The system telephone traffic can be arranged into different classes (e.g. a control data, a user data and a pilot frequency data). For each class, one or a plurality of OFDM code elements with appropriate sizes are chosen on the basis of the anticipated efficient loading amount used in the telephone traffic of the class. For example, the control data can be launched through using the first sized OFDM code element, the user data can be launched through using the first sized OFDM code element and the second sized OFDM code element, and the pilot frequency data can be launched through using the third sized OFDM code element and the first sized OFDM code element. In a sample design, the small OFDM code elements are used for sending the pilot frequency and the transmission channel of the controlling data, and the large OFDM code elements as well as the small OFDM code elements are used for sending the transmission channel of the user data.