Sigma-Delta-Over-Fiber With WDM Serial Connection for Distributed MIMO

In this paper, we investigate the first downlink with serially connected remote radio heads (RRHs) using sigma-delta-over-fiber (SDoF) for distributed MIMO (D-MIMO). Coarse wavelength-division multiplexing (CWDM) is used and one wavelength is assigned to each RRH. The serial connection architecture is implemented with passive off-the-shelf components. Specifically, signals are combined into one fiber cable by using an optical multiplexer (MUX) and signals are dropped at each RRH by using optical add-drop multiplexers (OADMS). The architecture implementation is scalable in terms of energy consumption and cost. We measure the sensitivity of SDoF towards the optical interference that occurs when several signals are combined into one fiber, and compare to alike measurements using analog radio-over-fiber (ARoF). We also address the challenge of different time-delays to each RRH, due to the different fiber lengths; we present a method for time-delay compensation, which makes the architecture using RRHs connected in series perform similarly to using RRHs connected in parallel. Finally, our measurements show that the architecture with serially connected RRHs can be used to serve two user equipment (UEs) simultaneously within the same time- and frequency resources, using multi-user MIMO technologies.