Contents I. Introduction
Optical wireless hierarchical transmission systems using infrared data communications and visible-light communications utilizing LED light have attracted much attention for the road to vehicle communications on the intelligent transport system. In the hierarchical transmission system, the broadcasting station transmits the hierarchical data contains wide-area information data (high-priority data) and local-area information data (low-priority data) simultaneously. Moreover, the hierarchical data is designed for satisfying the following two conditions: (1) a user at a short distance from the broadcasting station (i.e., nearby user) can demodulate both the high-priority information and low-priority information, and (2) a user at a large distance from the broadcasting station (i.e., distant user) can demodulate the high-priority data at least even if the distant user does not receive the low priority information. Therefore, the hierarchical transmission systems can provide the transmission reliability corresponding to priority of information. To realize the hierarchical transmission system for the optical wireless communications, many studies have been investigated [1]–[4]. In [1] [2], the hierarchical transmission system has been realized by using the image enhancement and image sensor as a receiver. By using the image sensor, this system can be reduced the influence of the back-ground noise because the image sensor can demodulate the signal and the back-ground noise individually. However, it is difficult to realize the high data transmission scheme compared with the schemes using the photo-diode (PD) detector. In [3]–[4], the hierarchical modulation schemes combine the different two modulation schemes have been proposed, and it is possible to demodulate by PD. Especially in [4], the amplitude sfhit keying with code number keying (ASK-CNK) using an extended prime code sequences (EPCS), and code shift keying with CNK (CSK-CNK) using EPCS have been evaluated.
