# Optical Interference Suppression Based on LCD-Filtering
## 1 Introduction  
In the field of visible light communication (VLC), interference reduction is a key component towards the realization of reliable communication links [1] (Chapter 9). Unlike conventional radio frequency communication, so-called direct detection receivers using one or several photo diodes (PDs) exhibit a much broader spectral response than required for detecting the useful signal. As a consequence, additional filtering is required to reduce interference. Owing to the wide-angle beam pattern of LEDs and the strong signal power attenuation increasing with distance, interfering sources and especially the solar irradiation often outclass the useful signal. Of course, it is possible to reduce the interference after detection by means of filtering in the electrical domain. In the simplest case, the interfering signal is of unmodulated nature as it can be expected for sunlight or contains only low frequency components such as the line frequency with its harmonics when electric lighting is present. In those cases, simple low-pass filtering can be applied while for stochastic interference signals algorithmic approaches such as successive interference cancellation (SIC) have to be used. However, this increases the computational complexity of the receiver, reduces the system performance and is sometimes unfeasible. Besides these limitations of electrical domain filtering, noise enhancement is inevitable since shot-noise is induced at the moment when the interfering signals are detected by the PD. Furthermore, receiver saturation due to high interference power levels has to be taken into account. The most prominent method for interference reduction is wavelength-dependent filtering [2]. Different filter types like colored glass filters and dichroic filters are available, sharing the principle challenge of suppressing unwanted spectral components while preserving the communication signal as completely as possible. The major limitation of dichroic filters is the angular derivation of the filter response [3]. This is a major problem for communication systems where the angle of arrival of the optical signal is not known a priori. In the case of colored glass filters, the filter transitions are not as sharp, leading to impaired transmissivity values when narrow passbands are required to match the LED spectral characteristic. Naturally, spectral filtering does not allow for suppressing interference in the wavelength regime of the desired signal. This is a critical limitation in LED-based communication systems with relatively wide transmitter spectra, e.g., when compared to laser diodes. Different filter architectures taking the direction of the incoming signals into account have been proposed. One approach consists of using a digital camera set-up [4; 5], where the individual PD pixels can be mapped to the incident angle of the incoming light rays such that filtering by the direction of the light sources is possible. This makes it possible to reduce the interference by selecting the detector pixels receiving the useful signal while dropping all others. However, unlike conventional PD receivers, this method requires a huge amount of photo cells to achieve a reasonable signal separation. This unavoidably leads to a reduction of the size of the individual detectors with an associated performance penalty to the useful signal. Moreover, this kind of direction-dependent filtering requires comparatively large hardware resources since only a small fraction of the available resources is used for signal detection. Imaging receivers [6; 7; 8] mapping the incident light rays on multiple PDs using lenses do not show this limitation. Owing to the static nature of the filters, channel decorrelation relies on a suitable positioning and orientation of the transmitters and receivers. Similar arguments apply to non-imaging angle-diversity receivers like the mirror-based approach described in [9]. Another possibility is presented by the authors in [10]. Here, a small number of PDs are combined with a repetitive static aperture.