## 1 Introduction  
The Fourth Industrial Revolution considers the replacement of humans by machines in certain tasks, or the development of new or more efficient tasks. Making use of robots and artificial intelligence, the Fourth Industrial Revolution is already deeply modifying society and organizations [1]. As seen in Figure 1 the Fourth Industrial Revolution comprises other parameters besides robots and artificial intelligence [2]. Robots need to communicate and to sense the environment (using sensors and communications), for which the Internet of things (IoT) is employed (all over the Internet protocol (IP)). The IoT generates massive quantities of data (big data) that will be processed with artificial intelligence to generate knowledge; that is, the data supports human decision-making, as well as decisions made by the robots. [3]. These new technologies will originate a deep modification of society with great impact on the human way of life, as well as on the employment market [4]. Societal modification includes more efficient mobility, based on autonomous cars, smart cities, home safety and automation, intelligent industries, agriculture, smart logistics, medical and lawyer counselling and the use of intelligent drones for a myriad of areas, including in defense (even micro drones used for tactical purposes) [5]. The future of mobility includes autonomous driving [6,7]. A car can be viewed as a robot that uses sensors and communications to interact with the environment, generating a large amount of data and using artificial intelligence to make decisions. Fifth generation (5G) communications play an important key in autonomous driving, using ultra-reliable low-latency communications (URLLC) [8]. As can be observed in Figure 2, URLLC was designed to support new services, such as remote surgery or autonomous vehicles, which are delay-sensitive services that require very low bit error rates (errors being almost unacceptable). 5G communications support vehicle-to-vehicle (V2V) communications without making use of a base station. Point-to-point communications are also considered in other 5G services. Figure 1: The context of the Fourth Industrial Revolution. Figure 2: Fifth generation (5G) use cases. Making use of massive multiple-input multiple-output (m-MIMO) technology [9; 10; 11] and millimeter wave (mm-wave) communications [12; 13] in a large scale, 5G communications provide higher spectral efficiency and capacity than fourth generation (4G) communications [14; 15]. It is worth noting that when using carrier frequencies around 60 GHz, instead of the traditional 2 or 3 GHz bands, mm-wave communications facilitate the implementation of m-MIMO technology, as the distance between antennas is greatly reduced and the size of the antennas is also highly reduced. Therefore, both the base station and the mobile terminals can accommodate hundreds of antenna elements [16; 17]. Cellular communications and different wireless local area networks (WLANs), namely IEEE 802.11 standards, have used similar technologies [18]. As in 5G communications, the use of m-MIMO and mm-wave technology is also comprised of 802.11 ad [12]. Mm-waves are specially well fitted for small cells [19], such as pico cells or femto cells, due to high propagation losses and high coherence bandwidths. In these scenarios, the throughput made available is greatly increased [16; 17]. This article aims to provide an overview of 5G and beyond, introducing the transmission techniques for current 5G communications and those expected of future developments. A brief study of non-orthogonal multiple access (NOMA) technology is conducted, viewed as a strong multiple access candidate for new releases of 5G, evidencing its added value in terms of spectral efficiency as it allows multiple users' signals to be transmitted simultaneously in the same carrier frequency. Moreover, sixth generation (6G) communication is also presented. The insertion of 5G and 6G within the Fourth Industrial Revolution is also dealt with.