SSBI-Free Direct-Detection System Employing Phase Modulation for Analog Optical Links

Analog optical transmission techniques such as radio-over-fiber (RoF) and multiple intermediate-frequency-over-fiber (multi-IFoF) have attracted a great deal of attention for use in 5G mobile fronthaul links. Recently, an IFoF transmission with a record CPRI-equivalent capacity of 1.53 Tb/s has been demonstrated by using the Kramers-Kronig (KK) receiver. However, one major issue associated with the KK receiver is its computational complexity. Also, since the minimum phase condition must be satisfied, the optimal carrier-to-signal power ratio (CSPR) should be large enough; however, this results in a low receiver sensitivity. To address these issues, we have recently proposed and demonstrated a signal-to-signal beat interference (SSBI)-free direct-detection (DD) system employing phase modulation (PM). Since a PM signal has a constant amplitude, its SSBI term becomes just a direct-current (DC) component. Therefore, the SSBI can be perfectly removed using a DC block without resorting to complicated digital-signal-processing (DSP)-based schemes such as the KK algorithm. In this article, we describe the principles of operation of the SSBI-free PM system in detail. We present a numerical study of performance comparisons between the SSBI-free and KK systems. The result reveals that the optimal CSPR is always 0 dB in the SSBI-free system, while it depends on optical-signal-to-noise-ratio (OSNR) levels in the KK system. The dispersion tolerance of the SSBI-free system is also investigated, showing that the SSBI-free system can support a 2-GHz 1024QAM-signal delivery in a typical deployment scenario of mobile fronthaul links. Finally, we present an experimental demonstration of a 2-GHz 1024QAM OFDM transmission with the SSBI-free system. To the best of our knowledge, this is the first demonstration of 1024QAM DD-based transmission for analog mobile fronthaul links.