Six Differences Between MU-MIMO and Massive MIMO

Multi-user MIMO (MU-MIMO) is not a new technology, but the basic concept of using multi-antenna base stations (BSs) to serve a multitude of users has been around since the late 1980s.

An example of how MU-MIMO was illustrated prior to Massive MIMO.

I sometimes get the question “Isn’t Massive MIMO just MU-MIMO with more antennas?” My answer is no, because the key benefit of Massive MIMO over conventional MU-MIMO is not only about the number of antennas. Marzetta’s Massive MIMO concept is the way to deliver the theoretical gains of MU-MIMO under practical circumstances. To achieve this goal, we need to acquire accurate channel state information, which in general can only be done by exploiting uplink pilots and channel reciprocity in TDD mode. Thanks to the channel hardening and favorable propagation phenomena, one can also simplify the system operation in Massive MIMO.

This is how Massive MIMO is often illustrated for line-of-sight operation.

Six key differences between conventional MU-MIMO and Massive MIMO are provided below.

Conventional MU-MIMO Massive MIMO
Relation between number of BS antennas (M) and users (K) MK and both are small (e.g., below 10) K and both can be large (e.g., M=100 and K=20).
Duplexing mode Designed to work with both TDD and FDD operation Designed for TDD operation to exploit channel reciprocity
Channel acquisition Mainly based on codebooks with set of predefined angular beams Based on sending uplink pilots and exploiting channel reciprocity
Link quality after precoding/combining Varies over time and frequency, due to frequency-selective and small-scale fading Almost no variations over time and frequency, thanks to channel hardening
Resource allocation The allocation must change rapidly to account for channel quality variations The allocation can be planned in advance since the channel quality varies slowly
Cell-edge performance Only good if the BSs cooperate Cell-edge SNR increases proportionally to the number of antennas, without causing more inter-cell interference

Footnote: TDD stands for time-division duplex and FDD stands for frequency-division duplex.

6 thoughts on “Six Differences Between MU-MIMO and Massive MIMO”

  1. So, with the above difference can we say that Massive MIMO is not supporting FDD? Please suggest.

    1. Massive MIMO of course works both in TDD (its canonical form) and FDD, but it works much better (=more reliably, higher capacity) in TDD because then the uplink-downlink channel reciprocity can be leveraged.

      You may also be interested in this comparison, https://arxiv.org/pdf/1704.00623.pdf

  2. In FDD Massive MIMO system, some experts have proposed a method called ‘Joint Spatial Division and Multiplexing’ to reduce the pilot overhead. What do you think is the shortcoming of this method?

    1. JSDM is a very nice concept, but it is not clear to what extent it can be applied in practice. As said in the original paper “The main idea of JSDM consists of partitioning the user population into groups with approximately the same channel covariance eigenspace”. Basically, we need to divide the world into regions where the covariance matrices are approximately equal, while they are substantially different between regions. The users in a given region will be one group. I’m not sure if that type of partitioning is possible in practice. I think that the covariance eigenspaces change gradually as you move around instead of changing more abruptly when you move from one region to another.

      That said, it is definitely important to take spatial channel correlation (channel eigenspaces) into account in the resource allocation in Massive MIMO.

  3. As a basic principle, what is the difference between Massive MIMO, MU-MIMO and SU-MIMO. Please in a simple manner, my confusion is: SU-MIMO is like 2×2 MIMO today where a single beam covers a particular sector in telecom site and as per scheduler, resource blocks are shooted from antenn in form of analogue signals with a particular time lap (0.5ms for 1RB). Now this shooting of RBs is divided as per application demand by the users. Similarly MU-MIMO is applied as short focused signal beams towards potential users but it is continuous, while 2×2 MIMO is having a time lap between shooting of RBs. Massive MIMO is approach of having transmission of signals from multiple antenna arrays but again here we can get max 8×8 MIMO parallel streams not like for 64×64 streams as it looks for massive MIMO, but these 8x8MIMO will be shooted from one set of antenna array panel in antenna and then 08 different antenna array panels to make total 64×64 antenna arrays so these 8×8 MIMO is shooted from 64×64 antenna array system periodically not parallel. Please elaborate if my concept is wrong. thanks really

    1. To answer these things in detail, I would have to write a book…

      To answer your question briefly:

      SU-MIMO: Transmission from a multi-antenna transmitter to a multi-antenna receiver. One can either send one or multiple simultaneously beams (also called layers or streams) to the receiver. The number of beams is limited by the minimum of the number of transmit antennas and receive antennas.

      MU-MIMO: Transmission from a multi-antenna transmitter to multiple receivers, each having either one or multiple antennas. The transmitter sends at least one simultaneous beam to each receiver. The opposite is also included: Simultaneous transmission from multiple transmitter to a single multi-antenna receiver.

      Massive MIMO: This is a name for MU-MIMO with very many antennas, typically at least 64 antennas at the base station. This base station is capable of sending 64 different beams simultaneously, but in typical operation it will probably use more like 10 beams since a core insight in the design of Massive MIMO is that is preferable to have many more antennas than beams. This makes the beams relatively narrow so there is empty space between them, leading to less interference.

      I recommend my video series about multiple antenna communications:
      https://www.youtube.com/watch?v=OA4viERrlzA&list=PLTv48TzNRhaKz0C-dCAwimXSypV_5UTxg

      And also this video: https://youtu.be/xGkyZw98Tug

Leave a Reply

Your email address will not be published. Required fields are marked *