Mari Carmen Aguayo-Torres

Impact of channel estimation error on adaptive modulation performance in flat fading

Presented here is an approach to analyzing the effect of imperfect channel estimation on adaptive modulation. The sensitivity of the main performance parameters to short-term and long-term estimation error sources is summarized in a set of new formulas that are either closed-form expressions or simple to compute numerically.

BER Analysis for Zero-Forcing SC-FDMA Over Nakagami-m Fading Channels

In this paper, we present an analytical study of the bit error rate (BER) for single-carrier frequency-division multiple access (SC-FDMA) transmission over frequency-selective fading channels when zero-forcing frequency-domain equalization is applied. SC-FDMA, which can be described as a precoded version of orthogonal frequency-division multiple access (OFDMA), is regarded as a promising candidate for next mobile communication systems due its favorable envelope characteristics and low peak-to-average-power ratio (PAPR), compared with that of OFDMA. We focus on Nakagami-m fading channels and provide a method to calculate BER values with a single numerical computation. We provide a closed-form expression for the BER with binary phase-shift keying (BPSK) and square M-ary quadrature amplitude modulation (M-QAM) under the assumption of independence among channel frequency responses for allocated subcarriers.

Analytical Modeling of Interference Aware Power Control for the Uplink of Heterogeneous Cellular Networks

Inter-cell interference is one of the main limiting factors in current heterogeneous cellular networks. Uplink fractional power control (FPC) is a well-known method that aims to cope with such a limiting factor and to save the battery life of the mobile terminals (MTs). In order to do that, the transmit power of each MT is adjusted as a function of a set of parameters that usually depend only on the link between MTs and serving base station (BS), such as the desired received power at the serving BS or the path loss between the MT and its serving BS. Contrary to these classical FPC schemes, in this paper, we use stochastic geometry to analyze a power control mechanism that keeps the interference generated by each MT under a given threshold. We also consider a maximum transmitted power and a partial compensation of the path loss. Our analysis reveals that such an interference aware method can reduce the average power consumption and increase the average spectral efficiency at once. In addition, the variance of the interference is reduced, thus improving the performance of adaptive modulation and coding schemes, since the interference can be better estimated.

YouTube QoE evaluation tool for Android wireless terminals

In this paper, we present an Android application which is able to evaluate and analyze the perceived quality of experience (QoE) for YouTube service in wireless terminals. To achieve this goal, the application carries out measurements of objective quality of service (QoS) parameters, which are then mapped onto subjective QoE (in terms of mean opinion score, MOS) by means of a utility function. Our application also informs the user about potential causes that lead to a low MOS as well as provides some hints to improve it. After each YouTube session, the users may optionally qualify the session through an online opinion survey. This information has been used in a pilot experience to correlate the theoretical QoE model with real user feedback. Results from such an experience have shown that the theoretical model (taken from the literature) provides slightly more pessimistic results compared to user feedback. Users seem to be more indulgent with wireless connections, increasing the MOS from the opinion survey in about 20% compared to the theoretical model, which was obtained from wired scenarios.

Closed-Form BER Expression for Interleaved SC-FDMA with M-QAM

One of the novelties of the forthcoming LTE technology is the use of SC-FDMA as the transmission technique in the uplink. In this paper a study of the BER in an interleaved SC-FDMA system over a fading channel is presented. This study provides the mathematical grounds to obtain a closed-form expression for the BER for BPSK and square M-QAM. Results are validated by means of comparison with simulations.

Optimized interpolator filters for timing error correction in DMT systems for xDSL applications

Discrete multitone (DMT) modulation is a multicarrier technique that allows the transmission of high speed data over band limited channels. This type of system is very sensitive to synchronization errors when used in digital subscriber loop applications (xDSL), due to the high number of carriers and high density constellations involved. This paper addresses the topic of all-digital timing error correction in a DMT system for xDSL applications, where the timing error correction procedure is based entirely on signal interpolation. An analytical study of the interpolator filter performance is carried out, arriving at an expression for the signal-to-distortion ratio (SDR) at the output of the receiver, as a merit figure. In this derivation, the fixed frequency domain equalizer (FEQ) plays an important role since it compensates for a great part of the distortion introduced by the interpolator. From this study, the design of the optimal interpolator filter in terms of SDR, based on a multirate approach with Kaiser window, is presented. Specific designs for ASIC and for DSP-based implementations are obtained. Performance results are excellent, yielding SDR values above 50 dB for all carriers while keeping the computational cost low.

Performance of LTE in Vehicle-to-Vehicle Channels

Vehicle-to-vehicle (V2V) communications have seen growing attention in the last few years. In this work, we address the use of the 3GPP Long Term Evolution (LTE) standard technology applied to V2V. We employ empirical models for the V2V channel, and show results from comprehensive computer simulations, based upon both the uplink (UL) and downlink (DL) standards for LTE. We provide results for 10 MHz and 20 MHz channels, in terms of BER, BLER, spectral efficiency and throughput, using multiple MIMO modes. Results show the attractive feasibility of the LTE technology in V2V communication systems.

Capacity with Probabilistic Delay Constraint for Rayleigh Channels

In this work, the maximum constant rate that can be supported with a probability epsiv f exceeding certain delay bound D t is evaluated. Following the works in (Wu, 2003), the procedure to derive this capacity with probabilistic delay constraint CD t,epsiv is described. A simple closed form analytical result is given for block fading channel and different adaptive rate policies. The extension to the general correlated Rayleigh fading is also provided based on a semi-analytical strategy and the previous result for independent samples. As expected, CD t,epsiv tends to the ergodic capacity for long allowed delays (D t rarr infin) and diminishes when the delay constraint is more strict. The expected delay violation probability is compared to simulations in order to validate our results.

LTE Performance over High Speed Railway Channel

Global System for Mobile Communications Railways (GSM-R) has been the de facto standard for wireless communications in the field of High Speed Railway (HSR). However due to the increasing requirements associated with HSR, Long Term Evolution for Railways (LTE-R) has been presented as the following wireless communication system. In this paper a complete performance analysis of LTE for HSR is carried out giving results for both Physical (PHY) and Medium Access Control (MAC) layer in order to identify appropriate Quality of Service (QoS) requirements. An adequate Rician channel model with a time-varying Doppler frequency offset has been integrated in a Downlink LTE simulator using Wireless Mobile SIMulator (WM-SIM) platform. The effect of Inter- Carrier Interference (ICI) degrades the Bit Error Rate (BER) whereas the reduced coherence time makes obsolete channel state information reducing the performance of adaptive modulation and coding (AMC). For this reason open loop Multiple Input Multiple Output (MIMO) techniques are attractive. Results illustrate the benefits of LTE in HSR scenario.

Spectral Efficiency of Interleaved SC-FDMA With Adaptive Modulation and Coding Over Nakagami-

Because it achieves similar performance to that of orthogonal frequency-division multiple access (OFDMA) with much more favorable envelope characteristics and lower peak-to-average power ratio (PAPR), single-carrier frequency-division multiple access (SC-FDMA) was chosen as the uplink transmission technology for Long Term Evolution (LTE) and LTE-Advanced systems. This choice has provoked a growing interest in this transmission technology; therefore, there is extensive literature on its performance in terms of bit error rate (BER) and PAPR. However, research on its spectral efficiency has been scarce until now. This paper aims to help fill this gap with an analytical study of the spectral efficiency in SC-FDMA when adaptive modulation and coding (AMC) and linear frequency-domain equalization are applied. This paper therefore computes the spectral efficiency for zero-forcing (ZF) and minimum-mean-square-error frequency-domain equalization (MMSE-FDE) for different Nakagami-m fading channels.