Dispersion-Tolerant DDO-OFDM System and Simplified Adaptive Modulation Scheme Using CAZAC Precoding

We apply the signal- and channel-independent constant amplitude zero autocorrelation sequence (CAZAC) precoding technique to a bandwidth-limited longer-reach DDO-OFDM system, and propose a simplified adaptive modulation scheme according to the equalized SNR per subcarrier after CAZAC precoding. Simulation results show that with the optimized modulation index, conventional 8.23-Gb/s double sideband (DSB) modulated QPSK-OFDM signal cannot realize 100 km standard single-mode fiber (SSMF) transmission at the given received optical power, while CAZAC precoding can enable 100 km SSMF transmission with dispersion-induced power penalty as low as 2.7 dB. The advantage becomes more obvious when CAZAC precoding is employed to 16.46-Gb/s 16QAM-OFDM systems. Thanks to the frequency diversity, OFDM system with CAZAC is proved to be more dispersion tolerant with improved receiver sensitivity. We also analyze the signal-to-noise ratio (SNR) of each individual subcarrier and its flatness is confirmed to be much improved independent of modulation formats and transmission distances when precoding is used. Based on the priori knowledge of flat SNR curve, we further propose a simplified adaptive modulation scheme using only three steps where bit loading can be applied with power loading omitted. Simulation and experimental results demonstrate that the BER performance can be further improved with negligible data rate reduction and system complexity aggravation using our proposed simplified adaptive modulation method.