# Optical Interference Suppression Based on LCD-Filtering
## 5 Conclusions  
In this work, we introduce a novel method for optical receive filtering that allows for actively steering the receive pattern of an optical detector. Accordingly, it is possible to utilize the angular information of the incoming signals as filter criteria leading to increased receiver performance in comparison with conventional filtering methods. The measurement and simulation results presented in this work support the practical relevance of the proposed interference suppression method. Thus, interference suppression is not restricted to masking other modulated sources but includes, among others, the reduction of interference-induced noise and the mitigation of multipath distortions. One further application is the suppression of self-interference in full-duplex communication, e.g., when the transmit signal of an optical underwater modem is reflected back by particles in the water column. Additionally, the possibility for MIMO decorrelation using an LCD filter is presented.  
As a by-product the proposed filter can further be used to extend the dynamic range of a given optical receiver by attenuating the optical signal, e.g., enabling the use of a sensitive detector in a short-range communication scenario.  
Regarding filter performance, the proposed direction-based filtering method implements a filter criteria independent from wavelength-based methods. Thus, enhancing the overall performance by combining both methods is possible. A comparison of the dynamic LCD-based filter to static approaches like imaging receivers strongly depends on the chosen parameters. Generally speaking, the static methods can offer superior performance while being limited to specific scenarios. Moreover, positioning and orientation is critical. The novel LCD filter, on the other hand, can be dynamically adapted. Thus, conventional angle-diversity filters can be beneficially applied in well-defined scenarios like free-space point-to-point links while the adaptive LCD filter approach shows advantages in mobile applications, for example. Unlike the state-of-the-art angle-diversity receivers, the LCD-based filter can be used for interference suppression using a single PD while the competing decorrelation methods require at least two PD receivers.  
A challenge is that the LC properties limit the optical performance. With the ongoing demand for energy-efficient and high-contrast ratio displays, especially for the use in mobile devices, further improvements of the LCD technology can be expected. This is supposed to lead to a further increase of the key performance parameters for optical filtering using the proposed method. However, the novel adaptive filtering is not limited to the LCD-based approach and is directly applicable to  
Figure 8: Comparison of LCD filter performance to conventional MIMO results presented in [13].  
alternative display technologies like MEMS-based digital microshutter displays [15] and transmissive electrophoretic [16] displays as well.  
Conceptualization, G.J.M.F. and A.K.; Writing--original draft, G.J.M.F.; Writing--review and editing, A.K. and P.A.H.; Supervision, P.A.H. This research received no external funding.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} BER & bit error rate \\ FOV & field-of-view \\ IM/DD & intensity modulation with direct detection \\ LC & liquid-crystal \\ LCD & liquid-crystal display \\ LED & light emitting diodes \\ LOS & line-of-sight \\ ML & maximum likelihood \\ MIMO & multiple-input multiple-output \\ PAM & pulse amplitude modulation \\ PD & photodiode \\ SIC & successive interference cancellation \\ SIR & signal-to-interference ratio \\ SM & spatial modulation \\ SMP & spatial multiplexing \\ SMU & source-measurement unit \\ TIA & transimpedance amplifier \\ VLC & visible light communication \\ \end{tabular}