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

Abstract

<p> Multiple-subcarrier modulated (MSM) indoor wireless optical systems provide highspeed bandwidth-efficient communication as well as multi-access capability at low cost. A major drawback of such systems is the high average optical power requirement. To address this problem, trellis codes are designed for data transmission realizing an average optical power reduction as high as 0.95 dB. A simultaneous peak optical power reduction up to 0.44 dB is also possible by employing this technique.</p> <p> Moreover, the unregulated bandwidth available in wireless optical channels is exploited to reduce the average optical power in MSM wireless optical systems. Data transmission is confined in a bandwidth located near DC, while reserved carrier signals are designed outside the data bandwidth to reduce the average optical power. Distorted out-of-band signals at higher frequency are removed at the receiver by low-pass filtering and not used for detection. To design the out-of-band carrier amplitudes, both a standard optimization algorithm over the set of real numbers and an exhaustive search over discrete constellation are proposed. It is shown that significant average optical power reduction as high as 2.56 dB is achieved.</p> <p> It is assumed that a symbol-by-symbol bias is used in MSM wireless optical systems since it offers significant optical power reduction. The received DC bias level provides a degree of diversity and is used at the receiver to improve detection. It is also used to design trellis codes with better distance properties. In this manner, an additional average optical power reduction up to 0.50 dB is achieved with a simultaneous peak power reduction of 0.46 dB for MSM wireless optical systems.</p>