The present disclosure describes methods, system, and devices for multiple carrier transmission mechanism. One method includes receiving a configuration of a value for a set of cells, and receiving a configuration of a search space of a multi-cell scheduling downlink control information (MC-DCI), wherein: the set of cells is configured for multi-cell scheduling and at least one of the set cells are scheduled by the MC-DCI on a physical downlink control channel (PDCCH) candidate on a scheduling cell, the value is used to determine control channel element (CCE) indexes of the search space corresponding to the PDCCH candidate, and the search space is configured on at least one of the following: the scheduling cell carrying the MC-DCI or a scheduled cell in the set of cells.