Adaptive digital predistortion of laser diode nonlinearity for wireless applications

Nonlinearity of a directly modulated laser diode imposes limitations in the performance of fiber based wireless access schemes. Previously, laser diode nonlinearity has been investigated in CATV related applications. In the CATV scenario, analog optoelectronic compensation schemes have been widely attempted due to the analog nature modulating signals. However, fiber based wireless schemes provide a unique opportunity for digital compensation. In this scenario, although the modulating signal is analog radio signal at several GHz, the information is conveyed through baseband digital symbols that have few Mbps bit rate. The nonlinear distortion of the laser, therefore, reflects on the amplitude and phase distortion of these vector modulated baseband information sequence and, correcting these distortions at baseband symbol rate is adequate. This fact is used in this paper to attempt an adaptive digital predistortion for wireless applications.