Digital pre-distortion in QAM formats based on adaptive complex multipliers

We propose and numerically validate a method to redistribute QAM symbols on the constellation diagram aiming to minimize channel transmission impairment effects. The pre-distortion method is based on the design of digital complex multipliers and it is tested by simulation in a Radioover-Fiber (RoF) system. Assuming different channel responses, ten types of redistributed formats are generated by applying the proposed digital pre-distortion method of data-symbols in the constellation diagram on classical 8-QAM and 16-QAM formats. The robustness of the redistributed modulation formats is evaluated in comparison with classical m-QAM formats in terms of the bit-error-rate (BER), for a transmission distance of 40 and 80 km over uncompensated single-mode fiber links at 1 and 10 Gb/s respectively. It is shown that 8-QAM Offset-π /4 at 80 km with a bit rate of 10 Gb/s attains a gain in log(BER) of 2.3 decades, and improves the reach up to 94.5 km when compared to classical 8-QAM. On the other hand, 16-QAM Offset-π/4 at 40 km achieves a gain of 1.7 decades improving the reach up to 50 km for the same error level having the classical 16-QAM as reference. With these results the future work will enable a methodology to design robust constellations against linear impairments of optical channel.