In an orthogonal frequency division multiple access (OFDMA) system including at least one base station and at least one wireless transmit/receive unit (WTRU) sub-carriers are allocated for data transmissions to multiple access WTRUs where sub-carriers are allocated according to a consecutive sub-carrier allocation (CSA) type and a distributed sub-carrier allocation (DSA) type. Pilot signals with distributed pilot sub-carriers are transmitted and measured at each WTRU to obtain a channel quality metric for each pilot sub-carrier. Each WTRU sends feedback to the base station reporting channel quality based on the measured channel quality metrics. An allocation type is selected and adaptively switched according to channel variations in time and frequency domain.