In an aspect, the disclosure is directed to a method and device for generating, by a higher L2 sublayer before receiving an UL grant, PDUs of the higher L2 sublayer, wherein each PDU of the higher L2 sublayer includes one or more SDUs of the higher L2 sublayer; generating, by a lowest L2 sublayer before receiving the UL grant, subheaders for each of SDUs of the lowest L2 sublayer, wherein each SDU of the lowest L2 sublayer is equivalent to each PDU of the higher L2 sublayer; performing, by the lowest L2 sublayer, a LCP procedure for the SDUs in response to receiving the UL grant; and generating, by the lowest L2 sublayer based on a result of the LCP procedure, a PDU of the lowest L2 sublayer by multiplexing a portion of the SDUs of the lowest L2 sublayer and the subheaders of the portion of the SDUs.