5G-NR MU-MIMO: What you need to master

5G-NR MU-MIMO (Multi user MIMO) implements spatial multiplexing of time frequency resources for multiple UEs in both uplink and downlink. Multiple UEs can share time-frequency resources achieving more channel orthogonality between UEs, leading to smaller interference, higher capacity and the spectral efficiency in both channels (uplink and downlink).

In following figure we can se how UEs 1,2,3,4,5,6,7,8 are using the same resources are the same time. UEs 9 and 10 are no pairing because they don’t share resources in time with any other UE.

In 5G-NR MU-MIMO, UE pairing procedure is critical. This process select multiple UEs for spatial multiplexing of time-frequency resources. During UE pairing, SRS signal quality and channel correlation need to be considered. The channel correlation is a calculation based on uplink feedback and also known as channel similarity. If two UEs are close to each other, the channel correlation is high. If two UEs are far away from each other, the channel correlation is low.

Also, if the SRS SINRs of UEs meet certain conditions and the channel correlation between UEs is low, interference between UEs reduces considerably and UE pairing can be performed for MU-MIMO. In this situation, MU-MIMO can fully utilize good channel conditions to increase the system capacity. Let’s review this in an example:

In previous image, UE1 and U2 are paired, same as UE3 with U4 and UE6 and UE5. This basically means that for each pair, each UE has good quality (SINR) and they are physically far away enough between each other but still connected to the same cell connected in a different antenna layer, so under this conditions UE pairing is possible.

The steps performed by the gNB to consider a UE for pairing are:

When a UE access the network, the gNB evaluates the SRS SINR of the UE based on a threshold established. If the value reach the threshold, gNB selects UEfor pairing, otherwise, it is not.
If at some point, the SRS SINR changes, then:
- The gNB evaluates the SRS SINR and compares it against another threshold + 3 dB, if it is above the threshold, the gNB consider the UE for pairing. This works as a hysteris to avoid some kind of ping pong effect between UEs considered for pairing or not.
- If the SRS SINR is below the previous target, gNB cannot consider UE for pairing.

From the gNB point of view, the UE pairing procedure is as follows:

Select UEs for pairing
Evaluate the SRS SINR and channel correlation
If the conditions passes:
- Check if the number of paired layers are less than the maximum configured for the cell
  - If true, pair the UE and contiue to select other UEs for pairing.
If any of the previous conditions do not pass, the UE cannot be pair and the gNB selects other UEs for pairing.

Important to mention, spatial multiplexing supports multiple UEs in following channels: PDSCH, PDCCH and PUSCH. In the following table you can see the number of layers available depending on the antenna MIMO. This could varies from vendor and software release.

gNodeb antenna number	Maximum Number of PDSCH MU-MIMO Layers	Maximum Number of PDCCH MU-MIMO Layers	Maximum Number of PUSCH MU-MIMO Layers
64T64R	16	4	8
32T32R	16	4	8
8T8R	4	Not supported	4

For 5G-NR MU-MIMO, in medium- and light-load scenarios, cell resources are not limited and enabling MU-MIMO may lead to fluctuations in the average UE throughput and cell data throughput. In heavy-load scenarios, cell resources are limited and enabling MU-MIMO can increase the average UE throughput and cell data throughput. The recommendation is that MU-MIMO should be enabled when the cell load is high. The cell load is high when the uplink or downlink PRB usage is high, for example, higher than or equal to 60%.

Under the same data volume condition, the increases in the average UE throughput and cell data throughput are related to the number of paired RBs and the average number of paired layers. The greater the numbers, the larger the increases.

Some possible impacts after enabling 5G-NR MU-MIMO:

If the CCE usage is high due to high traffic or large number of cell edge UEs before 5G-NR MU-MIMO activation, then after enabling downlink 5G-NR MU-MIMO, the number of UEs scheduled in the downlink increases, consuming more CCE resources. If CCE resources for the uplink are limited, the probability of CCE allocation failures for uplink data transmission increases and the uplink UE throughput and cell data throughput decrease. Similarly, after uplink 5G-NR MU-MIMO activation, the downlink UE throughput and cell data throughput decrease.
After downlink 5G/NR MU-MIMO takes effect, the number of uplink status reports increases. If there are not enough PUSCH PRB resources, the uplink UE throughput and cell data throughput decrease.
After uplink 5G-NR MU-MIMO takes effect, the number of downlink status reports increases. If there are not enough PDSCH PRB resources, the downlink UE throughput and cell data throughput decrease.
In medium- and heavy-load scenarios, the PRB usage may decrease. In low load scenarios, the performance is normal.
After 5G-NR MU-MIMO takes effect, the interference between paired UEs increases and the uplink and downlink BLERs of the cell increase.
After downlink 5G-NR MU-MIMO takes effect, the average downlink MCS index and average rank decrease.
When a UE accesses or handover to a 5G-NR MU-MIMO cell from the cell edge, the access success rate of the UE slightly decreases.

When enabling this feature, it is important to follow next actions for monitoring:

Average number of downlink MU-MIMO layers on each PRB in a cell.
Average number of uplink MU-MIMO layers on each PRB in a cell.
User Downlink Throughput
User Uplink Throughput
Cell Downlink Throughput
Cell Uplink Throughput