Feasibility Demonstration of a Mode-Division Multiplexed MIMO-Enabled Radio-Over-Fiber Distributed Antenna System

Abstract

In this paper the feasibility of mode-division multiplexing (MDM) for implementing multiple-input multiple-output (MIMO)radio-over-fiber (RoF)distributed antenna systems (DAS) is experimentally demonstrated, where the MIMO algorithm is able to reconstruct the data signals by overcoming distortion and cross-talk in both the free-space RF and optical fiber channels. This is achieved through RF characterization of an MDM RoF link, which is experimentally demonstrated to be capable of supporting at least $4 \times 4$ MIMO with 6 GHz channels over long lengths of MMF (2 km compared to $\sim$ 300 m typically found in MMF-based RoF DASs) and under tight fiber bending conditions (bend radius as low as 7.6 mm), which are commonly encountered in in-building fiber installations. First, experimental RF measurements are performed over a 2 km section of OM2 fiber, mode-multiplexed using a spatial light modulator (SLM) based MUX/DEMUX system, and it is shown to offer an RF condition number of <14 dB for up to $4 \times 4$ MIMO with 6 GHz channels, sufficient to enable good performance for most modern wireless protocols. Next, the effect of fiber curvature on RF performance is experimentally analysed using a 10 m section of OM3 fiber. It is seen that with a bend radius as low as 7.2 mm, a condition number of ${\sim}10$ dB can be achieved for up to a $6 \times 6$ MIMO system with 6 GHz channels. Although an SLM-based MUX/DEMUX is used here, condition number is not dependent on the exact mode-launching system used provided that orthogonal combinations of modes are excited, which is true of most MDM systems. It is therefore concluded that the RF characterization presented here demonstrates by proof-of-principle the feasibility of graded-index) MMF to support at least $4 \times 4$ MIMO with broadband channels via MDM over lengths up to 2 km and with fiber bend radii as low as 7.2 mm.