Publications

Journal Papers

H. Si, O. O. Koyluoglu, and S. Vishwanath, "Hierarchical polar coding for achieving secrecy over state-dependent wiretap channels without any instantaneous CSI," IEEE Transactions on Communications, vol.64, no.9, pp.3609-3623, Sep. 2016.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper presents a polar coding scheme to achieve secrecy in block fading binary symmetric wiretap channels without the knowledge of instantaneous channel state information (CSI) at the transmitter. For this model, a coding scheme that hierarchically utilizes polar codes is presented. In particular, on polarization of different binary symmetric channels over different fading blocks, each channel use is modeled as an appropriate binary erasure channel over fading blocks. Polar codes are constructed for both coding over channel uses for each fading block and coding over fading blocks for certain channel uses. In order to guarantee security, random bits are introduced at appropriate places to exhaust the observations of the eavesdropper. It is shown that this coding scheme, without instantaneous CSI at the transmitter, is secrecy capacity achieving for the simultaneous fading scenario. For the independent fading case, the capacity is achieved when the fading realizations for the eavesdropper channel is always degraded with respect to the receiver. For the remaining cases, the gap between lower and upper bounds is analyzed. Remarkably, for the scenarios where the secrecy capacity is achieved, the results imply that instantaneous CSI does not increase the secrecy capacity.

@ARTICLE{Si:Hierarchical16,
author={Si, H. and Koyluoglu, O. O. and Vishwanath, S.},
journal={IEEE Transactions on Communications},
title={Hierarchical polar coding for achieving secrecy over state-dependent wiretap channels without any instantaneous CSI},
volume={64},
no={9},
pages={3609-3623},
month={Sep.},
year={2016}
}
Y. Yoo, O. O. Koyluoglu, S. Vishwanath, and I. R. Fiete, "Multi-periodic neural coding for adaptive information transfer," Theoretical Computer Science, vol.633, pp.37-53, Jun. 2016.
[TCS] [pdf preprint] [abstract] [BibTeX]
Information processing in the presence of noise has been a key challenge in multiple disciplines including computer science, communications, and neuroscience. Among such noise-reduction mechanisms, the shift-map code represents an analog variable by its residues with respect to distinct moduli (that are chosen as geometric scalings of an integer). Motivated by the multi-periodic neural code in the entorhinal cortex, i.e., the coding mechanism of grid cells, this work extends the shift-map code by generalizing the choices of moduli. In particular, it is shown that using similarly sized moduli (for instance, evenly and closely spaced integers, which tend to have large co-prime factors) results in a code whose codewords are separated in an interleaving way such that when the decoder has side information regarding the source, then error control is significantly improved (compared to the original shift map code). This novel structure allows the system to dynamically adapt to the side information at the decoder, even if the encoder is not privy to the side information. A geometrical interpretation of the proposed coding scheme and a method to find such codes are detailed. As an extension, it is shown that this novel code also adapts to scenarios when only a fraction of codeword symbols is available at the decoder.

@ARTICLE{Yoo:Multi-periodic16,
author={Yoo, Y. and Koyluoglu, O. O. and Vishwanath, S. and Fiete, I. R.},
journal={Theoretical Computer Science},
title={Multi-periodic neural coding for adaptive information transfer},
volume={633},
pages={37-53},
month={Jun.},
year={2016}
}
O. O. Koyluoglu, A. S. Rawat, and S. Vishwanath, "Secure cooperative regenerating codes for distributed storage systems," IEEE Transactions on Information Theory, vol.60, no.9, pp.5228-5244, Sep. 2014.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
Regenerating codes enable trading off repair bandwidth for storage in distributed storage systems (DSS). Due to their distributed nature, these systems are intrinsically susceptible to attacks, and they may also be subject to multiple simultaneous node failures. Cooperative regenerating codes allow bandwidth efficient repair of multiple simultaneous node failures. This paper analyzes storage systems that employ cooperative regenerating codes that are robust to (passive) eavesdroppers. The analysis is divided into two parts, studying both minimum bandwidth and minimum storage cooperative regenerating scenarios. First, the secrecy capacity for minimum bandwidth cooperative regenerating codes is characterized. Second, for minimum storage cooperative regenerating codes, a secure file size upper bound and achievability results are provided. These results establish the secrecy capacity for the minimum storage scenario for certain special cases. In all scenarios, the achievability results correspond to exact repair, and secure file size upper bounds are obtained using min-cut analyses over a suitable secrecy graph representation of DSS. The main achievability argument is based on an appropriate precoding of the data to eliminate the information leakage to the eavesdropper.

@ARTICLE{6807720,
author={Koyluoglu, O. O. and Rawat, A. S. and Vishwanath, S.},
journal={IEEE Transactions on Information Theory},
title={Secure cooperative regenerating codes for distributed storage systems},
volume={60},
number={9},
pages={5228-5244},
month={Sep.},
year={2014}
}
H. Si, O. O. Koyluoglu, and S. Vishwanath, "Polar coding for fading channels: Binary and exponential channel cases," IEEE Transactions on Communications, vol.62, no.8, pp.2638-2650, Aug. 2014.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This work presents a polar coding scheme for fading channels, focusing primarily on fading binary symmetric and additive exponential noise channels. For fading binary symmetric channels, a hierarchical coding scheme is presented, utilizing polar coding both over channel uses and over fading blocks. The receiver uses its channel state information (CSI) to distinguish states, thus constructing an overlay erasure channel over the underlying fading channels. By using this scheme, the capacity of a fading binary symmetric channel is achieved without CSI at the transmitter. Noting that a fading AWGN channel with BPSK modulation and demodulation corresponds to a fading binary symmetric channel, this result covers a fairly large set of practically relevant channel settings. For fading additive exponential noise channels, expansion coding is used in conjunction to polar codes. Expansion coding transforms the continuous-valued channel to multiple (independent) discrete-valued ones. For each level after expansion, the approach described previously for fading binary symmetric channels is used. Both theoretical analysis and numerical results are presented, showing that the proposed coding scheme approaches the capacity in the high SNR regime. Overall, utilizing polar codes in this (hierarchical) fashion enables coding without CSI at the transmitter, while approaching the capacity with low complexity.

@ARTICLE{6871313,
author={Si, H. and Koyluoglu, O. O. and Vishwanath, S.},
journal={IEEE Transactions on Communications},
title={Polar coding for fading channels: Binary and exponential channel cases},
volume={62},
number={8},
pages={2638-2650},
month={Aug.},
year={2014}
}
A. S. Rawat, O. O. Koyluoglu, N. Silberstein, and S. Vishwanath, "Optimal locally repairable and secure codes for distributed storage systems," IEEE Transactions on Information Theory, vol.60, no.1, pp.212-236, Jan. 2014.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper aims to go beyond resilience into the study of security and local-repairability for distributed storage systems (DSSs). Security and local-repairability are both important as features of an efficient storage system, and this paper aims to understand the trade-offs between resilience, security, and local-repairability in these systems. In particular, this paper first investigates security in the presence of colluding eavesdroppers, where eavesdroppers are assumed to work together in decoding the stored information. Second, this paper focuses on coding schemes that enable optimal local repairs. It further brings these two concepts together to develop locally repairable coding schemes for DSS that are secure against eavesdroppers. The main results of this paper include: 1) an improved bound on the secrecy capacity for minimum storage regenerating codes; 2) secure coding schemes that achieve the bound for some special cases; 3) a new bound on minimum distance for locally repairable codes; 4) code construction for locally repairable codes that attain the minimum distance bound; and 5) repair-bandwidth-efficient locally repairable codes with and without security constraints.

@ARTICLE{6655894,
author={Rawat, A. S. and Koyluoglu, O. O. and Silberstein, N. and Vishwanath, S.},
journal={IEEE Transactions on Information Theory},
title={Optimal locally repairable and secure codes for distributed storage systems},
volume={60},
number={1},
pages={212-236},
month={Jan.},
year={2014}
}
A. El Gamal, O. O. Koyluoglu, M. Youssef, and H. El Gamal, "Achievable secrecy rate regions for the two-way wiretap channel," IEEE Transactions on Information Theory, vol.59, no.12, pp.8099-8114, Dec. 2013.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
The two-way wiretap channel is considered in this paper. Two legitimate users, Alice and Bob, wish to exchange messages securely in the presence of a passive eavesdropper Eve. In the full-duplex scenario, where each node can transmit and receive simultaneously, new achievable secrecy rate regions are obtained based on the idea of allowing the two users to jointly optimize their channel prefixing distributions and binning codebooks in addition to key sharing. The new regions are shown to be strictly larger than the known ones for a wide class of discrete memoryless and Gaussian channels. In the half-duplex case, where a user can only transmit or receive on any given degree of freedom, the idea of randomized scheduling is introduced and shown to offer a significant gain in terms of the achievable secrecy sum-rate. A practical setup is further developed based on a near field wireless communication scenario, and it is shown that one can exploit the two-way nature of the communication, via appropriately randomizing the transmit power levels and transmission schedule, to introduce significant ambiguity at a noiseless Eve.

@ARTICLE{6600802,
author={{El Gamal}, A. and Koyluoglu, O. O. and Youssef, M. and {El Gamal}, H.},
journal={IEEE Transactions on Information Theory},
title={Achievable secrecy rate regions for the two-way wiretap channel},
volume={59},
number={12},
pages={8099-8114},
month={Dec.},
year={2013}
}
K. Khalil, O. O. Koyluoglu, H. El Gamal, and M. Youssef, "Opportunistic secrecy with a strict delay constraint," IEEE Transactions on Communications, vol.61, no.11, pp.4700-4709, Nov. 2013.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
We investigate the delay limited secrecy capacity of the flat fading channel under two different assumptions on the available transmitter channel state information (CSI). The first scenario assumes perfect prior knowledge of both the main and eavesdropper channel gains. Here, upper and lower bounds on the delay limited secrecy capacity are derived, and shown to be tight in the high signal-to-noise ratio (SNR) regime. In the second scenario, only the main channel CSI is assumed to be available at the transmitter where, remarkably, we establish the achievability of a non-zero delay-limited secure rate, for a wide class of channel distributions, with a high probability. In the two cases, our achievability arguments are based on a novel two-stage key-sharing approach that overcomes the secrecy outage phenomenon observed in earlier works.

@ARTICLE{6630484,
author={Khalil, K. and Koyluoglu, O. O. and {El Gamal}, H. and Youssef, M.},
journal={IEEE Transactions on Communications},
title={Opportunistic secrecy with a strict delay constraint},
volume={61},
number={11},
pages={4700-4709},
month={Nov.},
year={2013}
}
O. O. Koyluoglu and H. El Gamal, "Polar coding for secure transmission and key agreement," IEEE Transactions on Information Forensics and Security, vol.7, no.5, pp.1472-1483, Oct. 2012.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
Achieving information theoretic security with practical coding complexity is of definite interest. This work first focuses on the key agreement problem. For this problem, a new cross-layer secure coding protocol over block fading channels is proposed. The proposed scheme requires only the statistical knowledge about the eavesdropper channel state information (CSI), and, utilizing a privacy amplification technique, reduces the problem of key agreement to a provably secure coding problem per block. Focusing on this secure coding problem, it is shown that polar codes, introduced by Arikan, achieve nonzero perfect secrecy rates for the binary-input degraded wiretap channel while enjoying a remarkably low encoding-decoding complexity. We further show that, in the special case of symmetric main and eavesdropper channels, this coding technique achieves the secrecy capacity. This approach is also extended to the multiple-access channel with a degraded eavesdropper where a nontrivial achievable secrecy region is established. This polar coding method is then utilized in the proposed key agreement protocol, where the secure coding per block is used to create an advantage for the legitimate nodes over the eavesdropper, which is then turned into a private key via the privacy amplification module.

@ARTICLE{6236066,
author={Koyluoglu, O. O. and {El Gamal}, H.},
journal={IEEE Transactions on Information Forensics and Security},
title={Polar coding for secure transmission and key agreement},
volume={7},
number={5},
pages={1472-1483},
month={Oct.},
year={2012}
}
O. O. Koyluoglu, C. E. Koksal, and H. El Gamal, "On secrecy capacity scaling in wireless networks," IEEE Transactions on Information Theory, vol.58, no.5, pp.3000-3015, May 2012.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This work studies the achievable secure rate per source-destination pair in wireless networks. First, a path loss model is considered, where the legitimate and eavesdropper nodes are assumed to be placed according to Poisson point processes with intensities $\lambda$ and $\lambda_e$, respectively. It is shown that, as long as $\lambda_e/\lambda=o((\log n)^{-2})$, almost all of the nodes achieve a perfectly secure rate of $\Omega(\frac{1}{\sqrt{n}})$ for the extended and dense network models. Therefore, under these assumptions, securing the network does not entail a loss in the per-node throughput. The achievability argument is based on a novel multi-hop forwarding scheme where randomization is added in every hop to ensure maximal ambiguity at the eavesdropper(s). Secondly, an ergodic fading model with $n$ source-destination pairs and $n_e$ eavesdroppers is considered. Employing the ergodic interference alignment scheme with an appropriate secrecy pre-coding, each user is shown to achieve a constant positive secret rate for sufficiently large $n$. Remarkably, the scheme does not require eavesdropper CSI (only the statistical knowledge is assumed) and the secure throughput per node increases as we add more legitimate users to the network in this setting. Finally, the effect of eavesdropper collusion on the performance of the proposed schemes is characterized.

@ARTICLE{6142080,
author={Koyluoglu, O. O. and Koksal, C. E. and {El Gamal}, H.},
journal={IEEE Transactions on Information Theory},
title={On secrecy capacity scaling in wireless networks},
volume={58},
number={5},
pages={3000-3015},
month={May},
year={2012}
}
O. O. Koyluoglu and H. El Gamal, "Cooperative encoding for secrecy in interference channels," IEEE Transactions on Information Theory, vol.57, no.9, pp.5682-5694, Sep. 2011.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper investigates the fundamental performance limits of the two-user interference channel in the presence of an external eavesdropper. In this setting, we construct an inner bound, to the secrecy capacity region, based on the idea of cooperative encoding in which the two users cooperatively design their randomized codebooks and jointly optimize their channel prefixing distributions. Our achievability scheme also utilizes message-splitting in order to allow for partial decoding of the interference at the nonintended receiver. Outer bounds are then derived and used to establish the optimality of the proposed scheme in certain cases. In the Gaussian case, the previously proposed cooperative jamming and noise-forwarding techniques are shown to be special cases of our proposed approach. Overall, our results provide structural insights on how the interference can be exploited to increase the secrecy capacity of wireless networks.

@ARTICLE{6006610,
author={Koyluoglu, O. O. and {El Gamal}, H.},
title={Cooperative encoding for secrecy in interference channels},
journal={IEEE Transactions on Information Theory},
volume={57},
number={9},
pages={5682-5694},
month={Sep.},
year={2011}
}
O. O. Koyluoglu, H. El Gamal, L. Lai, and H. V. Poor, "Interference alignment for secrecy," IEEE Transactions on Information Theory, vol.57, no.6, pp.3323-3332, Jun. 2011.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper studies the frequency/time selective K-user Gaussian interference channel with secrecy constraints. Two distinct models, namely the interference channel with confidential messages and the interference channel with an external eavesdropper, are analyzed. The key difference between the two models is the lack of channel state information (CSI) of the external eavesdropper. Using interference alignment along with secrecy precoding, it is shown that each user can achieve non-zero secure degrees of freedom (DoF) for both cases. More precisely, the proposed coding scheme achieves [(K-2)/(2K-2)] secure DoF with probability one per user in the confidential messages model. For the external eavesdropper scenario, on the other hand, it is shown that each user can achieve [(K-2)/(2K)] secure DoF in the ergodic setting. Remarkably, these results establish the positive impact of interference on the secrecy capacity region of wireless networks.

@ARTICLE{5773067,
author={Koyluoglu, O. O. and {El Gamal}, H. and Lifeng Lai and Poor, H. V.},
journal={IEEE Transactions on Information Theory},
title={Interference alignment for secrecy},
volume={57},
number={6},
pages={3323-3332},
month={Jun.},
year={2011}
}
O. O. Koyluoglu and H. El Gamal, "On power control and frequency reuse in the two user cognitive channel," IEEE Transactions on Wireless Communications, vol.8, no.7, pp.3546-3553, Jul. 2009.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper considers the generalized cognitive radio channel where the secondary user is allowed to reuse the frequency during both the idle and active periods of the primary user, as long as the primary rate remains the same. In this setting, the optimal power allocation policy with single-input single-output (SISO) primary and secondary channels is explored. Interestingly, the offered gain resulting from the frequency reuse during the active periods of the spectrum is shown to disappear in both the low and high signal-to-noise ratio (SNR) regimes. We then argue that this drawback in the high SNR region can be avoided by equipping both the primary and secondary transmitters with multiple antennas. Finally, the scenario consisting of SISO primary and multi-input multi-output (MIMO) secondary channels is investigated. Here, a simple zero-forcing approach is shown to significantly outperform the celebrated decoding- forwarding-dirty paper coding strategy (especially in the high SNR regime).

@ARTICLE{5165318,
author={Koyluoglu, O. O. and {El Gamal}, H.},
journal={IEEE Transactions on Wireless Communications},
title={On power control and frequency reuse in the two user cognitive channel},
volume={8},
number={7},
pages={3546-3553},
month={Jul.},
year={2009}
}

Conference Papers and Abstracts

S. Shivaramaiah, G. Calis, O. O. Koyluoglu, and L. Lazos, "Threshold-based file maintenance strategies for mobile cloud storage systems," in IEEE 2016 Global Communications Conference (Globecom 2016), Washington, DC, Dec. 2016.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
We study the data reliability problem for a community of devices forming a mobile cloud storage system. We consider the application of regenerating codes for maintaining a file within a geographically-limited area. Such codes require lower bandwidth to regenerate lost data fragments compared to file replication or reconstruction. We investigate threshold-based repair strategies where data repair is initiated after a threshold number of data fragments have been lost due to node mobility. We show that at a low departure-to-repair rate regime, a {\em lazy repair} strategy in which repairs are initiated after several nodes have left the system outperforms {\em eager repair} in which repairs are initiated after a single departure. This optimality is reversed when nodes are highly mobile. We further compare distributed and centralized repair strategies and derive the optimal repair threshold for minimizing the average repair cost per unit of time, as a function of underlying code parameters.

@INPROCEEDINGS{Shivaramaiah:Threshold-based16,
author = {Shivaramaiah, S. and Calis, G. and Koyluoglu, O. O. and Lazos, L.},
title = {Threshold-based file maintenance strategies for mobile cloud storage systems},
booktitle={Proc. IEEE 2016 Global Communications Conference (Globecom 2016)},
address={Washington, DC},
month={Dec.},
year={2016}
}
M. Ragone, S. Gianelli, D. Schwartz, L. Su, O. O. Koyluoglu, J. M. Fellous, "The role of hippocampal replay in a computational model of path learning," in Proc. Neuroscience 2016, San Diego, CA, Nov. 2016.
[SfN] [abstract] [BibTeX]
Hippocampal place cells play an important role in spatial navigation in rodents, but the exact mechanism by which networks of place cells store and process spatial information is poorly understood. ``Replay'' events, in which behaviorally relevant place cells are reactivated offline during sharp-wave ripples (SPWs), are likely critical to spatial information storage. However, the mechanisms by which cells are selected to fire within SPWs are unknown. In this study, we use a mobile robot and a biophysically realistic network model of place cells and interneurons with spike timing-dependent plasticity (STDP) to simulate hippocampal activity during spatial navigation. The robot is comparable in size to a rat and generates paths that are approximations of typical rat trajectories in spatial noise and velocity. These paths are used as inputs to the model and cause specific patterns of activation in the network, which, coupled with the STDP rule, generate a synaptic matrix that contains path-specific information. When SPWs are simulated in a trained network, its synaptic matrix lays constraints on which cells are reactivated, and may explain why some cells are selected for replay firing and others are not. The firing patterns of these cells during path traversal are then decoded in an ideal observer framework to estimate the path taken by the robot. We hypothesize that place cells that participate in trajectory replay events will convey more information about the path than non-replaying cells. We use the mean squared error (MSE) of the estimated location to test this hypothesis by comparing the MSE of decoding between different ensembles of place cells-all cells, replaying cells, and non-replaying cells. Recent work suggests that grid cells and place cells are spatially coherent during replay events. In a separate set of simulations, we investigate the effects of including modular or non-modular grid cell information to path decoding with the aforementioned place cell ensembles. We test the robustness of different code ensembles with nominal paths and paths that contain significant perturbations. Our study shows the extent to which replay is governed by the synaptic weights of the network as learning occurs across trials, and how replay may affect the encoding of a learned path in the face of perturbations.

@INPROCEEDINGS{Ragone:role16,
author = {Ragone, M. and Gianelli, S. and Schwartz, D. and Su, L. and Koyluoglu, O. O. and Fellous, J. M.},
title = {The role of hippocampal replay in a computational model of path learning},
booktitle={Neuroscience 2016},
address={San Diego, CA},
month={Nov.},
year={2016}
}
D. Schwartz and O. O. Koyluoglu, "A hybrid code from grid and place cells," in Proc. Neuroscience 2016, San Diego, CA, Nov. 2016.
[SfN] [abstract] [BibTeX]
Place cells in the hippocampus are active when an animal visits a certain location (referred to as the place field) within an environment, and remain silent otherwise. Grid cells in the medial entorhinal cortex (mEC) respond at multiple locations, where the firing fields exhibit a hexagonally symmetric periodic pattern. Both cell types have a dorso-ventral organization with larger firing fields distributed towards the ventral end. In addition, grid cells are clustered within discrete modules, wherein cells share scale and rotational phase, but differ in spatial phase offset. The joint activity of grid and place cell populations, as a function of location, forms a neural code for space. Different neural networks are constructed by varying the numbers of cells, grid modules, and grid scaling ratios. An ensemble of codes, for a given set of parameters, is generated by randomly selecting grid cell phases and place cell tuning curves. For each code in the ensemble, codewords are generated by stimulating a network with a discrete set of locations. The resulting code's resilience to neural noise is measured by the minimum pairwise distance between codewords, d. A larger d implies a more noise tolerant representation of space. Normalized code rank, on the other hand, measures dimensionality of the code space. Code rate, the number of locations represented per neuron, measures resolution of location. Analysis of these codes produces the following: For a fixed number of place cells, grid cell parameters may be chosen to produce a code with nearly any desired rank. For a fixed set of grid cell population parameters, increasing the number of place cells increases rank to a maximum, after which, inclusion of additional place cells lowers rank. For a fixed code rate, there is a sharp tradeoff between rank and d, i.e. maximizing either minimizes the other, and lower rates yield more desirable tradeoffs. There is a similar tradeoff between d and code rate. These findings hold for any scaling ratio between consecutive grid modules, L. An intermediate value of L yields a better tradeoff between d and rate. Increasing the number of grid modules increases d, but does not impact rank. Finally, these coding theoretic observations are revisited by measuring the performances of biologically realizable error correction algorithms (e.g., winner take all) implemented by a network of place and grid cell populations, as well as interneurons, which implement de-noising operations. Simulations demonstrate that de-noising mechanisms analyzed here can significantly reduce mean squared error (MSE) of location decoding. Furthermore, the modular organization of grid cells can be exploited to improve MSE.

@INPROCEEDINGS{Schwartz:hybrid16,
author = {Schwartz, D. and Koyluoglu, O. O.},
title = {A hybrid code from grid and place cells},
booktitle={Neuroscience 2016},
address={San Diego, CA},
month={Nov.},
year={2016}
}
Y. Chen, O. O. Koyluoglu, and A. Sezgin "Individual secrecy for the broadcast channel," in Proc. 2016 International Symposium on Information Theory and Its Applications (ISITA 2016), Monterey, CA, Oct. 2016.
[ISITA] [pdf preprint] [abstract] [BibTeX]
This paper studies the problem of secure communication over broadcast channels under the \emph{individual} secrecy constraints. That is, the transmitter wants to send two independent messages to two legitimate receivers in the presence of an eavesdropper, while keeping the eavesdropper ignorant of {\it each} message. A general achievable rate region is established by utilizing Marton's coding together with techniques such as rate splitting, Carleial-Hellman's secrecy coding, Wyner's secrecy coding and indirect decoding. Moreover, the individual secrecy capacity regions for some special cases are characterized, and an linear deterministic instance is exhibited to provide insights into the capacity regions under different secrecy constraints.

@INPROCEEDINGS{Chen:Individual16,
author = {Chen, Y. and Koyluoglu, O. O. and Sezgin, A.},
title = {Individual secrecy for the broadcast channel},
booktitle={2016 International Symposium on Information Theory and Its Applications (ISITA 2016)},
address={Monterey, CA},
month={Oct.},
year={2016}
}
Y. Chen, O. O. Koyluoglu, and A. J. Han Vinck, "On secure communication over the multiple access channel," in Proc. 2016 International Symposium on Information Theory and Its Applications (ISITA 2016), Monterey, CA, Oct. 2016.
[ISITA] [pdf preprint] [abstract] [BibTeX]
This paper studies the problem of secure communication over a $2$-transmitter multiple access channel (MAC) in the presence of an external eavesdropper. Two different secrecy constraints are considered: 1) individual secrecy (i.e., information leakage rate from each message to the eavesdropper is made vanishing) and 2) joint secrecy (i.e., information leakage rate from both messages to the eavesdropper is made vanishing). As a general result, the respective achievable secrecy rate regions are established. The single-letter characterizations of both regions involve three auxiliary random variables, one for time sharing and two for channel prefixing. Numerical results are presented to demonstrate the impact of different secrecy constraints and the advantage of channel prefixing in enlarging the achievable (individual/joint) secrecy rate regions.

@INPROCEEDINGS{Chen:secure16,
author = {Chen, Y. and Koyluoglu, O. O. and Han Vinck, A. J. },
title = {On secure communication over the multiple access channel},
booktitle={2016 International Symposium on Information Theory and Its Applications (ISITA 2016)},
address={Monterey, CA},
month={Oct.},
year={2016}
}
G. Calis and O. O. Koyluoglu, "On the maintenance of distributed storage systems with backup node for repair," in Proc. 2016 International Symposium on Information Theory and Its Applications (ISITA 2016), Monterey, CA, Oct. 2016.
[ISITA] [pdf preprint] [abstract] [BibTeX]
We consider a hierarchical DSS where the content is stored with coding in storage nodes and without coding in a backup node. We analyze the system where mobile nodes request the content from the storage nodes. Backup node is assumed to be accessible by storage nodes only in the case where repairs are required. Under this scenario, we derive the upper bound on the file size bound as well as establish critical points on the trade-off curve known as Minimum Storage Regenerating (MSR) and Minimum Bandwidth Regenerating (MBR) points. Next, we propose optimal code constructions by utilizing existing regenerating codes. Furthermore, we analyze the statistics of maintenance and data access costs under Poisson model for node failures and data requests. We derive expressions for expected values and variances of both such costs. Finally, numerical results are provided where we show that the most studied points in the literature (MSR and MBR) are not always optimal with respect to total cost. We also point out that variance of maintenance cost may be much important than variance of data access cost.

@INPROCEEDINGS{Calis:Maintenance16,
author = {Calis, G. and Koyluoglu, O. O.},
title = {On the maintenance of distributed storage systems with backup node for repair},
booktitle={2016 International Symposium on Information Theory and Its Applications (ISITA 2016)},
address={Monterey, CA},
month={Oct.},
year={2016}
}
A. S. Rawat, O. O. Koyluoglu, and S. Vishwanath, "Centralized repair of multiple node failures," in Proc. 2016 IEEE International Symposium on Information Theory (ISIT 2016), Barcelona, Spain, Jul. 2016.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper considers a distributed storage system, where multiple storage nodes can be reconstructed simultaneously at a centralized location. This centralized multi-node repair (CMR) model is a generalization of regenerating codes that allow for bandwidth-efficient repair of a single failed node. This work focuses on the trade-off between the amount of data stored and repair bandwidth in this CMR model. In particular, repair bandwidth bounds are derived for the minimum storage multi-node repair (MSMR) and the minimum bandwidth multi-node repair (MBMR) operating points. The tightness of these bounds are analyzed via code constructions. The MSMR point is characterized through codes achieving this point under functional repair for general set of CMR parameters, as well as with codes enabling exact repair for certain CMR parameters. The MBMR point, on the other hand, is characterized with exact repair codes for all CMR parameters for systems that satisfy a certain entropy accumulation property.

@INPROCEEDINGS{Rawat:Centralized16,
author = {Rawat, A. S. and Koyluoglu, O. O. and Vishwanath, S.},
title = {Centralized repair of multiple node failures},
booktitle={2016 IEEE International Symposium on Information Theory (ISIT 2016)},
address={Barcelona, Spain},
month={Jul.},
year={2016}
}
A. S. Rawat, O. O. Koyluoglu, and S. Vishwanath, "Progress on high-rate MSR codes: Enabling arbitrary number of helper nodes," in Proc. 2016 Information Theory and Applications Workshop (ITA 2016), La Jolla, CA, Jan. 2016.
[ITA 2016] [pdf preprint] [abstract] [BibTeX]
This paper presents a construction for high-rate MDS codes that enable bandwidth-efficient repair of a single node. Such MDS codes are also referred to as the minimum storage regenerating (MSR) codes in the distributed storage literature. The construction presented in this paper generates MSR codes for all possible number of helper nodes d as d is a design parameter in the construction. Furthermore, the obtained MSR codes have polynomial sub-packetization (a.k.a. node size) α. The construction is built on the recent code proposed by Sasidharan et al. [1], which works only for d=n−1, i.e., where all the remaining nodes serve as the helper nodes for the bandwidth-efficient repair of a single node. The results of this paper broaden the set of parameters where the constructions of MSR codes were known earlier.

@INPROCEEDINGS{Rawat:Progress16,
author = {Rawat, A. S. and Koyluoglu, O. O. and Vishwanath, S.},
title = {Progress on high-rate MSR codes: Enabling arbitrary number of helper nodes},
booktitle={2016 Information Theory and Applications Workshop (ITA 2016)},
address={La Jolla, CA},
month={Jan.},
year={2016}
}
B. Akgun, O. O. Koyluoglu, and M. Krunz, "Receiver-based friendly jamming with single-antenna full-duplex receivers in a multiuser broadcast channel," in Proc. 2015 IEEE Global Communications Conference (Globecom 2015), San Diego, CA, Dec. 2015.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper considers a broadcast channel with a multi-antenna transmitter (Alice) sending two independent confidential data streams to two legitimate users (Bob and Charlie) in the presence of a passive eavesdropper (Eve). To enhance their secrecy rates, Bob and Charlie are assumed to be capable of self-interference suppression (SIS). Alice, on the other hand, uses MIMO precoding to generate the two confidential information signals along with its own (Tx-based) friendly jamming. The interfering signals at Bob and Charlie are removed by employing the zero-forcing technique. This, however, leaves ``vulnerability regions'' around Bob and Charlie, which can be exploited by a nearby eavesdropper. We address this problem by augmenting Tx-based friendly jamming with Rx-based friendly jamming, generated by Bob and Charlie. For the resulting broadcast channel, a secrecy encoding scheme is developed to construct the signals intended to Bob and Charlie. The corresponding achievable secrecy sum-rate is characterized, and an optimization problem is formulated. A special case of this problem is investigated. Simulation results show the effectiveness of utilizing (Tx- and/or Rx-based) jamming, and the impact of the degree of SIS on physical-layer security.

@INPROCEEDINGS{Akgun:Receiver-based15,
author={Akgun, B. and Koyluoglu, O. O. and Krunz, M.},
title={Receiver-based friendly jamming with single-antenna full-duplex receivers in a multiuser broadcast channel},
booktitle={2015 IEEE Global Communications Conference (Globecom 2015)},
address={San Diego, CA},
month={Dec.},
year={2015}
}
Y. Chen, O. O. Koyluoglu, and A. Sezgin, "On the individual secrecy rate region for the broadcast channel with an external eavesdropper," in Proc. 2015 IEEE International Symposium on Information Theory (ISIT 2015), Hong Kong, China, Jun. 2015.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper studies the problem of secure communication over broadcast channels under the lens of individual secrecy constraints (i.e., information leakage from each message to an eavesdropper is made vanishing). It is known that, for the communication over the degraded broadcast channels, the stronger receiver is able to decode the message of the weaker receiver. In the individual secrecy setting, the message for the weaker receiver can be further utilized to secure the partial message that is intended to the stronger receiver. With such a coding spirit, it is shown that more secret bits can be conveyed to the stronger receiver. In particular, for the corresponding Gaussian model, a constant gap (i.e., 0.5 bits within the individual secrecy capacity region) result is obtained. Overall, when compared with the joint secrecy constraint, the results allow for trading-off secrecy level and throughput in the system.

@INPROCEEDINGS{Chen:individualsecrecy15,
author={Chen, Y. and Koyluoglu, O. O. and Sezgin, A.},
title={On the individual secrecy rate region for the broadcast channel with an external eavesdropper},
booktitle={2015 IEEE International Symposium on Information Theory (ISIT 2015)},
address={Hong Kong, China},
month={Jun.},
year={2015}
}
H. Si, O. O. Koyluoglu, and S. Vishwanath, "Achieving secrecy without any instantaneous CSI: Polar coding for fading wiretap channels," in Proc. 2015 IEEE International Symposium on Information Theory (ISIT 2015), Hong Kong, China, Jun. 2015.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper presents a polar coding scheme for fading wiretap channels that achieves reliability as well as security without the knowledge of instantaneous channel state information at the transmitter. Specifically, a block fading model is considered for the wiretap channel that consists of a transmitter, a receiver, and an eavesdropper; and only the information regarding the statistics (i.e., distribution) of the channel state information is assumed at the transmitter. For this model, a coding scheme that hierarchically utilizes polar codes is presented in order to address channel state variation. In particular, on polarization of different binary symmetric channels over different fading blocks, each channel use (corresponding to a possibly different polarization) is modeled as an appropriate binary erasure channel over fading blocks. Polar codes are constructed for both coding over channel uses for each fading block and coding over fading blocks for certain channel uses. In order to guarantee security, message bits are transmitted such that they can be reliably decoded at the receiver, and random bits are introduced to exhaust the observations of the eavesdropper. It is shown that this coding scheme, without instantaneous channel state information at the transmitter, is secrecy capacity achieving for the corresponding fading binary symmetric wiretap channel.

@INPROCEEDINGS{Si:Achieving15,
author={Si, H. and Koyluoglu, O. O. and Vishwanath, S.},
title={Achieving secrecy without any instantaneous CSI: Polar coding for fading wiretap channels},
booktitle={2015 IEEE International Symposium on Information Theory (ISIT 2015)},
address={Hong Kong, China},
month={Jun.},
year={2015}
}
Y. Chen, O. O. Koyluoglu, and A. Sezgin, "On the individual secrecy for Gaussian broadcast channels with receiver side information," in Proc. 2015 IEEE International Conference on Communications (ICC 2015), London, UK, Jun. 2015.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper studies the individual secrecy capacity region of the broadcast channel with receiver side information. First, an achievable rate region is established for the discrete memoryless case by employing superposition coding. Further, it is extended to the corresponding Gaussian case, where the individual secrecy capacity region is characterized in case of a weak or strong eavesdropper (compared to two legitimate receivers). For the case left, inner and outer bounds are established and the individual secrecy capacity region is characterized for the low and high SNR regimes. Note that the last case is distinctive due to the individual secrecy constraint, in the sense that positive rate pair is still possible although the eavesdropper may have the advantage against at least one of the legitimate receivers over the channel, unlike the situation if the joint secrecy constraint is imposed.

@INPROCEEDINGS{Chen:individual15,
author={Chen, Y. and Koyluoglu, O. O. and Sezgin, A.},
title={On the individual secrecy for Gaussian broadcast channels with receiver side information},
booktitle={Proc. 2015 IEEE International Conference on Communications (ICC 2015)},
address={London, UK,},
month={Jun.},
year={2015}
}
I. Aykin, O. O. Koyluoglu, and J.-M. Fellous, "Formation of dorso-ventral grid cell modules: The role of learning," in Proc. 2015 Computational and Systems Neuroscience Abstracts (Cosyne 2015), Salt Lake City, UT, Mar. 2015.
[Cosyne 2015] [abstract] [BibTeX]
Hippocampal place cells are active at specific locations, and grid cells in the medial entorhinal cortex show periodic grid-like firing as a function of location. Both types of cells are organized from dorsal to ventral levels in increasing spatial field sizes. The size gradient seems smooth for place cells, but modular for grid cells. Together, grid and place cells form a topographical map for navigational tasks. This map can be characterized by a weight matrix, where the entries correspond to synaptic connections between grid and place cells. Using a Hebbian plasticity rule with decay, these connections can be strengthened or weakened in accordance with the fields visited during behavior. Thus, a rat learning a path through specific locations will form a weight matrix that could act as a signature for the learned path.
Starting with a homogeneous, smooth distribution of firing field gradients of place and grid cells, we study the structure of the weight matrix when following actual paths recorded from rodents. Two conditions were considered: random foraging and learning of paths between specific rewarded locations. The resulting weight matrices show significant differences: a) Weight matrix corresponding to foraging shows a smooth connectivity pattern throughout the synaptic population; b) The weight matrix of the path with reward locations shows a clear pattern consisting of sub-modules organized along the dorso-ventral axis. This result suggests that grid cells synaptically group themselves in modules on a learned path. c) This modularity does not appear in place-place or grid-place connections. d) These results are compatible with, and may partially explain, the electrophysiological results obtained experimentally.
Overall, our plasticity-based framework is a novel computational model that suggests a mechanism for the formation of dorso-ventral modules in grid cells.

@INPROCEEDINGS{Aykin:Formation15,
author={Aykin, I. and Koyluoglu, O. O. and Fellous, J.-M.},
title={Formation of dorso-ventral grid cell modules: The role of learning},
booktitle={2015 Computational and Systems Neuroscience Abstracts (Cosyne 2015)},
address={Salt Lake City, UT},
month={Mar.},
year={2015}
}
G. Calis and O. O. Koyluoglu, "Repairable block failure resilient codes," in Proc. 2014 IEEE International Symposium on Information Theory (ISIT 2014), Honolulu, HI, Jun. 2014.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
In large scale distributed storage systems (DSS) deployed in cloud computing, correlated failures resulting in simultaneous failure (or, unavailability) of blocks of nodes are common. In such scenarios, the stored data or a content of a failed node can only be reconstructed from the available live nodes belonging to available blocks. To analyze the resilience of the system against such block failures, this work introduces the framework of Block Failure Resilient (BFR) codes, wherein the data (e.g., file in DSS) can be decoded by reading out from a same number of codeword symbols (nodes) from each available blocks of the underlying codeword. Further, repairable BFR codes are introduced, wherein any codeword symbol in a failed block can be repaired by contacting to remaining blocks in the system. Motivated from regenerating codes, file size bounds for repairable BFR codes are derived, trade-off between per node storage and repair bandwidth is analyzed, and BFR-MSR and BFR-MBR points are derived. Explicit codes achieving these two operating points for a wide set of parameters are constructed by utilizing combinatorial designs, wherein the codewords of the underlying outer codes are distributed to BFR codeword symbols according to projective planes.

@INPROCEEDINGS{Calis:Repairable14,
author={Calis, G. and Koyluoglu, O. O.},
title={Repairable block failure resilient codes},
booktitle={Proc. 2014 IEEE International Symposium on Information Theory (ISIT 2014)},
address={Honolulu, HI,},
month={Jun.},
year={2014}
}
Y. Chen, O. O. Koyluoglu, and A. Sezgin, "On achievable individual-secrecy rate region for broadcast channels with receiver side information," in Proc. 2014 IEEE International Symposium on Information Theory (ISIT 2014), Honolulu, HI, Jun. 2014.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
In this paper, we study the problem of secure communication over the broadcast channel with receiver side information, under the lens of individual secrecy constraints (i.e., information leakage from each message to an eavesdropper is made vanishing). Several coding schemes are proposed by extending known results in broadcast channels to this secrecy setting. In particular, individual secrecy provided via one-time pad signal is utilized in the coding schemes. As a result, we obtain an achievable rate region together with a characterization of the capacity region for special cases of either a weak or strong eavesdropper (compared to both legitimate receivers). Interestingly, the capacity region for the former corresponds to a line and the latter corresponds to a square with missing corners; a phenomenon occurring due to the coupling between user's rates. At the expense of having a weaker notion of security, positive secure transmission rates are always guaranteed, unlike the case of the joint secrecy constraint.

@INPROCEEDINGS{Chen:achievable14,
author={Chen, Y. and Koyluoglu, O. O. and Sezgin, A.},
title={On achievable individual-secrecy rate region for broadcast channels with receiver side information},
booktitle={Proc. 2014 IEEE International Symposium on Information Theory (ISIT 2014)},
address={Honolulu, HI,},
month={Jun.},
year={2014}
}
H. Si, O. O. Koyluoglu, and S. Vishwanath, "Lossy compression of exponential and laplacian sources using expansion coding," in Proc. 2014 IEEE International Symposium on Information Theory (ISIT 2014), Honolulu, HI, Jun. 2014.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
A general method of source coding is proposed in this paper, which enables one to reduce the problem of compressing an analog (continuous-valued) source to a set of much simpler problems, compressing discrete sources. Specifically, the focus is on lossy compression of exponential and Laplace sources, which are represented as set of discrete variables through a finite alphabet expansion. Due to the decomposability property of such sources, the resulting random variables post expansion are independent and discrete. Thus, these variables can be considered as independent discrete source coding problems, and the original problem is reduced to coding over these sources with a total distortion constraint. Any feasible solution to this resulting optimization problem corresponds to an achievable rate distortion pair of the original continuous-valued source compression problem. Although finding the optimal solution for a given distortion is not a tractable task, we show that, via a heuristic choice, our expansion coding scheme still presents a good performance in the low distortion regime. Further, by adopting low-complexity codes designed for discrete source coding, the total coding complexity can be reduced for practical implementations.

@INPROCEEDINGS{Si:Lossy14,
author={Si, H. and Koyluoglu, O. O. and Vishwanath, S.},
title={Lossy compression of exponential and laplacian sources using expansion coding},
booktitle={Proc. 2014 IEEE International Symposium on Information Theory (ISIT 2014)},
address={Honolulu, HI,},
month={Jun.},
year={2014}
}
A. S. Rawat, N. Silberstein, O. O. Koyluoglu, and S. Vishwanath, "Secure distributed storage systems: Local repair with minimum bandwidth regeneration," in Proc. 6th International Symposium on Communications, Control, and Signal Processing (ISCCSP'14), Athens, Greece, Apr. 2014.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper addresses the issue of securing information stored on a distributed storage system from a passive eavesdropping attack. The security notion is perfect secrecy, i.e., the system is said to be secure only if the mutual information between the stored information and the observations at the adversary is zero. The paper summarizes state of the art on securing repair-efficient distributed storage systems. Then, storage systems that employ locally repairable codes with minimum bandwidth regenerating codes as local codes (MBR-LRCs) are investigated. A secure file size upper bound and a construction of secure MBR-LRCs are provided. These two are shown to match under special cases, establishing the secrecy capacity of these systems.

@INPROCEEDINGS{Rawat:Secure14,
author={Rawat, A. S. and Silberstein, N. and Koyluoglu, O. O. and Vishwanath, S.},
title={Secure distributed storage systems: Local repair with minimum bandwidth regeneration},
booktitle={Proc. 6th International Symposium on Communications, Control, and Signal Processing (ISCCSP'14)},
address={Athens, Greece},
month={Apr.},
year={2014}
}
O. O. Koyluoglu, Y. Chen, and A. Sezgin, "Broadcast channel with receiver side information: Achieving individual secrecy," in Proc. 2014 International Zurich Seminar on Communications (IZS 2014), Zurich, Switzerland, Feb. 2014.
[IZS] [pdf preprint] [abstract] [BibTeX]
In this paper, we study the problem of secure communication over the broadcast channel with receiver side information, under the lens of individual secrecy constraints (i.e., information leakage from each message to an eavesdropper is made vanishing). Several coding schemes are proposed by extending known results in broadcast channels to this secrecy setting. In particular, individual secrecy provided via one-time pad signal is utilized in the coding schemes. As a preliminary result, we obtain a general achievable region together with a characterization of the capacity region for the case of a degraded eavesdropper.

@INPROCEEDINGS{Koyluoglu:Broadcast14,
author={Koyluoglu, O. O. and Chen, Y. and Sezgin, A.},
title={Broadcast channel with receiver side information: Achieving individual secrecy},
booktitle={Proc. 2014 International Zurich Seminar on Communications (IZS 2014)},
address={Zurich, Switzerland},
month={Feb.},
year={2014}
}
H. Si, O. O. Koyluoglu, and S. Vishwanath, "Polar coding for fading channels," in Proc. 2013 IEEE Information Theory Workshop (ITW 2013), Seville, Spain, Sep. 2013.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
A polar coding scheme for fading channels is proposed in this paper. More specifically, the focus is on the Gaussian fading channel with a BPSK modulation, where the equivalent channel is modeled as a binary symmetric channel with varying cross-over probabilities. To deal with variable channel states, a coding scheme of hierarchically utilizing polar codes is proposed. In particular, by observing the polarization of different binary symmetric channels over different fading blocks, each channel use corresponding to a different polarization is modeled as a binary erasure channel such that polar codes could be adopted to encode over blocks. It is shown that the proposed coding scheme, without instantaneous channel state information at the transmitter, achieves the capacity of the corresponding fading binary symmetric channel.

@INPROCEEDINGS{Si:Polar13,
author={Si, H. and Koyluoglu, O. O. and Vishwanath, S.},
title={Polar coding for fading channels},
booktitle={Proc. 2013 IEEE Information Theory Workshop (ITW 2013)},
address={Seville, Spain},
month={Sep.},
year={2013}
}

G. Kamath, N. Silberstein, N. Prakash, A. S. Rawat, V. Lalitha, O. O. Koyluoglu, P. V. Kumar, and S. Vishwanath, "Explicit MBR all-symbol locality codes," in Proc. 2013 IEEE International Symposium on Information Theory (ISIT 2013), Istanbul, Turkey, Jul. 2013.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
Node failures are inevitable in distributed storage systems (DSS). To enable efficient repair when faced with such failures, two main techniques are known: Regenerating codes, i.e., codes that minimize the total repair bandwidth; and codes with locality, which minimize the number of nodes participating in the repair process. This paper focuses on regenerating codes with locality, using pre-coding based on Gabidulin codes, and presents constructions that utilize minimum bandwidth regenerating (MBR) local codes. The constructions achieve maximum resilience (i.e., optimal minimum distance) and have maximum capacity (i.e., maximum rate). Finally, the same pre-coding mechanism can be combined with a subclass of fractional-repetition codes to enable maximum resilience and repair-by-transfer simultaneously.

@INPROCEEDINGS{Kamath:Explicit13,
author = {Kamath, G. and Silberstein, N. and Prakash, N. and Rawat, A. S. and Lalitha, V. and Koyluoglu, O. O. and Kumar, P. V. and Vishwanath, S.},
title = {Explicit MBR all-symbol locality codes},
booktitle={2013 IEEE International Symposium on Information Theory (ISIT 2013)},
address={Istanbul, Turkey},
month={Jul.},
year={2013}
}
A. S. Rawat, O. O. Koyluoglu, N. Silberstein, and S. Vishwanath, "Secure locally repairable codes for distributed storage systems," in Proc. 2013 IEEE International Symposium on Information Theory (ISIT 2013), Istanbul, Turkey, Jul. 2013.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper presents coding schemes for distributed storage systems (DSS) that are secure against eavesdroppers, while simultaneously enabling efficient node repair (regeneration). Towards this, novel upper bounds on secrecy capacity for minimum storage regenerating (MSR) codes and locally repairable codes (LRC) are derived. The eavesdropper model considered in this paper incorporates the ability to listen in on data downloaded during $\ell_2$ node repairs in addition to content stored on $\ell_1$ nodes. Finally, this paper presents coding schemes, based on precoding using Gabidulin codes, that achieve the upper bounds on secrecy capacity and characterize the secrecy capacity of DSS for various settings of system parameters.

@INPROCEEDINGS{Rawat:Secure13,
author = {Rawat, A. S. and Koyluoglu, O. O. and Silberstein, N. and Vishwanath, S.},
title = {Secure locally repairable codes for distributed storage systems},
booktitle={2013 IEEE International Symposium on Information Theory (ISIT 2013)},
address={Istanbul, Turkey},
month={Jul.},
year={2013}
}
O. O. Koyluoglu, A. S. Rawat, and S. Vishwanath, "The secrecy capacity of minimum bandwidth cooperative regenerating codes," in Proc. 2013 IEEE International Symposium on Information Theory (ISIT 2013), Istanbul, Turkey, Jul. 2013.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
Regenerating codes enable trading off repair bandwidth for storage in distributed storage systems (DSS). Due to their distributed nature, these systems are intrinsically susceptible to attacks, and they may be susceptible to multiple node failures. This paper analyzes storage systems that employ cooperative regenerating codes that are robust to passive eavesdroppers, and proposes codes achieving the secrecy capacity for the minimum bandwidth cooperative regenerating point. The achievability results correspond to exact repair, and secure file size upper bounds are obtained using mincut analyses over a suitable secrecy graph representation of DSS. The main achievability argument is based on appropriate precoding of the data using MRD (Gabidulin) codes to eliminate any information leakage to the eavesdropper.

@INPROCEEDINGS{Koyluoglu:Secrecy13,
author = {Koyluoglu, O. O. and Rawat, A. S. and Vishwanath, S.},
title = {The secrecy capacity of minimum bandwidth cooperative regenerating codes},
booktitle={2013 IEEE International Symposium on Information Theory (ISIT 2013)},
address={Istanbul, Turkey},
month={Jul.},
year={2013}
}
N. Silberstein, A. S. Rawat, O. O. Koyluoglu, and S. Vishwanath, "Optimal locally repairable codes via rank-metric codes," in Proc. 2013 IEEE International Symposium on Information Theory (ISIT 2013), Istanbul, Turkey, Jul. 2013.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper presents a new explicit construction for locally repairable codes (LRCs) for distributed storage systems which possess all-symbol locality and the largest possible minimum distance, or equivalently, can tolerate the maximum number of node failures. This construction, based on maximum rank distance (MRD) Gabidulin codes, provides new optimal vector and scalar LRCs. In addition, the paper also discusses mechanisms by which codes obtained using this construction can be used to construct LRCs with efficient local repair of failed nodes by combination of LRCs with regenerating codes.

@INPROCEEDINGS{Silberstein:Optimal13,
author = {Silberstein, N. and Rawat, A. S. and Koyluoglu, O. O. and Vishwanath, S.},
title = {Optimal locally repairable codes via rank-metric codes},
booktitle={2013 IEEE International Symposium on Information Theory (ISIT 2013)},
address={Istanbul, Turkey},
month={Jul.},
year={2013}
}
O. O. Koyluoglu and I. R. Fiete, "Information-theoretic limits on encoding over diverse populations," in 2013 Computational and Systems Neuroscience Abstracts (Cosyne 2013), Salt Lake City, UT, Feb. 2013.
[Cosyne 2013] [abstract] [BibTeX]
Population coding refers to a setting where a given stimulus is represented by the activities of a population of neurons. For instance, in orientation-tuned V1 neurons, each neuron fires near its preferred stimulus, with an activity profile given by the tuning curve. When combined with an estimator, these activities constitute a fully identified coding system in which the efficiency of the system is quantified by a measure of distortion (error) between the estimated stimulus and its actual value.
Here, we use an information-theoretic approach to bound distortion (in a mean-square sense) for populations of neurons: a stimulus $s$ is first sent to an encoder, that computes a vector-valued function of the stimulus, and each entry of the vector is then represented in a separate population code. We assume the total number of neurons is fixed at $Q$, and that the Fisher information in each neural population scales linearly with the number of neurons in that population (as seen for unimodal tuning curves with Poisson spike variability, among various examples). We consider two scenarios: The encoder simply passes out $s$, to one population of $Q$ total neurons; or, it passes out elements of the $N$-d vector $\vec{x}(s)$, to $N$ populations of $M=Q/N$ neurons each. For these scenarios, we use joint source-channel coding theory to bound how the information-theoretically minimal distortion will scale with $M,N$. We show that breaking the neurons into $N$ populations can, with appropriate encoding, result in distortions that scale as $M^{-N}$, whereas directly representing the stimulus in a single population, by necessity, produces distortions that scale as $1/(MN)$.
Our results show that diverse population encoding can result in potentially much lower distortion, and quantify how distortion scales with number of populations.

@INPROCEEDINGS{Koyluoglu:Information-theoretic13,
author={Koyluoglu, O. O. and Fiete, I. R.},
title={Information-theoretic limits on encoding over diverse populations},
booktitle={2013 Computational and Systems Neuroscience Abstracts (Cosyne 2013)},
address={Salt Lake City, UT},
month={Feb.},
year={2013}
}
A. S. Rawat, N. Silberstein, O. O. Koyluoglu, and S. Vishwanath, "Optimal locally repairable codes with local minimum storage regeneration via rank-metric codes," in Proc. 2013 Information Theory and Applications Workshop (ITA 2013), San Diego, CA, Feb. 2013.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper presents a new explicit construction for locally repairable codes (LRCs) for distributed storage systems. The codes possess all-symbols locality and maximal possible minimum distance, or equivalently, can tolerate the maximal number of node failures. This construction, based on maximum rank distance (MRD) Gabidulin codes, provides minimum distance optimal vector and scalar LRCs for a wide range of parameters. In addition, vector LRCs that allow for efficient local repair of failed nodes are considered. Towards this, the paper derives an upper bound on the amount of data that can be stored on DSS employing minimum distance optimal LRCs with given repair bandwidth, and presents codes which attain this bound by combining MRD and minimum storage regenerating (MSR) codes.

@INPROCEEDINGS{Rawat:Optimal13,
author={Rawat, A. S. and Silberstein, N. and Koyluoglu, O. O. and Vishwanath, S.},
title={Optimal locally repairable codes with local minimum storage regeneration via rank-metric codes},
booktitle={Proc. 2013 Information Theory and Applications Workshop (ITA 2013)},
address={San Diego, CA},
month={Feb.},
year={2013}
}
Y. Yoo, O. O. Koyluoglu, S. Vishwanath, and I. R. Fiete, "Dynamic shift-map coding with side information at the decoder," in Proc. 50th Annual Allerton Conference on Communication, Control, and Computing (Allerton 2012), Monticello, IL, Oct. 2012.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
Shift-map codes have been studied as joint source- channel codes for continuous sources. These codes are useful in delay-limited scenarios and also provide better tolerance to deviations of the signal-to-noise ratio (SNR) from a target SNR, compared to separate source and channel coding. This paper defines a generalized family of shift-map codes that share a strong connection with redundant residue number systems (RRNS), and are henceforth called RRNS-map codes. In the proposed coding scheme, side information about the source allows the decoder to consider only a fraction of the codebook for decoding, with no change in the encoding process. With an appropriately designed RRNS-map code, in this fraction of the codebook, the codewords are much better separated than the original codebook. As a result, RRNS-map codes achieve the same distortion in the mean square error sense as conventional shift-map codes without side information, but significantly outperform shift-map codes when side information is provided to the decoder. This coding scheme is ideally suited to applications where a simple and fixed encoding scheme is desired at the encoder, while the decoder is given access to side information about the source.

@INPROCEEDINGS{Yoo:Dynamic12,
author={Yoo, Y. and Koyluoglu, O. O. and Vishwanath, S. and Fiete, I. R.},
title={Dynamic shift-map coding with side information at the decoder},
booktitle={Proc. 50th Annual Allerton Conference on Communication, Control, and Computing (Allerton 2012)},
address={Monticello, IL},
month={Oct.},
year={2012}
}
O. O. Koyluoglu, K. Appaiah, H. Si, and S. Vishwanath, "Expansion coding: Achieving the capacity of an AEN channel," in Proc. 2012 IEEE International Symposium on Information Theory (ISIT 2012), Boston, MA, Jul. 2012.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
A general method of coding over expansions is proposed, which allows one to reduce the highly non-trivial problem of coding over continuous channels to a much simpler discrete ones. More specifically, the focus is on the additive exponential noise (AEN) channel, for which the (binary) expansion of the (exponential) noise random variable is considered. It is shown that each of the random variables in the expansion corresponds to independent Bernoulli random variables. Thus, each of the expansion levels (of the underlying channel) corresponds to a binary symmetric channel (BSC), and the coding problem is reduced to coding over these parallel channels while satisfying the channel input constraint. This optimization formulation is stated as the achievable rate result, for which a specific choice of input distribution is shown to achieve a rate which is arbitrarily close to the channel capacity in the high SNR regime. Remarkably, the scheme allows for low-complexity capacity-achieving codes for AEN channels, using the codes that are originally designed for BSCs. Extensions to different channel models and applications to other coding problems are discussed.

@INPROCEEDINGS{Koyluoglu:Expansion12,
author={Koyluoglu, O. O. and Appaiah, K. and Si, H. and Vishwanath, S.},
title={Expansion coding: Achieving the capacity of an AEN channel},
booktitle={2012 IEEE International Symposium on Information Theory (ISIT 2012)},
address={Boston, MA},
month={Jul.},
year={2012}
}
M. Fadel, A. Hindy, A. El-Keyi, M. Nafie, O. O. Koyluoglu, and A. M. Tulino, "Resource allocation for throughput enhancement in cellular shared relay networks," in Proc. 35th IEEE Sarnoff Symposium (Sarnoff 2012), Newark, NJ, May 2012.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
The downlink frame of a cellular relay network is considered, where a shared MIMO decode-and-froward relaying is used to serve the users at the edge of the cell. The relay employs zero-forcing beamforming to manage the interference among the mobile stations (MSs) at the edge of the cell. A non-cooperative scheme is considered where there is no coordination between the base stations (BSs) and the relay station (RS), and a power control algorithm for the RS is developed that maximizes the rate of the relayed users. A cooperative setting which allows the coordination of a power allocation between BSs and RSs is also considered. For this setting, based on the proposed achievable scheme, an optimization formulation is derived to maximize the total throughput of the MSs subject to a constraint on the total power of the system. The problem is solved iteratively as a sequence of geometric programs. Simulation results are provided showing that a significant increase in the network throughput can be achieved via the proposed schemes compared to a conventional cellular system with no relays.

@INPROCEEDINGS{Fadel:Resource12,
author={Fadel, M. and Hindy, A. and El-Keyi, A. and Nafie, M. and Koyluoglu, O. O. and Tulino, A. M.},
title={Resource allocation for throughput enhancement in cellular shared relay networks},
booktitle={35th IEEE Sarnoff Symposium (Sarnoff 2012)},
address={Newark, NJ},
month={May},
year={2012}
}
O. O. Koyluoglu and I. R. Fiete, "Information theoretic limits on performance in short-term memory tasks," in 2012 Computational and Systems Neuroscience Abstracts (Cosyne 2012), Salt Lake City, UT, Feb. 2012. (travel grant award)
[Cosyne 2012] [abstract] [BibTeX]
Short-term memory is limited. Early notions, that only a fixed number of items can be stored in memory, have been amended by recent experiments showing that memory systems can flexibly allocate memory resources across fewer or more items with a corresponding trade-off in recall accuracy. However, it remains unclear what factors limit working memory, and how it is best modeled to include parameters like item number, storage interval duration, and item complexity.
Here, we build a normative, or information-theoretically ideal, model for the short-term memory of a set of analog variables. We treat all inputs presented to the memory system on an equal footing, as information. Noisy neural dynamics, that degrade memory over the storage interval, are interpreted as passing the information through a noisy channel. We first show why this mapping is appropriate. Next, we derive fundamental limitations on short-term memory performance using information theoretic results on optimal communication over noisy channels. We show how the resulting performance -- which depends on optimal encoding and decoding of the stored information for the given channel noise -- scales with item number, storage time, and the dynamic range of the inputs. We apply these results to a visual working memory task, and compare predictions for memory performance with psychophysical findings. The functional dependence of recall on the parameters listed above is well-predicted by the normative model, suggesting that the brain performs very good information encoding before the storage period, and decoding after. As we show, these results stand in clear contrast to the predictions made if the brain were directly storing the uncoded variables in continuous attractor networks matched to the dimension and coding range of the variables.

@INPROCEEDINGS{Koyluoglu:Information12,
author={Koyluoglu, O. O. and Fiete, I. R.},
title={Information theoretic limits on performance in short-term memory tasks},
booktitle={2012 Computational and Systems Neuroscience Abstracts (Cosyne 2012)},
address={Salt Lake City, UT},
month={Feb.},
year={2012}
}
S. Vishwanath, O. O. Koyluoglu, H. Si, and K. Appaiah, "Coding over binary expansions," in Proc. 2012 Information Theory and Applications Workshop (ITA 2012), San Diego, CA, Feb. 2012.
[ITA 2012] [abstract] [BibTeX]
A general method of coding over expansions is proposed, which allows one to reduce the highly non-trivial problem of coding over continuous channels to much simpler discrete ones. More specifically, the focus is on the additive exponential noise (AEN) channel, for which the (binary) expansion of the (exponential) noise random variable is considered. It is shown that each of the random variables in the expansion corresponds to independent Bernoulli random variables (each having a mean given by a function of the corresponding level number and the mean of the underlying exponential random variable). This way, each of the expansion level (of the underlying channel) corresponds to a binary symmetric channel, and the coding problem is reduced to coding over these parallel channels.

@INPROCEEDINGS{Vishwanath:Coding12,
author={Vishwanath, S. and Koyluoglu, O. O. and Si, H. and Appaiah, K.},
title={Coding over binary expansions},
booktitle={Proc. 2012 Information Theory and Applications Workshop (ITA 2012)},
address={San Diego, CA},
month={Feb.},
year={2012}
}
O. O. Koyluoglu, R. Soundararajan, and S. Vishwanath, "State amplification under masking constraints," in Proc. 49th Annual Allerton Conference on Communication, Control, and Computing (Allerton 2011), Monticello, IL, Sep. 2011.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper considers a state dependent channel with one transmitter, Alice, and two receivers, Bob and Eve. The problem at hand is to effectively “amplify” the channel state sequence at Bob while “masking” it from Eve. The extent to which the state sequence cannot be masked from Eve is referred to as leakage, and the paper is aimed at characterizing the tradeoff-region between amplification and leakage rates for such a system. An achievable coding scheme is presented, wherein the transmitter enumerates the state sequence using two indices, and transmits one of the indices over the channel to facilitate the amplification process. For the case when Bob observes a “stronger” channel than Eve, the achievable coding scheme is enhanced with secure refinement. The optimal amplification-leakage rate difference, called as differential amplification capacity, is characterized for the degraded binary and the degraded Gaussian channels. For the degraded Gaussian model, extremal corner points of the tradeoff region are characterized. In addition, the gap between the outer bound and achievable rate-regions is determined, where it is shown that the gap is less than half a bit for a wide set of channel parameters.

@INPROCEEDINGS{Koyluoglu:State11a,
author={Koyluoglu, O. O. and Soundararajan, R. and Vishwanath, S.},
title={State amplification under masking constraints},
booktitle={Proc. 49th Annual Allerton Conference on Communication, Control, and Computing (Allerton 2011)},
address={Monticello, IL},
month={Sep.},
year={2011}
}
K. Appaiah, O. O. Koyluoglu, and S. Vishwanath, "Polar alignment for interference networks," in Proc. 49th Annual Allerton Conference on Communication, Control, and Computing (Allerton 2011), Monticello, IL, Sep. 2011.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
Polar coding has originally been introduced as a capacity achieving low complexity code for binary input symmetric channels. Polar codes can be understood as transformations that replace a probabilistic channel with parallel deterministic counterparts. This paper builds on this interpretation of polar codes, using it to perform alignment over the resulting deterministic channels to obtain gains for interference networks. It is important to note here that polar codes are not chosen with encoding and decoding complexity in mind, which is just a fortuitous side-benefit, but with the aim of transforming the original channels into a class of deterministic parallel channels over which interference-alignment is well-understood. A degraded one-sided interference network is chosen as the illustrative example. Polar alignment is shown to increase the achievable sum rate over known random coding schemes. The paper concludes with a brief discussion of possible extensions.

@INPROCEEDINGS{Appaiah:Polar11,
author={Appaiah, K. and Koyluoglu, O. O. and Vishwanath, S.},
title={Polar alignment for interference networks},
booktitle={Proc. 49th Annual Allerton Conference on Communication, Control, and Computing (Allerton 2011)},
address={Monticello, IL},
month={Sep.},
year={2011}
}
M. Shahmohammadi, O. O. Koyluoglu, T. Khattab, and H. El Gamal, "On the degrees of freedom of the cognitive broadcast channel," in Proc. 2011 IEEE International Symposium on Information Theory (ISIT 2011), Saint Petersburg, Russia, Jul. 2011.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
Cognitive broadcast channel, where two multiantenna transmitters communicate with their respective receivers, is considered. One of the transmitters is said to be cognitive (secondary) as it is assumed to know the messages of the other (primary) transmitter non-causally. The goal is to design cooperative schemes between the two transmitters, which impose only minimal changes to the primary broadcast channel (compared to the non-cognitive scenario). Towards this end, an achievable scheme is provided under which both intra cell and inter cell interferences at the primary receivers are aligned. The interference at the secondary receivers, on the other hand, is canceled by dirty paper coding. The corresponding achievable region and an outer bound region are provided in terms of the degrees of freedom (DoF) metric. Special cases shows the optimality of the proposed scheme in the high SNR regime for those cases. We also illustrate the advantage of the cognitive cooperation over the non-cognitive system by proving that the achieved sum DoF is strictly larger than the non-cognitive case.

@INPROCEEDINGS{6033915,
author={Shahmohammadi, M. and Koyluoglu, O. O. and Khattab, T. and {El Gamal}, H.},
title={On the degrees of freedom of the cognitive broadcast channel},
booktitle={Proc. 2011 IEEE International Symposium on Information Theory (ISIT 2011)},
address={Saint Petersburg, Russia},
month={Jul.},
year={2011}
}
O. Gungor, O. O. Koyluoglu, H. El Gamal, and C. E. Koksal, "Proactive source coding," in Proc. 2011 IEEE International Symposium on Information Theory (ISIT 2011), Saint Petersburg, Russia, Jul. 2011.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
A coding problem, over a slotted system, is introduced where the sender has to transmit one out of several packets to the receiver, but learns the request only at the beginning of each slot with prior statistical information about which packet is needed at the receiver. There is an associated cost of sending bits at each slot, and the goal is to minimize the expected cost of the communication. A proactive coding scheme is proposed, where the source proactively communicates with the receiver before the receiver requests the message. This way, by designing a cost optimal side information at the receiver, the scheme is able to minimize the expected cost of the communication. Numerical results are provided demonstrating the gains obtained by proactive coding over the conventional coding technique.

@INPROCEEDINGS{6033953,
author={Gungor, O. and Koyluoglu, O. O. and {El Gamal}, H. and Koksal, C. E.},
title={Proactive source coding},
booktitle={Proc. 2011 IEEE International Symposium on Information Theory (ISIT 2011)},
address={Saint Petersburg, Russia},
month={Jul.},
year={2011}
}
M. Shahmohammadi, O. O. Koyluoglu, T. Khattab, and H. El Gamal, "Joint interference cancellation and dirty paper coding for cognitive cellular networks," in Proc. 2011 IEEE Wireless Communications and Networking Conference (WCNC 2011), Cancun, Mexico, Mar. 2011.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
Downlink communication in a cellular network with a cognitive (secondary) cell is considered. In our model, the base station of the cognitive cell knows the messages of the other cell non-causally. We propose a new interference cancellation technique that zero forces the intra-cell interference in the primary cell by the help of the cognitive base station. In addition, as the primary messages are known at the cognitive base station, the interference caused by the primary base station on the secondary users are canceled using dirty paper coding (DPC). Moreover, we provide an outer bound on the achievable degrees of freedom (DoF) region and show that for some special cases the proposed signaling scheme is sum DoF optimal for the considered system when the cognitive cell operates at its maximum sum DoF. The benefit of the cognitive paradigm is also established using the derived outer-bound and showing that the achieved sum DoF is strictly larger than the case when cognitive message sharing is unavailable.

@INPROCEEDINGS{5779461,
author={Shahmohammadi, M. and Koyluoglu, O. O. and Khattab, T. and {El Gamal}, H.},
title={Joint interference cancellation and dirty paper coding for cognitive cellular networks},
booktitle={Proc. 2011 IEEE Wireless Communications and Networking Conference (WCNC 2011)},
address={Cancun, Mexico},
month={Mar.},
year={2011}
}
O. O. Koyluoglu and H. El Gamal, "Polar coding for secure transmission and key agreement," in Proc. 2010 IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC 2010), Istanbul, Turkey, Sep. 2010.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
Wyner's work on wiretap channels and the recent works on information theoretic security are based on random codes. Achieving information theoretical security with practical coding schemes is of definite interest. In this note, the attempt is to overcome this elusive task by employing the polar coding technique of Arikan. It is shown that polar codes achieve nontrivial perfect secrecy rates for binary-input degraded wiretap channels while enjoying their low encoding-decoding complexity. In the special case of symmetric main and eavesdropper channels, this coding technique achieves the secrecy capacity. Next, fading erasure wiretap channels are considered and a secret key agreement scheme is proposed, which requires only the statistical knowledge of the eavesdropper channel state information (CSI). The enabling factor is the creation of advantage over Eve, by blindly using the proposed scheme over each fading block, which is then exploited with privacy amplification techniques to generate secret keys.

@INPROCEEDINGS{5779461,
author={Koyluoglu, O. O. and {El Gamal}, H.},
title={Polar coding for secure transmission and key agreement},
booktitle={Proc. 2010 IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC 2010)},
address={Istanbul, Turkey},
month={Sep.},
year={2010}
}
E. Toher, O. O. Koyluoglu, and H. El Gamal, "Secrecy games over the cognitive channel," in Proc. 2010 IEEE International Symposium on Information Theory (ISIT 2010), Austin, TX, Jun. 2010.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
A secure communication game is considered for the cognitive channel with a confidential primary message, where the primary user is interested in maximizing its secure rate with lowest possible power consumption and the utility of the cognitive user is a weighted sum of the primary secrecy rate and the cognitive rate (corresponds to a spectrum law in favor of the legacy owners of the spectrum). An achievable rate region is derived for the channel with message splitting at the cognitive radio and noise forwarding. The game considers the case with no common message, but shows that even this limited scenario can still be beneficial. The established Nash Equilibrium (NE) shows that the cognitive user trades noise for bits. The results are also interesting in the sense that both users can benefit (by playing the distributed game) compared to their throughput resulting from the non-cooperative scenario.

@INPROCEEDINGS{5513708,
author={Toher, E. and Koyluoglu, O. O. and {El Gamal}, H.},
title={Secrecy games over the cognitive channel},
booktitle={Proc. 2010 IEEE International Symposium on Information Theory (ISIT 2010)},
address={Austin, TX},
month={Jun.},
year={2010}
}
O. O. Koyluoglu, C. E. Koksal, and H. El Gamal, "On the effect of colluding eavesdroppers on secrecy capacity scaling," in Proc. European Wireless 2010 (EW 2010), Lucca, Italy, Apr. 2010.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
In a powerful secrecy attack, eavesdroppers can collude, i.e., they can share their observations. Securing information in such a scenario will be an even more challenging task compared to non-colluding case. We here analyze the effect of eavesdropper collusion on the achievable performance in both the path loss and ergodic multi-path fading models. We provide two results: 1) For the Poisson point process model in a random extended network, if the legitimate nodes have unit intensity ($\lambda=1$) and the colluding eavesdroppers have an intensity of $\lambda_e = O((log n)^{-(2+p)})$ for any $p>0$, almost all of the nodes can achieve a secure rate of $\Omega(1/(\sqrt{n}))$ and 2) In the K-user Gaussian interference channel with E external colluding eavesdroppers, a secure degrees of freedom of $\eta=[1/2-E/K]^+$ per frequency-time slot is achievable for each user in the ergodic setting (in the absence of the eavesdropper channel state information).

@INPROCEEDINGS{5483463,
author={Koyluoglu, O. O. and Koksal, C. E. and {El Gamal}, H.},
title={On the effect of colluding eavesdroppers on secrecy capacity scaling},
booktitle={Proc. European Wireless 2010 (EW 2010)},
address={Lucca, Italy},
month={Apr.},
year={2010}
}
O. O. Koyluoglu, C. E. Koksal, and H. El Gamal, "On secrecy capacity scaling in wireless networks," in Proc. 2010 Information Theory and Applications Workshop (ITA 2010), La Jolla, CA, Feb. 2010.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
We study a random extended network, where the legitimate and eavesdropper nodes are assumed to be placed according to Poisson point processes in a square region of area n. It is shown that, when the legitimate nodes have unit intensity, $\lambda=1$, and the eavesdroppers have an intensity of $\lambda_e=O((log n)^{-2})$, almost all of the nodes achieve a perfectly secure rate of $\Omega{1/\sqrt{n}}$. The achievability argument is based on a novel multi-hop forwarding scheme where randomization is added in every hop to ensure maximal ambiguity at the eavesdropper(s). Remarkable, under these assumptions, securing the transmissions of nodes does not entail a loss in the per-node throughput in terms of scaling.

@INPROCEEDINGS{5454119,
author={Koyluoglu, O. O. and Koksal, C. E. and {El Gamal}, H.},
title={On secrecy capacity scaling in wireless networks},
booktitle={Proc. 2010 Information Theory and Applications Workshop (ITA 2010)},
address={La Jolla, CA},
month={Feb.},
year={2010}
}
A. El Gamal, O. O. Koyluoglu, M. Youssef, and H. El Gamal, "New achievable secrecy rate regions for the two way wiretap channel," in Proc. 2010 IEEE Information Theory Workshop (ITW 2010), Cairo, Egypt, Jan. 2010.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This work develops new achievable rate regions for the two way wiretap channel. In our setup, Alice and Bob wish to exchange messages securely in the presence of a passive eavesdropper Eve. In the full-duplex scenario, our achievability argument relies on allowing the two users to jointly optimize their channel prefixing distributions, such that the new channel conditions are favorable compared to that of Eve. Random binning and private key sharing over the channel are then used to exploit the secrecy advantage available in the equivalent cascade channel and to distribute the available secrecy rate among users. For the half-duplex case, we introduce the idea of randomized scheduling and establish the significant gain it offers in terms of the achievable secrecy sum-rate. We also quantify the gains that can be leveraged from the proposed schemes in the modulo-2 and Gaussian channels via numerical results in certain selected scenarios.

@INPROCEEDINGS{5503136,
author={El Gamal, A. and Koyluoglu, O. O. and Youssef, M. and {El Gamal}, H.},
title={New achievable secrecy rate regions for the two way wiretap channel},
booktitle={Proc. 2010 IEEE Information Theory Workshop (ITW 2010)},
address={Cairo, Egypt},
month={Jan.},
year={2010}
}
K. Khalil, M. Youssef, O. O. Koyluoglu, and H. El Gamal, "On the delay limited secrecy capacity of fading channels," in Proc. 2009 IEEE International Symposium on Information Theory (ISIT 2009), Seoul, Korea, Jun. 2009.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
In this paper, the delay limited secrecy capacity of the flat fading channel is investigated under two different assumptions on the available transmitter channel state information (CSI). The first scenario assumes perfect prior knowledge of both the main and eavesdropper channel gains. Here, upper and lower bounds on the secure delay limited capacity are derived and shown to be tight in the high signal-to-noise ratio (SNR) regime (for a wide class of channel distributions). In the second scenario, only the main channel CSI is assumed to be available at the transmitter. Remarkably, under this assumption, we establish the achievability of non-zero secure rate (for a wide class of channel distributions) under a strict delay constraint. In the two cases, our achievability arguments are based on a novel two-stage approach that overcomes the secrecy outage phenomenon observed in earlier works.

@INPROCEEDINGS{5205955,
author={Khalil, K. and Youssef, M. and Koyluoglu, O. O. and {El Gamal}, H.},
title={On the delay limited secrecy capacity of fading channels},
booktitle={Proc. 2009 IEEE International Symposium on Information Theory (ISIT 2009)},
address={Seoul, Korea},
month={Jun.},
year={2009}
}
O. O. Koyluoglu, M. Shahmohammadi, and H. El Gamal, "A new achievable rate region for the discrete memoryless X channel," in Proc. 2009 IEEE International Symposium on Information Theory (ISIT 2009), Seoul, Korea, Jun. 2009.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
We consider the discrete memoryless X channel, a communication model with two transmitters and two receivers in which every transmitter has a message for every receiver. We propose an achievable scheme, based on the message splitting and binning techniques, which results in the best inner bound on the capacity region of the X channel to date.

@INPROCEEDINGS{5206034,
author={Koyluoglu, O. O. and Shahmohammadi, M. and {El Gamal}, H.},
title={A new achievable rate region for the discrete memoryless X channel},
booktitle={Proc. 2009 IEEE International Symposium on Information Theory (ISIT 2009)},
address={Seoul, Korea},
month={Jun.},
year={2009}
}
O. O. Koyluoglu and H. El Gamal, "On the secrecy rate region for the interference channel," in Proc. 2008 IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC 2008), Cannes, France, Sep. 2008.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper studies interference channels with security constraints. The existence of an external eavesdropper in a two-user interference channel is assumed, where the network users would like to secure their messages from the external eavesdropper. The cooperative binning and channel prefixing scheme is proposed for this system model which allows users to cooperatively add randomness to the channel in order to degrade the observations of the external eavesdropper. This scheme allows users to add randomness to the channel in two ways: 1) Users cooperate in their design of the binning codebooks, and 2) Users cooperatively exploit the channel prefixing technique. As an example, the channel prefixing technique is exploited in the Gaussian case to transmit a superposition signal consisting of binning codewords and independently generated noise samples. Gains obtained form the cooperative binning and channel prefixing scheme compared to the single user scenario reveals the positive effect of interference in increasing the network security. Remarkably, interference can be exploited to cooperatively add randomness into the network in order to enhance the security.

@INPROCEEDINGS{4699954,
author={Koyluoglu, O. O. and {El Gamal}, H.},
title={On the secrecy rate region for the interference channel},
booktitle={Proc. 2008 IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC 2008)},
address={Cannes, France},
month={Sep.},
year={2008}
}
O. O. Koyluoglu, H. El Gamal, L. Lai, and H. V. Poor, "On the secure degrees of freedom in the K-user Gaussian interference channel," in Proc. 2008 IEEE International Symposium on Information Theory (ISIT 2008), Toronto, ON, Canada, Jul. 2008.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
This paper studies the K-user Gaussian interference channel with secrecy constraints. Two distinct network models, namely the interference channel with confidential messages and the one with an external eavesdropper, are analyzed. Using interference alignment along with secrecy pre-coding at each transmitter, it is shown that each user in the network can achieve non-zero secure degrees of freedoms (DoFs) in both scenarios. In particular, the proposed coding scheme achieves K-2/2K-2 secure DoFs for each user in the interference channel with confidential messages model, and K-2/2K secure DoFs in the case of an external eavesdropper. The fundamental difference between the two scenarios stems from the lack of channel state information (CSI) about the external eavesdropper. Remarkably, the results establish the positive impact of interference on the secrecy capacity of wireless networks.

@INPROCEEDINGS{4595013,
author={Koyluoglu, O. O. and {El Gamal}, H. and Lifeng Lai and Poor, H. V.},
title={On the secure degrees of freedom in the K-user Gaussian interference channel},
booktitle={Proc. 2008 IEEE International Symposium on Information Theory (ISIT 2008)},
address={Toronto, ON, Canada},
month={Jul.},
year={2008}
}
O. O. Koyluoglu and H. El Gamal, "On the utility of frequency reuse in cognitive radio channels," in Proc. 2007 IEEE International Symposium on Information Theory (ISIT 2007), Nice, France, Jun. 2007.
[IEEE Xplore] [pdf preprint] [abstract] [BibTeX]
We consider the generalized cognitive radio channel where the secondary user is allowed to reuse the frequency during the active periods of the primary user, as long as the primary rate remains the same. In this setting, the optimal power allocation policy with a single antenna secondary transmitter (and receiver) is explored. Interestingly, we show that the offered gain resulting from the frequency reuse during the active periods of the spectrum disappears in both the low and high signal-to-noise ratio (SNR) regimes. This drawback, however, is shown to disappear with multi-antenna nodes by using simple zero-forcing strategies at both ends of the secondary channel.

@INPROCEEDINGS{4557540,
author={Koyluoglu, O. O. and {El Gamal}, H.},
title={On the utility of frequency reuse in cognitive radio channels},
booktitle={Proc. 2007 IEEE International Symposium on Information Theory (ISIT 2007)},
address={Nice, France},
month={Jun.},
year={2007}
}