Optical Power Reduction for Multiple-Subcarrier Modulated Indoor Wireless Optical Channels

In this paper, the unregulated bandwidth available in wireless optical channels is exploited to reduce the average optical power in multiple-subcarrier modulated (MSM) systems. Data transmission is confined to a bandwidth located near DC, while out-of-band subcarrier signals are designed outside the data bandwidth to reduce the average optical power. Although the out-of-band signals at higher frequency are subject to severe corruption induced by multipath distortion, they are removed at the receiver and not used for detection. Optimizing the out-of-band carrier amplitudes over the set of real numbers yields gains as high as 2.6 dB over conventional MSM systems at the same bandwidth efficiency. We additionally apply in-band trellis coding and out-of-band signals, whose amplitudes are optimized over a discrete constellation. When no out-of-band signals are used, the system achieves an average optical power reduction of 0.9 dB over uncoded systems with a simultaneous peak power reduction of 0.4 dB. An additional average optical power reduction of 0.9 dB is realized at the expense of 4 out-of-band carriers and a moderate increase in peak power.