Xavier Gelabert

Small Cell densification requirements in high capacity future cellular networks

In order to cope with the expected increase in traffic demand, the large-scale deployment of Small Cells (SC) becomes a cost-efficient way to realize the foreseen capacity explosion in the near future. In this paper we investigate, for a particular forecast in the 2020, what are the requirements in terms of capacity per area unit and how SC densification along with other design criteria can fulfill such demands. A wide set of illustrative numerical results are provided showing key design criteria such as frequency reuse, deployment planning, macro-cell power-down and transmitted power reduction, which, in conjunction with SC densification, have strong impact on the capacity delivered by the network.

Supporting mobility in 5G: A comparison between massive MIMO and continuous ultra dense networks

This paper proposes an approach for providing 5G services to mobile users that is based on continuous ultra dense networks (C-UDNs). The proposed approach outperforms the widely accepted solution based on macro cells and massive MIMO systems (M-MIMO). In particular, we show that the performance of M-MIMO systems deployed on macro cells is significantly limited by channel aging. The proposed mobility solution based on uplink beacons overcomes the handover problem often linked to UDNs. Dense networks make it possible for users to transmit at a power lower than that of macro cells, thus making C-UDN more robust to pilot contamination and allowing for lower Channel State Information (CSI) latencies due to shorter reuse distance of UL pilots. Extensive numerical results from detailed systemlevel simulations are provided in order to compare and highlight the benefits of the proposed C-UDN mobility solution to that of a macro M-MIMO deployment.

Mobility performance and suitability of macro cell power-off in LTE dense small cell HetNets

It is widely accepted that future capacity demands will require the deployment of dense small cell networks most surely co-existing with legacy macro cell deployments forming a so-called heterogeneous network (HetNet). Whereas network capacity can benefit from these dense small cell deployments, mobility management becomes increasingly complex to handle, especially in high mobility outdoor urban environments. This paper addresses the performance evaluation of Long Term Evolution (LTE) handover mechanisms in dense small cell HetNets. We assess the potential challenges and we discuss the suitability of Macro cell power-off in “sufficiently dense” small cell deployments. Extensive numerical results are presented through detailed system level simulations.

A Heuristic Coordination Framework for Self-Optimizing Mechanisms in LTE HetNets

A self-organizing network (SON) is effectively realized by means of specific SON mechanisms (SONm), which relate to particular SON use cases (SONuc), these being defined by the Third-Generation Partnership Project (3GPP) and Next Generation Mobile Networks (NGMN). Typically, SONuc are grouped into self-configuration, self-optimization, and self-healing functions. Focusing on self-optimization, SONm therein aim at maintaining relevant key performance indicators (KPIs) above/below a specific target by actuating over an appropriate set of input parameters to achieve predefined network objectives. Typically, this problem is represented by several control loops where controllable input parameters are dynamically adjusted according to output measurable metrics and their corresponding target requirements. From an implementation viewpoint, the concept of a single SON “black box,” integrating multiple SONm, is very appealing. However, this approach may threaten the control that network operators (NOPs) have over their own network. As a result, a coordinated framework involving stand-alone SONm is proposed. Here, a SON controller (SONc) may be implemented responding to strategies, deciding at a given time which SONm actions have higher priority with respect to others. In the context of a heterogeneous network (HetNet) scenario, we propose and develop a simplified framework where stand-alone SONm react to either overshot or undershot KPI events by deciding to either increase or decrease corresponding influential parameter(s). By inspecting the arising interactions and possible conflicts between several SONm, we provide, for a specific evaluation scenario, a heuristic strategy-based solution for SON coordination, which may eventually trade off high-level NOP goals.

A Revenue-Maximizing Scheme for Radio Access Technology Selection in Heterogeneous Wireless Networks with User Profile Differentiation

In this paper, the problem of radio access technology (RAT) selection in heterogeneous wireless networks (HWNs) is tackled from an operator’s perspective, with the objective of maximizing the generated revenue. Two user profiles are considered with different priority levels. An integrated 3GPP Long Term Evolution (LTE) and Wireless Fidelity (WiFi) network is considered as an example of HWN, where LTE is used mainly for the high-priority class, while a portion of its resources, defined by a load threshold, can be shared by the low-priority class. A Markovian model is defined and validated by simulation. Subsequently, the value of the load threshold for resource sharing in LTE is investigated, and an optimization problem is formulated to find the optimal threshold for which the revenue is maximized.

Uplink reference signals enabling user-transparent mobility in ultra dense networks

This paper proposes a novel user-transparent mobility concept based on the transmission of uplink reference signals (beacons) using Zadoff-Chu signature sequences. Unlike legacy mobility schemes, the proposed scheme overcomes the need for the user to perform time and energy consuming downlink measurements and reporting them back to the network. This is particularly relevant for ultra dense network (UDN) deployments and high user densities as forecasted in the near future. The system requirements to support uplink beacons are presented and a detailed design for the signature sequence is provided to support a variety of system configurations and deployment cases. Numerical results illustrate the suitability and sufficiency of beacon resources as well as the reliability of the proposed beacon design in order to support a truly user-transparent and border-less mobility concept.

SECRET — Secure network coding for reduced energy next generation mobile small cells: A European Training Network in wireless communications and networking for 5G

SECRET is a collaborative European Training Network (ETN) committed to create an excellent educational training platform for Early Stage Researchers (ESRs) in the field of wireless communications and networking for 5G. The project is recently funded by the European Commission under the H2020 research and innovation program, through the Marie Curie People Program. This project targets to narrow the gap between current networking technologies and the foreseen requirements of future 2020 networking, through the recruitment and training of 17 ESRs. SECRET aims to strike a note by delivering higher capacity, ability to support more users, and lowering the cost per bit by adopting technology trends widely accepted to form part of the 5G roadmap, through the deployment of new disruptive “femtocell” type cells on demand, to what we refer to as mobile small cells. This will be complemented by a wireless high-speed fronthaul to bridge the small cell network to the core. Moreover, novel techniques will be investigated, including “network coding”, “cooperation”, and “energy-aware smart front-end”. Additionally, due to the confidential information that will be communicated over in future networks, a lightweight security framework built on secure network coding will be proposed.

Joint tracking of groups of users with uplink reference signals

In cellular networks user equipment (UE) need to be tracked so that they can be reached by incoming data and to keep context information such as encryption keys available for UE originated transmission. Typically UEs measure reference signal transmissions that are broadcasted by the network, and report to the network based on some criterion that allows the network to know the UE location with sufficient accuracy. An alternative approach is to let the UEs send out reference signals that the network can detect to track the UE location. This reduces the need for the UEs to measure and report, while it requires some resources for uplink transmission. In this paper we propose and evaluate a solution for jointly tracking groups of UEs that are moving together. The results show that UEs can be tracked efficiently with low resource consumption.

Energy efficiency of network-coding enabled mobile small cells

Abstract Energy efficiency becomes increasingly important due to the limited battery capacity in wireless devices while at the same time user throughput requirements are relentlessly increasing. In this paper, we study an energy efficient cooperation scheme which employs network coding to enhance the energy efficiency for mobile devices. Herein we propose that the mobile devices are clustered into mobile small cells with one of the mobile devices acting as a group head with basic transceiver, coding and relaying functionalities. Group heads coordinate the transmissions from the mobile devices in the mobile small cell to the networks base stations. The objective function of the cooperative scheme is to minimize mobile devices’ energy consumption subject to a certain bit error probability. The proposed network-coding based scheme has been evaluated by means of numerical simulations and compared to both a conventional direct transmit scheme, with no cooperation groups, and a cooperative relaying scheme. Results show that, with network-coded cooperation, energy efficiency may significantly increase provided the density of base stations and mobile devices is below a certain value. Above this value none of the compared cooperation schemes may improve energy efficiency, but rather power consumption is reduced only when mobile devices transmit via base stations in their close proximity.