Communication apparatus and method, computer program, and communication system

A communication apparatus, comprising:

• circuitry configured to:
• generate a plurality of media access control (MAC) information fields that include trigger information,
• wherein the trigger information indicates that uplink frame transmission is permitted, and
• wherein the plurality of MAC information fields are addressed to respective communication apparatuses of a plurality of communication apparatuses;
• transmit the plurality of MAC information fields at a downlink frame time to the plurality of communication apparatuses;
• and
• simultaneously receive, at an uplink frame time SIFS after the downlink frame time, a plurality of response frames compliant with the trigger information sent from each communication apparatus of the plurality of communication apparatuses, respectively,
• wherein the plurality of response frames have same frame length, and
• specify, by the trigger information, the same frame length for the plurality of response frames.