Coherent Optical OFDM Transmitter Design Employing Predistortion

In this letter, we analyze the transmitter design for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) employing predistortion. Digital clipping and digital predistortion processes are introduced to the radio frequency OFDM transmitter to compensate the nonlinearity effect introduced by the optical in-phase/quadrature modulator. We first provide a detailed description of signal processing involved in digital clipping and predistortion. We then carry out a numerical simulation to identify the performance dependence on clipping ratio, digital-to-analog converter (DAC) resolution, and modulation index. We find that for quadrature phase-shift keying modulation, 4-bit DAC resolution is sufficient for the CO-OFDM transmitter to incur only 0.3-dB penalty while maintaining the excess modulation insertion loss less than 6.0 dB. The performance improvement of the predistorted transmitter over nonpredistorted one is also discussed.