Secondary cell activation

A first apparatus comprising:

• at least one processor;
• and
• at least one memory including computer program codes;
• the at least one memory and the computer program codes are configured to, with the at least one processor, cause the first apparatus to:
• transmit, on a serving cell in an active status, a Media Access Control (MAC) layer command for activating at least one secondary cell to a second apparatus, the MAC layer command indicating a configuration on a procedure for activating the at least one secondary cell;
• receive an acknowledgement for the MAC layer command from the second apparatus;
• cause the second apparatus to synchronize, based on at least the configuration and the acknowledgement, with the first apparatus on the at least one secondary cell;
• and
• wherein the configuration indicates that an aperiodic tracking reference signal is to be used by the second apparatus for synchronizing with the first apparatus on the at least one secondary cell, and wherein the at least one memory and the computer program codes are configured to, with the at least one processor, cause the first apparatus to:
• in response to a first time offset between reception of the acknowledgement and transmission of the aperiodic tracking reference signal being indicated to the second apparatus,
• determine, based on the first time offset and the reception of the acknowledgement, a first slot for transmitting the aperiodic tracking reference signal;
• transmit, in the first slot, the aperiodic tracking reference signal to the second apparatus on the at least one secondary cell;
• and
• wait for a time period after transmitting the aperiodic tracking reference signal, such that the second apparatus is synchronized, based on the aperiodic tracking reference signal, with the first apparatus on the at least one secondary cell.