Machine learning (ML) algorithms have been widely used to improve the performance of telecommunications systems, including reconfigurable intelligent surface (RIS)-assisted wireless communication systems. The RIS can be considered a key part of the backbone of sixth-generation (6G) communication mainly due to its electromagnetic properties for controlling the propagation of the signals in the wireless channel. The ML-optimized (RIS)-assisted wireless communication systems can be an effective alternative to mitigate the degradation suffered by the signal in the wireless channel, providing significant advantages in the system’s performance. However, the variety of approaches, system configurations, and channel conditions make it difficult to determine the best technique or group of techniques for effectively implementing an optimal solution. This paper presents a comprehensive review of the reported frameworks in the literature that apply ML and RISs to improve the overall performance of the wireless communication system. This paper compares the ML strategies that can be used to address the RIS-assisted system design. The systems are classified according to the ML method, the databases used, the implementation complexity, and the reported performance gains. Finally, we shed light on the challenges and opportunities in designing and implementing future RIS-assisted wireless communication systems based on ML strategies.

VoIP is the multimedia service with the fastest growth on current cellular systems and this trend will continue in the future. Voice transmission by VoIP users with traditional dynamic scheduling schemes could become a bottleneck in the signaling channels due to the periodic transmission of signaling small packets. To solve this problem, persistent scheduling has been proposed. Since the resource allocation information is indicated in the first burst of persistent data no information resource allocation is required for the duration of the data bursts. This paper presents a performance study of the persistent scheduling based on the transmission of VoIP traffic, using as mathematical tools order statistics and teletraffic analysis. A combination of interference analysis and birth and death processes are performed to obtain the average transmission rate in the system using the Shannon equation. We obtain the order statistics of band SIRs (signal-to-interference ratio) by considering the co-channel interference. The order statistics are then used to analyze a best-m band feedback scheme, in which every user reports the index of their best-m bands to the BS. If multiple users present a band as their best band, two different aproaches are analysed: Random allocation and Maximum SIR allocation. The probability density function (pdf) of the SIR was obtained, it is observed that the Maximum SIR allocation has a significant gain of up to 20% in regard to random allocation. After that, the probability of using a specific modulation and coding scheme is obtained. In order to obtain the probability of forced termination and the average data rate, these probabilities are introduced in the birth and death process as a different arrival rates and service times. It is observed that the order statistics model the cellular systems more accurately. As an example for uniform user distribution and voice over VoIP traffic, the numerical results shown that the probability of forced termination is corrected by more than 70% in comparison without order statistics. More over, using the methodology developed with order statistics, an evaluation is made for a non-uniform user distribution with Skype traffic, considered as a very important service for 5G cellular systems.

In this paper, an easy-to-program mathematical expression to compute the Envelope Correlation Coefficient (ECC) between two antennas by using their radiated electric fields at the far-field region is derived. The main advantage of the resulting algorithm is its higher speed to obtain results compared to those used in commercial software. This novel approach presents a discrete equation that can be employed after obtaining the electric field in spherical coordinates by computer simulations. Thus, another alternative arises to report this metric for low or high radiation efficiency antennas, instead of the less accurate S-parameter-based equation, which does not ensure high accuracy when low radiation efficiency antennas are under study. A comparison between the presented algorithm and the results given by commercial software is shown and discussed, in which is clearly observed that the results are greatly convergent, but with the advantage of achieving a lesser time computing period by the proposed method.

Multiband spectrum sensing in a cooperative environment is a novel solution for efficient spectrum resource management under the cognitive radio networks (CRNs) paradigm. This paper presents a distinctive framework where a central entity collects power spectral density data from multiple geographically distributed secondary users and applies the Gramian angular field (GAF) summation method to transform the time-series data into image representations. A major contribution of this work is the integration of these GAF images with a convolutional neural network (CNN), enabling precise and real-time detection of primary user activity and spectrum occupancy. The proposed approach achieves 99.6% accuracy in determining spectrum occupancy, significantly outperforming traditional sensing techniques. The main contributions of this study are (i) the introduction of GAF-based image representations for cooperative spectrum sensing in CRNs; (ii) the development of a CNN-based classification framework for enhanced spectrum occupancy detection; and (iii) the demonstration of superior detection performance in dynamic, real-time environments.

This work presents the use of a novel impedance coupling technique and electrical length increase by using stub loading placed from the radiator to the ground plane. This method is applied to the design of a small four-element ultrawideband (UWB) MIMO antenna arranged in axial symmetry to achieve a compact array size while obtaining a bandwidth starting from a very low cutoff frequency compared to a conventional radiator operating at the same frequency. The four-element MIMO antenna, with an operational bandwidth of 1.9 GHz to 30 GHz, is based on a wideband monopole with a semicircular geometry, fed by a coplanar structure and an L-shaped half-ground plane section. To increase the electrical length of the structure and achieve a compact antenna design, reactive stub loading is introduced, placing it on the backside of the substrate, located orthogonally between the radiator and the L-shaped ground plane, obtaining a small-sized configuration. The axial symmetry is employed to increase the antennas’ isolation by taking advantage of the orthogonal positioning and making the radiated fields have a low correlation. The antenna array footprint measures 48 mm × 48 mm, corresponding to 0.3λ₀ × 0.3λ₀ at the lower cutoff frequency. The array exhibits a low envelope correlation coefficient (ECC) of around 0.033 at 2 GHz, and less than 0.001 at the rest of the bandwidth; a diversity gain (DG) of approximately 10; a stable total active reflection coefficient (TARC) below −10 dB; interport isolation between 20 and 40 dB; and an average gain of 2.8 dBi.

In this article, a combination of rectangular loop array and slot radiator for multiband applications is presented. The antenna is configured by arranging, concentrically, a set of rectangular loop radiators excited by electromagnetic coupling provided by a dumbbell slot. The size of the loops is calculated to obtain the desired resonant frequencies, which are almost independent of the adjacent rings. The exciting slot is designed to operate in a wideband frequency range to cover the upper desired resonance. In addition, to obtain directive radiation patterns, a reflector shaped like a box is introduced, giving a stable gain, radiation pattern shape, and port matching at the selected frequencies. The configuration presents great results, since to the authors’ knowledge, even a similar configuration given in the open literature presents some disadvantages compared to this one; moreover, not just any structure can be employed as the resonating elements, obtaining multiband behavior at the same time.