In this paper. a symmetric training sequence-based (STSB) copyright frequency offset estimation (FOE) scheme for coherent free-space optical (CFSO) communication is proposed. In the proposed technique. we exploit the training-sequence-based (TSB) copyright FOE algorithm to the CFSO communication system in the quadrature phase-shift keying (QPSK) modulation format. https://thebrickes.shop/product-category/sectional-component/