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.
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.
Six key differences between conventional MU-MIMO and Massive MIMO are provided below.
|Relation between number of BS antennas (M) and users (K)
|M ≈ K and both are small (e.g., below 10)
|M ≫ K and both can be large (e.g., M=100 and K=20).
|Designed to work with both TDD and FDD operation
|Designed for TDD operation to exploit channel reciprocity
|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
|The allocation must change rapidly to account for channel quality variations
|The allocation can be planned in advance since the channel quality varies slowly
|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.