Channel characterization for indoor visible-light communication (VLC) systems is revisited. The purpose of this work is to evaluate the channel frequency selectivity, or in other words, the significance of inter-symbol interference (ISI) at the receiver and the necessity of channel equalization to recover the transmitted data. We focus on the effect of the indoor channel by assuming no bandwidth constraint on the LEDs and by considering a simple intensity modulation technique (i.e., on-off keying), excluding discrete multi-tone modulation. The channel impulse response (CIR) is firstly simulated using the iterative sitebased method. Then, conventional metrics for evaluating channel frequency selectivity, i.e., the root-mean-square delay spread and channel frequency response, are investigated and their practical interest is discussed. We also consider the signal-to-ISI ratio (SIR), which we define based on the sampled (i.e., discretetime) signal at the receiver, and demonstrate its usefulness in determining the necessity of channel equalization at the receiver. We consider several link scenarios in a medium-size and a large room, including the special cases of a blocked line-of-sight (LOS) link and a tilted receiver. We show the significance of the LOS components of CIR in determining the channel frequency selectivity and also discuss the choice of the receiver filter and explain how it affects the SIR.
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