In order to enable DL-MIMO in OAI 5G softmodem, the prerequisite is to have `do_CSIRS = 1` in the configuration file. This allows the gNB to schedule CSI reference signal and to acquire from the UE CSI measurements to be able to schedule DLSCH with MIMO.
The following step is to set the number of PDSCH logical antenna ports. These needs to be larger or equal to the maximum number of MIMO layers requested (for 2-layer MIMO it is necessary to have at least two logical antenna ports).
This image shows an example of gNB 5G MIMO logical antenna port configuration. It has to be noted that logical antenna ports might not directly correspond to physical antenna ports and each logical antenna port might consist of a sub-array of antennas.
The basic element is a dual-polarized antenna, therefore the minimal DL MIMO setup with two logical antenna ports would consist of two cross-polarized antenna elements. In a single panel configuration, as the one in the picture, this element can be repeated vertiacally and/or horizontally to form an equi-spaced 1D or 2D array. The values N1 and N2 represent the number of antenna ports in the two dimensions and the supported configurations are specified in Section 5.2.2.2.1 of TS 38.214.
In 5G the basic element is a dual-polarized antenna, therefore the minimal DL MIMO setup with two logical antenna ports would consist of two cross-polarized antenna elements. In a single panel configuration, as the one in the picture, this element can be repeated vertically and/or horizontally to form an equi-spaced 1D or 2D array. The values N1 and N2 represent the number of antenna ports in the two dimensions and the supported configurations are specified in Section 5.2.2.2.1 of TS 38.214.
The DL logical antenna port configuration can be selected through configuration file. `pdsch_AntennaPorts_N1` can be used to set N1 parameter, `pdsch_AntennaPorts_N2` to set N2 and `pdsch_AntennaPorts_XP` to set the cross-polarization configuration (1 for single pol, 2 for cross-pol). To be noted that if XP is 1 but N1 and/or N2 are larger than 1, this would result in a non-standard configuration and the PMI selected would be the identity matrix regardless of CSI report. The default value for each of these parameters is 1. The total number of PDSCH logical antenna ports is the multiplication of those 3 parameters.
Finally the number of TX physical antenna in the RU part of the configuration file, `nb_tx`, should be equal or larger than the total number of PDSCH logical antenna ports.
The DL logical antenna port configuration can be selected through configuration file. `pdsch_AntennaPorts_N1` can be used to set N1 parameter, `pdsch_AntennaPorts_N2` to set N2 and `pdsch_AntennaPorts_XP` to set the cross-polarization configuration (1 for single pol, 2 for cross-pol). To be noted that if XP is 1 but N1 and/or N2 are larger than 1, this would result in a non-standard configuration and the PMI selected would be the identity matrix regardless of CSI report. The default value for each of these parameters is 1.
[Example of configuration file with parameters for 2-layer MIMO](https://gitlab.eurecom.fr/oai/openairinterface5g/-/blob/develop/targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb.sa.band77.fr1.273PRB.2x2.usrpn300.conf)
