FCT: DEE - Artigos em revista internacional com arbitragem científica
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- Sustainable robots for humanitarian deminingPublication . Santana, Pedro F.; Barata, José; Correia, Luís; DEE - Departamento de Engenharia Electrotécnica e de Computadores; DEE2010-C2 Robótica e Manufactura Integrada por Computador; SAGE PublicationsThis paper proposes a roadmap for the application of advanced technology (in particular robotics) for the humanitarian demining domain. Based on this roadmap, a portable demining kit to handle urgent situations in remote locations is described. A low-cost four-wheel steering robot with a biologically inspired locomotion control is the base of the kit. On going research on a method for all-terrain piloting, under the morphological computation paradigm is also introduced, along with the behavioural architecture underlying it, the Survival Kit. A multi-agent architecture, the DSAAR architecture, is also proposed as a way of promoting short time-to-market and soft integration of different robots in a given mission. A common denominator for all developments is the quest for sustainability with respect to (re-)engineering and maintainability effort, as well as economical and ecological impact. Failing to cope with these requirements greatly reduces the applicability of a given technology to the humanitarian demining domain. Finally it is concluded that biologically inspired design fits considerably well to support a sustainable demining paradigm.
- New PN even balanced sequences for spread-spectrum systemsPublication . Inácio, José A. L.; Gerald, José A. B.; Ortigueira, Manuel D.; DEE2010-B2 Sistemas; UNINOVA-Instituto de Desenvolvimento de Novas Tecnologias; DEE - Departamento de Engenharia Electrotécnica e de Computadores; HindawiA new class of pseudonoise even balanced (PN-EB) binary spreading sequences is derived from existing classical odd-length families of maximum-length sequences, such as those proposed by Gold, by appending or inserting one extra-zero element (chip) to the original sequences. The incentive to generate large families of PN-EB spreading sequences is motivated by analyzing the spreading effect of these sequences from a natural sampling point of view. From this analysis a new definition for PG is established, from which it becomes clear that very high processing gains (PCs) can be achieved in band-limited direct-sequence spread-spectrum (DSSS) applications by using spreading sequences with zero mean, given that certain conditions regarding spectral aliasing are met. To obtain large families of even balanced (i.e., equal number of ones and zeros) sequences, two design criteria are proposed, namely the ranging criterion (RC) and the generating ranging criterion (GRC). PN-EB sequences in the polynomial range 3
- Modeling and Identification of Nonlinear Effects in Massive MIMO Systems Using a Fifth-Order Cumulants-Based Blind ApproachPublication . Zidane, Mohammed; Dinis, Rui; DEE - Departamento de Engenharia Electrotécnica e de Computadores; MDPI - Multidisciplinary Digital Publishing InstitutePre-processing associated with massive multiple input-multiple output (MIMO) systems can lead to signals with high envelope fluctuations, which are very prone to nonlinear effects, especially when massive MIMO schemes are combined with orthogonal transform multiplexing (OFDM) modulations. If the nonlinear characteristics that affect a given system are known, we can design appropriate receivers that take into account the nonlinear effects introduced by the transmitter. Cubic systems are particularly important, not only because they can approximate many nonlinear effects (e.g., due to the power amplifier or clipping effects), but also because many more complex nonlinear characteristics in communication schemes can be replaced by equivalent lower-order nonlinear characteristics in general, and cubic characteristics in particular. To compensate the effects at the receiver side (e.g., by using the so-called Bussgang receivers), we need to estimate the nonlinear operation that was introduced at the transmitter, and this should be done blindly, without the need of training symbols. The paper contains a description of a mathematical approach for modeling and identification of nonlinear kernels in cubic systems. Based on theoretical tools of HOC in cubic systems, we build a new formula which relates the second-and fifth-order cumulants. Our performance results indicate that the proposed approach allows an accurate identification, yielding the desired kernels via fifth-order cumulants, and ensures a very good convergence, outperforming existing adaptive methods. This is achieved blindly, by exploiting the maximum information of the output system, making it suitable for many practical nonlinear effects.
- UavnomaPublication . Lima, Brena; Fachada, Nuno; Dinis, Rui; Costa, Daniel B.; Beko, Marko; DEE - Departamento de Engenharia Electrotécnica e de Computadores; Ubiquity PressUavnoma is a set of Python functions and a front-end script for modeling, studying, and analyzing the communication system composed by an unmanned aerial vehicle (UAV) and two ground users. We assume that the UAV acts as an aerial base station to serve the users according to non-orthogonal multiple access (NOMA) principles. For more practical insights, residual hardware impairments (RHI) and imperfect successive interference cancellation (ipSIC) are considered. More specifically, uavnoma allows the modelers to study and visualize the system performance in terms of achievable rate and outage probability. Additionally, the package produces figures and tables showcasing results from the specified performance metrics.
- A Novel Highly-Efficient Amplification Scheme for Wireless Communications in a CathLab EnvironmentPublication . Viegas, Pedro; Serra, Hugo; Guerreiro, João; Madeira, Ricardo; Borges, David; Dinis, Rui; Montezuma, Paulo; Oliveira, João Pedro; Campos, Luis M.; Beko, Marko; DEE - Departamento de Engenharia Electrotécnica e de Computadores; UNINOVA-Instituto de Desenvolvimento de Novas Tecnologias; Institute of Electrical and Electronics Engineers (IEEE)Wireless communication systems are being considered for medical applications to facilitate the doctors' operation and the quality of the medical procedures. A demonstrative example of this is the catheterization laboratory (CathLab), where it is desirable to replace the existent wired connections by wireless alternatives. However, there are some challenging requirements that need to be fulfilled by the wireless link, especially for intra-vascular ultra-sound (IVUS) systems, since the images acquired by the catheter should be transmitted with very high data rate and low latency, together with the highest possible amplification efficiency, to increase the battery life. The communication requirements can be achieved with latest the Wi-Fi standard IEEE 802.11ax (Wi-Fi 6). However, since Wi-Fi is based on orthogonal frequency division multiplexing (OFDM) waveforms, the transmitted signals present high envelope fluctuations, leading to amplification difficulties due to the nonlinear distortion effects and low energy efficiency. In this paper, we present an innovative amplification scheme named quantized digital amplification (QDA). It is shown that the QDA allows a quasi-linear amplification of IEEE 802.11ax signals while maintaining a very high energy efficiency. To demonstrate this, a QDA prototype and a set of performance results, regarding both the linearity of the transmitted signals and the energy efficiency, are presented.
- A State-Space Approach for Tracking Doppler Shifts in Radio Inter-Satellite LinksPublication . Pedrosa, Pedro; Castanheira, Daniel; Silva, Adão; DInis, Rui; Gameiro, Atílio; DEE - Departamento de Engenharia Electrotécnica e de Computadores; Institute of Electrical and Electronics Engineers (IEEE)The high Doppler effects in satellite communications make the equivalent channels fast varying, which leads to channel estimation difficulties and the need for high channel estimation overheads. In this paper we propose a state-space approach for tracking channel variations for satellite links with high Doppler frequency shifts. We consider the communication between satellite links where a multi-antenna receiving satellite is connected to several transmitting satellites, which can have substantially different Doppler drifts. The proposed channel tracking technique follows the individual Doppler drifts for the different satellites. Finally, our performance results indicate that the proposed channel tracking is able to cope with strong Doppler drifts, without the need for high channel estimation overheads, making it a promising technique for future systems using satellite mega-constellations.
- Low Complexity Hybrid Precoding Designs for Multiuser mmWave/THz Ultra Massive MIMO SystemsPublication . Pavia , João Pedro; Velez, Vasco; Ferreira, Renato; Souto, Nuno; Ribeiro, Marco; Silva, João; Dinis, Rui; DEE - Departamento de Engenharia Electrotécnica e de Computadores; MDPI - Multidisciplinary Digital Publishing InstituteMillimeter‐wave and terahertz technologies have been attracting attention from the wireless research community since they can offer large underutilized bandwidths which can enable the support of ultra‐high‐speed connections in future wireless communication systems. While the high signal attenuation occurring at these frequencies requires the adoption of very large (or the so-called ultra‐massive) antenna arrays, in order to accomplish low complexity and low power consumption, hybrid analog/digital designs must be adopted. In this paper we present a hybrid design algorithm suitable for both mmWave and THz multiuser multiple‐input multiple‐output (MIMO) systems, which comprises separate computation steps for the digital precoder, analog precoder and multiuser interference mitigation. The design can also incorporate different analog architectures such as phase shifters, switches and inverters, antenna selection and so on. Furthermore, it is also applicable for different structures, namely fully‐connected structures, arrays of subarrays (AoSA) and dynamic arrays of subarrays (DAoSA), making it suitable for the support of ultra‐massive MIMO (UM‐MIMO) in severely hardware constrained THz systems. We will show that, by using the proposed approach, it is possible to achieve good trade‐offs between spectral efficiency and simplified implementation, even as the number of users and data streams increases.
- A Low Complexity Channel Estimation and Detection for Massive MIMO Using SC-FDEPublication . Marques da Silva, Mário; Dinis, Rui; Guerreiro, João; DEE - Departamento de Engenharia Electrotécnica e de Computadores; MDPI - Multidisciplinary Digital Publishing Institute5G Communications will support millimeter waves (mm-Wave), alongside the conventional centimeter waves, which will enable much higher throughputs and facilitate the employment of hundreds or thousands of antenna elements, commonly referred to as massive Multiple Input–Multiple Output (MIMO) systems. This article proposes and studies an efficient low complexity receiver that jointly performs channel estimation based on superimposed pilots, and data detection, optimized for massive MIMO (m-MIMO). Superimposed pilots suppress the overheads associated with channel estimation based on conventional pilot symbols, which tends to be more demanding in the case of m-MIMO, leading to a reduction in spectral efficiency. On the other hand, MIMO systems tend to be associated with an increase of complexity and increase of signal processing, with an exponential increase with the number of transmit and receive antennas. A reduction of complexity is obtained with the use of the two proposed algorithms. These algorithms reduce the complexity but present the disadvantage that they generate a certain level of interference. In this article, we consider an iterative receiver that performs the channel estimation using superimposed pilots and data detection, while mitigating the interference associated with the proposed algorithms, leading to a performance very close to that obtained with conventional pilots, but without the corresponding loss in the spectral efficiency.
- Joint channel estimation and synchronization techniques for time-interleaved block-windowed burst OFDMPublication . Martins, João; Conceição, Filipe; Gomes, Marco; Silva, Vítor; Dinis, Rui; DEE - Departamento de Engenharia Electrotécnica e de Computadores; MDPI - Multidisciplinary Digital Publishing InstituteFrom a conceptual perspective, beyond-5G technologies promise to deliver very low latency, even higher data rates, and ultrareliable connections for future generations of communication systems. Modulation schemes based on orthogonal frequency-domain multiplexing (OFDM) can accommodate these requirements for wireless systems. Several hybrid OFDM-based systems, such as the time-interleaved block-windowed burst–OFDM (TIBWB–OFDM), are capable of achieving even better spectral confinement and power efficiency. This paper addresses the implementation of the TIBWB–OFDM system in more realistic and practical wireless link scenarios by addressing the challenges of proper and reliable channel estimation and frame synchronization. We propose to incorporate a preamble formed by optimal correlation training sequences such as the Zadoff–Chu (ZC) sequences. The added ZC preamble sequence is used to jointly estimate the frame beginning through signal-correlation strategies and a threshold decision device, and acquire channel-state information (CSI) by employing estimators on the basis of the preamble sequence and transmitted data. The employed receiver estimators show that it is possible to detect the TIBWB–OFDM frame beginning and highlight the robustness of the TIBWB–OFDM technique to imperfect channel estimations by showing that it can provide comparatively close BER performance to the one where the CSI is perfectly known.
- The Importance of Repetitions in Ultra-Dense NB-IoT NetworksPublication . Abu Sabah, Ayman T.; Rahman, Md Arifur; Oliveira, Rodolfo; Flizikowski, Adam; DEE - Departamento de Engenharia Electrotécnica e de Computadores; DEE2010-A1 Telecomunicações; Institute of Electrical and Electronics Engineers (IEEE)This letter investigates the influence of the number of repetitions adopted in the NB-IoT standard to increase cellular coverage. We characterize the SINR in both uplink and downlink channels of ultra-dense networks, which is then used to derive the coverage probability and the NB-IoT’s capacity. The performance results show the effectiveness of the number of repetitions to reduce the block error rate for different densities of transmitters.