I. Introduction
Distributed antenna systems (DASs) combined with radio-over-fiber (RoF) technology have been commonly recognized as a feasible solution for in-buildings to both increase the system capacity and extend the wireless coverage [1]–[5] . In a RoF-DAS, standardized radio signals, like wireless local-area network (WLAN) signals and cellular signals, could be directly modulated onto optical carriers and distributed over fibers to multiple simplified remote antenna units (RAUs). However, with the adoption of multiple-input multiple-output (MIMO) technology in wireless communication systems, DAS will thus require the ability to send multiple MIMO streams to each RAU, especially for future larger count of spatial streams [4], [5]. Although this could be realized simply by installing extra fibers, it will cause significant increase in the count of fiber connections, accordingly in the deployment cost and space occupation. Moreover, when it comes to the future dense deployment of in-building small cells, these problems will become more serious and complicated. To solve these issues, multiple methods have been proposed to multiplex the multiple spatial streams into a single fiber, such as subcarrier multiplexing (SCM) [6], polarization division multiplexing (PDM) [7], wavelength division multiplexing (WDM) [8], and the recent focus of space division multiplexing (SDM) [5], [9].