Carlos H. M. de Lima

Interference management for self-organized femtocells towards green networks

The femtocell concept is an emerging technology for deploying the next generation of the wireless networks, aiming at indoor coverage enhancement, increasing capacity and offloading the overlay macrocell traffic. One of the main challenges in short range femtocell networks is how to (re)configure the Home Node Bs (HNBs) in an autonomous manner so as to manage interference and diminish the energy consumption among nearby femtocells efficiently. In this paper, we investigate the indoor femtocell deployment making use of both the Frequency Division Duplexing (FDD) and the Time Division Duplexing (TDD) methods. In the FDD case, the HNBs share both the Uplink (UL) and Downlink (DL) channels with the macrocell without any cooperation to coordinate their access to the air interface. Conversely, in the TDD underlay case, femtocells only reuse the macrocell UL spectrum and cooperate with each other in order to minimize the interference among themselves, either with or without further coordination with the Macro User Equipment (MUE). The proposed solution is evaluated by means of system-level simulations using the Monte Carlo approach. Investigations have shown that the TDD underlay approach not only reduces the perceived interference levels, but also diminishes the outage probability.

Clusterization for Robust Geographic Routing in Wireless Sensor Networks

A cross-layer algorithm for geographic routing in wireless sensor networks (WSNs) is proposed, which is robust to dead-ends and resilient to topological variations due to network dynamics. The solution combines ideas of network tessellation (clusterization) with greedy forwarding, without suffering from the problems afflicting landmark-based alternatives. The clusterization algorithm is based on a discovered graph-spectral property and relies on connectivity information only. Cluster sizes can be varied, allowing for different trade-offs between packet delivery success ratio (PDSR) and average packet delivery latency (APDL) to be reached. Simulation results show that the technique can substantially improve the PDSR in networks where large concave holes (dead-ends) are present, with no or little impact on APDL.

A stochastic association mechanism for macro-to-femtocell handover

In this paper, we consider a macrocell site underlaid with self-organizing stand-alone femtocells and propose a mechanism for macro-to-femtocell handover, inspired by multi-stage Dutch auctions. The proposed solution introduces a new distributed handover procedure, which does not rely on any centralized coordination typically performed by the macro base stations in existent cellular systems. The applicability of this solution is assessed through system-level simulations using the Monte Carlo approach. When considering the duration of association procedures, results have shown that the distributed mechanism scales well with increasing density of femtocell sites. Additionally, macrocell users benefit from better link quality and less uncoordinated interference generated by the underlay femtocell tier. To illustrate, after performing handover, the co-channel interference experienced by a tagged macrocell user becomes less than one tenth of that in full interference scenario.

Analysis of Contention-based relay selection mechanisms in autonomous multi-hop networks

A fundamental question of interest in multi-hop networks is whether routing should be done in a smaller number of longer hops, or larger number of shorter hops. However, underneath these aspects of the routing problem itself is the number of contending nodes involved in the relay selection taking place at each hop so as to realize whichever routing strategy adopted. A larger number of nodes increases the likelihood of finding an adequate relay, whereas increasing the selection overhead, and vice-versa. In this article we investigate both the expected cost (in time consumed) incurred by the relay selection strategy, and the effectiveness of the forwarding strategy (in spatial advancements) as a function of the number of the contending nodes, when using Contention-based Geographic Forwarding (CGF) strategies to relay packets in multi-hop scenarios. In particular, Probability Generating Functions (PGFs) are utilized to compute the distribution of Contention Resolution Intervals (CRIs) and quantify overhead, while stochastic geometry is utilized to model topological aspects of the network and quantify expected progress. Both a totally random and an auction-based Relay Selection Algorithms (RSAs) are studied and compared, with the advantages of the auction-based approach established analytically. From an overhead point of view, the selection of the furthest and the nearest relays are equivalent, but our results indicate that the long-hop routing approach is less sensitive to the number of contending nodes than the short-hop alternative.

On the Secrecy of Interference-Limited Networks under Composite Fading Channels

This letter deals with the secrecy capacity of the radio channel in interference-limited regime. We assume that interferers are uniformly scattered over the network area according to a Point Poisson Process and the channel model consists of path-loss, log-normal shadowing and Nakagami-m fading. Both the probability of non-zero secrecy capacity and the secrecy outage probability are then derived in closed-form expressions using tools of stochastic geometry and higher-order statistics. Our numerical results show how the secrecy metrics are affected by the disposition of the desired receiver, the eavesdropper and the legitimate transmitter.

Coordination Mechanisms for Stand-Alone Femtocells in Self-Organizing Deployments

We investigate coordination mechanisms for controlling the co-channel interference generated by stand-alone femtocells in two-tier coexistence scenarios. Stochastic geometry is used to model network deployment scenarios, while the cumulants concept is utilized to characterize the probability distribution of the aggregate interference at a tagged user. The rationale for using coordination mechanisms is to opportunistically reuse resources without compromising ongoing transmissions on overlay macrocells, while still guaranteeing Quality of Service in both tiers. Results have shown that the analytical framework matches fairly well with numerical results obtained with Monte Carlo simulations. Yet coordination mechanisms improve performance of overlay macrocell network by substantially diminishing co-channel interference.

Contention-based Geographic Forwarding strategies for position-centric routing

This contribution addresses network routing algorithms exploiting geographic forwarding strategies to enhance the performance of distributed Random Multiple-Access (RMA) schemes in autonomous multi-hop wireless scenarios. Indeed, the Relay Selection (RS) mechanism among neighbors required to define an adequate relay at hop-basis is determinant to the overall system performance. Herein, the expected advancement towards the final destination achieved by the candidate relays following a specific Contention-based Geographic Forwarding (CGF) strategy - distinct forwarding decision regions - is quantified. Stochastic geometry and non-parametric order statistics concepts are exploited so as to characterize the expected progress at each hop when selecting a suitable relay. Two geographic forwarding areas, namely, sectoral and circular lens forwarding regions are used to derive priority functions so as to map nodes positions to the replying sequence of candidate relays. Results have shown that the expected distance of the n-th nearest eligible relay is larger when the convex lens decision region is employed. Additionally, the long-hop routing strategy is less sensitive then the short-hop routing alternative to increasing number of contending relay nodes.

Throughput of wireless networks with Poisson distributed nodes using location information

This paper provides a statistical characterization of the individual achievable rates and the spatial throughput of bipolar Poisson wireless networks. We derive closed-form expressions for the probability density function of the achievable rates under two decoding rules: treating interference as noise, and jointly detecting the strongest interfering signals treating the others as noise. Based on these rules and the bipolar model, we approximate the expected maximum spatial throughput, showing the best performance of the latter decoding rule. We show that, when the same decoding rule and density are considered, the cognitive spatial throughput always outperforms the other option.

Coordinated TDD-Underlay for Self-organizing Femtocells in Two-Tier Coexistence Scenarios

In this contribution, we investigate the concept of time division duplexing (TDD) mode as an alternative to underlay short-range femtocells on the uplink of legacy macrocell deployments. To mitigate the resulting co-channel interference, the underlaid femtocell tier uses a distributed mechanism which is based on regular busy tones and relies on minimal signaling exchange. Stochastic geometry is used to model practical scenarios by capturing network dynamics and channel variations. The impact of the fading correlation on the performance of the coordination mechanism is examined as well. Higher-order statistics through the cumulants concept are used to recover the distribution of the co-channel interference and evaluate the system performance in terms of the outage probability and average channel capacity. We observe that our analytical framework matches well with numerical results obtained using Monte Carlo simulations. In contrast to the uncoordinated frequency division duplexing mode, the coordinated TDD-underlay solution shows a reduction in the outage probability of nearly 80%, while the average spectral efficiency increases by approximately 90% in high loads.