Massive-MIMO in a Nutshell
It is no surprise that current and upcoming mobile wireless standards and methods are leveraging multiple technologies to increase the throughput to user equipment (UE) and the availability of high-speed data services. Some of the major challenges to overcome for high speed wireless data to be offered are limited spectrum availability, interference, and the sheer number of devices that will require servicing. Other associated challenges include energy efficiency, network infrastructure, and UE capabilities. One of the most significant enhancement technologies for mobile wireless, currently available with 4G LTE-Advanced, and targeted for future 5G applications, is multi-input multi-output (MIMO) antenna and RF front-end technology.
What MIMO enables is the use of an antenna array and intelligent processing features to create additional capacity using the optimum propagation channels, known as spatial multiplexing. Hence, a MIMO equipped base-station and UE could operate on several spatially multiplexed channels, and extending the available throughput to that device. Typical MIMO systems in handheld UE is 2 by 2, or 2×2 MIMO, which means that both the base station and UE have an antenna array with two transmit and two receive antennas–effectively doubling capacity. Some WiFi routers and other wireless systems currently have 4×4, 8×8, and even 16×16 MIMO systems, with other asymmetric combinations also existing for certain applications.
Given the future predictions of tens, hundreds, and even thousands of devices requiring high throughput data services, the concept of designing base stations with many multiples more transmit and receive antennas than with current MIMO systems has evolved into Massive-MIMO or Massive Multiuser MIMO (MU-MIMO).
The goal of Massive-MIMO is to provide base stations with a high number of transmit and receive streams, along with other network capacity augmentation technologies and methods, to improve peak downlink throughput, substantially improve uplink performance, and enhance coverage, specifically in densely populated and urban environments. Other than the obvious increase in network capacity, other benefits of Massive-MIMO could be an increase in spectral efficiency–especially for sub-6 GHz applications–, a reduction in energy consumption and battery life extension for UE, and the potential for less-complex scalability than with prior mobile wireless technologies. Moreover, Massive-MIMO may be a solution to providing the huge number of machine-type devices part of the Internet of Things (IoT) and Industry 4.0 trends with connectivity services, efficiently. Lastly, Massive-MIMO may also be a solution for ultra-reliable communication, as several physical links can be established to ensure uninterrupted communications to critical systems in aircraft, infrastructure, vehicles, and more.
Massive-MIMO provides significant improvements to spectral efficiency in low-mobility and no-mobility applications, but is less effective in high-mobility applications. The loss in spectral efficiency as a product of mobility is due to the drop in channel coherence and lower pilot availability reducing the multiplexing gain of a Massive-MIMO system servicing to the mobile UE. Other challenges with Massive-MIMO will likely be the availability of low-cost RF hardware and installation of network infrastructure that could power Massive-MIMO base stations.
Massive-MIMO is also commonly confused with large MIMO links with tens and hundreds of transmit and receive channels and full-dimension MIMO concepts, though these technologies are implementations of MIMO, they don’t serve a large number of users simultaneously.