I assume that most of the readers are already familiar with the basic concept of MIMO from LTE MIMO. In 5G/NR, we will use a very special types of MIMO called Massive MIMO to implement both sptial multiplexing(MIMO) and BeamForming.

In this page, I will describe mostly about 3GPP details on how the MIMO is operated in NR.

• How MIMO is configured ?
• MIMO Configuration Table : Antenna Port Configuration
• Table 7.3.1.2.2-1: Antenna port(s) (1000 + DMRS port), dmrs-Type=1, maxLength=1
• Table 7.3.1.2.2-2: Antenna port(s) (1000 + DMRS port), dmrs-Type=1, maxLength=2
• Table 7.3.1.2.2-3: Antenna port(s) (1000 + DMRS port), dmrs-Type=2, maxLength=1
• Table 7.3.1.2.2-4: Antenna port(s) (1000 + DMRS port), dmrs-Type=2, maxLength=2
• DMRS Pattern : Table 7.8.1.2.2-1
• DMRS Pattern : Table 7.8.1.2.2-2
• DMRS Pattern : Table 7.8.1.2.2-3
• DMRS Pattern : Table 7.8.1.2.2-4

How MIMO is configured ?

The major factors for MIMO configuration is the number of antenna and layers. In LTE, these factors are specified explicitely by RRC message (RRC Connection Setup or RRCConnectionSetupReconfiguration). However, in NR there is no explicit RRC parameter for the number of Antenna and the number of layers. In stead, both UE and gNB carries a few predefined tables defining the number of antenna port and layers.

Now you would have a few questions. Out of these multiple predefiend table, how a UE figure out which table to use ?

It is specified by the two dmrs related RRC parameters : dmrs-Type and maxLength. The mapping between the table and the RRC parameters is as follows.

< Antenna port(s) and number of layers >

Now you would have another question. Once a specific table is selected, how UE can figure out which row in the table is being used for each transmission from gNB ?

UE figures it out from a field called Antenna port(s) and number of layers in DCI 1_1. The number of DMRS ports indicates the number of Antenna being used in the tables.

MIMO Configuration Table : Antenna Port Configuration

< 38.212 - Table 7.3.1.2.2-1: Antenna port(s) (1000 + DMRS port), dmrs-Type=1, maxLength=1 >

Interpretation of this table in terms of MIMO are as follows.

• This table is associated with 38.211-Table 7.4.1.1.2-5 and 38.211-Table 7.4.1.1.2-1.
• Value (index) 0,1,3,4,5,6 = SISO
• Value (index) 2,7,8,11 = 2x2 MIMO
• Value (index) 9 = 3x3 MIMO
• Value (index) 10 = 4x4 MIMO

Interpretation of 'Number of front-load symbols' column are stated in 38.214 5.1.6.2 as below :

the UE may be configured with the maximum number of front-loaded DM-RS symbols for PDSCH by higher layer parameter maxLength given by DMRS-DownlinkConfig..

• if maxLength is set to 'len1', single-symbol DM-RS can be scheduled for the UE by DCI, and the UE can be configured with a number of additional DM-RS for PDSCH by higher layer parameter dmrs-AdditionalPosition, which can be set to 'pos0', 'pos1', 'pos2' or 'pos3'.
• if maxLength is set to 'len2', both single-symbol DM-RS and double symbol DM-RS can be scheduled for the UE by DCI, and the UE can be configured with a number of additional DM-RS for PDSCH by higher layer parameter dmrs-AdditionalPosition, which can be set to 'pos0' or 'pos1'.

< 38.212 - Table 7.3.1.2.2-2: Antenna port(s) (1000 + DMRS port), dmrs-Type=1, maxLength=2 >

< 38.212 - Table 7.3.1.2.2-3: Antenna port(s) (1000 + DMRS port), dmrs-Type=2, maxLength=1 >

< 38.212 - Table 7.3.1.2.2-4: Antenna port(s) (1000 + DMRS port), dmrs-Type=2, maxLength=2 >

DMRS Pattern : Table 7.8.1.2.2-1

DMRS Pattern : Table 7.8.1.2.2-2

DMRS Pattern : Table 7.8.1.2.2-3

DMRS Pattern : Table 7.8.1.2.2-4

Reference

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