Network node and method therein for assignment of Physical Cell Identities (PCIs) to Radio Base Stations (RBSs) in a wireless communication network. The method includes determining a most densely located RBS, based on the distances from each RBS to a certain number of other RBSs of the plurality of RBSs. The method further includes, for each RBS, i, for which a PCI is to be assigned: assigning a Secondary Synchronization Signal (SSS) to the RBSi from a set of SSSs, based on a distance from the RBSi to another RBS in the communication network associated with the same SSS, and a number of tiers between the RBSi and, at least, the other RBS. Further, a Primary Synchronization Signal (PSS) is assigned to the RBSi, based on the number of cells associated with the RBSi; and a PCI is assigned to the RBSi, based on the SSS and the PSS.