Patent ID: 8094553

Claim:
A base station using an OFDMA (Orthogonal Frequency Division Multiple Access) scheme for uplink communication from a plurality of radio terminals, comprising: a mapping information generator configured to generate mapping information indicating subcarriers assigned to the radio terminals for communications depending on the OFDMA scheme; a first transmitter configured to transmit the mapping information to the radio terminals; a receiver configured to receive OFDM signals from the radio terminals; a communication state detector configured to detect communication states of the radio terminals from the mapping information and the OFDM signals received from the radio terminals; a null information generator configured to determine whether or not null signals should be assigned to a part of the subcarriers assigned to the radio terminals from the communication states of the radio terminals and the mapping information, specify subcarriers to which null signals are to be assigned when it is determined that null signals should be assigned and generate null information for instructing to assign null signals to the specified subcarriers; a second transmitter configured to transmit the null information to the radio terminals determined to assign null signals; and an FFT section which performs a fast Fourier transform on a set of the OFDM signals received by the receiver from the radio terminals to get subcarrier signals; wherein the communication state detector detects subcarrier signals of each radio terminal based on the mapping information and detects communication states of each radio terminal from detected subcarrier signals of each radio terminal, the communication state detector detects an offset value indicating how far and in which direction the frequencies of subcarriers assigned to the radio terminal are deviated from proper frequencies as a communication state of the radio terminal, and when a difference between the offset value of the radio terminal and an offset value of a radio terminal adjacent to the radio terminal on the frequency domain exceeds an offset threshold, the null information generator determines to assign null signals to a part of subcarriers assigned to at least one of the radio terminal and the adjacent radio terminal.