Theoretical Characterization of Nonlinear Clipping Effects in IM/DD Optical OFDM Systems

This paper looks at the problem of theoretically characterizing the nonlinear biasing and clipping (BAC) effects on an optical Orthogonal Frequency-Division Multiplexing (OFDM) signal in intensity-modulated, direct-detected (IM/DD) optical systems. Due to the unipolarity of the IM/DD optical channel, a large DC bias and associated nonlinear clipping distortion (NLCD) is inevitable, resulting in a significant performance penalty. This NLCD can be well modelled as a linear deterministic attenuation plus an uncorrelated random additive clipping noise in the time domain. In the frequency domain, the NLCD results in an additive or impulsive noise on the received OFDM constellation. A total effective signal-to-noise ratio (SNR) formula is then presented which is a function of biasing power, modulation constellation and receiver SNR figure. This suggests that rather than eliminating all clippings, the system performance is indeed optimized with some deliberately introduced NLCD as a result of higher power efficiency. Analytical results are in agreement with simulations for various cases which help us to accurately and efficiently evaluate the performance of such systems.