A method of layer-to-demodulation reference signal, DMRS, port mapping, comprising: assigning, by a base station, a User Equipment, UE, with a corresponding data stream as a layer of a Multiple-Input-Multiple-Output, MIMO, downlink transmission; mapping, by the base station, the corresponding data stream from the layer to a corresponding DMRS port; and transmitting, by the base station, the corresponding data stream to the UE via the DMRS port, wherein the base station maps the corresponding data stream from the layer to a corresponding DMRS port dependent upon a value of a rank of the MIMO downlink transmission, wherein the rank is indicative of the number of layers of the MIMO downlink transmission, wherein, for the MIMO downlink transmission at the rank=1, the base station maps the layer to any DMRS port belonging to any of two DMRS code division multiplexing, CDM, group of DMRS ports; characterized in that, for the MIMO downlink transmission at the rank=2, when the MIMO downlink transmission is double-codeword transmission, the base station maps two layers to two DMRS ports belonging to the same DMRS CDM group, and when the MIMO downlink transmission is retransmission of a codeword, the base station maps two layers corresponding to the codeword to two DMRS ports belonging to the same DMRS CDM group; for the MIMO downlink transmission at the rank=3, when the MIMO downlink transmission is double-codeword transmission, the base station maps one layer corresponding to a codeword 1 to a first DMRS CDM group, and maps the other two layers corresponding to a codeword 2 to a second DMRS CDM group, and when the MIMO downlink transmission is retransmission of a codeword, the base station maps one layer corresponding to the codeword to the first DMRS CDM group, and maps the other two layers corresponding to the codeword to the second DMRS CDM group; and for the MIMO downlink transmission at the rank=4, when the MIMO downlink transmission is double-codeword transmission, the base station maps two layers corresponding to a codeword 1 to a first DMRS CDM group, and maps the other two layers corresponding to a codeword 2 to a second DMRS CDM group, and when the MIMO downlink transmission is retransmission of a codeword, the base station maps two layers corresponding to the codeword to the first DMRS CDM group, and maps the other two layers to the second DMRS CDM group. assigning, by a base station, a User Equipment, UE, with a corresponding data stream as a layer of a Multiple-Input-Multiple-Output, MIMO, downlink transmission; mapping, by the base station, the corresponding data stream from the layer to a corresponding DMRS port; and transmitting, by the base station, the corresponding data stream to the UE via the DMRS port, wherein the base station maps the corresponding data stream from the layer to a corresponding DMRS port dependent upon a value of a rank of the MIMO downlink transmission, wherein the rank is indicative of the number of layers of the MIMO downlink transmission, wherein, for the MIMO downlink transmission at the rank=1, the base station maps the layer to any DMRS port belonging to any of two DMRS code division multiplexing, CDM, group of DMRS ports; characterized in that, for the MIMO downlink transmission at the rank=2, when the MIMO downlink transmission is double-codeword transmission, the base station maps two layers to two DMRS ports belonging to the same DMRS CDM group, and when the MIMO downlink transmission is retransmission of a codeword, the base station maps two layers corresponding to the codeword to two DMRS ports belonging to the same DMRS CDM group; for the MIMO downlink transmission at the rank=3, when the MIMO downlink transmission is double-codeword transmission, the base station maps one layer corresponding to a codeword 1 to a first DMRS CDM group, and maps the other two layers corresponding to a codeword 2 to a second DMRS CDM group, and when the MIMO downlink transmission is retransmission of a codeword, the base station maps one layer corresponding to the codeword to the first DMRS CDM group, and maps the other two layers corresponding to the codeword to the second DMRS CDM group; and for the MIMO downlink transmission at the rank=4, when the MIMO downlink transmission is double-codeword transmission, the base station maps two layers corresponding to a codeword 1 to a first DMRS CDM group, and maps the other two layers corresponding to a codeword 2 to a second DMRS CDM group, and when the MIMO downlink transmission is retransmission of a codeword, the base station maps two layers corresponding to the codeword to the first DMRS CDM group, and maps the other two layers to the second DMRS CDM group.