José Antonio García-Naya

Experimental validation of Interference Alignment techniques using a multiuser MIMO testbed

Hardware platforms and testbeds are an essential tool to evaluate, in realistic scenarios, the performance of wireless communications systems. In this work we present a multiuser Multiple-Input Multiple-Output (MIMO) testbed made up of 6 nodes, each one with 4 antennas, which allows us to evaluate Interference Alignment (IA) techniques in indoor scenarios. We specifically study the performance of IA for the 3-user interference channel in the 5GHz band. Our analysis identifies the main practical issues that potentially degrade the IA performance such as channel estimation errors or collinearity between the desired signal and interference subspaces.

Experimental Evaluation of Analog Joint Source-Channel Coding in Indoor Environments

Recently, analog joint source-channel coding has been proposed as a means of achieving near-optimum performance for high data rates with a very low complexity. However, no experimental evaluation showing the practical feasibility of this scheme has been performed to date. In this paper, we describe a software-defined radio implementation of an analog joint source-channel coded wireless transmission system. Experimental evaluation carried out in an indoor environment making use of a wireless testbed show that the performance perfectly matches that originally reported by simulations in additive white Gaussian noise channels for signal-to-noise ratio values below 20 dB.

Blind channel identification in Alamouti coded systems: a comparative study of eigendecomposition methods in indoor transmissions at 2.4 GHz†

This paper focuses on blind channel estimation in Alamouti coded systems with one receiving antenna working in indoor scenarios where the flat fading assumption is reasonable. A comparative study of several channel estimation techniques in both simulated and realistic scenarios is presented. The tested methods exploit the orthogonality property of the Alamouti coded channel matrix, and are based on the eigendecomposition of a square matrix made up of second-order statistics (SOS) or higher order statistics (HOS) of the observed signals. An experimental evaluation is carried out on a testbed developed at the University of A Coruna (UDC) and operating at 2.4 GHz. The results show the superior performance of the SOS-based blind channel estimation technique in both line of sight (LOS) and non-LOS (NLOS) channels.

On the Feasibility of Time-Modulated Arrays for Digital Linear Modulations: A Theoretical Analysis

Wireless communications are widely based on linear digital modulation methods, which introduce the digital baseband information in the amplitude and/or in the phase of a carrier sinusoid. Time-modulated arrays, on the other hand, constitute an interesting technique capable of adapting the power radiation pattern of an antenna array. In this work, we study from a theoretical point of view the impact of time-modulated arrays on the transmission of linearly modulated digital communication signals. The work focuses on the requirements to safeguard the integrity of the signal and on the quantification of the useful power radiated within the desired band.

LTE Downlink Performance in High Speed Trains

Long Term Evolution (LTE) is expected to substitute the Global System for Mobile Communications (GSM) as the radio access technology for railway communications. Recently, especial attention has been devoted to high-speed trains since this particular environment poses challenging problems in terms of performance simulation and measurement. In order to severely decrease the cost and complexity of high-speed measurement campaigns, we have proposed a technique to induce effects caused by highly-time varying channels on Orthogonal Frequency-Division Multiplexing (OFDM) signals while conducting measurements at low speeds. In this work, we evaluate this technique by comparing the results of LTE measurements at different velocities as well as by simulations. Additionally, we use this technique to show the performance of LTE for high-speed train scenarios up to 600 km/h. To accomplish this, we use both a controlled high-speed measurement setup as well as a channel model developed according to the guidelines of the ITU Radiocommunication Sector (ITU-R) for the evaluation of radio interface technologies for IMT- Advanced systems.

Experimental validation of ICI-Aware OFDM receivers under time-varying conditions

Orthogonal Frequency Division Multiplexing (OFDM) communications under highly time-selective channels are severely affected by Inter-Carrier Interference (ICI). Estimation and cancellation of ICI in OFDM systems has been thoroughly studied, but few empirical measurements of the performance of such techniques have been done. We present a wireless communication testbed and a methodology to evaluate the performance of OFDM transmissions in real-world scenarios affected by large Doppler spreads. We show that it is possible to induce such large Doppler spreads while conducting experiments with a moving vehicle at a low speed.

Synchronization of wireless radio testbed measurements

Measuring the physical layer performance of a wireless radio transmission is straightforward: Firstly, a transmitter generates the signal-samples to be transmitted. Secondly, these samples are broadcasted in real-time and captured by the receiver. Thirdly, the received signal is evaluated and the desired figure of merit is calculated. If only isolated blocks of data are transmitted and the received signal is evaluated off-line to simplify implementation, the receiver has to know when to actually acquire the data. The situation becomes more complicated when several transmitters and receivers have to be synchronized in time. In this paper we will present a method to synchronize an arbitrary number of transmitters and receivers that is swift (e.g. 1 ms, the latency of an already exiting LAN infrastructure) as well as accurate (e.g. ±30 ns, the relative accuracy of GPS).

A Multicore SDR Architecture for Reconfigurable WiMAX Downlink

This paper describes a multicore Software Defined Radio (SDR) architecture devised to implement a fully reconfigurable downlink for WiMAX transceivers. The proposed architecture is made up of Commercial-Off-The-Shelf (COTS) modules available in the market and includes a DSP, three different models of FPGAs, DACs and ADCs. We show that the architecture is capable of supporting all the functionalities of the downlink sub frame of the Orthogonal Frequency Division Multiple Access (OFDMA) WiMAX physical layer, including Partial Usage of Sub carriers (PUSC) symbol structure and Forward Error Correction (FEC). The primary advantage of the design is the full reconfigurability at different levels: bandwidth, size of the FFT, modulation, code rate, etc. without modifying or restarting the system. We show that the five downlink profiles defined by the WiMAX Forum can be successfully implemented with the proposed achitecture.

A Testbed for Evaluating LTE in High-Speed Trains

LTE is expected to substitute GSM as the basis technology for railway communications. Recently, special attention has been deserved to HST as this particular environment (mainly due to the high speed condition) can severely impact wireless systems performance. Although several channel models have been derived during the few last years, most of them are not accurate enough as they are not supported by measurement campaigns. In this paper, the main requirements for HST environments are analyzed and a flexible, cost-affordable, and easily-scalable software and hardware architecture for a test bed suitable for assessing LTE at high speeds is proposed.

A methodology for repeatable, off-line, closed-loop wireless communication system measurements at very high velocities of up to 560 km/h

The impact of high velocities on the physical layer downlink performance of mobile radio communication systems is generally measured by placing a receiver in a car, train, or similar vehicle. While these so-called drive test measurements produce valuable results, they lack the flexibility, repeatability, and controllability usually required for initial testing of ideas and algorithms. In this paper, we present a methodology that allows for repeat-able, closed-loop, off-line-processed measurements at velocities up to 560 km/h (350 mph). The proposed laboratory set-up allows for precise controlling of velocity and average signal-to-noise ratio. For increased convenience during initial testing, the apparatus can be even used indoors.