Constellation design for next-generation hierarchically-modulated PON systems

In this paper, we study the performance of star quadrature amplitude modulation (QAM) signals with various constellations for hierarchically-modulated PON systems that overlay an over 20-Gbps PSK signal on a 10-Gbps on-off keying (OOK) signal; previous work examined only the performance of an 8-star QAM signal. A star QAM signal consists of a PSK signal with lower amplitude (inner-PSK signal) and a PSK signal with higher amplitude (outer-PSK signal). We propose to decrease the modulation level of the inner-PSK signal and increase that of the outer-PSK signal with the goal of improving the bit error rate performance in some conditions. Simulations indicate the minimum required received power for the various constellations examined: it is shown that effective design depends on the extinction ratio. For example, 10-star QAM improves the minimum required received power by 3 dB compared to 8-star QAM when the extinction ratio is 12 dB.