PPM Modulation And Coding Techniques Of Visible Light Communications Systems

In recent years, the development of wireless communications has encountered many difficulties, such as limited spectrum resources, the radio frequency (RF) radiation on the human body. With the fast development and the widely applications of light emitting diode (LED) technology in modern families, the corresponding indoor visible light communications (VLC) technologies, which use the LED lights for data transmission, have attracted a great attention. For competitive advantages in rich spectrum resources, green and safe environment, high energy conversion rate and other aspects, the VLC technologies have important research value in high-speed indoor wireless communications. This paper mainly focuses on the modulation and coding techniques of indoor VLC systems in order to efficiently optimize the performance of communications systems.Firstly, the paper introduces the basic theories and knowledge of VLC system, including system architec-ture, modulation techniques and channel coding schemes. Also we briefly present the research background and progress of VLC technologies and network architectures. The transmitter and receiver of a VLC sys-tem usually can be divided into coding,modulation and demodulation,decoding modules respectively. Then, various modulation schemes, such as OOK, PPM and MPPM, are analyzed and compared. We discuss and present the derivation of output log-likelihood ratio for the MPPM soft demodulator. Channel coding is an in-dispensable part of most communications systems. We mainly introduce the LDPC codes due to their superior performance and the important applications in wireless communications. Some iterative decoding algorithms of LDPC codes are discussed in the last part of chapter.Secondly, the optimization problem of LDPC coded MPPM schemes has been studied. The performance of communications systems with MPPM is primarily effected by bit-symbol mapping proposed. We present the LDPC coded MPPM systems using BICM-ID and explain their advantages and applications. A modified EXIT algorithm is proposed to evaluate the convergence performance of the BICM-ID scheme which com-bines the MPPM modulation and the LDPC coding. We propose a modified genetic algorithm to optimize the bit-symbol mapping. We use different fitness function to search sub-optimal solutions. Simulation results are given to show the feasibility and effectiveness of our method.Thirdly, the paper studies the optimization problem of MPAPM constellation. Compared to MPPM, the MPAPM scheme imports a new variable-amplitude and extends the constellation dimensions. It is suitable for the wireless communications with high-speed transmission rate. We convert MPAPM constellation opti-mization to a convex optimization problem and use matlab tools to solve it. According to the optimization results, MPAPM has a noticeable performance gain over OOK with the same spectral efficiency. If reduction in data transmission rate is considered, the energy unbalancing distributed MPAPM constellation can achieve a larger mean euclidean distance than that of the conventional balancing MPPM constellation. Therefore the MPAPM can obtain better performance compared with MPPM in the same date rate.Finally, the paper studies the dimming-support schemes in VLC systems. Dimming-support schemes can be divided into two class:modulation based schemes and coding based schemes. For modulation based schemes, we mainly introduce VOOK, VPPM, MPPM and hybrid MPPM scheme and analyze their advan-tages and disadvantages. Hybrid MPPM scheme is discussed in detail due to its advantages in dimming range, dimming resolution and data transmission rate. Coding based schemes use the puncturing of LDPC codes and the insertion of compensation symbols to control the dimming value. The adaptive FEC scheme with OOK modulation is also discussed. Then we try to replace OOK modulation with hybrid MPPM. Simulation results show that our new scheme has a remarkable superiority over adaptive FEC scheme with OOK modulation in different dimming values. Especially, our scheme can work well in extremely high and low dimming values.