Massive MIMO (massive multiple-input multiple-output) is a type of wireless communications technology in which base stations are equipped with a very large number of antenna elements to improve spectral and energy efficiency.

Massive MIMO systems typically have tens, hundreds, or even thousands of antennas in a single antenna array.

Other technologies such as beamforming and spatial multiplexing enable massive MIMO as one of the key technologies for 5G NR systems.

Benefits of Massive MIMO

• Improved coverage at cell edge: In the context of cellular communication (21:27), the closer the end user is to the base station, the stronger the signal. As the end user travels further away from the base station, they approach the cell edge where the signal gets weaker. Massive MIMO spatially directs transmissions to focus energy towards the end user, enabling better cell edge performance.
• Improved throughput: Using spatial multiplexing with MU-MIMO, wireless communications systems can simultaneously communicate with multiple user equipment (UEs) using the same time-frequency resources. This technology is often used in conjunction with massive MIMO to significantly improve spectral efficiency and aggregate throughput for the cell.
• Enabled by millimeter wave: Using millimeter wave frequencies (above 24 GHz), the signal power drops quickly due to path loss. As a result, millimeter wave transmissions enable massive MIMO to boost the signal power. The need for massive MIMO is more apparent in 5G systems (6:48) where new frequencies in millimeter wave (up to 52 GHz) have been introduced.

Challenges of Massive MIMO

• Modeling, simulation, and testing: With the introduction of 5G enabling technologies such as massive MIMO and millimeter wave, the challenges of modeling, simulation, and testing (32:59) are becoming more evident, especially if physical prototypes for radios employing these technologies are not yet available. Configuring these systems may require simulated results rather than results measured in the field (46:21).
• Power consumption: To achieve the required range needed for 5G millimeter wave transmissions, massive MIMO may require a large number of antenna elements. This demand increases the overall power and cost requirements of a system, although methods such as hybrid beamforming can be applied to reduce its power usage. 
• Channel reciprocity: Massive MIMO is designed for a time domain duplex (TDD) system, where transmission and reception occurs at the same center frequency. However, TDD requires additional calibration compared to its frequency domain duplex (FDD) counterpart in order to achieve channel reciprocity. This requirement is exacerbated by the deployment of many antennas introduced by massive MIMO.

Software tools such as MATLAB^® wireless communications products provide tools that help address these challenges.

Massive MIMO with MATLAB and Simulink

Using MATLAB and Simulink^® wireless communications products, you can:

• Design and synthesize complex antenna elements and massive MIMO phased arrays and subarrays
• Construct and partition hybrid beamforming (38:29) systems intelligently across digital and RF domains
• Validate spatial signal processing algorithms and channel models (24:25) including 5G NR CDL spatial channel model
• Verify link-level designs using standards-based simulations of 5G (32:59) systems