Andres Altieri

On Fundamental Trade-offs of Device-to-Device Communications in Large Wireless Networks

This paper studies the gains, in terms of served requests, attainable through out-of-band device-to-device (D2D) video exchanges in large cellular networks. A stochastic framework, in which users are clustered to exchange videos, is introduced, considering several aspects of this problem, i.e., the video caching policy, user matching for exchanges, and aspects regarding scheduling and transmissions. A family of admissible protocols is introduced: in each protocol the users are clustered by means of a hard-core point process and, within the clusters, video exchanges take place. Two metrics, quantifying the “local” and “global” fractions of video requests served through D2D are defined, and relevant trade-off regions involving these metrics, as well as quality-of-service constraints, are identified. A simple communication strategy is proposed and analyzed to obtain inner bounds to the trade-off regions and to draw conclusions on the performance attainable through D2D. To this end, the analysis of the time-varying interference that the nodes experience and the tight approximations of its Laplace transform are derived.

Cooperative strategies for interference-limited wireless networks

Consider the communication of a single user aided by a nearby relay involved in a large and dense wireless network where the nodes form an homogeneous Poisson point process. We assume that the network is working in the interference-limited regime. In this case the asymptotic error probability is bounded from above by the outage probability experienced by the user. We investigate the outage behavior for the well-known cooperative schemes, namely, decode-and-forward (DF) and compress-and-forward (CF). In this setting, the outage events are induced by both fading and the spatial proximity of neighbor nodes who generate the strongest interference and hence the worst communication case. Upper and lower bounds on the asymptotic error probability which are tight in some cases are derived. It is shown that there exists a clear trade-off between the network density and the benefits of user cooperation. These results are useful to evaluate performance and to optimize relaying schemes in the context of large wireless networks.

Analysis of a cooperative strategy for a large decentralized wireless network

This paper investigates the benefits of cooperation and proposes a relay activation strategy for a large wireless network with multiple transmitters. In this framework, some nodes cooperate with a nearby node that acts as a relay, using the decode-and-forward protocol, and others use direct transmission. The network is modeled as an independently marked Poisson point process, and the source nodes may choose their relays from the set of inactive nodes. Although cooperation can potentially lead to significant improvements in the performance of a communication pair, relaying causes additional interference in the network, increasing the average noise that other nodes see. We investigate how source nodes should balance cooperation versus interference to obtain reliable transmissions, and for this purpose, we study and optimize a relay activation strategy with respect to the outage probability. Surprisingly, in the high reliability regime, the optimized strategy consists on the activation of all the relays or none at all, depending on network parameters. We provide a simple closed-form expression that indicates when the relays should be active, and we introduce closed-form expressions that quantify the performance gains of this scheme with respect to a network that only uses direct transmission.

Cooperation versus interference in large wireless relay networks

This paper investigates the potential gain of cooperation in large wireless networks with multiple sources and relays, where the nodes form an homogeneous Poisson point process. The source nodes may choose their nearest neighbor from the set of inactive nodes as their relay. Although cooperation can potentially lead to significant improvements on the asymptotic error probability of a communication pair, relaying causes additional interference in the network, increasing the average noise. We address the basic question: how should source nodes optimally balance cooperation vs. interference to guarantee reliability in all communication pairs. Based on the decode-and-forward (DF) scheme at the relays, we derive closed-form approximations to the upper bounds on the error probability, averaging over all node positions. Surprisingly, in the small outage probability regime, there is an almost binary behavior that dictates - depending on network parameters - the activation or not of all relay nodes.

On the Outage Probability of the Full-Duplex Interference-Limited Relay Channel

In this paper, we study the performance, in terms of the asymptotic error probability, of a user that communicates with a destination with the aid of a full-duplex in-band relay. We consider that the network is interference-limited, and interfering users are distributed as a Poisson point process. In this case, the asymptotic error probability is upper bounded by the outage probability (OP). We investigate the outage behavior for well-known cooperative schemes, namely, decode-and-forward (DF) and compress-and-forward (CF) considering fading and path loss. For DF, we determine the exact OP and develop upper bounds that are tight in typical operating conditions. Also, we find the correlation coefficient between source and relay signals that minimizes the OP when the density of interferers is small. For CF, the achievable rates are determined by the spatial correlation of the interferences, and a straightforward analysis is not possible. To handle this issue, we show that the rate with correlated noises is at most one bit worse than with uncorrelated noises and thus find an upper bound on the performance of CF. These results are useful to evaluate the performance and to optimize relaying schemes in the context of full-duplex wireless networks.

A stochastic geometry approach to distributed caching in large wireless networks

This paper introduces a stochastic geometry model of a cellular network in which users exchange videos through out-of-band device-to-device (D2D) communications. Users are grouped into clusters, in which a user-based distributed cache is formed to satisfy in-cluster video requests through D2D, avoiding the base station. This paper studies how many of these requests could be served, considering two service metrics: the global relative density of served requests, and the mean ratio of served requests per cluster. The model considers random user placement, content request statistics, and transmission scheduling and impairments, like fading and path loss. A simple communications strategy to be used in the clusters, involving time-division multiple access and one-hop transmissions, is introduced and the metrics are evaluated in this setup, drawing conclusions on the performance of distributed-caching in large wireless networks.

Distributed detection of correlated random processes under energy and bandwidth constraints

We analyze a binary hypothesis testing problem built on a wireless sensor network (WSN). Using Large Deviation Theory (LDT), we compute the probability error exponents of a distributed scheme for detecting a correlated circularly-symmetric complex Gaussian process under the Neyman-Pearson framework. Using an analog scheme, the sensors transmit scaled versions of their measurements several times through a multiple access channel (MAC) to reach the fusion center (FC), whose task is to decide whether the process is present or not. In the analysis, we consider the energy constraint on each node transmission. We show that the proposed distributed scheme requires relatively few MAC channel uses to achieve the centralized error exponents when detecting correlated Gaussian processes.

Cooperative unicasting in large wireless networks

This paper investigates the potential gains of half-duplex unicast strategies in a large wireless network consisting of clusters in which a source node attempts to transmit a message with the aid of other inactive-potential relays- nodes within the cluster. The network is modeled as an independently marked Poisson point process and a slow-fading scenario is considered. A two-phase transmission protocol is studied and an achievable (upper) bound on the asymptotic error probability of any cluster in the network is derived in terms of the outage probability (OP). The proposed scheme takes advantage of the spatial distribution of the cooperating nodes to create a distributed virtual antenna array, thus improving the OP over direct transmission (DT) through cooperative diversity. Finally, we obtain a converse (lower) bound on the OP of any unicast strategy when restricting all admissible protocols to belong to the same-still quite general- class of half-duplex codes that includes the one studied.

On the balance between cooperation and interference in dense wireless networks

This paper explores the balance between cooperation through relay nodes and aggregated interference generation in large decentralized wireless networks using decode-and-forward. The source nodes in the network are modeled using a marked Poisson process. We consider the case in which only a single randomly located relay is added to one source in the network and study the outage probability gains obtained. Then, using a simple model, we study the case in which all sources can potentially use their nearest neighbor from the set of inactive nodes as relays, leading to a mixed transmission scheme in which some users employ decode-and-forward and others employ direct transmission. The optimal relay activation probability for the second case is found, observing that in the small outage probability regime it exhibits a binary behavior, being zero or one. Comparing both scenarios we conclude that activating more relays rapidly reduces the gains observed when only one source can use a relay. We derive closed-form approximations to the upper bounds on the error probability, averaging over all node positions and fading gains realizations, to support our claims.