1. A method for carrier frequency offset (CFO) estimation for coherent optical orthogonal-frequency-division-multiplexing (CO-OFDM) broadband systems, comprising: obtaining an initial estimate of a normalized CFO with an estimation range equal to +L/2 subcarrier subspacing using a single training symbol with L identical portions; storing the initial estimate in a memory device; and obtaining a maximum likelihood (ML) estimate of the CFO by performing a local grid search based on the initial estimate.