A method includes mapping, based on a first group-hopping associated with a first time resource, a first demodulation-reference signal (DM-RS) in a first symbol in the first time resource; mapping, based on a second group-hopping associated with the first time resource and with an offset, a second DM-RS in a second symbol in the first time resource; mapping, based on a third group-hopping associated with a second time resource, a third DM-RS in a third symbol in the second time resource; mapping, based on a fourth group-hopping associated with the second time resource and with an offset, a fourth DM-RS in a fourth symbol in the second time resource; and transmitting, to a second wireless user device, the first DM-RS, the second DM-RS, the third DM-RS, and the fourth DM-RS.