Power reduction techniques for multiple-subcarrier modulated diffuse wireless optical channels

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.