Massive MIMO toward 5G

The massive Multiple Input and Multiple Output (MIMO) technology has been proved to have enormous potentials to meet the continuously growing demand for higher data rate transmission-based communication systems [1,2]. Though the exponential growth in mobile communications remains the root cause of high data transfer need, the emergence of other services, such as the machine-to-machine communication, internet banking, internet of things (IoT), etc., are the other indispensable requirements of everyday life, that essentially prompted the need for exploiting new technologies capable to meet such growing demand of high data speed communication networks [3]. Within the context, the currently implemented e-learning in academic organizations and the “work-from-home” kind of concept in other professional set-ups could be witnessed well globally during the on-going span relevant to the Covid-19 aftermath. Indeed, the successful run of such operations essentially relies on the communication bandwidth and the rate of data transfer.

The current communication systems basically rely on the 4G cellular networks introduced by the International Telecommunication Union [4]. However, the exponentially increasing demand for internet traffic surpasses the capabilities of the 4G communication system – the fact that triggers the need for more fast 5G cellular data network implementation. The massive MIMO remains one of the key technology components of 5G communication network, wherein the system architecture exploits significantly enhanced number of antennas at the base transceiver stations, as compared to the systems implemented in other lower generation (e.g. 3G or 4G) communication networks [5]. The massive MIMO system possesses the efficacy to reduce the radiated amount of power by directing the wireless energy to the specified user, maintaining the extent of interference to the lowest possible level at the same time [6,7]. It is noteworthy that the MIMO technology is generally deployed to improve the quality of communication. These essentially involve the arrangement of multiple antenna elements placed in the proximity with enhanced isolation among the ports, thereby allowing MIMO antennas to provide higher channel capacity, as compared to the conventional wireless systems.

The need for afore discussed advancements in mobile communications makes the research area of 5G wireless data networks to be highly attracting among the R&D community to investigate new technologies to be deployed in this stream. Considering the importance of massive MIMO technology in the present-day enhancement need in communication standards, the joint editors-in-chief of the Journal of Electromagnetic Waves and Applications (JEMWA) had views of bringing out a Special Issue – Massive MIMO Toward 5G – on this topic, in order to incorporate certain research reports on the recent developments in the stated area under a cover. To the valued readers, we have to mention that this Special Issue contains some 11 papers. All focus on, in some way or the other, the mechanisms related to the promising perspectives of MIMO technology deployed in 5G communications.

Within the context, as the suitability of the type of antenna to be chosen in certain communication systems greatly matters to the performance of bit rate transmission in limited bandwidth, Mukherjee et al. present a review of the recent advances in dielectric resonator antennas (DRAs) that find applications in MIMO technology. The electrical and physical size miniaturization along with the bandwidth requirements for commercial needs remain of much importance in the background of MIMO architecture. Their work emphasizes on various such techniques to improve the bandwidth and radiation characteristics exploiting varieties of miniaturized forms of DRA-based structures.

One of the important features of MIMO antennas remains the isolation among various radiators [8]. In this stream, Tirado-Méndez et al. propose a miniaturized type of four-port wideband antenna with high inter-port isolation for MIMO applications. The operation of antenna exploits Kraus technique, wherein no abrupt transitions are included to obtain a wider bandwidth. The antenna geometry allows electromagnetic isolation of elements by limiting the propagation of surface current distribution of each radiator. As the authors claim, the proposed antenna prototype fulfils the requirements for MIMO applications with the advantages of its being simple, and yet achieving high operational qualities.

The paper by Singhwal et al. throws the concept of circular polarization (CP) agility in DRA-based MIMO antennas for 5G communications in the Sub-6 GHz band. In their work, the bandwidth could be controlled by inserting different cylindrical shells into the ring-type DRA. The antenna structure offers controllable CP and compactness, in comparison to other DRA-based MIMO antennas operating in the same spectral band. The simplicity in port isolation mechanism remains the added advantage of the structure. The authors present antenna prototypes, and compare the measured results with the simulated data obtained using the high-frequency structural simulator (HFSS).

In recent years, laptops got marked improvements in terms of design and performance. Lightweight, thin, and large display areas are some of the notable features of relatively good quality laptops with fast processing speed. Indeed, downsizing the space for antenna occupancy within the gadget remains one of the primary factors in tailoring the weight and size of the same. In line with this, Chen et al. contribute their work on a 3.50 GHz four-element MIMO antenna system to be mounted on the top edge of display ground plane of a 5G laptop with a high screen ratio. The antennas are configured in a way that the operating frequency band can completely support the desired 3.40–3.66 GHz for 5G applications. The authors report the measured isolation within the frequency band as exceeding 12 dB, and the antenna efficiency to be more than 40%, thereby claiming the features suitable for practical applications in thin- and narrow-border laptops.

In antenna characterization, the frequency reconfigurable property remains another greatly important aspect, in order to minimize the complexities [9,10]. In line with this, Hassan et al. touch upon a kind of two-element MIMO antenna with frequency reconfigurable characteristics for 5G applications. They utilize the radio frequency (RF) micro-electromechanical system (MEMS) technology-based switching mechanism [11], thereby eliminating the need for using additional decoupler for isolations. The obtained results indicate fairly good diversity performance in the 5G operating band, and therefore, determining the suitability of the same for 5G wireless communication systems. These authors also focus on the ways to achieve isolation enhancement and compactness of MIMO antennas. In such an attempt, they propose a new form of MIMO antenna array for wideband applications with 5.5 GHz wireless local area network (WLAN), utilizing parasitic decoupler in the configuration. The simulation and measurement results yield relatively high gain and isolation with band notch characteristics.

In the context of WLAN applications, Birwal et al. discuss about a four-port printed MIMO/diversity antenna, wherein all elements are printed on 1.6 mm thick FR-4 substrate in a coplanar configuration. In their work, the authors optimize the distance between antenna elements to achieve optimal isolation and diversity performance. The proposed antenna operates well at the 2.4 and 5.8 GHz frequencies, without the need for any external switching circuit. The authors claim the antenna structure doubles the available bandwidth, and also, provides a reliable Wi-Fi network avoiding overcrowding and interference, thereby allowing it to be a valuable candidate for WLAN-based MIMO/diversity applications.

The paper by Dkiouak et al. presents the design of dual-band MIMO antenna with high isolation for WLAN (5.3 GHz) and X-band satellite applications (7.5 GHz). The structure is comprised of two elements placed back-to-back that exhibit orthogonal polarization – the feature used to reduce the mutual coupling between the elements [12] as well as the size of the structure. The measured results exhibit the proposed antenna covering two frequency bands, namely 5.09–5.29 GHz and 7.38–7.87 GHz, and a high isolation around 19 dB at both the bands, toward the performance of the same. In another work, these authors investigate a different kind of antenna structure that emphasizes on the increase in isolation between the two radiating elements. The obtained results indicate the structure useful in WLAN applications.

Tiwari et al. introduce another kind of two-port compact MIMO antenna exhibiting dual-band characteristics. In such a configuration, the radiator assumes a dome-shaped monopole antenna element. As the authors state, the overall form of antenna is miniaturized in size, and the design yields notched band characteristics and improved isolation between the MIMO elements, operating in the 2.11–4.19 GHz and 4.98–6.81 GHz frequency bands.

In MIMO antenna structures, the used array configurations and feeding networks determine the operational performance [13]. In this stream, Staszek et al. present the design of simple feeding networks for two-beam antenna arrays that ensure similar radiation patterns in broadband frequency span. The authors propose feeding network comprised of two directional filters and broadband quadrature coupler, and demonstrate that such networks can implement balanced circuits yielding appropriate signal switching between two broadband radiating elements. Overall, the authors claim of achieving relatively good radiation characteristics with the use of much simpler feeding networks.

Finally, the potentials of MIMO technology-based antennas remain in increasing the usable bandwidth, and simultaneously avoiding the interference among the antenna elements. The other advantages of these have been discussed in the beginning of this editorial remarks. The papers incorporated in this Special Issue focus on the performance of a few different varieties of MIMO antennas, as obtained through simulations, and also, the measured results using the fabricated prototypes. The joint editors-in-chief expect the readers would find the Issue useful in conceptualizing many other new forms of MIMO antennas with even better performance characteristics, so far as the 5G mobile communication is concerned.