Multiple Frequency Offset Estimations in Multiuser OFDMA

In multiuser OFDMA uplink, different users usually have different carrier frequency offsets (CFOs). Estimation of these multiple CFOs is a crucial and challenging task. In this paper, a dedicated pilot OFDM symbol is designed to estimate the CFOs. Based on the maximum likelihood (ML) principle, a method is derived to estimate the CFOs of all users by optimizing a multi-dimensional (MD) cost function. To reduce complexity, the MD optimization problem is turned into independent one-dimensional (ID) optimization problems based on a specially designed pilot OFDM symbol. It is proved that the CFOs are identifiable by optimizing the ID cost functions. Although the ID optimization is substantially simpler than the MD optimization, it may still be vulnerable to local minima/maxima and sometimes too complex for wireless applications. A fast algorithm is then derived, which finds the CFOs in closed forms and needs very low computational complexity.