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We derive closed-form expressions for nonlinear transmission performance of dual-polarization closely spaced coherent optical orthogonal frequency-division multiplexing (CO-OFDM) systems. We find that the fiber nonlinear noise exhibits a signature of the flicker noise or 1/f noisebeyond the corner frequency that is inversely proportional to the total participating bandwidth. We derive a noise enhancement factor that captures the interference effect of nonlinear noises among different spans. For a 10 × 100-km standard single-mode-fiber (SSMF) link with no dispersion compensation at an optimal launch power density of - 15.9 dBm/GHz, the spectral efficiency of 9.90 b/s/Hz can be achieved for dual-polarization transmission, which is about 93% increase over single-polarization transmission. The closed-form expressions are also applicable to the closely spaced coherent single-carrier systems where the symbol rate is much larger than the dispersion walk-off bandwidth, and the optical dispersion is uncompensated.