1. An apparatus comprising:circuitry, which, in operation,
generates, for each of a plurality of antenna ports, a reference signal using information relating to a cyclic shift index, the cyclic shift index being associated with a first antenna port of the plurality of antenna ports; and
maps, for each of the plurality of antenna ports, the respective reference signals to frequency resources based on the cyclic shift index, wherein,
when the cyclic shift index is less than or equal to a determined number, the respective reference signals of each of the plurality of antenna ports are mapped to a first frequency resource; and
when the cyclic shift index is greater than the determined number, a portion of the respective reference signals is mapped to the first frequency resource and a remaining portion of the respective reference signals is mapped to a second frequency resource that is different from the first frequency resource, based on a port index number of the respective antenna port; and
a transmitter, which, in operation, transmits the mapped reference signals.