S. Ulukus

University of Maryland, College Park, Maryland, United States

Are you S. Ulukus?

Claim your profile

Publications (191)152.12 Total impact

  • [Show abstract] [Hide abstract]
    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.
    08/2014;
  • Jianwei Xie, Sennur Ulukus
    [Show abstract] [Hide abstract]
    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.
    04/2014;
  • O. Ozel, K. Shahzad, S. Ulukus
    [Show abstract] [Hide abstract]
    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 01/2014; 62(12):3232-3245. · 2.81 Impact Factor
  • Omur Ozel, Jing Yang, Sennur Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: We consider an energy harvesting transmitter sending messages to two users over parallel and fading Gaussian broadcast channels. Energy required for communication arrives (is harvested) at the transmitter and a finite-capacity battery stores it before being consumed for transmission. Under off-line knowledge of energy arrival and channel fading variations, we obtain the trade-off between the performances of the users by characterizing the maximum departure region in a given interval. We first analyze the transmission with an energy harvesting transmitter over parallel broadcast channels. We show that the optimal total transmit power policy that achieves the boundary of the maximum departure region is the same as the optimal policy for the non-fading broadcast channel, which does not depend on the priorities of the users, and therefore is the same as the optimal policy for the non-fading scalar single-user channel. The optimal total transmit power can be found by a directional water-filling algorithm. The optimal splitting of the power among the parallel channels is performed in each epoch separately. Next, we consider fading broadcast channels and obtain the transmission policies that achieve the boundary of the maximum departure region. The optimal total transmit power allocation policy is found using a specific directional water-filling algorithm for fading broadcast channels. The optimal power allocation depends on the priorities of the users unlike in the case of parallel broadcast channels. Finally, we provide numerical illustrations of the optimal policies and maximum departure regions for both parallel and fading broadcast channels.
    Computer Communications. 07/2013; 36(12):1360-1372.
  • Source
    Jianwei Xie, Sennur Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: We determine the exact sum secure degrees of freedom (d.o.f.) of the K-user Gaussian interference channel. We consider three different secrecy constraints: 1) K-user interference channel with one external eavesdropper (IC-EE), 2) K-user interference channel with confidential messages (IC-CM), and 3) K-user interference channel with confidential messages and one external eavesdropper (IC-CM-EE). We show that for all of these three cases, the exact sum secure d.o.f. is K(K-1)/(2K-1). We show converses for IC-EE and IC-CM, which imply a converse for IC-CM-EE. We show achievability for IC-CM-EE, which implies achievability for IC-EE and IC-CM. We develop the converses by relating the channel inputs of interfering users to the reliable rates of the interfered users, and by quantifying the secrecy penalty in terms of the eavesdroppers' observations. Our achievability uses structured signaling, structured cooperative jamming, channel prefixing, and asymptotic real interference alignment. While the traditional interference alignment provides some amount of secrecy by mixing unintended signals in a smaller sub-space at every receiver, in order to attain the optimum sum secure d.o.f., we incorporate structured cooperative jamming into the achievable scheme, and intricately design the structure of all of the transmitted signals jointly.
    05/2013;
  • Source
    Omur Ozel, Khurram Shahzad, Sennur Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: We consider data transmission with an energy harvesting transmitter which has a hybrid energy storage unit composed of a perfectly efficient super-capacitor (SC) and an inefficient battery. The SC has finite space for energy storage 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 the offline throughput maximization problem by a deadline 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 obtained by applying directional water-filling algorithm multiple times.
    05/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We consider the capacity of an energy harvesting communication channel with a finite-sized battery. As an abstraction of this problem, we consider a system where energy arrives at the encoder in multiples of a fixed quantity, and the physical layer is modeled accordingly as a finite discrete alphabet channel based on this fixed quantity. Further, for tractability, we consider the case of binary energy arrivals into a unit-capacity battery over a noiseless binary channel. Viewing the available energy as state, this is a state-dependent channel with causal state information available only at the transmitter. Further, the state is correlated over time and the channel inputs modify the future states. We show that this channel is equivalent to an additive geometric-noise timing channel with causal information of the noise available at the transmitter.We provide a single-letter capacity expression involving an auxiliary random variable, and evaluate this expression with certain auxiliary random variable selection, which resembles noise concentration and lattice-type coding in the timing channel. We evaluate the achievable rates by the proposed auxiliary selection and extend our results to noiseless ternary channels.
    05/2013;
  • Source
    Berk Gurakan, Omur Ozel, Jing Yang, Sennur Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: In energy harvesting communications, users transmit messages using energy harvested from nature during the course of communication. With an optimum transmit policy, the performance of the system depends only on the energy arrival profiles. In this paper, we introduce the concept of energy cooperation, where a user wirelessly transmits a portion of its energy to another energy harvesting user. This enables shaping and optimization of the energy arrivals at the energy-receiving node, and improves the overall system performance, despite the loss incurred in energy transfer. We consider several basic multi-user network structures with energy harvesting and wireless energy transfer capabilities: relay channel, two-way channel and multiple access channel. We determine energy management policies that maximize the system throughput within a given duration using a Lagrangian formulation and the resulting KKT optimality conditions. We develop a two-dimensional directional water-filling algorithm which optimally controls the flow of harvested energy in two dimensions: in time (from past to future) and among users (from energy-transferring to energy-receiving) and show that a generalized version of this algorithm achieves the boundary of the capacity region of the two-way channel.
    IEEE Transactions on Communications 03/2013; · 1.75 Impact Factor
  • Raef Bassily, Sennur Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, we investigate the roles of cooperative jamming (CJ) and noise forwarding (NF) in improving the achievable secrecy rates of a Gaussian wiretap channel (GWT). In particular, we study the role of a deaf helper in confusing the eavesdropper in a GWT channel by either transmitting white Gaussian noise (cooperative jamming) or by transmitting a dummy codeword of no context yet drawn from a codebook known to both the destination and the eavesdropper (noise forwarding). We first derive the conditions under which each mode of deaf cooperation improves over the secrecy capacity of the original wiretap channel and show that a helping node can be either a useful cooperative jammer or a useful noise forwarder but not both at the same time. Secondly, we derive the optimal power allocation for both the source and the helping node to be used in each of the two modes of deaf helping. Thirdly, we consider the deaf helper selection problem where there are N relays present in the system and it is required to select the best K deaf helpers, K ≥ 1, that yield the maximum possible achievable secrecy rate. For the case of K=1, we give the optimal selection strategy with optimal power allocation. The computational complexity of the optimal selection strategy when K > 1 is relatively large, especially for large values of K and N. Thus, we propose a suboptimal strategy for the selection problem when K > 1. We derive the complexity of the proposed selection strategies and show that, for K > 1, our suboptimal strategy, which works in a greedy fashion, enjoys a significantly less computational complexity than the optimal strategy. Nevertheless, as demonstrated by numerical examples, our suboptimal strategy gives rise to reasonable performance gains in terms of the achievable secrecy rate with respect to the case of K=1.
    IEEE Transactions on Signal Processing 03/2013; 61(6):1544-1554. · 2.81 Impact Factor
  • Source
    Jianwei Xie, Sennur Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: We consider the Gaussian wiretap channel with M helpers, where no eavesdropper channel state information (CSI) is available at the legitimate entities. The exact secure d.o.f. of the Gaussian wiretap channel with M helpers with perfect CSI at the transmitters was found in [1], [2] to be M/(M+1). One of the key ingredients of the optimal achievable scheme in [1], [2] is to align cooperative jamming signals with the information symbols at the eavesdropper to limit the information leakage rate. This required perfect eavesdropper CSI at the transmitters. Motivated by the recent result in [3], we propose a new achievable scheme in which cooperative jamming signals span the entire space of the eavesdropper, but are not exactly aligned with the information symbols. We show that this scheme achieves the same secure d.o.f. of M/(M+1) in [1], [2] but does not require any eavesdropper CSI; the transmitters blindly cooperative jam the eavesdropper.
    02/2013;
  • [Show abstract] [Hide abstract]
    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 01/2013; 30(5):16-28. · 3.37 Impact Factor
  • Jianwei Xie, S. Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, we study the sum secure degrees of freedom (d.o.f.) of two-unicast layered wireless networks. Without any secrecy constraints, the sum d.o.f. of this class of networks was studied by and shown to take only one of three possible values: 1, 3/2 and 2, for all network configurations. We consider the setting where, in addition to being reliably transmitted, each message is required to be kept information-theoretically secure from the unintended receiver. We show that the sum secure d.o.f. can only take one of five possible values: 0, 2/3, 1, 3/2, 2, for all network configurations. To determine the sum secure d.o.f., we divide the class of two-unicast layered networks into several sub-classes, and propose an achievable scheme based on the specific structure of the networks in each sub-class. Our achievable schemes are based on real interference alignment, cooperative jamming, interference neutralization and cooperative jamming neutralization techniques.
    IEEE Journal on Selected Areas in Communications 01/2013; 31(9):1931-1943. · 3.12 Impact Factor
  • Jianwei Xie, S. Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: We determine the exact sum secure degrees of freedom (d.o.f.) of the K-user Gaussian interference channel. We consider three different secrecy constraints: 1) K-user interference channel with one external eavesdropper (IC-EE), 2) K-user interference channel with confidential messages (IC-CM), and 3) K-user interference channel with confidential messages and one external eavesdropper (IC-CM-EE). We show that for all of these three cases, the exact sum secure d.o.f. is K(K-1)/2K-1. We show converses for IC-EE and IC-CM, which imply a converse for IC-CM-EE. We show achievability for IC-CM-EE, which implies achievability for IC-EE and IC-CM. We develop the converses by relating the channel inputs of interfering users to the reliable rates, and by quantifying the secrecy penalty in terms of the eavesdroppers' observations. Our achievability uses structured signalling, channel prefixing via structured cooperative jamming, and asymptotic real interference alignment.
    Information Theory Proceedings (ISIT), 2013 IEEE International Symposium on; 01/2013
  • Jianwei Xie, S. Ulukus
    [Show abstract] [Hide abstract]
    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; 01/2013
  • Source
    Jianwei Xie, Sennur Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the secure degrees of freedom (d.o.f.) of one-hop wireless networks by considering four fundamental Gaussian network structures: wiretap channel, broadcast channel with confidential messages, interference channel with confidential messages, and multiple access wiretap channel. The secure d.o.f. of the canonical Gaussian wiretap channel with no helpers is zero. It has been known that a strictly positive secure d.o.f. can be obtained in the Gaussian wiretap channel by using a helper which sends structured cooperative signals. We show that the exact secure d.o.f. of the Gaussian wiretap channel with a helper is 1/2. Our achievable scheme is based on real interference alignment and cooperative jamming, which renders the message signal and the cooperative jamming signal separable at the legitimate receiver, but aligns them perfectly at the eavesdropper preventing any reliable decoding of the message signal. Our converse is based on two key lemmas. The first lemma quantifies the secrecy penalty by showing that the net effect of an eavesdropper on the system is that it eliminates one of the independent channel inputs. The second lemma quantifies the role of a helper by developing a direct relationship between the cooperative jamming signal of a helper and the message rate. We extend this result to the case of M helpers, and show that the exact secure d.o.f. in this case is M/(M+1). We then generalize this approach to more general network structures with multiple messages. We show that the sum secure d.o.f. of the Gaussian broadcast channel with confidential messages and M helpers is 1, the sum secure d.o.f. of the two-user interference channel with confidential messages is 2/3, the sum secure d.o.f. of the two-user interference channel with confidential messages and M helpers is 1, and the sum secure d.o.f. of the K-user multiple access wiretap channel is K(K-1)/(K(K-1)+1).
    IEEE Transactions on Information Theory 09/2012; · 2.62 Impact Factor
  • Source
    Ersen Ekrem, Sennur Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: This document is withdrawn due to an error in Lemma 4.
    02/2012;
  • Source
    Ersen Ekrem, Sennur Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the vector Gaussian CEO problem, where there are an arbitrary number of agents each having a noisy observation of a vector Gaussian source. The goal of the agents is to describe the source to a central unit, which wants to reconstruct the source within a given distortion. The rate-distortion region of the vector Gaussian CEO problem is unknown in general. Here, we provide an outer bound for the rate-distortion region of the vector Gaussian CEO problem. We obtain our outer bound by evaluating an outer bound for the multi-terminal source coding problem by means of a technique relying on the de Bruijn identity and the properties of the Fisher information. Next, we show that our outer bound strictly improves upon the existing outer bounds for all system parameters. We show this strict improvement by providing a specific example, and showing that there exists a gap between our outer bound and the existing outer bounds. Although our outer bound improves upon the existing outer bounds, we show that our outer bound does not provide the exact rate-distortion region in general. To this end, we provide an example and show that the rate-distortion region is strictly contained in our outer bound for this example.
    02/2012;
  • R. Bassily, S. Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, we investigate the roles of cooperative jamming (CJ) and noise forwarding (NF) in improving the achievable secrecy rates of a Gaussian wiretap channel (GWT) when the helper node is equipped with multiple antennas. We decompose the channel from the helper to the eavesdropper into two orthogonal components: one is aligned in the direction of the channel between the helper and the legitimate receiver (direct component) and the other is in the orthogonal direction to the channel between the helper and the legitimate receiver (orthogonal component). We then propose a strategy in which the helper uses the orthogonal component to transmit pure Gaussian noise as in the CJ strategy while he uses the direct component for either CJ or NF depending on the given channel conditions. We explicitly derive the optimal power control policy for this strategy and give the achievable secrecy rates when the direct component is used to perform CJ or NF. We hence derive the channel conditions where CJ is better than NF over the direct component and vice-versa. Finally, we consider the reversely degraded multiple antenna relay-eavesdropper channel. We show that a simple strategy in which the relay jams with full power along the orthogonal component and transmits nothing in the direct component achieves a secrecy rate that approaches the secrecy capacity of this channel as the relay's average power goes to infinity. Moreover, we show that this result holds almost surely even if the relay-eavesdropper's channel state information is unavailable.
    IEEE Transactions on Information Forensics and Security 01/2012; 7(6):1855-1864. · 1.90 Impact Factor
  • R. Bassily, Sennur Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, we study the role of cooperative relays to provide and improve secure communication rates through decode-and-forward (DF) strategies in a full-duplex multiple relay network with an eavesdropper. We consider the DF scheme as a basis for cooperation and propose several strategies that implement different versions of this scheme suited for cooperation with multiple relays. Our goal is to give an efficient cooperation paradigm based on the DF scheme to provide and improve secrecy in a multiple relay network. We first study the DF strategy for secrecy in a single relay network. We propose a suboptimal DF with zero forcing (DF/ZF) strategy for which we obtain the optimal power control policy. Next, we consider the multiple relay problem. We propose three different strategies based on DF/ZF and obtain their achievable secrecy rates. The first strategy is a single hop strategy whereas the other two strategies are multiple hop strategies. In the first strategy, we show that it is possible to eliminate all the relays' signals from the eavesdropper's observation (full ZF), however, the achievable secrecy rate is limited by the worst source-relay channel. Our second strategy overcomes the drawback of the first strategy, however, with the disadvantage of enabling partial ZF only. Our third strategy provides a reasonable compromise between the first two strategies. That is, in this strategy, full ZF is possible and the rate achieved does not suffer from the drawback of the first strategy. We conclude our study by a set of numerical results to illustrate the performance of each of the proposed strategies in terms of the achievable rates in different practical scenarios.
    Journal of Communications and Networks 01/2012; 14(4):352-363. · 0.75 Impact Factor
  • Source
    Raef Bassily, Sennur Ulukus
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, we first study the Decode-and-Forward strategy for secrecy in a single-relay network. We propose a suboptimal Decode-and-Forward with Zero Forcing (DF/ZF) strategy for which we obtain the optimal power control policy. Next, we consider the multiple relay problem. We propose three different strategies based on DF/ZF. The first strategy is a single-hop strategy in which all the relays decode the source message at the same time, then perform beamforming such that all the relays' signals are eliminated from the eavesdropper's observation (full zero-forcing). We give the achievable rate by this strategy and derive the optimal power control policy. We show that, in this strategy, the relays which are far from the source create a bottleneck and limit the achievable rate. The second strategy is a multiple hop strategy that overcomes the drawback of the first strategy, however, with the disadvantage of enabling partial zero-forcing only, assuming that all the relays are required to transmit fresh information in every transmission block. The third strategy is also a multiple hop strategy in which full zero-forcing is possible and the rate achieved does not suffer from the drawback of the first strategy.
    01/2012;

Publication Stats

3k Citations
152.12 Total Impact Points

Institutions

  • 2003–2013
    • University of Maryland, College Park
      • Department of Electrical & Computer Engineering
      Maryland, United States
  • 2008–2010
    • Bahçeşehir University
      İstanbul, Istanbul, Turkey
    • Stanford University
      • Department of Electrical Engineering
      Stanford, CA, United States
  • 2006–2007
    • Isik University
      İstanbul, Istanbul, Turkey
  • 2002
    • University of Maryland, Baltimore
      Baltimore, Maryland, United States
    • Pennsylvania State University
      • Department of Electrical Engineering
      University Park, MD, United States
  • 1997–2002
    • Rutgers, The State University of New Jersey
      • Department of Electrical and Computer Engineering
      New Brunswick, NJ, United States
  • 2001
    • Lehigh University
      • Department of Electrical and Computer Engineering
      Bethlehem, PA, United States
    • Princeton University
      Princeton, New Jersey, United States
  • 2000–2001
    • AT&T Labs
      Austin, Texas, United States