| The high power LED has the characteristics of higher electro-optical conversion efficiency, energy saving, more reliable, longer life and higher speed modulation, which has been widely used in our daily life and is recognized as the next generation of green lighting products. The high power LED for indoor lighting technology combining with the optical communication technology produces a completely new technology which is referred as visible light communication(VLC). Compared with the traditional radio communication, VLC has high transmission power, no elector-magnetic interference, no need for spectrum resources, and other advantages. This thesis focused on the existing indoor visible light communication issues to make preliminary research, and we finished some work that included the LED characteristics, the channel model, the communications modulation and coding, and the diversity receiving technology for MIMO channel model.First, the thesis introduced the indoor high power LED communication system components and we analyzed the characteristics of luminous intensity of the LED. In order to use white LED as a lighting source and communication part, a new modulation scheme called reverse dual header pulse interval modulation(RDH-PIM) was proposed for indoor visible light communications based on the analysis of the structures of on-off keying(OOK), pulse position modulation(PPM), digital pulse position modulation(DPIM), and dual header pulse interval modulation(DH-PIM). After analyzing and comparing the symbol structure, bandwidth requirement and average transmission power, slot error rate was derived. Simulation results showed that OOK had the minimum bandwidth, while RDH-PIM had the highest average transmission power and its bandwidth efficiency was obviously better than PPM and DPIM.Second, we used the weak turbulence channel which the light intensity scintillation obeying the lognormal distribution as the background model. Combining the characteristics of atmospheric channel, the bit error rate(BER) calculating formula of uncoded system was derived based on different modulations, then as a benchmark, under the case of independent with identical distribution, the average bit error rates of RS and LDPC coded system were derived respectively. The decoding algorithm was analyzed according to the system model, and it was simulated in the atmospheric turbulence channel. Simulation results showed that, the channel coding technology had better BER performance than the uncoded system and it could be efficient to get the code gain and to improve jamming-rejection capability of system.Finally, in order to overcome the channel fade effect caused by the atmospheric turbulence, we proposed to use the diversity receiving technology to improve the reliability of MIMO systems. The calculating formula of bit error rate of the system based on intensity detection PPM using three linear combining technologies, i.e., the maximal ratio combining(MRC), equal gain combining(EGC) and selection combining(Sel C) were compared by means of numerical simulation. The results showed that the improvement of system by MRC was the best, followed by EGC and Sel C was poor. However, Sel C was simpler and more convenient. And the technology of spatial diversity receiver combination was efficient to improve the performance of FSO and had strong ability of resistance to atmospheric channel fade. |
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