A radio-over-fibre transmitter comprising: an optical splitter arranged to receive an optical carrier signal having a carrier optical frequency, and split it into a plurality of portions; electro-optic modulation apparatus each arranged to receive a respective optical carrier signal portion and a respective modulated radio frequency subcarrier signal, and arranged to modulate the respective optical carrier signal portion with the respective modulated radio frequency subcarrier signal and arranged to suppress onward transmission of the respective optical carrier signal portion, to form a respective carrier suppressed optical subcarrier signal; an optical combiner arranged to receive the carrier suppressed optical subcarrier signals and one of the optical carrier signal portions and arranged to combine them to form a subcarrier multiplexed optical signal; and polarisation apparatus arranged to ensure that the carrier suppressed optical subcarrier signals and said optical carrier signal portion each have the same polarisation state at the optical combiner.