A method for wireless communications implemented by a user equipment, UE, comprising: receiving a demodulation reference signal, DMRS, sequence grouping from a base station, the DMRS sequence grouping identifying one or more groups of DMRS sequences, each group of DMRS sequences corresponding to a set of DMRS sequences from a plurality of DMRS sequences; selecting a DMRS sequence for transmission of DMRS based on the DMRS sequence grouping; and transmitting uplink, UL, data along with the DMRS for UL transmission using contention-based protocol, wherein the DMRS is transmitted based on the selected DMRS sequence; wherein selecting the DMRS sequence for transmission of DMRS based on the DMRS sequence grouping comprises concatenating a plurality of base sequences to construct a composite DMRS sequence, wherein the plurality of base sequences includes at least two sequences selected from the one or more groups of DMRS sequences, the method further comprising: transmitting the composite DMRS sequence in one or more of slots of a plurality subframes, wherein a first DMRS sequence is transmitted in a first slot and a second DMRS sequence is transmitted in a second slot, and wherein the first DMRS sequence has a first base sequence index and the second DMRS sequence has a second base sequence index; performing a cyclic shift hopping on the first DMRS sequence based on the second base sequence index; and performing a cyclic shift hopping on the second DMRS sequence based on the first base sequence index. receiving a demodulation reference signal, DMRS, sequence grouping from a base station, the DMRS sequence grouping identifying one or more groups of DMRS sequences, each group of DMRS sequences corresponding to a set of DMRS sequences from a plurality of DMRS sequences; selecting a DMRS sequence for transmission of DMRS based on the DMRS sequence grouping; and transmitting uplink, UL, data along with the DMRS for UL transmission using contention-based protocol, wherein the DMRS is transmitted based on the selected DMRS sequence; wherein selecting the DMRS sequence for transmission of DMRS based on the DMRS sequence grouping comprises concatenating a plurality of base sequences to construct a composite DMRS sequence, wherein the plurality of base sequences includes at least two sequences selected from the one or more groups of DMRS sequences, the method further comprising: transmitting the composite DMRS sequence in one or more of slots of a plurality subframes, wherein a first DMRS sequence is transmitted in a first slot and a second DMRS sequence is transmitted in a second slot, and wherein the first DMRS sequence has a first base sequence index and the second DMRS sequence has a second base sequence index; performing a cyclic shift hopping on the first DMRS sequence based on the second base sequence index; and performing a cyclic shift hopping on the second DMRS sequence based on the first base sequence index. transmitting the composite DMRS sequence in one or more of slots of a plurality subframes, wherein a first DMRS sequence is transmitted in a first slot and a second DMRS sequence is transmitted in a second slot, and wherein the first DMRS sequence has a first base sequence index and the second DMRS sequence has a second base sequence index; performing a cyclic shift hopping on the first DMRS sequence based on the second base sequence index; and performing a cyclic shift hopping on the second DMRS sequence based on the first base sequence index.