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Research On Key Technologies Of Modulation For High Speed Visible Light Communications

Posted on:2016-03-29Degree:DoctorType:Dissertation
Country:ChinaCandidate:J F LiFull Text:PDF
GTID:1108330482957876Subject:Communication and Information System
Abstract/Summary:PDF Full Text Request
As the mobile internet, mobile multimedia and other new services appearing, the demand of wireless high-speed service is growing. However, wireless electromagnetic spectrum is gradually occupied by all kinds of wireless communication technologies, meanwhile, the spectrum less than 10 GHz is almost completely expended. Therefore, a variety of emerging communication technologies spring up at the historic moment, such as millimeter wave, terahertz waves and visible light waves, etc. The most typical technology is the visible light communication based on light-emitting diodes (LED). The visible light communication combines indoor illumination and communication technology, which has become research hotspot in recent years. To improve the transmission data rate, we can use multi-dimensional resources of LED to achieve more efficient modulation. The current researches focus on the spatial and frequency domain resources of LED. For the spatial resources, the technology of multi-input multi-output (MIMO) can enhance data rate for visible light communications. However, the spatial correlation of sub-channels is very strong for LED array, which limits the system data- rate of MIMO. In frequency domain of utilization of resources, by general OFDM techniques, the frequency band is divided into sub-channels, so as to achieve frequency division multiplexing. However, the LED nonlinearity limits the data rate of OFDM system. Therefore, how to take full advantages of spatial and frequency domain resources according to the characteristics of LED, is a major challenge for modulation techniques of visible light communications. Therefore, in this dissertation, the research is focused on the LED modulation, and research on the spatial modulation technologies and eliminate LED nonlinearity noise for visible light communication. The main contributions of this dissertation can be summarized below:Firstly, a novel modulation scheme called spatial superposed pulse amplitude modulation (SPAM) is proposed, which is based on the spatial characteristics of the LED and can improve the transmission rate. For indoor scenario, the primary purpose for LED source is general lighting. The general commercial single-chip LED power is generally 0.1-1W. With the commercially available LED, an amount of chips is needed to ensure sufficient brightness for the room illumination. The LED array is always designed as lighting source in actual scenario, in which the LED chips are placed in space regularly. Therefore, we can utilize this spatial character of LED array in modulation mechanism. When such optical intensity information sends in LOS, the base-band signal could transmit directly. Therefore, multi signals transmitting in parallel can overlay linearly in free space. According to the geometric characteristics of the LED array, a new type of spatial modulation techniques is researched to increase the transmission rate. The space linear superposition of theoretical analysis and simulation results are provided in this dissertation. Finally, experimental results show that, compared with OOK, the system data rate of the SPAM modulation is improved by 50% under the same conditions, meanwhile, the SPAM has lower implementation costs and complexity.Secondly, a polarity separating optical OFDM (PSO-OFDM) modulation technology is proposed, which employs the LED array to mitigate the dynamic range for OFDM modulation. Meanwhile, the PSO-OFDM can improve the system data rate. At present, two OFDM techniques are reported in the literature:direct-current-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical (ACO-OFDM) in visible light communication system. DCO-OFDM has high spectrum efficiency, but it needs a larger linear dynamic range of LED to sustain great nonlinear effects. The ACO-OFDM endures lower nonlinear, but its spectrum efficiency is low. The performance of PSO-OFDM modulation technology proposed combines the advantage of the two methods, which has very high spectrum efficiency and low nonlinear. Finally, we build an experiment plane to verify the modulation technologies mentioned about. The experimental results show that the maximal transmission rates of DCO-OFDM, ACO-OFDM and POS-OFDM are 0.85 Gb/s,0.84 Gb/s and 1.1 Gb/s, respectively.Thirdly, a novel hybrid time-frequency domain equalization scheme is proposed to mitigate the white light emitting diode (LED) nonlinearity in visible light communication systems based on OFDM, which can enhance the modulation index for OFDM signal. In the visible light communication systems, the transmitter LED is often a very large nonlinear. When the OFDM signal is loaded on the LED, if OFDM signal is beyond the scope of the LED linear, the LED nonlinearity will make OFDM sub-carrier produce crosstalk, the signal spectrum bandwidth of internal and external noise are introduced, so as to make the system performance deterioration. Experimental results show that the low-frequency subcarriers withstand high nonlinearity. Therefore, the pre-equalization enhancing the high frequency part of the OFDM signal and suppressing the low frequency intensity can mitigate the impact of LED nonlinearity to some extent. Then, a hybrid time-frequency domain equalization scheme is uesed to mitigate the LED nonlinearity. In the scheme we proposed, the linear distortion is compensated by the frequency domain equalization (FDE) and the nonlinear distortion is equalized by the N-TDE. With the nonlinear compensation, not only the in-band noise, but also the out-band noise can be eliminated. The maximum transmission rate of the system from 0.9 Gb/s upgrade to 1.4Gb/s.
Keywords/Search Tags:Light Emitting Diodes (LED), Visible Light Communications, Modulation, Orthogonal Frequency Division Multiplexing (OFDM)
PDF Full Text Request
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