Contents 1. Introduction
As the mobile devices significantly involve into our daily life, the associated communication technologies have been desperately evolving in the past decade. The capacity demands for 5G enhanced mobile broadband (eMBB) can easily be fulfilled by upgrading the wireless channels to millimeter wave regimes [2]. Moreover, radio-over-fiber (RoF), which transmits radio-frequency signals through optical fiber, has gradually become a potential candidate in mobile fronthaul networks to meet the bandwidth and latency requirements of the cross-haul transmissions. [1] To further enhance transmission efficiency in optical domain, polarization-division-multiplexing (PDM) is a promising solution to double the data rate. However, due to the circular symmetry nature of the optical fiber, signals' polarization states are roaming all the time during transmission, especially when the external environments change. Thus, it's impractical to simply demultiplex the orthogonally polarized signals with a passive polarization beam splitter (PBS) at the receivers. Conventional PDM demultiplexing typically employs a coherent detection scheme or applies a polarization tracking mechanism to eliminate crosstalk for direct detection [3]–[4], which are too complicated, nor is cost efficient, for future 5G mass-deployed millimeter wave (mmW) base stations scenario [5]. Another polarization demultiplexing technique by combining both PDM and MIMO channel estimations was also proposed to mitigate the influences caused by polarization crosstalk [6]. However, this approach requires very complicated data assignments and can't successful demultiplex the signal at a specific polarization, i.e., linearly polarization at 45° with respect to the two orthogonal axes of PBS.
