Relay node (110), main unit (113) for a relay node and method in a main unit (113) for a relay node (110), which main unit (113) is connectible to a first radio unit (111) and to a second radio unit (112), for synchronising wireless communication over the second radio unit (112) with wireless communication over the first radio unit (111). The method comprises transmitting a synchronisation signal at the second radio unit (112), receiving the signal at the first radio unit (111), to compute a first timing difference corresponding to the signal propagation time and to adjust the downlink transmission timing at the second radio unit (112) according to the first timing difference. Similar signalling, estimation of timing difference and adjustment is made for signals to be received from the user equipment (130) at the second radio unit (112).