I am not sure about downlink but for uplink data due to poor radio channel condition ue may need to re-transmit packets for real time application packet (like VoIP) which is not acceptable.
To be decoded successfully a transmitted packet, UE sends same transport block four time consequently with different coding which is also known as TTI bundling. eNodB sends HARQ for a bundle.
RRC layer at eNodeB enable/disable TTI bundling for an UE. please check MAC-main config which is carried by RRC Connection Reconfiguration message.
I think only uplink scheduler has to care care of since TTI is associated with UL-SCH configuration but not sure.