1. A method performed by a base station serving a plurality of wireless devices, the method comprising: sending a request for grouping information indicating one or more clustering criterion and receiving the grouping information in response to the request; grouping one or more of the plurality of wireless devices together based at least in part on clustering information corresponding to each of the plurality of wireless devices and according to the one or more clustering criterion, wherein the grouping information comprises each one of the wireless devices in a group being distinct from each other according to the associated clustering information and the one or more clustering criterion, and wherein the one or more clustering criterion comprises a spatial relationship between each of the one or more wireless devices and the base station; assigning at least one pilot signal to the one or more wireless devices based on the grouping information, wherein at least two of the one of more wireless devices in said group are assigned the same pilot signal; and transmitting the at least one pilot signal.