In this paper, two novel techniques are proposed to reduce the average optical power in wireless optical multiple- subcarrier modulated (MSM) systems, namely in-band trellis coding and out-of-band carrier design. Data transmission is confined to a bandwidth located near DC. By expanding the signal set and coding over the increased degrees of freedom, an in-band trellis coding technique achieved an average optical power reduction up to 0.95 dBo over conventional MSM systems while leaving the peak optical power nearly unaffected. With a symbol-by-symbol bias, the received DC level can be detected to provide a degree of diversity at the receiver. In this manner, an additional average optical power reduction up to 0.50 dBo together with a peak power reduction of 0.46 dBo is achieved. Moreover, the unregulated bandwidth available in wireless optical channels is exploited and out-of-band carrier signals are designed outside the data bandwidth to reduce the average optical power. Average optical power reduction as high as 2.56 dBo is realized at the expense of 4 out-of-band carriers and an increase in the peak optical power. Finally, combining the three techniques achieves the best average optical power reduction of 2.63 dB optical.

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.