Miguel Angel Lagunas

In this study we introduce Logarithmic Frequency Shift Keying (Log-FSK), a novel frequency modulation for over-the-air computation (AirComp). Log-FSK leverages non-linear signal processing to produce AirComp in the frequency domain, this is, the maximum frequency of the received signal corresponds to the sum of the individual transmitted frequencies. The demodulation procedure relies on the inverse Discrete Cosine Transform (DCT) and the extraction of the maximum frequency component. Log-FSK enables the computation of functions beyond the sum by incorporating nomographic function representation. Furthermore, unlike existing AirComp modulations, Log-FSK allows to compute several functions in a single transmission. We evaluate the capabilities of the scheme in an additive white Gaussian noise (AWGN) and flat-fading channels. To demonstrate its practicality, we present specific applications and experimental results showcasing the effectiveness of Log-FSK AirComp within linear Wireless Sensor Networks (WSN). Our numerical results show that Log-FSK outperform linear analog modulations in terms of MSE and power consumption.

The expressiveness of neural networks highly depends on the nature of the activation function, although these are usually assumed predefined and fixed during the training stage. Under a signal processing perspective, in this paper we present Expressive Neural Network (ENN), a novel model in which the non-linear activation functions are modeled using the Discrete Cosine Transform (DCT) and adapted using backpropagation during training. This parametrization keeps the number of trainable parameters low, is appropriate for gradient-based schemes, and adapts to different learning tasks. This is the first non-linear model for activation functions that relies on a signal processing perspective, providing high flexibility and expressiveness to the network. We contribute with insights in the explainability of the network at convergence by recovering the concept of bump, this is, the response of each activation function in the output space. Finally, through exhaustive experiments we show that the model can adapt to classification and regression tasks. The performance of ENN outperforms state of the art benchmarks, providing above a 40% gap in accuracy in some scenarios.

Satellite Internet of Things (Sat-IoT) is a novel framework in which satellites integrate sensing, communication and computing capabilities to carry out task-oriented communications. In this paper we propose to use the Long Range (LoRa) modulation for the purpose of estimation in a Sat-IoT scenario. Then we realize that the collisions generated by LoRa can be harnessed in an Over-the-Air Computing (AirComp) framework. Specifically, we propose to use LoRa for Type-based Multiple Access (TBMA), a semantic-aware scheme in which communication resources are assigned to different parameters, not users. Our experimental results show that LoRa-TBMA is suitable as a massive access scheme, provides large gains in terms of mean squared error (MSE) and saves scarce satellite communication resources (i.e., power, latency and bandwidth) with respect to orthogonal multiple access schemes. We also analyze the satellite scenarios that could take advantage of the LoRa-TBMA scheme. In summary, that angular modulations, which are very useful in satellite communications, can also benefit from AirComp.

This paper studies the cosine as basis function for the approximation of univariate and continuous functions without memory. This work studies a supervised learning to obtain the approximation coefficients, instead of using the Discrete Cosine Transform (DCT). Due to the finite dynamics and orthogonality of the cosine basis functions, simple gradient algorithms, such as the Normalized Least Mean Squares (NLMS), can benefit from it and present a controlled and predictable convergence time and error misadjustment. Due to its simplicity, the proposed technique ranks as the best in terms of learning quality versus complexity, and it is presented as an attractive technique to be used in more complex supervised learning systems. Simulations illustrate the performance of the approach. This paper celebrates the 50th anniversary of the publication of the DCT by Nasir Ahmed in 1973.

With the integration of communication and computing, it is expected that part of the computing is transferred to the transmitter side. In this paper we address the general problem of Frequency Modulation (FM) for function approximation through a communication channel. We exploit the benefits of the Discrete Cosine Transform (DCT) to approximate the function and design the waveform. In front of other approximation schemes, the DCT uses basis of controlled dynamic, which is a desirable property for a practical implementation. Furthermore, the proposed modulation allows to recover both the measurement and the function in a single transmission. Our experiments show that this scheme outperforms the double side-band (DSB) modulation in terms of mean squared error (MSE). This can also be implemented with an agnostic receiver, in which the function is unkown to the receiver. Finally, the proposed modulation is compatible with some of the existing transmission technologies for sensor networks.

This work presents a resource allocation algorithm in K-user, M-subcarrier and NT-antenna systems for on-line scheduling. To exploit temporal diversity and to reduce complexity, the ergodic sum rate is maximized instead of the instantaneous one. Dual optimization is applied to further diminish complexity together with a stochastic approximation, which is more suitable for online algorithms. Weighted sum rate is considered so that users can be either prioritized by higher layers or differentiated by proportional rate constraints. The performance and complexity of this algorithm is compared with well-known benchmarks and also evaluated under real system conditions for the MIMO Broadcast channel.

This paper deals with the problem of multibeam satellite pre-coding design under spectrum sharing constraints. These regulation restrictions allow the coexistence with other wireless services such as terrestrial mm-wave wireless local loop systems. This work focuses on the case where the satellite operator can use a certain frequency band whenever the signal power strength is limited over the coverage area. The precoding design is optimized considering this restriction by means of formulating a robust optimization. Numerical results show the trade-off between the achievable rates of the satellite segment and the regulation violation outage.

This paper presents the effect of broadband matching in simultaneous information and power transfer along with an implementation. The narrowband characteristic of antennas has limited the applications of simultaneous information and power transfer. The performance improvements in terms of channel capacity and power delivery under broadband matching have been demonstrated. Electromagnetic simulation and multiobjective optimization are performed to analyze the tradeoff between the channel capacity and power delivery in different matching conditions. The performance gain using the matching networks has been demonstrated and analyzed.

Due to the scarcity of the spectrum, there is a current tendency to offer novel shared access licenses in order to increase the overall spectral efficiency. Under this context, license shared access (LSA) has appeared as an alternative to the exclusive license and license-exempt mechanisms, which so far have dominated the wireless communications. LSA provides shared access to a given set of users under certain conditions which must be optimized in order to foster the shared use of the spectrum, while maintaining certain restrictions on the generated interference. In this paper we consider a LSA system where both satellite and terrestrial wireless service providers share a given frequency band. Basically, the constraints in the LSA scenario reduce to the control of the received signal power within the coverage area. We focus on the precoding scheme in multi-beam satellite systems restricting the received power of the satellite user terminals to be under certain threshold. The objective is that the terrestrial wireless service provider observes the satellite signal power strength under a certain value, providing a peaceful coexistence between both services. Two traditional linear precoding techniques are properly modified in order to satisfy the receiver power constraints. The optimum precoder results to be zero forcing (ZF), in terms of maximum achievable sum-rate, nevertheless the so-called minimum mean square error (MMSE) exhibits better power efficiency than the ZF, in terms of bps/Hz unit power used at the transmitter.

A plethora of papers can be found in the scientific literature talking about time reversal. The term time reversal exists in the publications in a variety of different fields, such as ultrasonics, acoustics, wireless communications, electromagnetics, antennas and propagation, optics, physics, and philosophy. The term time reversal by itself sounds very interesting, and can provide many interpretations that are fascinating. However, due to a loose use of this term, it may in many cases lead to fallacious interpretations and conclusions if it is not interpreted in strict scientific terms. The main goal of this paper is to explain the appropriate definition of the term time reversal in electrical engineering in general, and in electromagnetics for wireless communications, to be specific. However, the motivation of the work presented in this paper is not to raise a controversy about time reversal and its related research. Rather, the aim of this work is to highlight the basic electrical engineering fundamentals that are necessary to study time reversal and to correctly define it, and to then accordingly interpret the results. Hence, the true and fallacious benefits of electromagnetic time reversal will be exposed. To serve this purpose, the paper is divided into two parts. First, a detailed literature review is presented. The true history of the term time reversal in electrical engineering is traced. In this part, most of the reported definitions of time reversal and its claimed capabilities are summarized. Some problems with the application of time reversal in electromagnetics are discussed, both theoretically and practically. All of the time-reversal papers talk about reciprocal networks, so the fact of the non-reciprocity of a single-antenna system in the time domain (the way we generally interpret it) is presented, to prove theoretically that there is a problem with how we interpret time reversal in electromagnetics. Finally, practical examples are shown, where time reversal is used in a wireless system and in an acoustic de-reverberation problem. The exact capabilities of time reversal in electrical engineering are exposed in those experiments.