Various communication systems may benefit from improved uplink data transmission. A method may include determining an assignment, at a user equipment, of a first set of physical resource blocks in an unlicensed spectrum to a physical random access channel. The physical random access channel occupies at least one of the physical resource blocks, and wherein the at least one of the physical resource blocks occupied by the physical random channel are distributed into clusters in a frequency domain. The method may also include determining an assignment, when the user equipment transmits data, of a second set of at least one remaining physical resource block of the physical resource blocks to at least one uplink channel. In addition, the method can include transmitting random access preambles through the physical radio access channel or data through of the at least one the uplink channel from the user equipment to a network entity.