## 3 Architecture and Experiment of the Bi-Directional Free-Space VLC  
Figure 2 shows the proposed system architecture of the bi-directional free-space VLC system, in which the LDOF acts as an optical antenna supporting multiple moveable clients. In order to increase the VLC system's flexibility, the LDOF could be installed at a remote location, and the DL data are sent from the head-end office or CO via free-space VLC. The LDOF at the client side acts as an optical antenna to re-transmit the DL signal to different moveable clients. In principle, the system can support a large number of Rxs simultaneously as long as there is enough space along the LDOF circumference. The UL signal at another wavelength is sent via the LDOF and free space towards the CO.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Optical or Mechanical Properties** & **Feature** \\ \hline Diffusion Length & 1 m \\ Numerical Aperture (NA) & \textgreater{}0.46 \\ FOV Around Fiber Circumference & 360\({}^{\circ}\) \\ FOV Along Fiber & 120\({}^{\circ}\) \\ Operating Wavelength & 420 to 700 nm \\ Core Diameter & 170 \(\pm\) 3 μm \\ Clading Diameter & 230 + 0/\(-\)10 μm \\ Proof Test: Tensile Strength & 100 kpsi \\ Operating Temperature & \(-\)20 to + 105 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table}
Table 2: Characteristics of the LDOF.  
Figure 2: System architecture of the bi-directional free-space VLC system, in which the LDOF acts as an optical antenna supporting multiple moveable clients. Col.—collimators; DM—dhichroic mirror; CO—central office; UL—uplink; DL—downlink.  
Figure 3 shows the experimental setup of the free-space VLC system with bi-direction transmissions supporting multiple moveable Rxs. The LDOF is manufactured by Corning(r). As discussed in Section 2, nanostructures are added to the inner core to produce light diffusion. The DL and UL Txs are a 633 nm red LD (Thorlab(r), HL63163 DG) and a 520 nm green LD (Thorlab(r), PL520), respectively. The use of 633 nm and 520 nm wavelengths is based on the transmission characteristics of the dichroic mirror (DM, [PERSON](r) #86-386). The DM can well separate these two wavelengths at the CO to minimize the crosstalk. Two pulse-pattern generators (PPGs) are used to drive the DL and UP LDs at the same time to produce optical on-off-keying (OOK) signals via bias tees with proper direct-current (DC) biases. At the CO, a DM is employed to separate the red DL and green UL signals. Collimators (Col.) are used to couple optical signals into and out of the LDOF. At the CO, the green UL signal is received by an avalanche photodetector (APD, Thorlab(r), APD210). On the client side, an APD (Thorlab(r), APD110A) with a red optical filter (OF) is employed to receive the red DL signal from the LDOF. The client APD can slide along the whole LDOF to receive the DL signal, and the performance will be discussed in the next section. As discussed above, in principle, the system can support a large number of client APDs simultaneously as long as there is enough space along the LDOF circumference. Finally, the received DL and UL OOK eye diagrams are captured by a digital sampling oscilloscope (DSO) (Agilent(r), 86100A), and their BER is measured by a BER tester (Anritsu(r), MP1800A).