In this letter, we propose the transmit power controlled adaptive downlink frequency symbol spreading OFDM (TPC-AMS/FSS-OFDM) system. In the TPC-AMS/FSS-OFDM, each S/P transformed signal is spread by orthogonal spreading codes and combined in the transmitter, so the detected signals obtain the same SINR for each frequency symbol spreading block in the receiver. In this case, we can assign the same modulation level and transmit power for each frequency symbol spreading block for next transmission. Thus, the proposed system not only increases throughput performance but also reduces the total transmit power, FBI and MLI.

In AMS/OFDM systems, a base station controls the modulation level of each subcarrier with feedback information (FBI), and then, adaptive modulated packets are transmitted from the base station to the mobile station. In this case, the mobile station requires modulation level information (MLI) to demodulate the received packet. The MLI is generally transmitted as a data symbol, so the throughput is degraded. To overcome this problem and increase the total throughput, in this paper, we propose superimposed frequency symbol based adaptive OFDM with frequency spreading and equalization. In the proposed system, each S/P transformed signal is spread by orthogonal spreading codes and combined. This means that each subcarrier holds several superimposed S/P transformed signals with the same power rate. In this case, the frequency-selective faded subcarriers obtain the same power rate for each S/P transformed signal. Therefore, the detected signals also obtain the same SINR, and as a result, we can assign the same modulation level for each frequency symbol spreading block. Hence, the proposed system requires only one piece of FBI and MLI for each frequency symbol spreading block, as compared with conventional adaptive OFDM.