Method and arrangement in a relay node, for selecting communication mode. The relay node is configured to communicate with a base station via a donor antenna over a first link and to communicate with a user equipment via a coverage antenna over a second link. The method comprises obtaining an isolation value, based on the radio wave isolation between the relay donor antenna and the relay coverage antenna. The obtained isolation value is compared with an isolation threshold level value. The relay node is configured to communicate in full duplex mode if the obtained isolation value exceeds the isolation threshold level value, otherwise in half duplex mode. Information concerning the configured duplex mode of the relay node is transmitted to the base station.Also a method and arrangement in a base station is described.