Cecilia G. Galarza

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.

Capacity-approaching block-based transceivers with reduced redundancy

We present a transceiver structure for a frequency selective channel that allows the introduction of reduced redundancy. At the same time we optimize the transmitter and receiver in this structure to maximize the information rate. We show that we can decouple the problem of optimizing the transmitter and the receiver in two different problems by using to the data processing inequality of information theory. The problem of finding the transmitters is a difficult non-linear optimization problem. For that reason we present two simple algorithmic procedures in order to obtain suboptimal solutions. We present simulation results that show that the proposed design outperforms other existing ones in the literature. We also show that the proposed scheme is asymptotically optimal in the sense that it approaches the channel capacity.

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.

On the block error probability of finite-length codes in decentralized wireless networks

The block error probability of large decentralized wireless networks for fixed code length and rate is investigated. The network model consists of a large number of nodes, distributed according to a homogeneous Poisson point process, emitting their messages independently from the others. The transmitted signals are attenuated due to both path loss and fading. First, we show that in the asymptotic regime, when the code length is long enough, and users communicate with Gaussian codebooks, the error probability behaves as the well known outage probability. Here outage events are induced by fading and interference. Second, in the non-asymptotic regime, we bound the block error probability as a function of the code length and the rate. Applications of these results arise in the context of decentralized wireless networks, e.g. mobile ad hoc networks (MANETs).

A Novel Modular Positive-Sequence Synchrophasor Estimation Algorithm for PMUs

A novel modular positive-sequence estimation algorithm for phasor measurement units (PMUs) is described in this paper with a focus on the restrictions imposed by the IEEE C37.118.1–2011 standard. The first stage consists in a three-phase demodulator which allows us to separate the positive-sequence from the negative-sequence signal in the frequency domain and to eliminate the zero-sequence signal. The second stage is a prefilter that mitigates noise and interference, thus relaxing the filtering requirements of the following stage. The suitability of a linear-phase FIR filter is shown and a comparison of single and multistage designs is presented. On the third stage, a digital state-space-based extension of a synchronous reference frame-phase-locked loop is used for tracking of amplitude, phase, frequency, and rate of change of frequency. It is shown that a phase predictor inside the loop is required. The fourth stage is a compensation algorithm which takes into account the narrowband nature of the input signal to perform an accurate compensation of the filter effects on the signal of interest. Analytical properties of the system are then presented, providing insight into the main factors that affect global performance. Finally, a strict evaluation of the system is presented for both M and P class PMU.

Optimal Resource Allocation for Detection of a Gaussian Process Using a MAC in WSNs

We analyze a binary hypothesis testing problem built on a wireless sensor network (WSN) for detecting a stationary random process distributed both in space and time with a circularly-symmetric complex Gaussian distribution under the Neyman-Pearson (NP) framework. Using an analog scheme, the sensors transmit different linear combinations of their measurements through a multiple access channel (MAC) to reach the fusion center (FC), whose task is to decide whether the process is present or not. Considering an energy constraint on each node transmission and a limited amount of channel uses, we compute the miss error exponent of the proposed scheme using Large Deviation Theory (LDT) and show that the proposed strategy is asymptotically optimal (when the number of sensors approaches infinity) among linear orthogonal schemes. We also show that the proposed scheme obtains meaningful energy saving in the low signal-to-noise ratio regime, which is the typical scenario of WSNs. Finally, a Monte Carlo simulation of a 2-dimensional process in space validates the analytical results.