Examinando por Autor "Femenias Nadal, Guillem"

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Analysis and Optimization of FFR-Aided OFDMA-Based Heterogeneous Cellular Networks
(IEEE, 2016-08-10) García Morales, Jan; Femenias Nadal, Guillem; Riera Palou, Felip
Two-tier networks combining an operator-managed infrastructure of macrocell base stations combined with a user-deployed network of femtocells have recently emerged in the context of modern wireless standards as a solution to meet the ambitious performance requirements envisaged in 4G/5G architectures. Most often, these systems require interference coordination schemes that allow near universal frequency reuse while maintaining a considerably high signal-to-interference-plus-noise ratio levels across the coverage area. In particular, fractional frequency reuse (FFR) and its variants are deemed to play a fundamental role in the next generation of cellular systems. This paper develops an analytical framework targeting the downlink performance evaluation of FFR-aided orthogonal frequency division multiple access-based two-tier heterogeneous networks. In the considered scenario, macrocell and femtocell tiers are assumed to be uncoordinated and co-channel deployed, thus representing a worst-case scenario in terms of inter-tier interference. The proposed framework allows the evaluation of the impact produced by both inter- and co-tier interferences on the performance of either the macro-users (MUs) or the femto-users. Analytical results are used to optimize the FFR parameters as a function of, for example, the density of MUs per cell, the resource block scheduling policy, the density of femto base stations per area unit, or the degree of isolation provided by wall penetration losses. Moreover, different optimization designs of the FFR component are proposed that allow a tradeoff between throughput performance and fairness by suitably dimensioning the FFR inner and outer areas and the corresponding frequency allocation.
Energy-Efficient Access-Point Sleep-Mode Techniques for Cell-Free mmWave Massive MIMO Networks With Non-Uniform Spatial Traffic Density
(IEEE, 2020-07-27) García Morales, Jan; Femenias Nadal, Guillem; Riera Palou, Felip
Cell-free massive multiple-input multiple-output (MIMO) is a novel beyond 5G (B5G) and 6G paradigm that, through the use of a common central processing unit (CPU), coordinates a large number of distributed access points (APs) to coherently serve mobile stations (MSs) on the same time/frequency resource. By exploiting the characteristics of new less-congested millimeter wave (mmWave) frequency bands, these networks can improve the overall system spectral and energy efficiencies by using low-complexity hybrid precoders/decoders. For this purpose, the system must be correctly dimensioned to provide the required quality of service (QoS) to MSs under different traffic load conditions. However, only heavy traffic load conditions are usually taken into account when analysing these networks and, thus, many APs might be underutilized during low traffic load periods, leading to an inefficient use of resources and waste of energy. Aiming at the implementation of energy-efficient AP switch on/off strategies, several approaches have been proposed in the literature that only consider rather unrealistic uniform spatial traffic distribution in the whole coverage area. Unlike prior works, this paper proposes energy efficient AP sleep-mode techniques for cell-free mmWave massive MIMO networks that are able to capture the inhomogeneous nature of spatial traffic distribution in realistic wireless networks. The proposed framework considers, analyzes and compares different AP switch ON-OFF (ASO) strategies that, based on the use of goodness-of-fit (GoF) tests, are specifically designed to dynamically turn on/off APs to adapt to both the number and the statistical distribution of MSs in the network. Numerical results show that the use of properly designed GoF-based ASO strategies under a non-uniform spatial traffic distribution can serve to considerably improve the achievable energy efficiency.
Higher Order Sectorization in FFR-Aided OFDMA Cellular Networks: Spectral- and Energy-Efficiency
(IEEE, 2019-01-11) García Morales, Jan; Femenias Nadal, Guillem; Riera Palou, Felip
It is well known that per macro-site spectral efficiency (SE) can be increased through higher order sectorization (HOS) by radially partitioning the coverage area of each site into multiple sectors and reusing the spectral resources in each sector and across all sites. In order to further reinforce its benefits, HOS can be combined with fractional frequency reuse (FFR) techniques to improve the SE and/or energy efficiency (EE) of the network. This paper presents an analytical framework that is used to assess the sectorization performance in terms of both the SE and EE in the downlink of HOS/FFR-aided orthogonal frequency-division multiple access (OFDMA)-based macro-cellular networks. Tractable mathematical expressions are derived for the round robin, the proportional fair, and the maximum signal-to-interference-plus-noise ratio scheduling rules and the corresponding capacities. The results show the impact of the sectorization gain on the system performance for different cell-edge frequency reuse factor values. Furthermore, an optimization problem for the HOS/FFR-aided OFDMA-based network is addressed, allowing a tradeoff between the EE performance and fairness by suitably dimensioning the FFR inner and outer areas and the corresponding frequency allocation to each of these regions.
Multi-Layer FFR-Aided OFDMA-Based Networks Using Channel-Aware Schedulers
(IEEE, 2017-12-29) García Morales, Jan; Femenias Nadal, Guillem; Riera Palou, Felip; Thompson, John
In orthogonal frequency-division multiple access (OFDMA) networks, the use of universal frequency reuse improves overall cell capacity at the cost of very high levels of inter-cell interference particularly affecting the users located in the cell-edge regions. In order to provide a better quality of experience to cell-edge users while still achieving high spectral efficiencies, conventional fractional frequency reuse (FFR) schemes split the cells into inner and outer regions (or layers) and allocate disjoint frequency resources to each of these regions by applying higher frequency reuse factors to the outer regions. Recently, multi-layer FFR-aided OFDMA-based designs, splitting the cell into inner, middle, and outer layers, have been proposed with the aim of further improving the throughput fairness among users. This paper presents an analytical framework allowing the performance evaluation and optimization of multi-layer FFR-aided OFDMA-based networks. Tractable mathematical expressions of the average spectral efficiency are derived and used to pose optimization problems allowing network designers to achieve the optimal trade-off between spectral efficiency and fairness. Analytical and simulation results clearly show that, irrespective of the channel-aware scheduler in use, multi-layer FFR-schemes can outperform the conventional two-layer FFR architectures.
On the Design of OFDMA-Based FFR-Aided Irregular Cellular Networks With Shadowing
(IEEE, 2018-02-12) García Morales, Jan; Femenias Nadal, Guillem; Riera Palou, Felip
Owing to its high capabilities in terms of spectral efficiency and flexibility, orthogonal frequency division multiple access (OFDMA) has played a crucial role towards the success of 4G cellular systems and an increasing number of actors in the 5G arena strongly advocate for its continuation. OFDMA-based architectures do not introduce intracell interference but, due to the use of very aggressive frequency reuse plans, they must implement some form of inter-cell interference (ICI) control to warrant prescribed levels of quality of service, specially to users located near the cell edge. An efficient technique for mitigating ICI in OFDMA networks is the well-known fractional frequency reuse (FFR) scheme. In FFR, a signal-to-interference-plus-noise ratio threshold is used to categorize mobile stations (MSs) as cell-center or cell-edge MSs. Furthermore, a different number of frequency resources are allocated to cellcenter and cell-edge areas according to a prescribed frequency reuse plan. This paper presents an analytical characterization of FFR-aided OFDMA-based multi-cellular networks that, unlike most previous studies, incorporates shadowing effects and, furthermore, considers that base stations are irregularly deployed. This analytical approach can incorporate different scheduling rules and can underpin different designs for which the optimal FFR parameters can be derived. The proposed framework allows the performance evaluation and optimization of any cell in the system by considering the specific network topology, the user association and categorization processes, the spatial density of users and the characteristics of both the fast multipath fading and the spatially correlated slow shadow fading.
Performance Analysis and Optimisation of FFR-Aided OFDMA Networks Using Channel-Aware Scheduling
(Springer, 2016-05-06) García Morales, Jan; Femenias Nadal, Guillem; Riera Palou, Felip
Modern cellular standards typically incorporate interference coordination schemes allowing near universal frequency reuse while preserving reasonably high spectral efficiencies over the whole coverage area. In particular, fractional frequency reuse (FFR) and its variants are deemed to play a fundamental role in the next generation of cellular deployments (B4G/5G systems). This paper presents an analytical framework allowing the downlink performance evaluation of FFR-aided OFDMA networks when using channel-aware scheduling policies. Remarkably, the framework contemplates the use of different rate allocation strategies, thus allowing to assess the network behaviour under ideal (capacity-based) or realistic (throughput-based) conditions. Analytical performance results are used to optimise the FFR parameters as a function of, for instance, the resource block scheduling policy or the density of UEs per cell. Furthermore, different optimisation designs of the FFR component are proposed that allow a tradeoff between throughput performance and fairness by suitably dimensioning the FFR-defined cell-centre and cell-edge areas and the corresponding frequency allocation to each region. Numerical results serve to confirm the accuracy of the proposed analytical model while providing insight on how the different parameters and designs affect network performance.
Statistical Analysis and Optimization of a Fifth-Percentile User Rate Constrained Design for FFR/SFR-Aided OFDMA-Based Cellular Networks
(IEEE, 2017-12-13) García Morales, Jan; Femenias Nadal, Guillem; Riera Palou, Felip
Interference mitigation strategies are deemed to play a key role in the context of the next generation (B4G/5G) of multicellular networks based on orthogonal frequency division multiple access. Fractional and soft frequency reuse (FFR, SFR) constitute two powerful mechanisms for intercell interference coordination that have been already adopted by emerging cellular deployments as an efficient way to improve the throughput performance perceived by cell-edge users. This paper presents a novel optimal fifth-percentile user rate constrained design for FFR/SFR-based networks that, by appropriately dimensioning the center and edge regions of the cell, rightly splitting the available bandwidth among these two areas while assigning the corresponding transmit power, allows a tradeoff between cell throughput performance and fairness to be established. To this end, both the cumulative distribution function of the user throughput and the average spectral efficiency of the system are derived assuming the use of the ubiquitous proportional fair scheduling policy. The mathematical framework is then used to obtain numerical results showing that the novel proposed design clearly outperforms previous schemes in terms of throughput fairness control due to a more rational compromise between average cell throughput and cell-edge ICIC.
SWIPT-Enhanced Cell-Free Massive MIMO Networks
(IEEE, 2021-05-25) Femenias Nadal, Guillem; García Morales, Jan; Riera Palou, Felip
Simultaneous wireless information and power transfer (SWIPT) has been advocated as a highly promising technology to provide near- perpetual operation to low-powered wireless devices in Internet-of-Things (IoT)-based wireless networks. In this paper, a SWIPT-enhanced cell-free massive MIMO network is proposed. In such a network, a large set of spatially distributed access points (APs) interconnected via a central processing unit (CPU) can collaboratively serve a large number of both energy harvesting mobile stations (MSs) (requiring wireless energy transfer) and conventional MSs (not requiring wireless energy transfer) on the same time-frequency resources. We consider spatially correlated Rician fading channels and the use of different precoding schemes that are based on different channel estimators differing on the assumed knowledge of the line-of-sight component. Mathematically manageable expressions are derived for the harvested energy during the downlink (DL) energy harvesting phase and the achievable spectral and energy efficiencies during the uplink (UL) payload transmission phase. A coupled UL/DL optimization problem is formulated aiming at finding the power control coefficients that maximize the minimum of the weighted achievable UL signal-to-interference-plus-noise ratios (SINRs) of all MSs. Extensive numerical results are presented that serve to highlight the existing trade-offs among the achievable spectral and energy efficiencies, the harvested energy, the energy dedicated to UL pilot transmission or the system configuration.