=1,2$ where $I_{1}, I_{2}\in \{\mathsf{P}, \mathsf{D}, \mathsf{N}\}$, and the forms $\mathsf{P}$, $\mathsf{D}$ and $\mathsf{N}$ correspond to perfect and instantaneous, completely delayed, and no CSIT, respectively. Thus, the overall CSIT can alternate between $9$ possible states corresponding to all possible values of $I_{1}I_{2}$, with each state occurring for $\lambda_{I_{1}I_{2}}$ fraction of the total duration. The main contribution of this paper is to establish the secure degrees of freedom (s.d.o.f.) region of the MISO BCCM with alternating CSIT with the symmetry assumption, where $\lambda_{I_{1} I_{2}}=\lambda_{I_{2}I_{1}}$. The main technical contributions include developing a) novel achievable schemes for MISO BCCM with alternating CSIT with security constraints which also highlight the synergistic benefits of inter-state coding for secrecy, b) new converse proofs via local statistical equivalence and channel enhancement; and c) showing the interplay between various aspects of channel knowledge and their impact on s.d.o.f.
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    ABSTRACT: This article summarizes recent contributions in the broad area of energy harvesting wireless communications. In particular, we provide the current state of the art for wireless networks composed of energy harvesting nodes, starting from the information-theoretic performance limits to transmission scheduling policies and resource allocation, medium access and networking issues. The emerging related area of energy transfer for self-sustaining energy harvesting wireless networks is considered in detail covering both energy cooperation aspects and simultaneous energy and information transfer. Various potential models with energy harvesting nodes at different network scales are reviewed as well as models for energy consumption at the nodes.
    IEEE Journal on Selected Areas in Communications 01/2015; 33(3). DOI:10.1109/JSAC.2015.2391531 · 4.14 Impact Factor
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    Yi-Peng Wei, Sennur Ulukus
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    ABSTRACT: Information-theoretic work for wiretap channels is mostly based on random coding schemes. Designing practical coding schemes to achieve information-theoretic security is an important problem. By applying the two recently developed techniques for polar codes, we propose a polar coding scheme to achieve the secrecy capacity of the general wiretap channel.
  • Source
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    ABSTRACT: We consider a binary energy harvesting communication channel with a finite-sized battery at the transmitter. In this model, the channel input is constrained by the available energy at each channel use, which is driven by an external energy harvesting process, the size of the battery, and the previous channel inputs. We consider an abstraction where energy is harvested in binary units and stored in a battery with the capacity of a single unit, and the channel inputs are binary. Viewing the available energy in the battery as a state, this is a state-dependent channel with input-dependent states, memory in the states, and causal state information available at the transmitter only. We find an equivalent representation for this channel based on the timings of the symbols, and determine the capacity of the resulting equivalent timing channel via an auxiliary random variable. We give achievable rates based on certain selections of this auxiliary random variable which resemble lattice coding for the timing channel. We develop upper bounds for the capacity by using a genie-aided method, and also by quantifying the leakage of the state information to the receiver. We show that the proposed achievable rates are asymptotically capacity achieving for small energy harvesting rates. We extend the results to the case of ternary channel inputs. Our achievable rates give the capacity of the binary channel within 0.03 bits/channel use, the ternary channel within 0.05 bits/channel use, and outperform basic Shannon strategies that only consider instantaneous battery states, for all parameter values.
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    ABSTRACT: We consider a binary energy harvesting channel (BEHC) where the encoder has unit energy storage capacity. We first show that an encoding scheme based on block indexing is asymptotically optimal for small energy harvesting rates. We then present a novel upper bounding technique, which upper bounds the rate by lower-bounding the rate of information leakage to the receiver regarding the energy harvesting process. Finally, we propose a timing based hybrid encoding scheme that achieves rates within 0.03 bits/channel use of the upper bound; hence determining the capacity to within 0.03 bits/channel use.
    2014 IEEE International Symposium on Information Theory (ISIT); 06/2014
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    ABSTRACT: We consider the block Rayleigh fading multiple-input multiple-output (MIMO) wiretap channel with no prior channel state information (CSI) available at any of the terminals. The channel gains remain constant in a coherence time of T symbols, and then change to another independent realization. The transmitter, the legitimate receiver and the eavesdropper have nt, nr and ne antennas, respectively. We determine the exact secure degrees of freedom (s.d.o.f.) of this system when T ≥ 2 min(nt, nr). We show that, in this case, the s.d.o.f. is exactly (min(nt, nr) - ne)+(T - min(nt, nr))/T. The first term can be interpreted as the eavesdropper with ne antennas taking away ne antennas from both the transmitter and the legitimate receiver. The second term can be interpreted as a fraction of s.d.o.f. being lost due to the lack of CSI at the legitimate receiver. In particular, the fraction loss, min(nt, nr)/T, can be interpreted as the fraction of channel uses dedicated to training the legitimate receiver for it to learn its own CSI. We prove that this s.d.o.f. can be achieved by employing a constant norm channel input, which can be viewed as a generalization of discrete signalling to multiple dimensions.
    ICC 2014 - 2014 IEEE International Conference on Communications; 06/2014
  • Pritam Mukherjee, Ravi Tandon, Sennur Ulukus
    2014 IEEE International Symposium on Information Theory (ISIT); 06/2014
  • Berk Gurakan, Sennur Ulukus
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    ABSTRACT: We consider the energy harvesting diamond channel, where the source and two relays harvest energy from nature, the relays help deliver the source's messages via signal cooperation, and the source has the option of wirelessly transferring some of its energy to the relays via energy cooperation. We find the optimal offline transmit power allocations and energy transfer policies that maximize the end-to-end throughput. For the case of no energy cooperation, we decompose the problem into inner and outer maximization problems, and solve the overall problem iterating between the two. We show that the class of procrastinating policies, where energy is transferred only when it will be immediately used, is optimal. We then show that the problem with energy cooperation is equivalent to a problem without energy cooperation with suitably modified rate expressions. We show that, in this system, if the source sends more energy to a relay, then it sends less data, showing us how data and energy should flow together optimally in this network.
    2014 IEEE International Symposium on Information Theory (ISIT); 06/2014
  • Omur Ozel, Khurram Shahzad, Sennur Ulukus
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    ABSTRACT: We consider data transmission with an energy harvesting transmitter that has hybrid energy storage with a perfect super-capacitor (SC) and an inefficient battery. The SC has finite storage space while the battery has unlimited space. The transmitter can choose to store the harvested energy in the SC or in the battery. The energy is drained from the SC and the battery simultaneously. In this setting, we consider throughput optimal offline energy allocation problem over a point-to-point channel. In contrast to previous works, the hybrid energy storage model with finite and unlimited storage capacities imposes a generalized set of constraints on the transmission policy. As such, we show that the solution generalizes that for a single battery and is found by a sequential application of the directional water-filling algorithm. Next, we consider offline throughput maximization in the presence of an additive time-linear processing cost in the transmitter's circuitry. In this case, the transmitter has to additionally decide on the portions of the processing cost to be drained from the SC and the battery. Despite this additional complexity, we show that the solution is obtained by a sequential application of a directional glue pouring algorithm, parallel to the costless processing case. Finally, we provide numerical illustrations for optimal policies and performance comparisons with some heuristic online policies.
    IEEE Transactions on Signal Processing 06/2014; 62(12):3232-3245. DOI:10.1109/TSP.2014.2321733 · 3.20 Impact Factor
  • Jianwei Xie, Sennur Ulukus
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    ABSTRACT: We examine the separability of the parallel multiple access wiretap channel. Separability, when exists, is useful as it enables us to code separately over parallel channels, and still achieve the optimum overall performance. It is well-known that the parallel single-user channel, parallel multiple access channel (MAC) and parallel broadcast channel (BC) are all separable, however, the parallel interference channel (IC) is not separable in general. In this paper, we show that, while MAC is separable MAC wiretap channel is not separable in general. We prove this via a specific linear deterministic MAC wiretap channel. We then show that even the Gaussian MAC wiretap channel is inseparable in general. Finally, we show that, when the channel gains are drawn from continuous distributions, and when the secure degrees of freedom (s.d.o.f.) region is considered, then the Gaussian MAC wiretap channel is almost surely separable.
    2014 IEEE International Symposium on Information Theory (ISIT); 06/2014
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    ABSTRACT: We determine the capacity of a discrete memoryless communication channel with an energy harvesting transmitter and its battery state information available at the transmitter and the receiver. This capacity is an upper bound for the problem where side information is available only at the transmitter. Since channel output feedback does not increase the capacity in this case, we equivalently study the resulting finite-state Markov channel with feedback. We express the capacity in terms of directed information. Additionally, we provide sufficient conditions under which the capacity expression is further simplified to include the stationary distribution of the battery state. We also obtain a single-letter expression for the capacity with battery state information at both sides and an infinite-sized battery. Lastly, we consider achievable schemes when side information is available only at the transmitter for the case of an arbitrary finite-sized battery. We numerically evaluate the capacity and achievable rates with and without receiver side information.
    2014 IEEE International Symposium on Information Theory (ISIT); 06/2014
  • Source
    Jianwei Xie, Sennur Ulukus
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    ABSTRACT: The sum secure degrees of freedom (s.d.o.f.) of two fundamental multi-user network structures, the K-user Gaussian multiple access (MAC) wiretap channel and the K-user interference channel (IC) with secrecy constraints, have been determined recently as K(K-1)/(K(K-1)+1) [1,2] and K(K-1)/(2K-1) [3,4], respectively. In this paper, we determine the entire s.d.o.f. regions of these two channel models. The converse for the MAC follows from a middle step in the converse of [1,2]. The converse for the IC includes constraints both due to secrecy as well as due to interference. Although the portion of the region close to the optimum sum s.d.o.f. point is governed by the upper bounds due to secrecy constraints, the other portions of the region are governed by the upper bounds due to interference constraints. Different from the existing literature, in order to fully understand the characterization of the s.d.o.f. region of the IC, one has to study the 4-user case, i.e., the 2 or 3-user cases do not illustrate the generality of the problem. In order to prove the achievability, we use the polytope structure of the converse region. In both MAC and IC cases, we develop explicit schemes that achieve the extreme points of the polytope region given by the converse. Specifically, the extreme points of the MAC region are achieved by an m-user MAC wiretap channel with (K-m) helpers, i.e., by setting (K-m) users' secure rates to zero and utilizing them as pure (structured) cooperative jammers. The extreme points of the IC region are achieved by a (K-m)-user IC with confidential messages, m helpers, and N external eavesdroppers, for m>=1 and a finite N. A byproduct of our results in this paper is that the sum s.d.o.f. is achieved only at one extreme point of the s.d.o.f. region, which is the symmetric-rate extreme point, for both MAC and IC channel models.
  • Jianwei Xie, Sennur Ulukus
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    ABSTRACT: We show that the sum secure degrees of freedom (d.o.f.) of the K-user Gaussian multiple access (MAC) wiretap channel is K(K-1)/K(K-1)+1. Our achievability is based on real interference alignment and structured cooperative jamming. Each user divides its message into K - 1 sub-messages, and sends a linear combination of signals carrying these sub-messages together with a structured cooperative jamming signal. All cooperative jamming signals are aligned in a single dimension at the legitimate receiver allowing for reliable decoding of the message carrying signals by the legitimate receiver. Each cooperative jamming signal is aligned with K-1 message signals at the eavesdropper limiting the information leakage rate to the eavesdropper. We provide a matching converse establishing the exact sum secure d.o.f. of the Gaussian MAC wiretap channel as K(K-1)/K(K-1)+1.
    Information Theory Proceedings (ISIT), 2013 IEEE International Symposium on; 11/2013
  • Omur Ozel, Khurram Shahzad, Sennur Ulukus
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    ABSTRACT: We consider data transmission with an energy harvesting transmitter with non-negligible processing circuitry power and a hybrid energy storage unit composed of an ideal super-capacitor (SC) and an inefficient battery. The SC has finite space for energy storage while the battery has unlimited space. The transmitter stores the harvested energy either in the SC or in the battery and the energy is drained from the SC and the battery simultaneously. In this setting, we address the offline throughput maximization problem over a point-to-point channel. We show that the solution is obtained by a sequential application of the directional glue-pouring algorithm.
    2013 Asilomar Conference on Signals, Systems and Computers; 11/2013
  • Pritam Mukherjee, Ravi Tandon, Sennur Ulukus
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    ABSTRACT: It is well known that parallel interference channels (ICs) are in general inseparable. Existing results illustrate the benefit of joint encoding across sub-channels by exploiting the asymmetry within the individual sub-channels. In this paper, we show, perhaps surprisingly, that symmetric parallel ICs are also inseparable. To this end, a class of parallel ICs is considered in which each individual sub-channel is a symmetric linear deterministic IC (LDIC). The capacity region of this class of parallel ICs is characterized. As a consequence, we find the necessary and sufficient conditions for inseparability of this class of parallel ICs.
    2013 51st Annual Allerton Conference on Communication, Control, and Computing (Allerton); 10/2013
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    ABSTRACT: Wireless communications systems are particularly vulnerable to security attacks because of the inherent openness of the transmission medium. In this article, we focus on guaranteeing confidentiality against eavesdropping attacks where an unauthorized entity aims to intercept an ongoing wireless communication, and we provide a comprehensive summary of recent advances in the area of physical-layer security that guarantees confidentiality by using cooperative techniques unique to the wireless medium. These cooperative techniques consist of carefully designed coding and signaling schemes that are able to harness the properties of the physical layer and to ensure some level of information-theoretic security.
    IEEE Signal Processing Magazine 09/2013; 30(5):16-28. DOI:10.1109/MSP.2013.2260875 · 4.48 Impact Factor

Publication Stats

4k Citations
195.47 Total Impact Points

Institutions

  • 2007–2014
    • Loyola University Maryland
      Baltimore, Maryland, United States
  • 2003–2014
    • University of Maryland, College Park
      • Department of Electrical & Computer Engineering
      CGS, Maryland, United States
  • 2010
    • Bahçeşehir University
      İstanbul, Istanbul, Turkey
  • 2008
    • Stanford University
      • Department of Electrical Engineering
      Stanford, CA, United States
  • 2006–2007
    • Isik University
      İstanbul, Istanbul, Turkey
  • 2002
    • Pennsylvania State University
      • Department of Electrical Engineering
      University Park, MD, United States
  • 1997–2002
    • Rutgers, The State University of New Jersey
      • • Wireless Information Network Laboratory (WINLAB)
      • • Department of Electrical and Computer Engineering
      Нью-Брансуик, New Jersey, United States
  • 2001
    • Lehigh University
      • Department of Electrical and Computer Engineering
      Bethlehem, PA, United States
    • Princeton University
      Princeton, New Jersey, United States
  • 1998–2001
    • AT&T Labs
      Austin, Texas, United States