# Testbed for Experimental Characterization of Indoor Visible Light Communication Channels
## 1 Introduction  
The dramatic demand for ubiquitous wireless data transfer in recent years has led to a renewed interest for alternatives to swamped radio communications, making visible light communication (VLC) an excellent, sustainable and energy-efficient candidate [1]. This technology is based on the use of visible (white) light-emitting diode (WLED) lamps to simultaneously illuminate but also provide data communication. In spite of the inherent advantages of this kind of wireless optical communication (WOC), mainly the possibility of working with high power transmission (visible light radiation at illumination levels is little harmful to the human eye) in addition to the fact that these communications are confined to a very small area (therefore enabling multiple high data rate communication cells in reduced space which are usually known as attocells [2]), there still exist many problems to solve [3].  
Impressive Gigabit-per-second data rates at visible wavelengths have been reported in controlled laboratory experiments with perfect alignment between receivers and emitters [4, 5], although these situations are not easily applicable to commercial solutions. Therefore, in many recent theoretical and experimental studies, the achievable data rates are less optimistic [6, 7, 8]. Another important issue is the necessity of a direct line-of-sight (LOS) between the emitter lamp and the receiver for a feasible communication, which is not always possible in a real environment where receiver orientation and shadowing effects are decisive in communication quality [9, 10].  
Downlink multi-user communications in indoor environments is one of the main current topics of VLC [11; 12]. There exist many works oriented to characterize this indoor visible optical channel [13; 14; 15]. However, many studies have focused on demonstrating modulation methods or configuration networks for VLC, usually applying simpler channel models to evaluate their proposals (e.g., just considering the LOS component of the arriving signal), leading to overestimating the achievable data rates. Other works that include experimental demonstrations of their previous theoretical analysis usually develop testbeds, which are little representative of real indoor environments. This provokes a great gap between theoretical estimations [16] and real environment tests [17].  
This work aims to develop an experimental testbed to provide deeper insight on the effects of non-LOS components of the radiated light and imperfect alignment between emitter lamps and receiver in indoor multi-user visible light communications from an experimental (not theoretical) point of view. The rest of the paper is organized as follows: Section 2 describes the main characteristics of indoor VLC channels. In Section 3, the testbed designed for experimental evaluation of VLC systems is presented, describing its physical characteristics and the implemented communication scheme. Section 4 shows the simulation results performed so as to assess the suitability of the mockup to evaluate indoor VLC scenarios. Section 5 presents the experimental results obtained with the manufactured mockup and, finally, the main conclusions of this work are summarized in Section 6.