Research On Key Technologies In Wireless Visible Light Communications

Wireless visible light communication (VLC) is an optical wireless communication technology which uses visible light between 400 and 800 THz (780-375 nm) as a data communications medium. VLC leads to good electromagnetic compatibility, no spectrum licensing requirement, improved communication quali-ty(under certain circumstances), enhanced information security and hence receives extensive attention from academia and industry. The inherent modulation bandwidth of lighting device, e.g. light emitting diode (LED), can be exploited to provide a dual role as a communication device. Deducting the power consumption of lighting itself, energy consumption for wireless transmission is significantly lower than the conventional R-F wireless communication systems. Wireless visible light communication technology, one of the hot topics in wireless communications recently, has been attracted more and more attention. On the one hand, whether the VLC enjoys a same channel capacity as conventional RF wireless communication technology is unknown; on the other hand, high data rate achievable coding theory and modulation make it a strong candidate for present and future access network technology. LED devices are being used in large scale in indoor lighting, traffic lights, etc. Compared with conventional lighting methods, LED lighting is a promising technology to higher power efficiency of general illumination, lower heat generation, and smaller size. With the popularity of LED devices, wireless VLC technology will be widely used in "last meter solution" and have profound and broad influences on wireless communications. Under this background, the investigation of channel modeling, con-figuration and optimization of electro-optical resources, efficient modulation and transceiver design should be brought to the attention. This dissertation investigates the above several technical problems for wireless VLC system. Specifically, we list our main contributions as follows:In Chapter 2, we investigate channel capacity for optical wireless communication system based on light-wave transmission mechanism. Firstly, we provide a closed-form capacity equation in the Poisson channel model, and furthermore propose the lower bound of channel capacity when the average power is considered. Secondly, we pay our attention to the impact of the peak and average transmission power constraints on the channel capacity. New upper bounds and lower bounds are derived with one or both of the constraints. Com-puter simulations convinced the accuracy of our proposed bounds compared with conventional ones. Finally, we analyze the clipped signal characterization from ACO-OFDM and DCO-OFDM system and propose the derivations of maximum achievable transmission data rate.In Chapter 3, we investigate channel equalization for optical wireless communications. This chapter analyzes major source of inter-symbol interference (ISI) in wireless optical communication system. We pro-pose a fractional spaced equalization (FSE)scheme to deal with narrow modulation bandwidth of indoor LED light source. FSE will increase the signal sampling rate as well as the filter tap resolution of fractional spaced channel equalization. We then propose filter tap coefficients solutions by zero-forcing algorithm and decision feedback algorithm. The numerical results convinced the performance enhancement of our proposed schemes compared with conventional equalization schemes.We pay our attention to transmitter optimization in Chapter 4. We analyze the principles of three optical OFDM modulations. Most of the existing works only consider a lower clipping because of the unipolar signal in intensity modulation/direct detection (IM/DD) system. However, PAPR has been an important issue for an OFDM transmission system. One should provide a reasonable transmitting power to meet the requirements of the linear transfer characterization of the LED light source by upper and lower asymmetric clipping. We also optimize the bias current power in DCO-OFDM system in order to overcome the bit error rate (BER) performance degradation which is caused by clipping distortion. We optimize the overall system performance by minimizing the signal clipping distortion function for all sub-carriers with guaranteed asymmetric clipping constrains. The formulated problem is a combinatorial optimization problem, where the objective function is non-convex and we design a searching algorithm for global optimum solution. Computer simulation results show that an optimum bias current power is very necessary for transmitter in DCO-OFDM system. It also can mitigate the system performance degradation in some extent.In Chapter 5, the problem of receiver optimization of optical OFDM system is considered. We use error vector magnitude (EVM) to represent the distortion level and provide the EVM analytical expression of clipped signal. After this, the derivation of BER is proposed base on maximum likelihood detection. We then propose two improved iterative algorithms. The first one update the original estimate sequences by iterative estimate of residual clipping noise; the second one whited the clipping noise by weighted factor and make the maximum likelihood estimation algorithm satisfies the condition of the best linear detection. Computer simulation confirmed the iterative receiving algorithm presented in this chapter can improve system performance effectively.