AbstractIt’s known that different physical impairments can occur in coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. Not only linear impairments but also nonlinear impairments can cause significant signal degradations. It is important to analyze the physical layer impairment (PLI) model which should consider these impairments. In this paper, a new PLI model is developed, where amplified spontaneous emission (ASE) categorized to linear impairments and four-wave mixing (FWM) categorized to nonlinear impairments are regarded as the main noise sources. The performance of developed PLI model in terms of Q factor versus number of subcarriers and Q factor versus fiber length are extensively simulated. Numerical results show that when the OFDM signal passes through fiber link, the noise power expansion occurs. This expansion which is related to the fiber length, number of subcarriers as well as input power, can cause higher degradation in OFDM signal. Through precise analysis, optimum parameters of the transmission system can be chosen. Simulations prove that Q factor of CO-OFDM, whose number of subcarriers is 128, fiber length is 90 km and input power is −2 dBm, is up to abound 10 dB. In addition, in order to verify the developed PLI model, comparisons of the situation with and without considering FWM effects have also been made respectively. Results show that the performance of the model conforms to the actual conditions.