Simultaneous optical transmission of pulse amplitude modulation and discrete multitone signals in passive optical link using derivative and subtractive equalization

Abstract

A derivative and subtractive equalization (DSE), which simultaneously transmits both four-level pulse amplitude modulation (PAM-4) and discrete multitone (DMT) signals encoded by quadrature amplitude phase shift keying (QPSK) with the same bandwidth, is proposed. The proposed DSE recovers the DMT signal by tracking the change of derivative value between adjacent sampling points and then subtracting the recovered PAM-4 signal after derivative value tracking from the received signals. A 20-km optical link is implemented to experimentally verify the proposed DSE technique. The bit error rate (BER) of the PAM-4 signal (1 Gbaud) and the error vector magnitude (EVM) of QPSK signal (1 Gbaud) are measured in order to examine the difference in transmission performance before and after using the proposed DSE technique. The 20% EVM of QPSK signal and the 5.4 × 10 − 5 BER of PAM-4 signal is observed at the 25% power ratio of DMT signal to PAM-4 signal. It is found that the signal-to-noise ratio of input PAM-4 and DMT signal should be 25 and 40 dB, respectively, with the 7% forward error correction to successfully transmit them at the same time. The 4-dB power penalty is observed before and after using DSE technique. These experimental results tell us that the transmission capacity of 4 times the bandwidth can be achieved using the proposed DSE technique.