In orthogonal frequency-division multiple access (OFDMA)

closely spaced multiple subcarriers are assigned to different users for parallel signal transmission.An interleaved subcarrier-assignment scheme is preferred because it provides maximum frequency diversity and increases the capacity in frequency-selective fading channels.Carrier-frequency offsets (CFOs) between the transmitter and the receiver destroy the orthogonality and introduce ICI and multiuser interferences.A subspace-based two stages searching algorithm is proposed in this paper for estimating the CFOs of all users simultaneously in the uplink of Interleaved OFDMA systems using only one OFDMA block.Compared with existing CFO-estimation algorithms

it does not need the a priori info

such as the number of users and their occupied subchannels

so it is suitable for the case that subchannels randomly assigned.Simulation results illustrate the high accuracy and efficiency of the proposed algorithm.