A base station classifies at least some of radio communication apparatuses belonging to the same cell into a plurality of groups. The base station assigns to a first group a first signal sequence that is orthogonal to a cyclically shifted version thereof, and assigns to a second group a second signal sequence that is orthogonal to a cyclically shifted version thereof and that is different from the first signal sequence and from the cyclically shifted version of the first signal sequence. The radio communication apparatus belonging to the first group performs spread modulation on data using the first signal sequence or a signal sequence obtained by cyclically shifting the first signal sequence. The radio communication apparatus belonging to the second group performs spread modulation on data using the second signal sequence or a signal sequence obtained by cyclically shifting the second signal sequence.