Massive MIMO: Advantages and Disadvantages

This page explores the advantages and disadvantages of Massive MIMO compared to traditional MIMO.

What is Massive MIMO?

Introduction: The term “Massive MIMO” combines “Massive” and “MIMO.” “Massive” refers to the large number of antennas used in a Base Station (BS) antenna array. “MIMO” refers to multiple spatially separated mobile users served by the antenna array simultaneously using the same time/frequency resources. Essentially, Massive MIMO is beamforming with a significant number of antennas.

Traditional MIMO uses a smaller number of spatially separated antennas for transmitting and receiving electromagnetic (EM) waves. It utilizes techniques like Space-Time Block Coding (STBC) and Spatial Multiplexing (SM) to transmit data symbols using these antennas.

5G Massive MIMO Simulation

The figure above illustrates the baseband and RF modules used in massive MIMO, along with the antenna array elements. Each RF transceiver is connected to two antenna elements. One antenna element uses vertical polarization, and the other uses horizontal polarization. Massive MIMO is often abbreviated as mMIMO. It combines radio transceivers (or remote radio heads) with a large number of antenna elements (e.g., 16, 32, 64, or 96) within a single active antenna unit, boosting the system’s performance.

Massive MIMO can be implemented using two duplexing schemes: FDD and TDD. FDD (Frequency Division Duplexing) uses different frequencies for downlink (DL) and uplink (UL) transmissions. TDD (Time Division Duplexing) uses the same frequency for both downlink and uplink. TDD benefits from channel reciprocity, where channel estimation from the uplink can be used for downlink beamforming, reducing overhead.

Single User MIMO vs Multi User MIMO

SU-MIMO vs MU-MIMO

Antenna arrays are used to serve mobile users in two main ways: SU-MIMO (Single User MIMO) and MU-MIMO (Multi User MIMO). The figure above is based on Wi-Fi 6 (IEEE 802.11ax), but the core concepts are used in 4G/LTE and 5G NR (New Radio) technologies as well.

In SU-MIMO, all the streams from the antenna array are focused on a single user. In MU-MIMO, different streams are directed towards different users. Moreover, each of these streams can serve more than one mobile subscriber or user with the help of TDD. Massive MIMO primarily uses MU-MIMO to increase spectral efficiency.

Massive MIMO Antenna Array

5G Massive MIMO Antenna Array

In a conventional antenna array, each column typically uses 8 to 12 antenna elements to create a vertically fixed directional pattern. In a 64T64R massive MIMO system (64 Transmitters, 64 Receivers), there are 64 RF transceivers, each mapped to 2 antenna elements. All 128 antenna elements create an antenna pattern allowing both horizontal and vertical beamforming.

Benefits or Advantages of Massive MIMO over MIMO

Here are some of the key advantages of Massive MIMO:

• Higher Spectral Efficiency and Capacity: Massive MIMO significantly increases spectral efficiency, enabling support for a greater number of users per 5G cell tower or antenna array.
• Improved Throughput: It boosts average sector throughput (up to 6 times), enhances cell edge throughput (up to 3 times), and increases peak sector throughput (up to 8 times).
• Better Coverage: Massive MIMO provides improved coverage.
• Reduced Interference: By focusing beamforming towards the desired user, Massive MIMO reduces interference with surrounding users, resulting in improved overall performance.
• Higher Energy Efficiency: Massive MIMO concentrates radiated energy on mobile phone users or UEs, leading to better energy efficiency.

Drawbacks or Disadvantages of Massive MIMO

Here are some of the drawbacks of Massive MIMO:

• Higher Cost: Massive MIMO units are considerably more expensive than traditional radio units.
• Complex Antenna Designs: Massive MIMO antenna designs are more complex, requiring more effort and time during the assembly process.
• Feedback Overhead (FDD): Using FDD in massive MIMO can lead to significant feedback overhead. This overhead increases with the number of antenna elements. TDD is often preferred due to its channel reciprocity.
• Packing Density: The small spacing between antennas requires packing a lot of hardware into a smaller space. For example, a 32T32R system requires 64 RF paths with antenna spacing of approximately 4.2 cm at a frequency of about 3.5 GHz.
• Heat Dissipation: The greater number of RF paths and antennas lead to increased power dissipation, requiring careful thermal management.
• Complex Signal Processing: Massive MIMO requires complex signal processing algorithms at the receiver to mitigate inter-user interference.

Conclusion

Understanding the benefits and limitations of Massive MIMO requires comparing it to traditional MIMO and grasping the specific terminology associated with mMIMO.