Algoritmos en transmisión y recepción para OFDM en entornos multi-usuario

Desde su incursión en la vida de los ciudadanos de forma cotidiana, las comunicaciones móviles han ido adquiriendo cada vez más protagonismo. Lo que hace unos años era un elemento de lujo o causa de envidias, hoy es algo tan habitual como coger el autobús. Este auge en las comunicaciones móviles ha llevado consigo un aumento del número de usuarios de estos servicios, así como una mayor exigencia por parte de éstos en cuanto a velocidad de transmisión, seguridad, servicios, calidad ... Por este motivo, se han ido introduciendo nuevas redes, así como mejoras en las ya existentes. Actualmente estamos asistiendo al inicio de la implantación de las redes denominadas de tercera generación (3G). No obstante, estas redes 3G fueron de nidas a mediados de los años 90. En la actualidad nos encontramos en la de nición de lo que será la cuarta generación móvil (4G) a la que probablemente preceda una intermedia, tal vez Super 3G. Muchas son las tecnologías candidatas para esta Super 3G o para la 4G, pero es OFDM (Multiplexación por División en Frecuencias Ortogonales - Orthogonal Frequency Division Multiplexing) una de las más prometedoras para terminar siendo la tecnología elegida, debido principalmente a la robustez que ofrece frente al multi-trayecto, la flexibilidad que proporciona por el hecho de dividir el ancho de banda en sub-portadoras que pueden ser moduladas independientemente para adaptarse a las condiciones específicas de los canales móviles y al aprovechamiento de la diversidad multi-usuario cuando se combina en OFDMA (Acceso múltiple por división en frecuencias ortogonales - Orthogonal Frequency Division Multiple Access). Sin embargo, para que estos sistemas OFDMA sean una realidad en redes futuras como la Super 3G o la 4G, es preciso reducir la complejidad que ellos conllevan, en aspectos como la sincronización multi-usuario o la implementación de la modulación adaptativa. Esta Tesis está enfocada a reducir la complejidad de implementación para que la tecnología OFDMA pueda ser un hecho en las futuras redes inalámbricas. La sincronización en sistemas OFDMA, principalmente en escenarios ad-hoc plantea un reto importante a la hora de implementarla en este tipo de redes. Dado que existe una literatura extensa dedicada a resolver el problema de la sincronización en entornos OFDM mono-usuario, en esta Tesis se propone un procedimiento para reducir el problema de la sincronización multiusuario al caso mono-usuario, lo que permite la re-utilización de todas las técnicas propuestas en la literatura. Además de las buenas prestaciones incluso en situaciones de saturación de la red, permite un ahorro de la energía necesaria para la realización de las tareas de sincronización. Asimismo, a la vez que se resuelve la sincronización multi-usuario, también en esta Tesis se aporta un algoritmo para realizar el seguimiento de la desviación de frecuencia en sistemas tanto OFDM como OFDMA, de una forma sencilla y eficiente. Este algoritmo utiliza las sub-portadoras piloto de los símbolos, y ofrece unas prestaciones razonables, incluso cuando el número de estas sub-portadoras piloto es reducido. La modulación adaptativa u otras técnicas avanzadas de comunicaciones como MIMO (Multiple Input Multiple Output) o el conformado de haz (beamforming), que se hacen más atractivas cuando se combinan con OFDM u OFDMA, requieren de cierta realimentación hacia el transmisor por parte del receptor, además de un conocimiento del canal bastante preciso. En esta Tesis se aportan algoritmos para la compresión de esta información (que puede ser muy elevada dependiendo de la granularidad de la adaptación y la variabilidad del canal), lo que permite un mejor aprovechamiento de los recursos radio, y simpli ca la implementación de este tipo de sistemas. Se consiguen reducciones superiores a cuatro veces la tasa original, lo que implica grandes ventajas en todos los sentidos. Además, se realiza un estudio teórico sobre cómo afectan los errores en la estimación de canal en la capacidad del sistema, obteniéndose dos cotas numéricas para ésta. Gracias a estas cotas se puede concluir que las técnicas actuales de estimación de canal son su cientemente buenas como para que las pérdidas producidas por los errores en esta estimación no sean muy significativas; No obstante, estas pérdidas aumentan a medida que se incrementa la relación señal a ruido, y, por tanto, es conveniente seguir mejorando la estimación de canal si se pretende que estos sistemas trabajen a altas relaciones señal a ruido. Por último, destacar que las simulaciones llevadas a cabo para examinar las prestaciones de los algoritmos desarrollados han proporcionado datos prácticos que ayudarán al diseño e implementación de futuras redes móviles 4G.Nowadays mobile communications are common in our society. In less than two decades they have changed the way people understand personal communications. This interest is still growing and makes users (every day larger in number) ask for more and more services, speed, security and quality, at least comparable to the wired technologies. For this reason, new networks have been introduced in the scene, as well as improvements in the already existing ones. At the moment we are witnessing the beginning of the deployment of the so-called third generation networks (3G). However, these 3G networks were defined in the middle of years 90. At the present time we are in the definition of what will be the fourth mobile generation, which is called the 4G and probably an intermediate stage as Super 3G could be defined first. Many different technologies are in consideration, but OFDM (Orthogonal Frequency Multiplexing Division) is one of most promising candidates to end up becoming the technology to be used, due mainly to the robustness that it others to the multi-path channel, the flexibility that it provides by the fact that it divides the bandwidth into sub-carriers which can be modulated independently to adapt the signal to the specific channel conditions and to the advantage of the multi-user diversity when it is used to multiplex several users in the form of OFDMA (Orthogonal Frequency Division Multiple Access). Nevertheless, in order for these OFDMA systems to be a reality in future networks as Super 3G or 4G, the reduction in complexity has to be considered, in aspects such as the multi-user synchronization or the implementation of the Adaptive Modulation. This doctoral Thesis is focused on reducing the implementation complexity in both multi-user synchronization and Adaptive Modulation so that OFDMA technology can be a fact in the near future radio networks. The synchronization in OFDMA systems, specially in ad-hoc scenarios, raises an important challenge at the time of implementing this technology. Since an extensive literature is devoted to solve the problem of the synchronization in single-user OFDM systems, in this Thesis we have designed and proposed a procedure to reduce the multi-user synchronization problem to the single-user case, and therefore it allows the reuse of all the already proposed techniques in the literature. The procedure exhibits a good performance even in network saturation situations, and in addition it does not incur in a high power consumption when performing the synchronization tasks. Besides, solving the multi-user synchronization problem, the Thesis also proposes an algorithm for frequency offset tracking in an efficient and simple way, for both OFDM and OFDMA systems. This algorithm uses the pilot sub-carriers scattered in each OFDM symbol, and o ers reasonable performance, even when the number of these pilot sub-carriers is small. Adaptive Modulation and other advanced techniques as MIMO (Multiple Input Multiple Output) or beamforming become more attractive when they are combined with OFDM or OFDMA. However they require certain feedback information from receiver to the transmitter, and in addition, a quite precise knowledge about channel conditions. In this Thesis several algorithms for compressing the feedback information have been developed. This feedback information can be very highly demanding (depending on the granularity of the adaptation and the variability of the channel), and therefore this compression allows a better use of the resources, and simplifies the implementation of this kind of systems. Reductions of more than four times the original flow are obtained, which implies great advantages. In addition, a theoretical study is made on how errors in channel estimation affect the channel capacity, and several upper-bounds (one of them very tight) for this effect are obtained. One of the conclusions is that the actual state of the art in channel estimation techniques is good enough to be used without too much loss; However, the loss increases as the signal to noise ratio increases, and therefore, it is suggested to continue improving channel estimation algorithms for improving performance in high signal to noise ratio scenarios. Finally, thanks to simulations and designs carried out in order to examine the performance of the developed algorithms, some practical data are provided that will help in the future design of new mobile networks as 4G

DCO-OFDM Signals With Derated Power for Visible Light Communications Using an Optimized Adaptive Network-Based Fuzzy Inference System

Direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) signals used in visible light communications suffer from high peak-to-average-power ratio (PAPR) or cubic metric (CM). It strongly degrades the performance due to the great back-off necessary to avoid the clipping effect in the light-emitting diode. Thus, PAPR and CM reduction techniques become crucial to improve the system performance. In this paper, an adaptive network-based fuzzy inference system (ANFIS) is used to obtain efficient DCO-OFDM signals with a low power envelope profile. First, signals specially designed for DCO-OFDMwith very low CM, as the ones obtained from the raw cubic metric (RCM)-active constellation extension method, are used to train the fuzzy systems in time and frequency domains. Second, after the off-line training, the ANFIS can generate a real-valued signal in a one-shot way with 8.9 dB of RCM reduction from the original real-valued signal, which involves a gain in the input power back off larger than 2.8 dB, an illumination-to-communication conversion efficiency gain of more than 35% and considerable improvements in bit error rate

CCSDS 131.2-B-1 transmitter design on FPGA with adaptive coding and modulation schemes for satellite communications

Satellite communications are a well-established research area in which the main innovation of last decade has been the use of multi-carrier modulations and more robust channel coding techniques. However, in recent years, novel advanced signal processing has started being developed for these communications due to the increase in the signal processing capacity of transmitters and receivers. Although signal processing capabilities are increasing, they are still constrained by large limitations because these techniques need to be implemented in real hardware, thus making complexity a matter of critical importance. Therefore, this paper presents the design and implementation of a transmitter with adaptable coding and modulation on a field-programmable-gate-array (FPGA). The main motivation came from the standard CCSDS 131.2-B-1 which recommends that such a novel transmitter which has to date not been implemented in a real system The system was modeled by MATLAB with the purpose of being programmed in VHDL following the AXI-stream protocol between components. Behavioral simulation results were obtained in VIVADO and compared with MATLAB for verification purposes. The transmitter logical circuit was synthesized in a FPGA Zynq Ultrascale RFSoC ZU28DR, showing low resource consumption and correct functioning, leading us to conclude that the deployment of new communication systems in state-of-the-art hardware in satellite communications is justified.The research was funded by Projects IRENE (PID2020-115323RB-C33) (MINECO/AEI/FEDER, UE) and MFOC (Madrid Flight on Chip "Innovation Cooperative Projects Comunidad of Madrid" HUBS 2018/ Madrid Flight on Chip)

Lights and Shadows: A Comprehensive Survey on Cooperative and Precoding Schemes to Overcome LOS Blockage and Interference in Indoor VLC

Visible light communications (VLC) have received significant attention as a way of moving part of the saturated indoor wireless traffic to the wide and unregulated visible optical spectrum. Nowadays, VLC are considered as a suitable technology, for several applications such as high-rate data transmission, supporting internet of things communications or positioning. The signal processing originally derived from radio-frequency (RF) systems such as cooperative or precoding schemes can be applied to VLC. However, its implementation is not straightforward. Furthermore, unlike RF transmission, VLC present a predominant line-of-sight link, although a weak non-LoS component may appear due to the reflection of the light on walls, floor, ceiling and nearby objects. Blocking effects may compromise the performance of the aforementioned transmission schemes. There exist several surveys in the literature focused on VLC and its applications, but the management of the shadowing and interference in VLC requires a comprehensive study. To fill this gap, this work introduces the implementation of cooperative and precoding schemes to VLC, while remarking their benefits and drawbacks for overcoming the shadowing effects. After that, the combination of both cooperative and precoding schemes is analyzed as a way of providing resilient VLC networks. Finally, we propose several open issues that the cooperative and precoding schemes must face in order to provide satisfactory VLC performance in indoor scenarios.This work has been supported partially by Spanish National Project TERESA-ADA(TEC2017-90093-C3-2-R) (MINECO/AEI/FEDER, UE), the research project GEOVEOLUZ-CM-UC3Mfunded by the call “Programa de apoyo a la realización de proyectos interdisciplinares de I+D parajóvenes investigadores de la Universidad Carlos III de Madrid 2019-2020” under the frame ofthe Convenio Plurianual Comunidad de Madrid-Universidad Carlos III de Madrid and projectMadrid Flight on Chip (Innovation Cooperative Projects Comunidad of Madrid - HUBS 2018/MadridFlightOnChip). Additionally, it has been supported partially by the Juan de la CiervaIncorporación grant IJC2019-040317-I and Juan de la Cierva Formación grant (FJC2019-039541-I/AEI/10.13039/501100011033)

Practical Guidelines for Approaching the Implementation of Neural Networks on FPGA for PAPR Reduction in Vehicular Networks

Nowadays, the sensor community has become wireless, increasing their potential and applications. In particular, these emerging technologies are promising for vehicles' communications (V2V) to dramatically reduce the number of fatal roadway accidents by providing early warnings. The ECMA-368 wireless communication standard has been developed and used in wireless sensor networks and it is also proposed to be used in vehicular networks. It adopts Multiband Orthogonal Frequency Division Multiplexing (MB-OFDM) technology to transmit data. However, the large power envelope fluctuation of OFDM signals limits the power efficiency of the High Power Amplifier (HPA) due to nonlinear distortion. This is especially important for mobile broadband wireless and sensors in vehicular networks. Many algorithms have been proposed for solving this drawback. However, complexity and implementations are usually an issue in real developments. In this paper, the implementation of a novel architecture based on multilayer perceptron artificial neural networks on a Field Programmable Gate Array (FPGA) chip is evaluated and some guidelines are drawn suitable for vehicular communications. The proposed implementation improves performance in terms of Peak to Average Power Ratio (PAPR) reduction, distortion and Bit Error Rate (BER) with much lower complexity. Two different chips have been used, namely, Xilinx and Altera and a comparison is also provided. As a conclusion, the proposed implementation allows a minimal consumption of the resources jointly with a higher maximum frequency, higher performance and lower complexity.This work has been partly funded by projects TERESA-ADA (TEC2017-90093-C3-2-R) (MINECO/AEI/FEDER, UE) and ELISA (TEC2014-59255-C3-3-R)

Learning Mobile Communications Standards through Flexible Software Defined Radio Base Stations

Mobile communications are today widespread and contribute to the development of our society. Every day new devices include some means of wireless transmission, which is becoming ubiquitous with the Internet of Things. These systems are standardized by international organizations such as the IEEE, 3GPP, and ETSI, among others. Even though knowledge of wireless standards is key to the understanding of these systems, wireless communications are quite often taught in engineering degrees in a traditional way, without much emphasis on the standardization. Moreover, strong focus is often placed on the theoretical performance analysis rather than on practical implementation aspects. In contrast, most of the current applications make extensive use of mobile data, and the global users' satisfaction is highly correlated with the mobile data throughput. Thus, modern wireless engineers need to have deep insight on the standards that define the mobile transmission systems, and this knowledge is not acquired following the traditional theoretical teaching schemes. In this article, a new learning approach is described. This novel paradigm is based on a new flexible hardware/software platform (FRAMED-SOFT), which is also detailed. Although the article is focused on two wireless standards, GSM and UMTS, the work discussed in this article can easily be extended to other standards of interest, such as LTE and beyond, WiFi, and WiMAX

Resource Allocation for Cooperative Transmission in Optical Wireless Cellular Networks With Illumination Requirements

This work has been partially funded by the Spanish MECD FPU fellowship program granted to the author Borja Genovés Guzmán, the Catalan Government under Grant 2017-SGR-1479, and the Spanish Government under the national project ’TERESA-ADA’ with ID no. TEC2017-90093-C3-2-R and TEC2017-90093-C3-1-R (MINECO/AEI/FEDER, UE)

Robust Cooperative Multicarrier Transmission Scheme for Optical Wireless Cellular Networks

Visible light communication (VLC) is a promising technology to achieve high data rates in heterogeneous scenarios. However, VLC strongly depends on the existence of a line-of-sight (LoS) link between transmitter and receiver to guarantee a good data rate performance, which is often a condition that is difficult to satisfy in practice. In this letter, a novel cooperative multicarrier transmission scheme is proposed, where neighboring attocells smartly cooperate to decrease the probability of blockage in the LoS link. This approach is compared to single-cell transmission schemes, obtaining notable gains in both received signal-to-interference-plus-noise ratio and cell data rate when blockage of the LoS link occurs toward the nearest base station.This work was supported in part by the Catalan Government under Grant 2014SGR1567 and in part by the Spanish Government under the National Project "ELISA" with ID TEC2014-59255-C3-3/1-R. The work of B. Genovés Guzmán was supported in part by the Spanish MECD FPU Fellowship Program

Downlink Performance of Optical OFDM in Outdoor Visible Light Communication

Visible light communication (VLC) is a promising ubiquitous design alternative for supporting high data rates. Its application has been primarily oriented to indoor scenarios, but the proliferation of light-emitting diodes in the streets warrants its investigation in outdoor scenarios as well. This paper studies the feasibility of VLC in a conventional outdoor scenario, when optical orthogonal frequency division multiplexing techniques are employed. The presence of sunlight reduces the system's performance, hence sophisticated adaptive techniques must be applied. Closed-form expressions of the signal-to-noise ratio and of the mean cell data rate are derived and our simulations demonstrate their accuracy. Besides, the outage probability when adaptive modulation and coding schemes are employed is analytically expressed. It is shown that, when modulation bandwidth adaptation is carried out depending on the time of day and the illuminance from ambient light, the mean cell data rate is increased and the outage probability is reduced.This work was supported in part by the Spanish National ELISA Project under Grant TEC2014-59255-C3-3-R, the TERESA-ADA Project under MINECO/AEI/FEDER, UE Grant TEC2017-90093-C3-2-R and the 5RANVIR Project under MINECO/AEI/FEDER, UE Grant TEC2016-80090-C2-1-R. The work of B. Genovés Guzmán was supported by the Spanish MECD FPU Fellowship Program. The work of M. C. Aguayo-Torres was supported by the Universidad de Málaga. The work of H. Haas was supported in part by EPSRC through the Established Career Fellowship Extension under Grant EP/R007101/1 and in part by the Wolfson Foundation and the Royal Society. The work of L. Hanzo was supported in part by EPSRC under Project EP/Noo4558/1 and Project EP/PO34284/1, in part by the Royal Society's GRFC Grant, and in part by the European Research Council's Advanced Fellow Grant QuantCom

Superimposed training-based channel estimation for miso optical-OFDM vlc

In this paper, we investigate a novel channel estimation (CE)method for multiple-input and single-output (MISO) systems in visible lightcommunication (VLC). Direct current biased optical orthogonal frequencydivision multiplexing (DCO-OFDM) is commonly used in VLC where halfof the available subcarriers are spent to guarantee a real-valued outputafter the inverse fast Fourier transform operation. Besides, dedicated subcarriers are typically used for CE, thus, many resources are wasted andthe spectral efficiency is degraded. We propose a superimposed trainingapproach for CE in MISO DCO-OFDM VLC scenarios. Analytical expressions of mean squared error (MSE) and spectral efficiency are derived whenthe least squares estimator is considered. This analysis is valid for outdoorand indoor scenarios. For the CE error, simulation results of MSE showa perfect match with analytical expressions. Moreover, results prove thatthis technique guarantees a larger spectral efficiency than previous schemeswhere dedicated pilots were used. Finally, the optimal data power allocationfactor is also analytically derived.This work was supported in part by the National Secretary of Higher Education, Science, Technology, and Innovation (SENESCYT) in Ecuador and in part by the Spanish National Project TERESA-ADA (TEC2017-90093-C3-2-R) (MINECO/AEI/FEDER, UE). The work of B. G. Guzmán was supported by the Spanish MECD FPU Fellowship Program