Contents I. Introduction
The mobile traffic has rapidly grown [1]. To overcome emerging issues for wireless access, such as higher speed transmission, offload of mobile traffic, lower latency, and so on, the collaboration between optical and radio communications will be indispensable for future wireless access. For the collaboration, physically seamless connection and multi-level signal transmission become important technical demands. To realize these demands, a combination of radio-over-fiber (RoF) and optical coherent detection techniques is a suitable candidate. However, the unpredictable phase fluctuation of light sources, such as laser phase noise and frequency offset between lasers in the transmitter and the receiver, may seriously impact the system performance. To overcome this problem, we have proposed a laser-phase-fluctuation-insensitive optical coherent detection scheme assisted by a digital signal processing (DSP) technique for radio-over-fiber (RoF) systems [2]. In this system, a “two-tone” local light is used for an optical coherent detection of both the carrier and the modulated components of RoF signal, where a frequency separation of two-tone local light is the same as that of RoF signal. To distinguish the modulated signal and unmodulated carrier components in the process of optical coherent detection, they have to be separated in the optical domain. Recently, we have also proposed a new optical coherent detection scheme with an “offset-frequency-spaced two-tone” local light, which is also in principle insensitive to the laser phase fluctuation and is assisted by the DSP technique [3], [4]. In this scheme, it is not required to separate the modulated signal and unmodulated carrier components in the optical domain because they can be easily separated in the electrical domain after the photo-detection. Therefore, the system configuration is expected to be simpler than that in the previous scheme. In the demonstration of our schemes, the state-of-polarization (SOP) of the received RoF signal and the two-tone local light was manually matched to get the maximum output after the photo-detection. However, the SOP mismatch is the essential problem for the optical coherent detection. To overcome this problem, a polarization diversity is an important technique. Here, we are thinking that there are some polarization diversity techniques, which are applicable to our optical coherent detection scheme.
