Research On OFDM Modulation For Visible Light Communication

The main characteristics of visible light communication (VLC) system which is based on Orthogonal Frequency Division Multiplexing (OFDM) technology are:First of all, VLC system usually adopts the intensity modulation/direct detection (IM/DD) method, the OFDM signal must be positive and real. Secondly, the linear dynamic range of Light Emitting Diode (LED) is very limited. If the amplitude of the input signal is too small, it will lead to cut-off distortion easily, however, it will cause saturation distortion if the amplitude is too large. Thirdly, the PAPR of the OFDM system is too high, which will affect the performance of the system seriously. This paper carries out research work based on the above several aspects. The main purpose is to realize the transmission system with high information transfer rate, good communication quality and strong anti-interference ability. The main research content and the innovation of this paper are as follows:1. Analysis and improvement of the OFDM modulation technology in VLC system. According to different characteristics of VLC system and radio frequency communication system, this paper introduces some optical OFDM modulation methods which are suitable for VLC system, and compares their performance characteristics. At the same time, an improved method of Flip-OFDM technology is proposed, which can reduce the effect of the system noise and improve the bit error rate performance of the system.2. A transmission system based on Amplitude-Phase Shift Keying (APSK) modulation combined with OFDM technology (APSK-OFDM) is introduced in VLC system in order to decrease the effect of the nonlinear characteristics of LED and improve the spectrum efficiency of the system. Compared with the traditional rectangular QAM modulation, APSK signal envelope fluctuation is relatively small, which puts forward the relatively low requirement on the nonlinearity of LED. The simulation result shows; that the system performance of APSK-OFDM modulation is better than QAM-OFDM modulation when the nonlinear characteristic of LED is considered. At the same time, this paper researches on the relationship between relative radius of the different APSK signal constellation and the bit error rate performance of the system. As each sub-channel suffers from different fading conditions in visible light OFDM system, this paper puts forward the method that combines APSK-OFDM modulation with adaptive technologyr to make fall use of the system bandwidth resource. This method allocates system resources reasonably according to different sub-channel conditions, so as to improve the whole performance of VLC system.3. A new method is proposed to reduce the peak to average power ratio (PAPR) of the visible light OFDM system. Firstly, the probability distribution of PAPR in visible light OFDM system is analysed to provide a theoretical basis for the following research. Secondly, several typical algorithms are introduced to reduce the PAPR of the OFDM system:companding transform method, selective mapping (SLM) and partial transmit sequence (PTS), and then their performance is simulated and analysed. The PAPR performances of these methods are different, and it is difficult to achieve desired result if using a single method. It is necessary to combine with other methods to further reduce the PAPR of the system. At the same time, the relationship of the transmission efficiency and PAPR in VLC system is researched. Therefore, this paper proposes the method that combined Diserete-Fourier-Transform Spread (DFT-S) with SLM and PTS respectively. Analysis results show that in the case of the system performance is similar, the complexity of the joint algorithm is much less than that of the single algorithm, and it can increase the transmission efficiency at the same time.