Disclosed is a method performed by an IRU (210) of a base station system (200), the base station system comprising the IRU (210), a BBU (230) connected to the IRU (210), and a first RH (221) connected to the IRU (210) via a packet data network (240). The first RH (221) is arranged for wireless transmission in RF of a plurality of antenna carriers to UEs (250). The method comprises receiving, from the BBU (230), a plurality of first digital representations of the plurality of antenna carriers of the first RH (221), each first digital representation representing one antenna carrier, the plurality of first digital representations being received in a baseband frequency range, frequency multiplexing the plurality of first digital representations of the plurality of antenna carriers into a second digital representation over a first bandwidth, and transmitting the second digital representation to the first RH (221).