This paper considers a cache-aided device-to-device (D2D) system where the users are equipped with cache memories of different size. During low traffic hours, a server places content in the users' cache memories, knowing that the files requested by the users during peak traffic hours will have to be delivered by D2D transmissions only. The worst-case D2D delivery load is minimized by jointly designing the uncoded cache placement and linear coded D2D delivery. Next, a novel lower bound on the D2D delivery load with uncoded placement is proposed and used in explicitly characterizing the minimum D2D delivery load (MD2DDL) with uncoded placement for several cases of interest. In particular, having characterized the MD2DDL for equal cache sizes, it is shown that the same delivery load can be achieved in the network with users of unequal cache sizes, provided that the smallest cache size is greater than a certain threshold. The MD2DDL is also characterized in the small cache size regime, the large cache size regime, and the three-user case. Comparisons of the server-based delivery load with the D2D delivery load are provided. Finally, connections and mathematical parallels between cache-aided D2D systems and coded distributed computing (CDC) systems are discussed.

This paper studies device to device (D2D) coded-caching with information theoretic security guarantees. A broadcast network consisting of a server, which has a library of files, and end users equipped with cache memories, is considered. Information theoretic security guarantees for confidentiality are imposed upon the files. The server populates the end user caches, after which D2D communications enable the delivery of the requested files. Accordingly, we require that a user must not have access to files it did not request, i.e., secure caching. First, a centralized coded caching scheme is provided by jointly optimizing the cache placement and delivery policies. Next, a decentralized coded caching scheme is developed that does not require the knowledge of the number of active users during the caching phase. Both schemes utilize non-perfect secret sharing and one-time pad keying, to guarantee secure caching. Furthermore, the proposed schemes provide secure delivery as a side benefit, i.e., any external entity which overhears the transmitted signals during the delivery phase cannot obtain any information about the database files. The proposed schemes provide the achievable upper bound on the minimum delivery sum rate. Lower bounds on the required transmission sum rate are also derived using cut-set arguments indicating the multiplicative gap between the lower and upper bounds. Numerical results indicate that the gap vanishes with increasing memory size. Overall, the work demonstrates the effectiveness of D2D communications in cache-aided systems even when confidentiality constraints are imposed at the participating nodes and against external eavesdroppers.

In this work, we consider a cache-aided network where the users share the end-caches. In particular, a user has access to only one of the caches and the number of caches is less than the number of users. We propose a coded placement scheme that exploits the asymmetry in the number of users associated with each cache. Some of the signals sent to the overloaded caches facilitate the decoding of the coded subfiles stored at the underloaded caches. We present an explicit caching scheme and fully characterize the coded placement gain for two-cache systems. Then, we generalize our scheme to larger networks, where the optimal parameters are characterized by solving a linear program. We observe that, with the proposed scheme, as the asymmetry in the users' connectivity increases, the gain from coded placement is more evident.

This work considers a network consisting of a server and a layer of relay nodes equipped with cache memories which aim to deliver content to end nodes that also have cache memories. The server and the end nodes consider the intermediate relay caches to be untrusted with the content. As a result, the server must design strategies to place content in relay caches not only to serve end users, but also to ensure that any a subset of them, even when colluding, cannot gain any information about the contents of the server database. The end users randomly connect to a subset of these untrusted caches at the beginning of the delivery phase via multicast links. For this network model, a coded caching scheme is developed by jointly optimizing the cache placement and delivery phases using secure regenerating codes. In addition, the scheme is extended to the setup of combination networks with untrusted relays, where the untrusted relays are connected to the end users via unicast links. The study highlights the benefits of cooperating with untrusted caches by designing the end users' caches to provide multicast opportunities in order to minimize the delivery load.

We consider information theoretic security in a two-hop combination network where there are groups of end users with distinct degrees of connectivity served by a layer of relays. The model represents a network set up with users having access to asymmetric resources, here the number of relays that they are connected to, yet demand security guarantees uniformly. We study two security constraints separately and simultaneously: secure delivery where the information must be kept confidential from an external entity that wiretaps the delivery phase; and secure caching where each cache-aided end-user can retrieve the file it requests and cannot obtain any information on files it does not. The achievable schemes we construct are multi-stage where each stage completes requests by a class of users.

In cache-aided networks, the server populates the cache memories at the users during low-traffic periods in order to reduce the delivery load during peak-traffic hours. In turn, there exists a fundamental tradeoff between the delivery load on the server and the cache sizes at the users. In this paper, we study this tradeoff in a multicast network, where the server is connected to users with unequal cache sizes and the number of users is less than or equal to the number of library files. We propose centralized uncoded placement and linear delivery schemes which are optimized by solving a linear program. Additionally, we derive a lower bound on the delivery memory tradeoff with uncoded placement that accounts for the heterogeneity in cache sizes. We explicitly characterize this tradeoff for the case of three end-users, as well as an arbitrary number of end-users when the total memory size at the users is small, and when it is large. Next, we consider a system where the server is connected to the users via rate-limited links of different capacities and the server assigns the users' cache sizes subject to a total cache budget. We characterize the optimal cache sizes that minimize the delivery completion time with uncoded placement and linear delivery. In particular, the optimal memory allocation balances between assigning larger cache sizes to users with low capacity links and uniform memory allocation.

This paper considers a device-to-device (D2D) coded caching system where the users have differing cache sizes. During low traffic hours, the server places subsets of the files at the users' cache memories, in a manner that enables serving the users' requests via D2D transmissions during peak traffic hours. The objective is to jointly design the users' cache contents and the D2D transmissions in order to minimize the D2D delivery load. In particular, we seek to identify the optimal uncoded placement and linear delivery schemes. We propose a novel lower bound on the D2D delivery load under uncoded placement, which enables us to explicitly characterize the minimum D2D delivery load under uncoded placement for several cases of interest.

We consider the wiretap channel when a (secure) cache memory is added to the legitimate receiver. With the goal of utilizing coded caching for improving secrecy, during the cache placement phase, the receiver caches a function of the files, and the secret key shared with the transmitter subject to the memory constraint. The signals transmitted to serve the receiver's request during the delivery phase are observed by an eavesdropper over a wiretap channel. We characterize the secrecy capacity of the wiretap channel with a cache, i.e., the maximum achievable file size while keeping the overall database secure, for both the discrete memoryless and the Gaussian channels. The optimal caching scheme maximizes the utilization from the transmission over the delivery phase by sharing sufficient amount of keys between the legitimate communication nodes during the placement phase. Interestingly, we demonstrate that the existence of cache memory is an enabler of secure communication, i.e., the secrecy capacity remains positive, even when the main channel is degraded with respect to the eavesdropper channel.

This paper considers a two-hop network architecture known as a combination network, where a layer of relay nodes connects a server to a set of end users. In particular, a new model is investigated, where the intermediate relays employ caches in addition to the end users. First, a new centralized coded caching scheme is developed that utilizes maximum distance separable coding, jointly optimizes cache placement and delivery phase, and enables decomposing the combination network into a set virtual multicast sub-network. It is shown that if the sum of the memory of an end user and its connected relay nodes is sufficient to store the database, then the server can disengage in the delivery phase and all the end users' requests can be satisfied by the caches in the network. Lower bounds on the normalized delivery load using genie-aided cut-set arguments are presented along with second hop optimality. Next, recognizing the information security concerns of coded caching, this new model is studied under three different secrecy settings: 1) secure delivery where we require an external entity that must not gain any information about the database files by observing the transmitted signals over the network links; 2) secure caching, where we impose the constraint that end users must not be able to obtain any information about files that they did not request; and 3) both secure delivery and secure caching, simultaneously. We demonstrate how network topology affects the system performance under these secrecy requirements. Finally, we provide numerical results demonstrating the system performance in each of the settings considered.

This paper considers centralized coded caching, where the server not only designs the users' cache contents, but also assigns their cache sizes under a total cache memory budget. The server is connected to each user via a link of given finite capacity. For given link capacities and total memory budget, we minimize the worst-case delivery completion time by jointly optimizing the cache sizes, the cache placement and delivery schemes. The optimal memory allocation and caching scheme are characterized explicitly for the case where the total memory budget is smaller than that of the server library. Numerical results confirm the savings in delivery time obtained by optimizing the memory allocation.

We study a two-hop cache-aided network, where a layer of relay nodes connects a server and a set of end users, i.e., a combination network. We consider the case where both the relay nodes and the end users have caching capabilities. We provide upper and lower bounds which are applicable to any combination network, noting that previous work had focused on models where the relays do not have caches as well as schemes that were suitable for a special class of combination networks. Utilizing maximum distance separable (MDS) codes, we jointly optimize the placement and the delivery phases, demonstrating the impact of cache memories in alleviating the delivery load over the two hop communications. Moreover, we show how cooperation between the relay nodes and the end users can effectively replace the server during the delivery phase whenever the total memory at each end user and its connected relay nodes is sufficient to store the database.

Coded caching can improve fundamental limits of communication, utilizing storage memory at individual users. This paper considers a centralized coded caching system, introducing heterogeneous cache sizes at the users, i.e., the users' cache memories are of different size. The goal is to design cache placement and delivery policies that minimize the worst-case delivery load on the server. To that end, the paper proposes an optimization framework for cache placement and delivery schemes which explicitly accounts for the heterogeneity of the cache sizes. We also characterize explicitly the optimal caching scheme, for the case where the sum of the users' cache sizes is smaller than or equal to the library size.

We consider a two-source two-destination two-hop relay network, where all data communication must be kept secret from the relay node. The model considered is the simplest primitive that embodies a multi-transmitter multi-receiver network that needs to communicate sharing an untrusted relay node. We focus on two scenarios. In the first scenario, each source aims to send two messages to be kept secret from the relay: a common message that should be decoded by both destinations, and a private message that should be decoded by the first destination while kept secret from the second one. We define an achievable rate region by utilizing stochastic encoding at the sources, the Gaussian noise cooperative jamming from the destinations, and compress-and-forward at the relay. In the second scenario, each source aims to send a confidential message to its intended destination, which should be kept secret from the other one as well as the relay. We define an achievable rate region using a combination of nested lattice codes and random binning at the sources, structured cooperative jamming from destinations, and scaled compute-and-forward at the relay. We also derive genie-aided outer bounds on the secrecy rate regions. We present numerical results that demonstrate the performance of the proposed achievable schemes. Overall, this paper provides insights into how to utilize an untrusted relay to communicate to destinations with different levels of security clearance, and how intentional interference is an enabler of communication.

We consider a Gaussian network consisting of a source that aims to communicate to its legitimate destination via an untrusted relay node in the presence of an external eavesdropper. The source wishes to send two independent messages to the destination: one message must be kept secret from the external eavesdropper only, while the other message must be kept secret from the external eavesdropper and the untrusted relay both. We identify achievable secure rates under these layered secrecy constraints. Considering a two-hop half-duplex setup, we employ the destination as a cooperative jammer in the first phase in order to help provide secrecy from the relay and the external eavesdropper, and the source as a cooperative jammer in the second phase in order to detriment the external eavesdropper. The source encodes its messages using stochastic encoding and security embedding coding. We provide the secrecy analysis and present numerical results to demonstrate the performance of the proposed achievability technique. Our study points to the value of the source serving as a cooperative jammer as well as the need for power control policies at the legitimate nodes in order to ensure secrecy in this system.

We consider a distributed storage system where data storage nodes are equipped with energy harvesting transmitters. In particular, F files are stored over n storage nodes using regenerating codes. The main operations of the distributed storage system are serving the file requests of data collectors and repairing the content of storage nodes that fail or leave the system. Each operation has an associated energy expenditure. Under the intermittent energy arrival profile, we study the problem of maximizing the number of retrieved files given a deadline. Additionally, we consider the problem of minimizing the repair time of a failed node. Both optimization problems turn out to be equivalent to binary programs, for which we provide a tractable solution in two steps. First, we determine necessary and sufficient conditions on the harvested energy that ascertain the feasibility of retrieving (repairing) M files in T time slots. Using these conditions, we develop two algorithms that reduce the formulated optimization problems to a single feasibility problem. Then, we solve the feasibility problem using forward and backward algorithms. Additionally, we study the online setup where only causal knowledge of energy arrivals is available at the network nodes. We present numerical results on the short and long term performance of the system operations under the proposed algorithms.

Abstract In this paper, we derive the capacity of the deterministic relay networks with relay messages. We consider a network that consists of five nodes, four of which can only communicate via the fifth one. However, the fifth node is not merely a relay as it may exchange private messages with the other network nodes. First, we develop an upper bound on the capacity region based on the notion of a single‐sided genie. In the course of the achievability proof, we also derive the deterministic capacity of a four‐user relay network (without private messages at the relay). The capacity achieving schemes use a combination of two network coding techniques: the simple ordering scheme and detour scheme. In the simple ordering scheme, we order the transmitted bits at each user such that the bi‐directional messages will be received at the same channel level at the relay, while the basic idea behind the detour scheme is that some parts of the message follow an indirect paths to their respective destinations. This paper, therefore, serves to show that user cooperation and network coding can enhance throughput, even when the users are not directly connected to each other. Finally, we make a conjecture about the capacity region of the generalK‐node relay network with relay messages. Copyright © 2016 John Wiley & Sons, Ltd.

Online social communities often exhibit complex relationship structures, ranging from close friends to political rivals. As a result, persons are influenced by their friends and foes differently. Network applications can benefit from accompanying these structural differences in propagation schemes. In this paper, we study the optimal influence propagation policies for networks with positive and negative relationship types. We tackle the problem of minimizing the end-to-end propagation cost of influencing a target person in favor of an idea by utilizing the relationship types in the underlying social graph. The propagation cost is incurred by social and physical network dynamics such as frequency of interaction, the strength of friendship and foe ties, propagation delay or the impact factor of the propagating idea. We extend this problem by incorporating the impact of message deterioration and ignorance. We demonstrate our results in both a controlled environment and the Epinions dataset. Our results show that judicious propagation schemes lead to a significant reduction in the average cost and complexity of influence propagation compared to naïve myopic algorithms.

In this paper, we study the degrees of freedom (DoF) of the MIMO relay networks. We start with a general Y channel, where each user has Miantennas and aims to exchange messages with the other two users via a relay equipped with N antennas. Then, we extend our work to a general 4-user MIMO relay network. Unlike most previous work which focused on the total DoF of the network, our aim here is to characterize the achievable DoF region as well. We develop an outer bound on the DoF region based on the notion of one sided genie. Then, we define a new achievable region using the Signal Space Alignment (SSA) and the Detour Schemes. Our achievable scheme achieves the upper bound for certain conditions relating Mi's and N.

This paper considers a Gaussian multiple access channel aided by a relay. Specifically, the relay facilitates communication between multiple sources and a destination to which the sources have no direct link. In this set up, the relay node is considered to be untrusted, i.e., honest but curious, from whom the source messages need to be kept secret. We identify an achievable secrecy rate region utilizing cooperative jamming from the destination, and using compress-and-forward at the relay. Additionally, an outer bound on the secrecy rate region is derived. Numerical results indicate that the outer bound is tight in some cases of interest.

In this paper, we completely characterize the deterministic capacity region of a four-node relay network with no direct links between the nodes, where each node communicates with the three other nodes via a relay. Towards this end, we develop an upper bound on the deterministic capacity region, based on the notion of a one-sided genie. To establish achievability, we use the detour schemes that achieve the upper bound by routing specific bits via indirect paths instead of sending them directly.

We study the capacity region of a deterministic 4-node network, where 3 nodes can only communicate via the fourth one. However, the fourth node is not merely a relay since it can exchange private messages with all other nodes. This situation resembles the case where a base station relays messages between users and delivers messages between the backbone system and the users. We assume an asymmetric scenario where the channel between any two nodes is not reciprocal. First, an upper bound on the capacity region is obtained based on the notion of single sided genie. Subsequently, we construct an achievable scheme that achieves this upper bound using a superposition of broadcasting node 4 messages and an achievable “detour” scheme for a reduced 3-user relay network.