The estimation of the atrial activity (AA) signal from electrocardiogram (ECG) recordings is an important step in the noninvasive analysis of atrial fibrillation (AF), the most common sustained arrhythmia encountered in clinical practice. This problem admits a blind source separation (BSS) formulation that has been recently posed as a tensor factorization, using the Hankel-based block term decomposition (BTD), which is particularly well-suited to the estimation of exponential models like AA during AF. However, persistent forms of AF are characterized by short R-R intervals and very disorganized (or weak) AA, making it difficult to model AA directly and perform its successful extraction through Hankel-BTD. To overcome this drawback, the present work proposes a tensor approach to estimate QRS complexes and subtract them from the ECG, resulting in a signal that, ideally, only contains the AA component. Such an approach tackles the problem of blind separation of rational functions, which models QRS complexes explicitly. The data tensor admitting a BTD is built from Löwner matrices generated from each lead of the observed ECG. To this end, this paper formulates a variant of the recently proposed constrained alternating group lasso (CAGL) algorithm that imposes Löwner structure on the decomposition blocks. This is done by performing an orthogonal projection, which we explicitly derive, at each iteration of CAGL. Results from experiments with synthetic data show the consistency of the proposed Löwner-constrained AGL (LCAGL) in extracting the desired sources. Experimental results obtained on a population of 20 patients suffering from persistent AF show that the proposed variant outperforms other tensor-based methods in terms of atrial signal estimation quality from ECG records as short as a single heartbeat.

In this work, a resource allocation problem is investigated for a relay-based multiuser cooperative orthogonal frequency division multiple access (OFDMA) uplink system, considering a very large antenna array at the base station and nonlinear power amplifiers (PAs) in both user and relay nodes. Firstly, a theoretical characterization of the considered scenario is performed. Then, some analytical expressions for the nonlinear distortion (NLD) variance and the PA constant of a third-order polynomial PA are derived, as well as the instantaneous signal-to-noise ratio (SNR) of the considered system is calculated. Then, a low-complexity suboptimal relay selection and subcarrier allocation technique is proposed for the considered scenario based on the developed expressions. Numerical simulation results are presented to validate the derived expressions and to evaluate the performance of the proposed technique.

A three-hop AF MIMO relay system with tensor coding at the source and the relays is considered in this paper. The signals received at destination form a fifth-order tensor that satisfies a high-order nested Tucker decomposition, characterized by the concatenation of three Tucker models. We propose a receiver based on an alternating least square algorithm to jointly estimate the symbol matrix and the channels of each hop. Monte Carlo simulation results are provided to illustrate the behavior of the proposed system and of the semi-blind receiver. These simulation results show a performance closest to the one of the zero-forcing receiver, yielding a significant SER improvement due to the relay-assisted link when compared to the direct link.

Orthogonal frequency division multiplexing (OFDM) is considered as a key technology for future wireless communication systems. One of the main disadvantages of OFDM systems is the high peak‐to‐average power ratio (PAPR) of the transmitted signals. When a non‐linear power amplifier (PA) is present, the high PAPR may cause the introduction of non‐linear distortions. On the other hand, cooperative communications have emerged as a promising technology for wireless networks. A theoretical analysis of an amplify‐and‐forward OFDM system taking into consideration the non‐linear distortions introduced by a PA is done. More specifically, approximate analytical expressions for the outage probability and average symbol error rate (SER) are derived and studied under several special situations. The performance analysis sheds light on the influence of PA non‐linearity on the outage probability and the average SER of an OFDM system. Particularly, it is shown that the non‐linear PA has a more significant effect over the average SER for medium and high signal‐to‐noise ratio (SNR) values and for small‐order constellations. The analytical results are validated by means of computer simulations for different system configurations and SNR levels.

We consider the uplink of a cooperative DS-CDMA communication system, where each user communicates with the base station through a direct link and with the help of relays. A multiuser detection approach for the joint estimation of spatial signatures, channel gains and transmitted symbols of all the users is proposed. Such an approach is derived from a trilinear tensor model for the received signal that combines the source-destination and relay-destination links. The proposed multiuser receiver provides an extension of the traditional trilinear DS-CDMA receiver to cooperative relaying systems, by including the users' relay-assisted links into the trilinear model. Moreover, in contrast to a previous work by the authors, where the relays of a cluster transmit over orthogonal channels, the proposed receiver operates in a more attractive scenario where all the relays simultaneously transmit towards the base station without requiring orthogonal codes. Simulation results illustrate the performance of the proposed receiver in comparison with alternative approaches.

In this letter, we present new blind receivers for uplink multiuser cooperative diversity systems. Considering amplify-and-forward (AF), fixed decode-and-forward (FDF), and selective decode-and-forward (SDF) relaying protocols, the proposed receivers exploits a unified formulation of the received signal as a CANDECOMP/PARAFAC (CP) model with dimensions$receive\ antenna\times cooperative\ branch\times symbol\ period$. Under the assumption that channel state information (CSI) is not available neither at the relays nor at the base station, the proposed receiver jointly and blindly estimates the transmitted symbols and channel parameters. In addition to avoiding the use of pilots symbols, the CP-based receiver can operate with less base station antennas than users or, alternatively, with a single relay per user.

Two semi-blind tensor based receivers for uplink radio over fiber (ROF) nonlinear instantaneous channels in a multiuser code division multiple access (CDMA) environment are proposed in this paper. These receivers are based on the PARAFAC decomposition of a tensor (three-way array) composed of received signals with space, time and code diversities. A great advantage of the tensor approach is that it allows joint blind estimation of the channel and transmitted signals under weak uniqueness conditions compared with existing methods, which is particularly suitable when identifying nonlinear systems. Two estimation algorithms are proposed and illustrated by means of computer simulations.