FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION [See section 10, Rule 13] TRANSMISSION EQUIPMENT AND METHOD FOR SENDING CONTROL MESSAGES; PANASONIC CORPORATION, A CORPORATION ORGANIZED AND EXISTING UNDER THE LAWS OF JAPAN, WHOSE ADDRESS IS 1006, OAZA KADOMA, KADOMA-SHI, OSAKA 5718501, JAPAN THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED. DESCRIPTION Technical Field The present invention relates to a transmission apparatus and control information transmission method for transmitting control information for resource assignment in a multiple access wireless communication system. Background Art Currently, wireless mobile communication systems are evolving towards the fourth generation (i.e. 4G networks). The evolution to 4G promises substantial increase in system requirements (e.g. an increase in the number of users, user bandwidths, and mobility). Several new technologies are planning to be used to make possible this increase in system requirements. One of these technologies is orthogonal frequency division multiple access (OFDMA), which is a wireless technique proposed for WiMAX (IEEE 802.16e) and 3GPP LTE (Long Term Evolution). The orthogonal frequency division multiplexing (OFDM) is a technique of multiplexing and transmitting transmission data using a plurality of subcarriers. In an OFDM system, first, a data stream is divided into several parallel sub-streams. The data rate then is lowered. That is to say, with these parallel substreams, the symbol duration increases compared to the original data stream. The plurality of parallel substreams are then modulated and later mapped on separate subcarriers. The increased symbol duration improves the robustness of OFDM to channel delay spread. Furthermore, the introduction of cyclic prefixes (CPs) can completely eliminate inter-symbol interference as long as the CP duration is longer than the channel delay spread. In addition, OFDM modulation can be realized using the efficient inverse Fast Fourier transform (IFFT), which enables a large number of sub-carriers to be provided with lower complexity. In an OFDM communication system, resources are defined by means of OFDM symbols in the time domain or by means of subcarriers in the frequency domain. That is, OFDMA is a multiple-access scheme that makes possible multiplexing transmission by mapping data streams from a plurality of users upon time resources and frequency resources. For example, non-patent literature 1 discloses a conventional wireless communication apparatus that communicates using an OFDM communication scheme. FIG.l illustrates an example of an OFDMA frame structure. FIG.l is an illustration of a downlink