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.

4 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.

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