The next generation of mobile networks (5G) is expected to achieve high data rates, reduce latency, as well as improve the spectral and energy efficiency of wireless communication systems. Several technologies are being explored to be used in 5G systems. One of the main promising technologies that is seen to be the enabler of 5G is massive multipleinput multiple-output (mMIMO) systems. Numerous studies have indicated the utility of mMIMO in upcoming wireless networks. However, there are several challenges that needs to be unravelled. In this paper, the latest progress of research on challenges in mMIMO systems is tracked, in the context of mutual coupling, antenna selection, pilot contamination and feedback overhead. The results of a systematic mapping study performed on 63 selected primary studies, published between the year 2017 till the second quarter of 2020, are presented. The main objective of this secondary study is to identify the challenges regarding antenna design and channel estimation, give an overview on the state-of-the-art solutions proposed in the literature, and finally, discuss emerging open research issues that need to be considered before the implementation of mMIMO systems in 5G networks.
INTRODUCTIONThe next-generation of mobile networks, also known as the fifth-generation (5G) is foreseen to deliver a solution for bandwidth-hungry application, due to the dramatic increase in traffic demand of users. 5G aims to meet much higher data rates with fast connectivity, more robust reliability, spectral efficiency (SE) and energy efficiency (EE) [1]. However, there are technical requirements and challenging issues that 5G needs to overcome so that it can be implementable. One of the major challenges is that the new generation must support much higher data rate (approximately 1000× compared to legacy networks), which makes it essential to go to higher frequencies, 30-300 GHz (Mm-Wave) spectrum, to allow more bandwidth. The use of the mm-wave spectrum requires the use of many antennas allowing more throughput gain in the spatial dimension to overcome high path loss. Furthermore, smaller cells provided by the use of these many antenna are needed to avoid interference [2]. Recently, massive multiple-input multiple-output (mMIMO) has become the most promising wireless technology and the This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.