“precode_y” represent the symbols of a user after MIMO precoding. These symbols are obtained by multiplying “layer_x” with the precoding matrix. “layer_x” again is obtained from “tx_user_symbols” via layer mapping, and “tx_user_symbols” are the QAM modulated user symbols.
“tx_symbols” contains the data of all scheduled users.
None of these variables are however associated to resource elements. The actual mapping onto resource elements happens in “y_tx_assembled”.