Gestión de interferencias en sistemas Femtocelda

Hasta ahora las comunicaciones móviles han estado basadas en grandes estaciones base dedicadas a ofrecer servicios de voz y datos en una determinada zona, ya sea en escenarios urbanos cubriendo superficies con un radio de centenares de metros o en escenarios rurales en zonas de hasta varios kilómetros de radio. La aparición de nuevas tecnologías móviles como UMTS o WiMAX y el desarrollo de los terminales actuales, han hecho necesaria una actualización en los servicios ofrecidos por los operadores, obligando a aumentar tanto la frecuencia como el ancho de banda utilizado, lo que conlleva un uso mayor del cada vez más saturado espectro radioeléctrico. Desde el mercado de las telecomunicaciones se ha señalado a los sistemas femtocelda como solución a los problemas de cobertura, ancho de banda y falta de personalización en los servicios 3G ofrecidos actualmente, a la vez que se acelera la convergencia fijo-móvil. Básicamente una femtocelda es una pequeña estación base de baja potencia para el uso en hogares o negocios. Dicha estación base se integra en la red móvil mediante una conexión de banda ancha, normalmente ADSL, de modo que cuando el usuario entre en su casa sea su propia red la encargada de ofrecerle cobertura 3G, sustituyendo automáticamente la ofrecida por el operador móvil correspondiente. Respecto al producto final, éste tendrá el aspecto de un router WiFi tradicional, de hecho, fabricantes como NetGear han comenzado a crear hardware incorporando ambas tecnologías, y podrán ser instaladas por el propio usuario en cualquier lugar de su domicilio sin necesidad de haber realizado un plan de frecuencias previamente. La introducción de sistemas femtocelda ofrece ventajas tanto a los operadores móviles como a los usuarios. Por un lado, al operador le permite mejorar su cobertura en interiores, así como la liberación de parte del tráfico que circula en las macroceldas, terminando con la necesidad actual de aumentar el número de estaciones base. Mientras, los usuarios podrán solucionar los problemas de cobertura en sus hogares o negocios, a la vez que aumenta la velocidad de su enlace 3G con una reducción en el precio de su tarifa de datos. Sin embargo, las femtoceldas también conllevan una serie de desventajas, la más evidente consiste en que no es una tecnología madura y la apuesta de los operadores en su utilización acarreará una serie de gastos en hardware, ofertas, publicidad... Pero no es este factor el principal problema de la tecnología femtocelda, el verdadero talón de Aquiles consiste en la gestión de interferencias. Tanto las antenas encargadas de ofrecer cobertura en cada domicilio, como sus terminales deben convivir con las antenas macrocelda y sus usuarios. Por ejemplo, en el caso de que un usuario macrocelda utilice la misma frecuencia que el terminal femtocelda aparece una interferencia en enlace ascendente de ambos elementos, reduciendo la calidad de los servicios ofrecidos e incluso creando una “zona muerta” en torno al punto de acceso, dentro de la cual resultará imposible acceder a la cobertura macrocelda. Destacar la aplicación de diversas técnicas para minimizar el efecto nocivo de estas interferencias, desde mecanismos sencillos como la utilización de control de ganancia en las antenas receptoras, a la utilización de sistemas MIMO, o la coordinación entre estaciones base, de modo que los grupos de usuarios no sean totalmente cerrados y cada antena distribuya una parte de su potencia disponible a fin de buscar el bien común. Finalmente, en cuanto al futuro de las comunicaciones inalámbricas, los operadores consideran que el crecimiento del mercado a través de la vía del crecimiento de usuarios se encuentra saturado, cobrando mayor importancia los servicios que puedan prestar así como el volumen de negocio que pueda surgir de éstos. Es en este aspecto donde la aparición de sistemas femtocelda toma un papel principal, en el que muchos operadores esperan encontrar el modo de reducir el tráfico en sus antenas macrocelda a la vez que abren un nuevo mercado orientado a la movilidad de dispositivos en entornos interiores.Ingeniería de Telecomunicació

Green strategies for Block Diagonalization-Network MIMO with Fairness

This proceeding at: Workshop on Small Cell Cooperative Communications, Lyon (Francia), 2-Mayo 2012.The event Web site at http://ic1004-iplan-2012.conf.citi-lab.fr/Future cellular networks will be more dense and heterogeneous. A typical deployment will be based on micro, pico and femto cells. Under this scenario, novel interference management techniques such as network MIMO will be mandatory. However, it is usual to combine them with power allocation strategies to maximize the sum of the rates or minimize the power consumption. Those strategies are unfair at user level in heterogeneous networks and do not provide a "green" measure. In this work we propose some more equitable alternatives to the classical power allocation schemes and a strategy based on a green metric. Finally we compare the performance, in user spectral efficiency and power consumption terms, of these strategies in micro and femto cell deployments.This work has been partly funded by projects GRE3N TEC2011-29006-C03-03 and COMONSENS CSD2008-00010.Publicad

Blind interference alignment for cellular networks

Mención Internacional en el título de doctorManaging the interference is the main challenge in cellular networks. Multiple-Input Multiple-Output (MIMO) schemes have emerged as a means of achieving high-capacity in wireless communications. The most efficient MIMO techniques are based on managing the interference instead of avoiding it by employing orthogonal resource allocation schemes. These transmission schemes require the knowledge of the Channel State Information at the Transmitter (CSIT) to achieve the optimal Degrees of Freedom (DoF), also known as multiplexing gain. Providing an accurate CSIT in cellular environments involves high-capacity backhaul links and accurate synchronization, which imply the use of a large amount of network resources. Recently, a Blind Interference Alignment (BIA) scheme was devised as a means of achieving a growth in DoF regarding the amount of users served without the need for CSIT in the Multiple-Input Single-Output (MISO) Broadcast Channel (BC). It is demonstrated that BIA achieves the optimal DoF in the BC without CSIT. However, the implementation of BIA in cellular networks is not straightforward. This dissertation investigates the DoF and the corresponding sum-rate of cellular networks in absence of CSIT and their achievability by using BIA schemes. First, this dissertation derives the DoF-region of homogenous cellular networks with partial connectivity. Assuming that all the Base Stations (BSs) cooperate in order to transmit to all users in the network, we proposed an extension of the BIA scheme for the MISO BC where the set of BSs transmits as in a network MIMO. It is shown that the cooperation between BSs results futile because of the lack of full connectivity in cellular networks. After that, this dissertation presents several transmission schemes based on the network topology. By differentiating between users that can treat this interference optimally as noise and those who need to manage the interference from neighbouring BSs, a network BIA scheme is devised to achieve the optimal DoF in homogeneous cellular networks. Second, the use of BIA schemes is analyzed for heterogeneous cellular networks. It is demonstrated that the previous BIA schemes based on the network topology result nonoptimal in DoF because of the particular features of the heterogenous cellular networks. More specifically, assuming a macro-femto network, cooperation between both tiers leads to a penalty for macro users while femto users do not exploit the particular topology of this kind of network. In this dissertation, the optimal linear DoF (lDoF) in a two-tier network are derived subject to optimality in DoF for the upper tier. It is demonstrated that, without CSIT or any cooperation between tiers, the lower tier can achieve nonzero DoF while the upper tier attains the optimal DoF by transmitting independently of the lower tier deployment. After that, a cognitive BIA scheme that achieves this outer bound is devised for macro-femto cellular networks. The third part of this dissertation is focused on the implementation of BIA in practical scenarios. It is shown that transmission at limited SNR and coherence time are the main hurdles to overcome for practical implementations of BIA. With aim of managing both constraints, the use of BIA together with orthogonal approaches is proposed in this work. An improvement on the inherent noise increase of BIA and the required coherence time is achieved at expenses of losing DoF. Therefore, there exists a trade-off between multiplexing gain, sum-rate at finite SNR and coherence time in practical scenarios. The optimal resource allocation for orthogonal transmission is obtained after solving a very specific optimization problem. To complete the characterization of the performance of BIA in realistic scenarios a experimental evaluation based on a hardware implementation is presented at the end of this work. It is shown that BIA outperforms the sum-rate of schemes based on CSIT such as LZFB because of the hardware impairments and the costs of providing CSIT in a realist implementation.Programa Oficial de Doctorado en Multimedia y ComunicacionesPresidente: Luc Vandendorpe.- Secretario: María Julia Fernández-Getino García.- Vocal: Ignacio Santamaría Caballer

Optimization of the Receiving Orientation Angle for Zero-Forcing Precoding in VLC

We study the performance of linear zero-forcing (ZF) precoding in multiuser multiple-input single-output visible light communications (VLC) when we are able to select the receiving orientation angle (ROA) of each user. For radio-frequency communications, the non-line-of-sight rich scattering environment usually ensures the linear independence among user's channels. However, this condition is less likely to happen in VLC systems, degrading the performance of ZF precoding. In this work, we propose a variable ROA (vROA) photodetector able to modify its orientation vector in order to generate semi-orthogonal channel responses among users. We derive the algorithm for determining the orientation of the vROA photodetector of each user, obtaining optimal and suboptimal solutions with high and low complexity, respectively. Simulation results show that the performance of ZF precoding is improved considerably by managing the users ROA.This work has been supported by the Spanish National Project TERESAADA (TEC2017-90093-C3-2-R) (MINECO/AEI/FEDER, UE) and the research project GEOVEOLUZ-CM-UC3M.Publicad

Visible Light Communications for IoT services based on high-power LEDs in Industry 4.0

Proceeding of: IEEE Conference on Standards for Communications and Networking (CSCN 2022), 28-30 November 2022, Thessaloniki, GreeceIn the framework of Industry 4.0, visible light communications (VLC) are proposed for providing connectivity in those environments where radio-frequency (RF) transmission achieves a poor performance or it is even banned [1] . Specifically, VLC are potentially useful for providing Internet of Things (IoT) services while consuming a small portion of the transmission resources, considering a user-centric approach and subject to a low-cost implementation [2] . Recently, the European Commission warned about the need for employing low-cost and energy-efficient LEDs for future VLC systems to maintain the reduction in the energy consumption [3] . As a consequence, the VLC-IoT hardware implementations must consider commercial LEDs, which are subject to non-linearities, reduced and unknown bandwidth and other impairments for data transmission, while maintaining the energy efficiency for illumination purposes. In this work, a low-cost VLC-IoT implementation is presented based on commercial high-power LEDs. The proposed configuration is focused on providing connectivity in those environments where RF transmission is not efficient so that an external access point (AP) distributes the connectivity through a backhaul link that feeds the set of optical access points (APs).This work has received funding from the European Union Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie ETN TeamUp5G, grant agreement No. 813391, and from Spanish National Project IRENE-EARTH (PID2020-115323RB-C33) (MINECO/AEI/FEDER, UE). The work of Máximo Morales-Céspedes is also supported by the Juan de la Cierva Incorporación under Grant IJC2019-040317-I

Performance Analysis of RIS-Assisted FSO Communications over Fisher-Snedecor F Turbulence Channels

The Fisher Snedecor (F-S) F distribution has recently been introduced as a tractable turbulence-induced (TI) fading model that fits well with the experimental data. This paper provides a performance evaluation of a free-space optical (FSO) re-configurable intelligent surface (RIS)-assisted communications (ACs) link over the F-S F TI fading channels, assuming the intensity modulation direct detection (IM DD) technique. In particular, novel and closed-form (C-F) analytical expressions for the probability density function (PDF) and cumulative distribution function (CDF) of the end-toend signal-to-noise ratio (SNR) in terms of Gaussian hyper-geometric functions are efficiently derived. Capitalizing on the obtained results, novel C-F analytical expressions for the moment generating function (MMGF), outage probability (OP), average bit error rate (BER) and ergodic channel capacity (Cgamma) of the FSO RIS-ACs system over the F-S F TI fading channels are provided and numerically evaluated under the various TI fading severity conditions. Furthermore, the second-order (S-O) statistical expressions for the level crossing rate (LCR) and average fade duration (AFD) are obtained and thoroughly examined for various FSO RIS-ACs system model parameters.This research was funded by funding from UC3M and the European Unions Horizon 2020 programme under the Marie Skodowska-Curie grant agreement no. 801538 and by project IRENE-EARTH (PID2020-115323RB-C33/AEI/10.13039/501100011033

Tangential Power Allocation NOMA scheme for Visible Light Communications

Proceeding of: IEEE Conference on Standards for Communications and Networking (CSCN 2022), 28-30 November 2022, Thessaloniki, GreeceNon-orthogonal multiple access (NOMA) has been proposed in both radio-frequency (RF) and visible light communications (VLC) to both improve the achievable rate and overcome the constrains in the number of users of orthogonal multiple access (OMA) schemes. Despite the advantages of NOMA, there still exist some issues that require more investigation such as power allocation schemes. This is issue is more remarkable in VLC due to the small and confined coverage footprint of each optical access point. In this poster, we propose a novel methodology denoted by tangential power allocation (TPA) for NOMA in VLC. Basically, the power allocation coefficients are calculated based on the tangential point on the NOMA rate region that is parallel to the OMA region. It is shown that TPA achieves greater performance in terms of achievable rate and fairness in comparison with conventional NOMA schemes.This work was funded by the European Union(EU) Horizon 2020 research and innovation program under the Marie Sklodowska-Curie ETN TeamUp5G, grant agreement No. 813391

Achievable throughput with Block Diagonalization on OFDM indoor demonstrator

The proceeding at: 21st European Signal Processing Conference (EUSIPCO 2013), took place 2013, September 09-13, in Marrakech, Septiembre 2013.Block Diagonalization (BD) is a linear precoding transmission technique able to achieve full multiplexing gain in multiple antenna systems. In this work we present a Multiple-Input Multiple-Output (MIMO) implementation based on Orthogonal Frequency Division Multiplexing (OFDM) made up of a transmitter with 4 antennas and 2 users equipped with 2 antennas each one, which allows us to evaluate the performance of BD in indoor scenarios. First, the theoretic achievable rates are obtained for the measured channel in an offline evaluation. After that, the bit error rate performance is evaluated regarding the system sum throughput. To the best of our knowledge, this is the first time that BD performance is validated using a multiuser MIMO testbed.This work has been partially funded by research projects COMONSENS (CSD2008-000 1 0), and GRE3N (TEC20 11-29006-C03-02).Publicad

User-Centric Blind Interference Alignment Design for Visible Light Communications

Visible light communications (VLC) are considered as a key technology for future wireless communications. In order to mitigate the interference, several transmit precoding (TPC) schemes have been proposed for VLC. However, beyond the need for channel state information and backhaul links, the TPC schemes are subject to additional constraints given by the features of the optical channel such as ensuring a real and non-negative transmitted signal or a low correlation among users. Besides, the traditional network centric (NC) design, i.e., considering only the position of the transmitters, leads to rigid transmission schemes for VLC networks due to the small and confined coverage of the optical transmitters.In this paper, we consider blind interference alignment (BIA) schemes for VLC, which solve the aforementioned issues, based on the concept of reconfigurable photodetector. In this context, we propose a user-centric (UC) clustering strategy based on the K-means algorithm where the users are treated as an active element of the network instead of a mere endpoint. For the proposed UC design, we derive two BIA schemes based on the connectivity of the clusters; a straightforward scheme considering each cluster as a broadcast channel referred to as KM-sBIA and a scheme that is flexible to the connectivity of each user within the cluster referred to as KM-topBIA. The simulation results show that the proposed schemes outperform the use of classical TPC or other BIA-based schemes considering both NC and UC approach.This work was supported by the Spanish National Project TERESA-ADA (MINECO/AEI/FEDER, UE) under Grant TEC2017-90093-C3-2-

Load balancing in hybrid VLC and RF networks based on blind interference alignment

Visible light communications (VLC) are proposed for increasing the spectral efficiency and the number of devices served in indoor environments, while providing illumination through light emitting diodes (LED). For VLC, each optical access point (AP) provides a small and confined area of coverage. Since several sources of light are usually deployed in overlapping fashion in order to provide satisfactory illumination, VLC are limited by inter-cell interference. Moreover, transmission from a specific optical AP can be blocked by the elements of the scenario. On the other hand, radio-frequency (RF) systems such as WiFi are usually available in most of the indoor scenarios. In this work, we first propose a dynamic cell formation method for grouping the optical APs in multiple optical cells that cover a footprint each minimizing the inter-cell interference. After that, we use transmission based on blind interference alignment (BIA) in each optical cell. Considering the coexistence with RF systems based on orthogonal frequency division multiplexing (OFDM), a load balancing algorithm is proposed for managing the resources of the resulting hybrid VLC/RF network and determining the user association to each system. However, the complexity of this optimization problem is excessively high for practical VLC/RF networks. In order to obtain a suboptimal but tractable solution, we propose a decentralized optimization method based on Lagrangian multipliers. Simulation results show that the proposed scheme outperforms other approaches for user grouping and managing the resources of hybrid VLC/RF networks.This work was supported in part by the Spanish National Project TERESA-ADA under Grant TEC2017-90093-C3-2-R and Grant MINECO/AEI/FEDER, UE, and in part by the research project GEOVEOLUZ-CM-UC3M funded by the call Programa de apoyo a la realización de proyectos interdisciplinares de I+D para jóvenes investigadores de la Universidad Carlos III de Madrid 2019-2020 under the frame of the Convenio Plurianual Comunidad de Madrid, Universidad Carlos III de Madri