José A. López-Salcedo

Opportunistic relay selection with outdated CSI: outage probability and diversity analysis

In this paper, we analyze the outage probability and diversity order of opportunistic relay selection in a scenario based on decode and forward and where the available channel state information (CSI) is outdated. The study is conducted analytically by obtaining a closed-form expression for the outage probability, which is defined as the probability that the instantaneous capacity is below a target value. We derive high-SNR approximations for the outage probability. By doing so, we demonstrate that the diversity order of the system is reduced to 1 when CSI is outdated, being this behavior independent of the level of CSI accuracy. A physical explanation for this extreme loss of diversity is provided along with numerical results to support the analytical study.

Challenges in Indoor Global Navigation Satellite Systems: Unveiling its core features in signal processing

Accurately determining ones position has been a recurrent problem in history [1]. It even precedes the first deep-sea navigation attempts of ancient civilizations and reaches the present time with the issue of legal mandates for the location identification of emergency calls in cellular networks and the emergence of location-based services. The science and technology for positioning and navigation has experienced a dramatic evolution [2]. The observation of celestial bodies for navigation purposes has been replaced today by the use of electromagnetic waveforms emitted from reference sources [3].

Achievable localization accuracy of the positioning reference signal of 3GPP LTE

Robustness of nominal Global Navigation Satellite Systems (GNSS) performance can be enhanced by means of complimentary systems, such as the Long Term Evolution (LTE). Particularly, the LTE standard specifies a dedicated downlink signal for positioning purposes, i.e. the positioning reference signal (PRS). This paper presents the achievable localization accuracy of the PRS signal for different interference LTE scenarios by means of the Crámer-Rao bound (CRB) for time delay estimation, in order to assess the LTE positioning capabilities.

Unified Framework for the Synchronization of Flexible Multicarrier Communication Signals

This paper presents a unified framework for the formulation of synchronization algorithms dealing with the general class of Filter-Bank Multi-Carrier (FBMC) communication signals. This is a wide family of signaling formats that includes Orthogonal Frequency Division Multiplexing (OFDM), for instance, as just one of many particular cases. One of the main contributions of this work is the proposal of a novel matrix signal model for so-called flexible FBMC signals, in which no restrictions are imposed on the signal design parameters (i.e., pulse shaping, symbol rate, carrier spacing, sampling frequency, etc.), unlike the type of multicarrier signals currently deployed. As an example of application of the proposed matrix signal model, blind joint time-delay and frequency estimators will be derived under both the Conditional Maximum Likelihood (CML) and the low-SNR Unconditional Maximum Likelihood (UML) principles, for any FBMC signal propagating through an arbitrary multipath channel. These estimators will be specialized first for the case of critically sampled cyclic-prefix OFDM (CP-OFDM) signals in frequency flat fading, leading to simple architectures amenable to a hardware implementation. Later on, and for the low-SNR UML principle, the special case of critically sampled CP-OFDM in multipath channels will be addressed, where a novel synchronizer will be proposed.

Massive MIMO for Wireless Sensing With a Coherent Multiple Access Channel

We consider the detection and estimation of a zero-mean Gaussian signal in a wireless sensor network with a coherent multiple access channel, when the fusion center (FC) is configured with a large number of antennas and the wireless channels between the sensor nodes and FC experience Rayleigh fading. For the detection problem, we study the Neyman-Pearson (NP) detector and energy detector (ED) and find optimal values for the sensor transmission gains. For the NP detector, which requires channel state information (CSI), we show that detection performance remains asymptotically constant with the number of FC antennas if the sensor transmit power decreases proportionally with the increase in the number of antennas. Performance bounds show that the benefit of multiple antennas at the FC disappears as the transmit power grows. The results of the NP detector are also generalized to the linear minimum mean-squared error estimator. For the ED, which does not require CSI, we derive optimal gains that maximize the deflection coefficient of the detector, and we show that a constant deflection can be asymptotically achieved if the sensor transmit power scales as the inverse square root of the number of FC antennas. Unlike the NP detector, for high sensor power, the multi-antenna ED is observed to empirically have significantly better performance than the single-antenna implementation. A number of simulation results are included to validate the analysis.

Survey on Robust Carrier Tracking Techniques

In most wired and wireless systems, carrier tracking is an essential task that allows the receiver to precisely synchronize with the carrier of the incoming signal. Stringent carrier tracking requirements are imposed in systems that are sensitive to carrier mismatches, such as orthogonal frequency division multiplexing (OFDM), digital communication receivers employing high-order constellations, and terrestrial- or satellite-based positioning systems, just to mention a few. In the recent years, even more critical requirements are being imposed due to the emergence of new applications and services that are pushing traditional systems to operate in much more challenging conditions than the ones for which they were originally designed. The presence of severe fading, signal outages, abrupt phase changes and high user dynamics, are currently compromising the validity of well-known and long-established traditional carrier tracking techniques, thus calling for the development of new robust carrier tracking algorithms. In this paper, we provide a detailed survey on the five main strategies that can be adopted to cope with the technical challenges of robust carrier tracking. These strategies range from some basic optimizations of current tracking loops, to the use of Kalman filter-based architectures, or the application of innovative carrier tracking techniques based on particle filters or compressive sensing. We will also review some open-loop techniques, which are widely adopted in burst-mode communications receivers, as an alternative and potential candidate solution for robust carrier tracking in harsh conditions.

Comparative results analysis on positioning with real LTE signals and low-cost hardware platforms

Long Term Evolution (LTE) networks are rapidly deploying around the world, covering the needs of high data rates demanded by many applications. Still, less attention is paid on the positioning capabilities specified in the LTE standard. Thus, an experimental LTE positioning receiver is presented to assess the positioning accuracy in commercial LTE deployments. This receiver is based on a software defined radio (SDR) and a low-cost radio-frequency (RF) front-end, such as the universal software radio peripheral (USRP) or a DVB-T dongle with the Realtek RTL2832U chipset. These two platforms are then used to capture and post-process real LTE signals generated in the laboratory. The positioning results obtained show the viability on the use of this experimental SDR LTE positioning receiver with low-cost hardware platforms for commercial LTE networks.

Joint maximum likelihood time-delay estimation for LTE positioning in multipath channels

This paper presents a joint time-delay and channel estimator to assess the achievable positioning performance of the Long Term Evolution (LTE) system in multipath channels. LTE is a promising technology for localization in urban and indoor scenarios, but its performance is degraded due to the effect of multipath. In those challenging environments, LTE pilot signals are of special interest because they can be used to estimate the multipath channel and counteract its effect. For this purpose, a channel estimation model based on equi-spaced taps is combined with the time-delay estimation, leading to a low-complexity estimator. This model is enhanced with a novel channel parameterization able to characterize close-in multipath, by introducing an arbitrary tap with variable position between the first two equi-spaced taps. This new hybrid approach is adopted in the joint maximum likelihood (JML) time-delay estimator to improve the ranging performance in the presence of short-delay multipath. The JML estimator is then compared with the conventional correlation-based estimator in usual LTE conditions. These conditions are characterized by the extended typical urban (ETU) multipath channel model, additive white Gaussian noise (AWGN) and LTE signal bandwidths equal to 1.4, 5 and 10 MHz. The resulting time-delay estimation performance is assessed by computing the cumulative density function (CDF) of the errors in the absence of noise and the root-mean-square error (RMSE) and bias for signal-to-noise ratio (SNR) values between −20 and 30 dB.

Simple Closed-Form Approximation to Ricean Sum Distributions

This letter focuses on the approximation of Ricean sum distributions of L independent random variables. Although no closed-form expression is available for these exact distributions, a simple but accurate closed-form approximation is proposed. The expression is based on the Central Limit Theorem (CLT) for which an appropriate correction term is added. The result is a rather tight fit with the exact distribution for a wide range of input values and Ricean K-factors.

Asymptotic equivalence between the unconditional maximum likelihood and the square-law nonlinearity symbol timing estimation

This paper provides a systematic approach to the problem of nondata aided symbol-timing estimation for linear modulations. The study is performed under the unconditional maximum likelihood framework where the carrier-frequency error is included as a nuisance parameter in the mathematical derivation. The second-order moments of the received signal are found to be the sufficient statistics for the problem at hand and they allow the provision of a robust performance in the presence of a carrier-frequency error uncertainty. We particularly focus on the exploitation of the cyclostationary property of linear modulations. This enables us to derive simple and closed-form symbol-timing estimators which are found to be based on the well-known square timing recovery method by Oerder and Meyr. Finally, we generalize the OM method to the case of linear modulations with offset formats. In this case, the square-law nonlinearity is found to provide not only the symbol-timing but also the carrier-phase error.