Efficiency of Power Loading Strategies for Visible Light Communication

Optical Wireless Communication via LEDs faces the problem that LEDs are band- limited but show a gentle roll off above the 3 dB bandwidth. This requires an appropriate power loading of the different frequency components of the signal spectrum. OFDM is an attractive modulation method to appropriately or even optimally distribute the available power among its sub- carriers. It is now commonly accepted and experimentally verified that the LED transfer function declines exponentially with frequency. This motivated us to derive the theoretical capacity of this channel and evaluate to what extent waterfilling and uniform power loading strategies perform w.r.t. this capacity bound. For waterfilling, we managed to derive a novel closed- formula for the relation between the power budget and the achievable data rate and for the optimum bandwidth to be used. Yet, in practice these adaptive loading schemes need to achieve good performance with only discrete choices for QAM constellations. To approach the effect of waterfilling, Hughes-Hartogs proposed an iterative scheme. We generalized this to also find optimal signal bandwidth, power and constellation per sub- carrier for uniform power loading. We found that, for LEDs, waterfilling does not yield any significantly larger rate than the simpler uniform loading.