Massive MIMO (M-MIMO) Basics, Advantages, and Applications

This page covers the fundamentals of Massive MIMO (M-MIMO). It details the advantages and disadvantages of Massive MIMO, along with its operational aspects and applications.

MIMO stands for Multiple Input Multiple Output. It refers to the use of multiple antennas for both transmission and reception of electromagnetic waves. There are two primary techniques for transmitting complex data symbols using multiple antennas:

• STBC (Space Time Block Coding): Data symbols are transmitted at a time instant $t = t_1$, and modified copies of the same data symbols are re-transmitted at $t = t_2$. This minimizes errors at the cost of redundant data symbols. The redundancy helps correct errors, eliminating the need for re-transmission.

• SM (Spatial Multiplexing): Different data symbols are transmitted at all time instants without any redundant copies. This increases data rates. SM relies on pilot subcarriers for channel estimation and equalization. Pilot subcarriers are embedded between data symbols before transmission.

Some systems, like Mobile WiMAX, employ both STBC and SM to leverage the benefits of both coverage and capacity improvements. Channel estimation can be performed at the Base Station (BS) or Mobile Station (MS). In Massive MIMO, the BS estimates the channel response based on pilot subcarriers received from the MSs.

In MIMO, there are two approaches based on how base station antennas serve mobile subscribers:

• Single-user MIMO: All data streams from the base station antennas are directed towards a single user.

• Multi-user MIMO: Different data streams, generated by combining signals from different antennas, are directed towards different users. One stream can even serve multiple users.

Massive MIMO utilizes the multi-user MIMO concept, as depicted in Figure 1 above. Various antenna configurations, such as spherical, cylindrical, distributed, linear, and rectangular, are employed in massive MIMO-based wireless cellular systems.

In Massive MIMO, a large number of antennas (typically 32 to 64) are used at the base station to simultaneously serve tens of users or mobile subscribers (MSs) in the same time-frequency grid. M-MIMO offers numerous benefits compared to conventional MIMO systems. The difference can be understood by studying the operation of both systems along with their advantages and disadvantages. Massive MIMO may also be referred to as “Large Scale Antenna Systems,” “Hyper MIMO,” “Very Large MIMO,” “ARGOS,” and “Full Dimension MIMO.”

Massive MIMO Advantages

The following are the advantages of Massive MIMO (M-MIMO) systems:

• High spectrum efficiency: Achieved through large multiplexing gain and antenna array gain.

• High energy efficiency: Concentrated radiated energy directed towards the MS/UE.

• High reliability: Due to large diversity gain.

• Weak inter-user interference and enhanced physical security: Orthogonal MS channels and extremely narrow beams contribute to this.

• Simple scheduling scheme.

• Robustness to individual element failure: Due to the large number of antenna array elements.

• Cost-effective development: Massive MIMO can be developed using low power and inexpensive components.

• Reduced Latency: Enables a significant reduction in latency on the air interface.

Massive MIMO Disadvantages

The following are the disadvantages of Massive MIMO (M-MIMO) systems:

• Pilot contamination: Due to limited orthogonal pilot subcarriers within bounded coherent intervals and bandwidth.

• High signal processing complexity: Due to the large number of antennas and multiplexing of UEs (or Mobile subscribers).

• Sensitive to beam alignment: The use of extremely narrow beams makes the system susceptible to MS movement or antenna array swaying.

• Channel reciprocity assumption: Relies on the channel reciprocity assumption in TDD mode of Massive MIMO.

Massive MIMO (M-MIMO) Applications

The following are the applications of Massive MIMO (M-MIMO):

• 4G LTE
• LTE Advanced
• Advanced WLAN versions (e.g., 802.11ac, 802.11ad)