Impact Analyses of High-Order Light Reflections on Indoor Optical Wireless Channel Model and Calibration

This paper analyzes the impact of high order light reflections on indoor optical wireless communication (IOWC) channel models. Based on observing the results of computer simulations, a calibration method is proposed to reduce model errors. Channel models are generated by tracing and adding up diffuse light reflections and sequential sub-reflections along its traveling path. As computation complexity increases significantly with the number of reflection orders considered, researchers traditionally, though incorrectly, take the contribution of first a few reflection orders, most commonly three, to represent the complete channel. Discarded high-order reflections bring no significant performance difference to low-speed transmission systems; however, major contemporary IOWC research institutions focus on high-speed Gigabits per second (Gbps) communications and the model errors resulting from discarded high-order reflections are no longer negligible. This is where the importance of our proposed method lies. root-mean-square (RMS) delay-spread, for instance, is severely underestimated by neglecting higher-order reflections. We simulate an IOWC system in an ordinary 6 m × 6 m × 3 m room and calculate the contributions of each order of reflections at 841 locations. It shows the RMS delay-spread estimation using the first three orders underestimates the true value by 15.3% on the average and by at most 26.6% as maximum. To limit error within half a symbol period, 1 Gbps and 10 Gbps systems tolerate underestimations up to 13.7% and 1.4%, respectively. These must be achieved by applying first five and nine orders. To maintain the computation efficiency of low-order reflection models and improve their accuracies, we propose a statistical calibration method. It reduces average model error of first three reflection orders from 15.7% to 4.3%. The numbers of orders required by 1 and 10 Gps systems are individually reduced to 3 and 7.