In an exemplary embodiment there are determined which radio resource are to be used for contention based access, and the allocated radio resource is accessed according to a probability function that varies in dependence on at least one of interference measured on the allocated radio resource and a received power level for the allocated radio resource. By example the radio resource is allocated by a cellular base station for use in D2D communications; the base station provides configuration parameters for the D2D communications via common or dedicated signaling; the received power level is a target received power level that is received via broadcast system information or dedicated control signaling; and/or the received power level is itself a function of pathloss on a wireless link to the cellular base station. Embodiments are presented for method, apparatus, and tangibly stored computer program.