Abstract
Using light emitting diodes (LED) for the purpose of simultaneous communication and illumination is known as visible light communication (VLC). Interference by ambient light sources is among the most critical challenges. Owing to the wideband VLC spectrum, the efficiency of wavelength-dependent optical filtering is limited, especially in the presence of sunlight. Multi-user VLC causes additional interference, since LEDs are characterized by a wide viewing angle. Although algorithm-based interference suppression is a feasible method, receiver saturation and especially noise enhancement are two challenges that can only by addressed effectively by filtering in the optical domain prior to the photodetector. In this publication, we propose the use of a liquid-crystal display (LCD) as receiver-side filter unit. The main advantage of this technology is the possibility to focus the field-of-view of the receiver on a specific light source and thereby suppress interference. Interference by ambient light, modulated interference and multi-aperture interference are introduced and signal-to-interference ratio improvements are derived using experimental results for a given LCD characteristic. By deriving the bit error rate for MIMO communications, the potential of the proposed interference reduction method is demonstrated.
Highlights
In the field of visible light communication (VLC), interference reduction is a key component towards the realization of reliable communication links [1] (Chapter 9)
Simple low-pass filtering can be applied while for stochastic interference signals algorithmic approaches such as successive interference cancellation (SIC) have to be used. 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 photo diodes (PDs)
The higher gains for spatial multiplexing (SMP) show that the impairment of the modified constellation diagram for spatial modulation (SM) is outbalanced by the additional degree of freedom
Summary
In the field of visible light communication (VLC), interference reduction is a key component towards the realization of reliable communication links [1] (Chapter 9). 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. 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. 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.
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