A data transmission method performed by a user equipment, the method comprising: • receiving a first reference signal set; • obtaining a channel estimate based on the first reference signal set; • selecting according to a system equation of a communication system comprising the user equipment and the base station and according to the channel estimate an optimal rank r * of the user equipment, wherein the rank r * is a rank indicator, RI; • obtaining based on the rank r * a quantity of transport layers used to transmit the data, wherein each transport layer is a transport layer between the base station and the user equipment; wherein the quantity of transport layers is either three or four; • determining, for each symbol transmitted on each layer of each resource element of the determined quantity of transport layers, a signal to interference plus noise ratio, SINR; • determining for each symbol transmitted on each layer of each resource element a corresponding symbol of a codeword based on a first codeword-to-layer mapping and determining for each symbol in the codewords a SINR, wherein the first codeword-to-layer mapping comprises at least the following mappings: Quantity of transport layers N layer Quantity of codewords N CW Codeword-to-transport layer mapping i = 0,1,..., M symb layer − 1 3 3 x ( i) = d ( i) M symb layer = M symb 0 = M symb 1 = M symb 2 x ( i) = d ( i) x ( i) = d ( i) 4 4 x ( i) = d ( i) M symb l ⁢ ayer = M symb 0 = M symb 1 = M symb 2 = M symb 3 x ( i) = d ( i) x ( i) = d ( i) x ( i) = d ( i) wherein N layer indicates the quantity of transport layers, N CW indicates a quantity of codewords, x ( i), x ( i), x ( i), x ( i) are respectively symbols transmitted on the transport layers, d ( i), d ( i), d ( i), d ( i) are respectively symbols comprised in the codewords, and M symb 0, M symb 1, M symb 2, M symb 3 are respectively quantities of symbols transmitted on the transport layers; • determining, for each codeword, based on the SINRs of all symbols in the codeword, a SINR of the codeword; • determining, for each codeword, a channel quality indicator, CQI, based on the SINR of the corresponding codeword; • sending channel state information, CSI comprising the CQI for each codeword and the RI to the base station, receiving a second reference signal set sent by the base station; and • receiving based on the second reference signal set and a second codeword-to-layer mapping, data sent by the base station, wherein a quantity of layers to which at least one codeword is mapped in the first codeword-to-layer mapping is less than or equal to a quantity of layers to which a codeword is mapped in the second codeword-to-layer mapping, and the second codeword-to-layer mapping comprises a second mapping between at least one codeword and the at least one transport layer. • receiving a first reference signal set; • obtaining a channel estimate based on the first reference signal set; • selecting according to a system equation of a communication system comprising the user equipment and the base station and according to the channel estimate an optimal rank r * of the user equipment, wherein the rank r * is a rank indicator, RI; • obtaining based on the rank r * a quantity of transport layers used to transmit the data, wherein each transport layer is a transport layer between the base station and the user equipment; wherein the quantity of transport layers is either three or four; • determining, for each symbol transmitted on each layer of each resource element of the determined quantity of transport layers, a signal to interference plus noise ratio, SINR; • determining for each symbol transmitted on each layer of each resource element a corresponding symbol of a codeword based on a first codeword-to-layer mapping and determining for each symbol in the codewords a SINR, wherein the first codeword-to-layer mapping comprises at least the following mappings: Quantity of transport layers N layer Quantity of codewords N CW Codeword-to-transport layer mapping i = 0,1,..., M symb layer − 1 3 3 x ( i) = d ( i) M symb layer = M symb 0 = M symb 1 = M symb 2 x ( i) = d ( i) x ( i) = d ( i) 4 4 x ( i) = d ( i) M symb l ⁢ ayer = M symb 0 = M symb 1 = M symb 2 = M symb 3 x ( i) = d ( i) x ( i) = d ( i) x ( i) = d ( i) wherein N layer indicates the quantity of transport layers, N CW indicates a quantity of codewords, x ( i), x ( i), x ( i), x ( i) are respectively symbols transmitted on the transport layers, d ( i), d ( i), d ( i), d ( i) are respectively symbols comprised in the codewords, and M symb 0, M symb 1, M symb 2, M symb 3 are respectively quantities of symbols transmitted on the transport layers; • determining, for each codeword, based on the SINRs of all symbols in the codeword, a SINR of the codeword; • determining, for each codeword, a channel quality indicator, CQI, based on the SINR of the corresponding codeword; • sending channel state information, CSI comprising the CQI for each codeword and the RI to the base station, receiving a second reference signal set sent by the base station; and • receiving based on the second reference signal set and a second codeword-to-layer mapping, data sent by the base station, wherein a quantity of layers to which at least one codeword is mapped in the first codeword-to-layer mapping is less than or equal to a quantity of layers to which a codeword is mapped in the second codeword-to-layer mapping, and the second codeword-to-layer mapping comprises a second mapping between at least one codeword and the at least one transport layer.