No abstract

Multiple-subcarrier modulated (MSM) indoor wireless optical systems provide high speed 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.Moreover, the unregulated bandwidth available in wireless optical channels is ex ploited to reduce the average optical power in MSM wireless optical systems. Data transmission is confined in a bandwidth located near DC, while reserved carrier sig nals 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 am plitudes, 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.It is assumed that a symbol-by-symbol bias is used in MSM wireless optical sys tems 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 simulta neous peak power reduction of 0.46 dB for MSM wireless optical systems.lV

In this work we apply techniques for power reduction in electrical multiple-subcarrier modulated (MSM) systems to wireless optical systems. The signal set is expanded by adding a zero amplitude constellation point to each frequency channel. A peak and average optical power gain are realized by coding over these increased degrees of freedom. We develop search techniques to find good mappings between transmitted symbols and trellis edges both at low and high bandwidth efficiencies. With a symbolby-symbol bias, an average optical power reduction of 0.9 dB over uncoded systems is achieved with a simultaneous peak power reduction of 0.4 dB at the same bandwidth efficiency.