Multilevel pulse-position modulation based on balanced incomplete block designs

In this paper, two new modulation schemes using multilevel pulse-position modulation (PPM) for application in unipolar optical wireless systems are presented. Balanced incomplete block designs (BIBD) are used for constructing the symbol alphabets. Each symbol is obtained by combining multiple codewords of a BIBD code. In one scheme the symbols have equal energies, and therefore, no threshold is needed to make a decision on the received signal. The other modulation has better performance yet higher complexity. Since cyclic BIBDs are used for constructing the symbols, the transmitters and receivers have simple structures, and can be implemented using shift registers. These schemes can achieve high spectral-efficiencies, and are therefore suitable for systems with bandlimited sources or highly dispersive channels, where intersymbol interference (ISI) has a significant impact on the performance. We also show that using the same receiver structure, the constellation size can be increased by including the complements of the codewords. The performance of the proposed schemes are compared to other modulation schemes for both LED-based non-dispersive and dispersive free-space optical (FSO) systems.