Pedro Suarez-Casal

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.

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.

Experimental assessment of WiMAX transmissions under highly time-varying channels

Modern wireless communication systems, such as WiMAX, rely on Orthogonal Frequency Division Multiplexing, a very robust modulation against frequency-selectivity, but very sensitive to time-selectivity, which is a typical feature in mobility scenarios. In this work, experimental measurements in real scenarios are carried out by employing WiMAX signals including the standard-compliant pilot signals. Highly time-varying conditions are induced by enlarging the symbol period by time interpolation in the digital domain. In addition, the performance of Inter-Carrier Interference cancellation techniques, which take advantage of the WiMAX pilot structure, are tested to evaluate their behaviour under highly time-varying conditions.

Design and implementation of an OFDMA-TDD physical layer for WiMAX applications

This work describes the design, implementation, and performance evaluation of an orthogonal frequency division multiple access (OFDMA) time-division duplexing (TDD) physical layer (PHY) compliant with the worldwide interoperability for microwave access (WiMAX) standard using a costeffective software-defined radio (SDR) platform containing field programmable gate array (FPGA) and digital signal processor (DSP) modules. We show that the proposed SDR architecture is capable of supporting the wide variety of configuration options described in the WiMAX standard while fulfilling the stringent requirements of WiMAX OFDMA TDD PHYs. The architecture allows for the implementation of all TDD functionalities in the downlink and the uplink at both the base station and the mobile station. The proposed design is shown to efficiently use the available FPGA and DSP resources. We also carried out specific experiments that take into account the frame and the downlink map messages detection over ITU-R wireless channel models to illustrate the performance of the proposed design. Finally, we discuss the utilization of the proposed hardware architecture to implement the wirelessMAN-advanced air interface.

KLT-based estimation of rapidly time-varying channels in MIMO-OFDM systems

MIMO-OFDM is a transmission method that is very sensitive to the channel time-variations arising in high mobility scenarios due to the emergence of Inter-Carrier Interference (ICI). In this paper we propose a channel estimation method using a basis expansion model that incorporates the knowledge of the channel Doppler spectrum, taking into account the statistics of the averaged coefficients of the channel. The advantage of the proposed approach is that it does not require a specific pilot subcarrier structure to estimate the ICI, and outperforms existing methods in the literature.

Analog distributed coding of correlated sources for fading multiple access channels

In this work, we address the analog transmission of correlated information over fading Multiple Access Channels (MACs) using analog Joint Source Channel Coding (JSCC). We consider module-like mappings to encode the source data and the utilization of different orthogonal access schemes. The user information is individually mapped at each transmitter and the receiver exploits the source correlation and the properties of the module mappings to decode the received symbols. We also propose a Maximum-A-Posteriori (MAP) method which achieves similar performance to that of the optimal decoding with significantly lower complexity. The obtained results confirm the potential of using analog JSCC techniques to transmit correlated data over fading MACs.

Parametric analog mappings for correlated Gaussian sources over AWGN channels

We address the transmission of bivariate Gaussian sources using analog Joint Source Channel Coding (JSCC). The analog mappings are specifically designed to exploit the correlation between the source symbols. A parametric mapping based on sinusoidal functions is proposed and its performance is compared to that of the optimal non parametric mappings and other applicable analog JSCC mappings, and also to the theoretical bound. The obtained results show that the performance of the parametric mapping closely approaches the optimal performance, and it is very similar to that of the non parametric one.

Analog Transmission of Spatio-Temporal Correlated Sources Over MAC With Modulo Mappings

Modulo mappings is an appealing scheme for the analog transmission of spatially correlated discrete-time analog source symbols over multiple access channels. In this letter, we show that the modulo mappings are also useful to exploit temporal correlation with zero-delay and without impairing encoding/decoding computational complexity. Spatio-temporal source correlation is exploited with a Kalman filter-based receiver, coupled with a sphere decoder, that takes into account the non-linearities of the modulo mappings. We also explain how to optimize the mapping parameters.

Hybrid digital-analog joint source channel coding for broadcast multiresolution communications

Multilayer coding represents an appealing solution for the broadcasting of common data when we want to ensure the receivers decode the signal with different distortion levels depending on the quality of the received signal. In addition, the combination of digital and analog techniques to encode the source data allows to solve the limitation of these approaches when they are applied separately. In this work, we consider a Hybrid Digital-Analog (HDA) multilayer system where the digital layers are employed to satisfy different Quality of Service (QoS) requirements and an analog layer refines the estimates of the source symbols. The resulting scheme provides good performance for different expansion factors and number of layers, and it also presents good scaling for all SNR values.