The spot-diffusing geometry is one of the attractive configurations considered in the literature. It provides a better signal-to-noise ratio (SNR) than the conventional diffuse system (CDS), but its SNR can be degraded due to shadowing, signal blockage and mobility. Three methods: imaging reception, beam angle and beam power adaptation are introduced to the design of spot-diffusing OW systems to effectively mitigate the degradation due to mobility in the presence of ambient light noise, multipath propagation, and shadowing. The performance of our systems was evaluated through channel and noise modeling. The CDS SNR performance improves by more than 20 dB when an imaging receiver with maximum ratio combining (MRC) replaces a non-imaging receiver. A 24 dB SNR gain can be achieved when spot-diffusing is employed with an imaging MRC receiver instead of the imaging MRC CDS. In an imaging spot-diffusing system, the SNR is independent of the transmitter position and can be maximized at all receiver locations when our new methods (beam angle and beam power adaptation) are implemented. Regardless of the transmitter position, beam angle adaptation can target the spots at the optimum location that yields the best SNR at the receiver. A significant SNR improvement of 36 dB in the imaging spot-diffusing performance can be achieved when angle adaptation is introduced. Further SNR improvement of 4 dB can be obtained if the power is adaptively distributed among the spots. Furthermore, an increase in the channel bandwidth from 43 MHz (non-imaging CDS) to 8.19 GHz can be achieved through the combination of these methods (imaging reception, spot-diffusing, beam angle and beam power adaptation). The increase in channel bandwidth and SNR can enable the OW system to achieve higher data rates and 2.5 Gbit/s and 5 Gbit/s mobile OW systems are shown to be feasible. The results also prove that the influence of shadowing and signal blockage can be sufficiently combated through the use of these methods.
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