Abstract: A physical random access channel (PRACH) waveform design to satisfy the requirements on the occupied channel bandwidth in eLAA wireless communications network is provided. In one alternative, PRACH comprises multiple segments in the time/frequency domain in one subframe. In another alternative, PRACH comprises multiple segments in the time/frequency domain across two or more subframes by using inter-subframe hopping. In one advantageous aspect, PDCCH triggers the dynamic allocation of the PRACH resource to facilitate the listen-before-talk (LBT) channel access procedure in unlicensed carriers. The aperiodic and dynamic allocation of PRACH resource improves efficiency and PRACH success rate.