Systems, methods, and instrumentalities (e.g. aspects of entities, interfaces and procedures in a wireless transmit/receive unit (WTRU) and/or network layers LI, L2, 13) are disclosed for low latency medium access control (MAC) protocol data unit (PDU) assembly in wireless systems, such as 5G flexible radio access technology (RAT) (5gFLEX). Latency may be reduced, for example, by WTRU determination of network transmission parameters and signaling prior to a transmission grant. A WTRU may receive a modulation and coding scheme (MCS), resource range, etc. prior to a grant, e.g., for use in future grants. Data blocks may be incrementally created/encoded prior to a grant. Data units may be segmented, assembled and multiplexed, for example, based on data block sizes that allow MAC and radio link control (RLC) processing prior to a grant. Flexible grant sizes may be provided for early generation of transport blocks before a grant. A minimum guaranteed transport block size (TBS) may be signaled to permit early generation of a MAC PDU. Transmission parameters may be selected prior to a grant, for example, using blind decoding or a DCI reception procedure.