Wavelength Resource Sharing in Bidirectional Optical Mobile Fronthaul

In this paper, we propose high-capacity spectrally-efficient optical mobile fronthaul (MFH) architectures featuring wavelength resource sharing, where multiple cell sites share one dense wavelength division multiplexing (DWDM) channel instead of occupying a number of separated DWDM channels. In the downlink (DL), subcarrier multiplexing (SCM) is integrated with DWDM passive optical network. The key technical innovation lies in the reception of uplink (UL) signals being optically aggregated with ultrasmall spacing. We introduce two novel schemes for spectrally-efficient and low-complexity upstream transmissions. The first method is based on SCM in remote radio heads (RRHs) and coherent detection in baseband units (BBUs), achieving channel spacing of as small as 100 MHz. For the first time, successful coherent reception of LTE signal, whose symbol rate (15 ksps) is much smaller than the laser linewidth, is achieved by pairing a radio frequency pilot tone from each RRH. Bidirectional transmission of four 20-MHz 16-QAM-OFDM LTE-like signals with 100-MHz spacing aggregated on one wavelength is demonstrated. For the second proposed UL scheme, we report a low-complexity ultradense WDM over DWDM allocation with 1-GHz wavelength spacing and novel phase-noise-insensitive heterodyne detection using MHz-linewidth lasers and an off-the-shelf envelope detector. We experimentally demonstrate the transmission and simple detection of multichannel 100/200 Mb/s OOK or 200 Mb/s 16-QAM-OFDM signals with 1-GHz spacing over 30-km standard single mode fiber using 5-MHz-linewidth lasers. By enabling ultrahigh spectral efficiency, high receiver sensitivity, and low-complexity, the proposed approach achieves a significant breakthrough for future high-capacity mobile fronthaul systems.