Numerical Study of a Hybrid Optical DMT/DFT-S QAM Modulation

A hybrid modulation offers the peak-to-average power ratio (PAPR) robustness of discrete Fourier transform spread (DFT-S) quadrature amplitude modulation (QAM) with the bit rate optimization of discrete multi-tone (DMT) modulation. We examine via simulation under what circumstances this hybrid can increase achievable bit rate. Hybrid PAPR reduction allows us to increase the peak-to-peak voltage at the modulator electrical input to increase the signal mean power at the modulator output. We propose a methodology to identify the optimal driving strategy. We optimize the bit rate for the available spectrum, i.e., the spectral efficiency, taking into account the bandwidth limited nature of the transmitter. The final optimization we propose is the partition of the available spectrum into a lower frequency band for DFT-S QAM and a higher frequency band for DMT. The modulation level of the DFT-S QAM is also optimized. We compare the optimal hybrid performance versus DMT performance for a range of bit rates for a given modulation bandwidth. Improved performance comes at the cost of greater DSP complexity for the hybrid solution. We compare the number of complex multipliers required to implement hybrid versus DMT for both dispersive and non-dispersive systems.