MIMO OTA testing based on transmit signal processing

Usually, multiple-input-multiple-output (MIMO) testbeds are combined with channel emulators for testing devices and algorithms under controlled channel conditions. In this work, we propose a simple methodology that allows over-the-air (OTA) MIMO testing using a MIMO testbed solely, avoiding the use of channel emulators. The MIMO channel is emulated by linearly combining the signals at the testbed transmitter. The method is fully flexible, so it is able to emulate any equivalent baseband narrowband MIMO channel by adequately selecting the weights of the linear combination. We derive closed-form expressions for the computation of such weights. To prove its feasibility, the method has been implemented and tested over a commercial MIMO testbed.This work was supported by the Spanish Government, Ministerio de Ciencia e Innovacain (MICINN), under projects COSIMA (TEC2010-19545-C04-03) and COMONSENS (CSD2008-00010, CONSOLIDER-INGENIO 2010)

Performance analysis of SNR-based scheduling policies in asymmetric broadcast ergodic fading channel

We analyze the performance of SNR-based scheduling algorithms in broadcast ergodic fading channels where multiuser selection diversity is exploited. At each channel state, the user with the highest weighted signal-to-noise ratio is selected to be transmitted. The use of weights associated to the users allows us to control the degree of fairness among users and to arrange them according to a prescribed quality of service. These weights parametrize the scheduling algorithms so each set of weights corresponds to a specific scheduling algorithm. Assuming Rayleigh fading broadcast channel, we derive a closed-form expression for the achievable user’s rates as a function of the scheduling algorithm, the channel fading statistics of each user, and the transmit power. With the help of this expression, we solve some interesting inverse problems. For example, for a given arbitrary channel statistics we obtain the optimum scheduling algorithm to achieve a prescribed set of users’ rates with minimum transmit power

Experimental evaluation of flexible duplexing in multi-tier MIMO networks

In this paper, we present an experimental evaluation of the performance benefits provided by flexible duplexing, an access technique that allows uplink and downlink cells to coexist within the same time-frequency resource blocks. In order to replicate a wireless multi-tier network composed of 1 macro-cell and 2 small cells, a measurement campaign has been conducted using an indoor wireless testbed comprised of a total of 6 multiple-input multiple-output (MIMO) software-defined radio (SDR) devices. Since each cell has a single active user, each uplink/downlink configuration can be identified with a different interference channel, over which interference alignment (IA) is used as an inter-cell interference management technique and compared to other existing methods. The obtained results show that flexible duplexing clearly outperforms the conventional time-division duplex (TDD) access approach, where all cells operate synchronized either in uplink or dowlink mode. Additionally, interference alignment consistently provides better results in most of the interference regimes when compared to minimum means quare error (MMSE)-based schemes. The impact of channel estimate quality on the different communication strategies is also studied. It is worth highlighting that the presented over-the-air (OTA) experiments represent the first implementation of IA with real-time precoding and decoding.The work of Jacobo Fanjul, Jesús Ibáñez and Ignacio Santamaria has been supported by the Ministerio de Economía, Industria y Competitividad (MINECO) of Spain, and AEI/FEDER funds of the E.U., under grant TEC2016-75067-C4-4-R (CARMEN), grant PID2019-104958RB-C43 (ADELE), and FPI grant BES-2014-069786. The work of José A. García-Naya has been funded by the Xunta de Galicia (ED431G2019/01), the Agencia Estatal de Investigación of Spain (TEC2016-75067-C4-1-R, RED2018-102668-T), and ERDF funds of the E.U. (AEI/FEDER, UE)

Expresiones cerradas de capacidad de sistemas MIMO 2X2 en canales arbitrarios

Recent works have shown the potential performance MIMO systems based on dual-polarized antennas at transmitter and receiver. These works assume Rayleigh or Ricean channel models. In this paper we provide closed-form expressions of the ergodic capacity for arbitrary channels, being the Rayleigh and Ricean two particular cases. These expressions are function of the specific physical characteristics of the propagation environment and antennas, therefore they directly connect the ergodic capacity with the physical characteristics of the MIMO channel

Expresiones analíticas de capacidad de sistemas MIMO-OSFBC

Orthogonal frequency-division multiplexing (OFDM) combined with orthogonal space-frequency block coding (OSFBC) has been shown to be a simple and efficient means to exploit the inherent spatial diversity of multiple-input-multipleoutput (MIMO) configurations in frequency-selective channels. In this paper we derive simple analytical closed-form expressions for the ergodic and outage capacity of OSFBC-OFDM systems assuming that the channel is unknown at the transmitter. The resulting expressions are simple functions of the spatial correlation matrices at the channel taps. They clearly reveal the dependence of the capacity on the channel and system parameters. Numerical results show the excellent accuracy of the derived expressions

Modeling nonlinear power amplifiers in OFDM systems from subsampled data: a comparative study using real measurements

A comparative study among several nonlinear high-power amplifier (HPA) models using real measurements is carried out. The analysis is focused on specific models for wideband OFDM signals, which are known to be very sensitive to nonlinear distortion. Moreover, unlike conventional techniques, which typically use a single-tone test signal and power measurements, in this study the models are fitted using subsampled time-domain data. The in-band and out-of-band (spectral regrowth) performances of the following models are evaluated and compared: Saleh’s model, envelope polynomial model (EPM), Volterra model, the multilayer perceptron (MLP) model, and the smoothed piecewise-linear (SPWL) model. The study shows that the SPWL model provides the best in-band characterization of the HPA. On the other hand, the Volterra model provides a good trade-off between model complexity (number of parameters) and performance

Capacidad ergódica de sistemas MIMO basados en antenas con polarización dual

MIMO systems based on dual-polarized antennas at transmitter and receiver constitute an interesting alternative to conventional MIMO configurations. This paper analyzes the ergodic capacity of such systems in urban micro- and picocellular environments. The MIMO channel is modeled by using a site-specific ray-tracing propagation tool. This technique permits to analyze the impact of environmental parameters, like antennas location and orientation, on the system performance. Ergodic capacity estimations in a specific urban environment are presented

Caracterización banda ancha de canal MIMO a 5GHz en interiores

This paper presents a simple characterization methodology for broadband Multiple-Input Multiple-Output (MIMO) channels in indoor environments. The overall procedure involves the transmission of a set of phase-optimized complex exponentials within the band of interest, the subsequent acquisition through the receive antennas, and the frequency and amplitude estimation of the transmitted exponentials with high-accuracy methods based on the Fast Fourier Transform (FFT). In addition to its simplicity, other advantage of the proposed methodology is that hard tasks such as synchronization and detection are avoided. Measurements using the proposed method have been obtained using a 4x4 flexible dual band (2.4/5 GHz) MIMO testbed

Evaluation criteria for Spanish as a foreign language: an analysis of indexes and properties in current curricula

Ante la prioridad y necesidad de homogeneidad y coherencia en la evaluación de las competencias en lenguas para la homologación y transparencia en los diplomas y las certificaciones, los autores se remiten –por una parte- a los referentes curriculares actuales: el Marco común europeo de referencia para las lenguas: enseñanza, aprendizaje, evaluación (2002) y, por su carácter subsidiario y actualizado, el Plan Curricular del Instituto Cervantes (2006). Por otra parte, y con la finalidad de optimizar las acciones relativas a la evaluación del ELE en particular, elaboran un dispositivo cuya modelización exponen en sus dos niveles: concepción y aplicación. Dicho dispositivo -mediante los procedimientos y las estrategias propias del análisis de contenido como método de indagación- está orientado a proporcionar a los profesionales implicados un repertorio de criterios de evaluación, definidos de forma pertinente y utilitaria, que sirva de guía para la toma de decisiones en cada situación de enseñanza/ aprendizaje del ELE.Bearing in mind the priority and the need of the homogeneity and coherence of the languages competences assessment in order to get the homologation and transparency of the diplomas and certification, the authors refer to the present curricula references: Common European Framework of Reference for Languages: Learning, Teaching, Assessment (2002) and Plan Curricular del Instituto Cervantes (2006) because of their subsidiary and updated character. On the other hand, the authors will create a devise which is defined into two different levels: conception and application, in order to optimize the actions on the SFL assessment. By means of the procedures and strategies of the content analysis as a research method, this device will give the professionals a repertoire of assessment criteria, defined in useful and relevant way as a guide in order to take decisions in each SFL learning /teaching situations

Detection of radio receivers: an experimental evaluation approach

This paper considers the problem of detecting extremely low power emissions due to local oscillator (LO) leakage. This challenging problem arises in cognitive radio (CR) networks when trying to detect a primary user in reception mode (the hidden node problem), and also in physical layer security applications when trying to determine the presence of an eavesdropper. In contrast to the numerous works on the detection of an active radio transmitter, the problem addressed in this paper still requires further research. As a matter of fact, previous studies focus on the theoretical analysis of the detection strategies, and the reported performances do not take into account the additional limitations imposed by the existing hardware technologies. In this paper, we provide an experimental evaluation of two different detectors based on the averaged periodogram or the total energy, respectively. Moreover, we propose practical guidelines to overcome the hardware limitations and maximize the detection performance. Our experimental measurements using Universal Software Radio Peripheral (USRP) boards reveal that the widely employed energy detector is outperformed by the periodogrambased detector