UE Pairing Procedure in Multi-user MIMO (MU-MIMO)
UE pairing is a procedure of selecting 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 calculated based on uplink feedback and can be considered 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.
If the SRS SINRs of UEs meet certain conditions and the channel correlation between UEs is low, interference between UEs can be effectively mitigated 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.
When a UE initially accesses the network, if the SRS SINR of the UE is greater than or equal to the value of the defined threshold, the UE can participate in pairing; otherwise, it cannot.
After the UE accesses the network, if its SRS SINR changes, then:
If the SRS SINR is greater than the value of the Configured threshold, the UE can participate in pairing.
If the SRS SINR is less than the value of the Configured threshold, the UE cannot participate in pairing.
The gNodeB selects appropriate UEs for pairing based on the preceding rules and schedules the UEs.
The below video illustrates the pairing procedure.
If UEs have poor SRS signal quality (for example, low SINRs or large signal fluctuations) or high channel correlation, inter-UE interference cannot be effectively eliminated.
In this situation, MU-MIMO may decrease the system throughput. Therefore, such UEs will not be selected for pairing.
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