A method includes allocating length-2n spreading codes to reference signals contained on first and second groups of resource elements in a first physical resource block and to copies of the reference signals contained on a selected group from the first or second groups and a third group of resource elements in a second physical resource block. Each length-2n spreading code is determined using a first length-n spreading code allocated to the first group and a second length-n code allocated to the second group, or using whichever one of the first or second length-n spreading codes is allocated to the selected group and a third length-n spreading code allocated to the third group. Symbols for the reference signals and their copies are spread and transmitted on the first and second physical resource blocks.