Contents I. Introduction
The mobile traffic has exponentially increased [1] [1]. Not only to effectively aggregate and stably transmit a large amount of such traffic among separated places but also to make efficient use of radio resource, the collaboration between optical and radio communications will be indispensable for future radio access networks. For the collaboration. a combination of radio- over-fiber (RoF) and optical coherent detection techniques is a suitable candidate for realizing these demands. However, in general, 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 RoF systems [2], [3]. 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, polarization diversity is an important technique. Here, we are exploring some polarization diversity techniques, which are applicable to our unique optical coherent detection scheme. Recently, a combination technique of constellations recovered from two orthogonal polarization components has been newly proposed as one of polarization diversity techniques in offset-frequency-spaced two-tone optical coherent transmission of radio-over-fiber signal [4].
