A Novel Encoding Scheme for Improving the Bandwidth Efficiency of DPPM

The differential pulse position modulation (DPPM) is one of the popular power-efficient schemes for supporting visible light communication in underwater environments and across the air-water interface. Despite such an advantage, DPPM does not efficiently utilize the available channel capacity. This paper aspires to tackle such shortcomings by striking a better balance between power and bandwidth efficiency. Particularly, L-DPPM is considered where a block of M input data bits is mapped into one of the L distinct waveforms containing only one ‘on’ chip. A novel encoding algorithm and frame structure are proposed in order to shorten the time between consecutive symbols and consequently improve the bit rate of L-DPPM. The idea is based on avoiding bit patterns that contribute the most to bandwidth inefficiency. The proposed algorithm explores a number of bit patterns remapping through simple complement and shifting operations. A detailed frame structure with all necessary control bits is provided. Overall, boosting the bandwidth efficacy comes at the expense of a slight increase in control bit count and transmission power. The simulation results demonstrate the effectiveness of the proposed encoding algorithm and provide guidelines for determining M for best performance.