Synthesis of a GS-16QAM signal for a simplified optical ISB system and direct detection with the K-means clustering algorithm

Abstract

An independent sideband (ISB) is a promising scheme for short-reach and metro applications because of its high spectral efficiency, low complexity, and tolerance to chromatic dispersion. Here, we develop a signal synthesis scheme to further reduce the complexity of ISB direct-detection (DD) systems. Two lower-order quadrature amplitude modulation (QAM) sideband signals are generated digitally, then the left sideband (LSB) and right sideband (RSB) are modulated with regular quadrature phase shift keying (QPSK) and geometrically shaped shifted QPSK (GS-S-QPSK), respectively. Then, the two independent sideband signals are received in a single photodiode to synthesize a GS-16QAM signal. The LSB and RSB signals can be separated and demodulated by a digital signal process (DSP) instead of using two optical bandpass filters. The proposed scheme significantly reduces the complexity of the ISB-DD receiver, thus saving system cost. Three different GS-S-QPSK signals are evaluated, with the square GS-S-QPSK achieving the best bit error rate (BER) with its optimal shaping factor. Considering signal–signal beating interference, a sub-blind K-means clustering algorithm is used to improve the BER performance, and the results indicate that it can achieve a large received optical power (ROP) gain at a threshold of 3.8 × 10−3.