Embodiments herein disclose a method in a radio base station (12) for allocating a radio resource to be used for communication in a radio communications network, which radio base station (12) serves a first cell (13) of a first radio access technology and a second cell (15) of a second radio access technology. The first cell (13) and second cell (15) are comprised in the radio communications network, and which radio base station (12) controls a radio resource that supports communication over a frequency band The method comprises determining (1010) a load in at least one of the first cell (13) and second cell (15), and allocating (1020) the radio resource supporting communication over the frequency band to the first radio access technology of the first cell (13) or the second radio access technology of the second cell (15) based on the determined load.