Recently, within the framework of 5G, there has been a high level of interest in Massive MIMO as one of the technological solutions for 5G radio. Massive MIMO refers to a significant number of antenna elements at a base station – namely, 64, 128, 256 or even more. To pack this number of antenna elements in reasonably sized antenna panels, Massive MIMO must be used for high frequencies. Therefore, it is even more interesting in combination with the millimeter wave (mmW) region of the spectrum. In this spectrum range the wavelength is very short, so the antenna elements can be very small, and the separation between them can also be small. However, Massive MIMO systems are not limited to mmWave. They can also be used for LTE-Advanced Pro and 5G Radio at 3.5GHz.
The key benefits of Massive MIMO
The figure above shows the benefits that a Massive MIMO system has over the regular MIMO systems, which are currently used in cellular networks (with the typical configuration of 2T2R, 4T4R or 8T8R, where T is the transmitting side, and R is the receiving side). They are shortly described in more detail below:
Accurate beamforming (top left picture) – The more antenna elements in an antenna panel there are, the more precise and narrow the beams created by the panel can be. This enables precise transmission and tracking of the UE. Additionally, this approach is energy efficient, due to the relatively low amount of power being dissipated into unhelpful directions. This is also beneficial from the perspective of interference, which is low for all of the directions other than the desired ones.
Channel hardening (top right picture) – A large number of antennas provide a large number of individual radio channels. Therefore, while one single channel could be fluctuating (due to a fading effect), the combined signals received from a massive number of antennas result in a stable channel, and in less fading. This means that there is a lower requirement for channel estimation accuracy and less of a need for frequency selective scheduling.
Higher system capacity (bottom left picture) – In the MIMO systems, the more antennas there are, the more individual spatial streams that can be created, which means multiple users can be served at the same time (this is also known as MU-MIMO). Thus, if there is a large number of antennas, the frequency resources are high, increasing the system’s overall capacity and maximizing the number of users that can be served at the same time. This is especially important to note in the case where the UEs have a low number of antennas. By utilizing the Massive MIMO system, the overall capacity can be distributed onto different users’ links. This, of course, works for both transmission ways, (i.e. multiple single/double antenna UEs can transmit at the same time in the UL, so that the Massive MIMO panel at the base station is able to receive them simultaneously.)
Higher link throughput (bottom right picture) – The more antennas there are, the more spatial streams that can be provided. However, unlike the previous aspect, the set of different spatial streams is targeting a single user, equipped with multiple receive antennas. This increases their individual throughput (also known as their SU-MIMO).
The use of Massive MIMO systems is highly beneficial, as discussed above, due to a number of reasons: it’s a stable channel, has a high system capacity and single user capacity, and is precise with beamforming. While these benefits are described separately, they should be seen as connected. For example, in practice, we aren’t likely to see UEs with 64-256 antenna elements in the near future. Therefore, typically the combination of the high link throughput of individual UEs, equipped with a smaller number of received antennas (e.g. 2, 4, or 8) with a higher system capacity gained by transmitting to multiple UEs (where each of them has 2/4/8 receive antennas) is a practical use case. Additionally, for both SU- and MU-MIMO, beamforming will probably be used, and for this, multiple antennas will be applied, so the channel hardening effect will be observed.
This concept is applied by the current 3GPP developments of 5G radio interfaces, called New Radio (NR), in the form of beamforming with a large number of antennas. In this, the beam management for DL and UL beamforming includes beam determination, measurement, reporting and beam sweeping .
 3GPP TR 38.802 v14.1.0, “Study on New Radio Access Technology Physical Layer Aspects”, 06.2017
Marcin Dryjanski received his M.Sc. degree in telecommunications from the Poznan University of Technology in Poland in June 2008. During the past 10 years, Marcin has served as R&D Engineer, Lead Researcher, R&D Consultant, Technical Trainer and Technical Leader. He has been providing expert level courses on LTE/LTE-Advanced for leading mobile operators and vendors. Marcin was a workpackage leader in EU-funded research projects aiming at radio interface design for 5G including FP-7 5GNOW and FP-7 SOLDER. He co-authored several research papers targeting LTE-Advanced Pro and 5G radio interface design, and is a co-author of a book entitled “From LTE to LTE-Advanced Pro and 5G”, (M. Rahnema, M. Dryjanski, Artech House 2017). Marcin is the co-founder of Grandmetric, heading the field of mobile wireless systems. In this role, Marcin provides consulting services and training courses on LTE and 5G related topics. Marcin is also a trainer at 5G-Courses.com and co-author of the Towards 5G – Research and Standardization online course.