Research On Key Technologies Of Indoor Visible Light Communication Systems

With the increase of mobile users, the electromagnetic spectrum occupied bythe radio frequency (RF) is increasingly crowded. Communication technologieswith higher frequency and shorter wavelength light as its transmission medium ofcommunication has attracted much attention from the research community becauseof its unregulated huge (Terahertz) bandwidth, large communication capacity, highdata transfer rate and strong ability of anti-electromagnetic interference and so on.Based on adding an auxiliary function of data transmission to an illumination lightsource, an indoor visible light communication (VLC) system transmits informationthrough a high speed switching of the illumination light source, and uses aphotosensitive device to achieve the information carried in the optical signal. VLCsystem provides a new wideband access to indoor wireless communication network,and is an effective complement to the building of indoor wireless communicationsystem. Therefore, the key technologies of indoor VLC systems, such as channelmodel, modulation schemes, and channel code, have been studied in thisdessertation to further improve the performance of the system and lay a foundationfor the practical application of indoor system.Firstly, after in-deep analyzing the channel characteristics, a channel modelfor indoor VLC systems using light emitting diode (LED) array as the illuminationsource are proposed, which composed of the topology and wiring of theseillumination source. The imperfections of current channel model are in-depthstudied through analyzing the characteristics of the line of sight link and the nonline of sight link of indoor VLC systems. Factors such as the topology of LEDarrays, paths between communication access point (CAP) and LED array accesspoint (LAAP), paths between LEDs and LAAP, and the existence of multiplereflective paths are taken into consideration in the first time, through which animprovement of the model of VLC channel link is made and a mathematicalmodel of the impulse function of indoor VLC is derived in details. According totheoretical analysis and simulation results of the channel model proposed, themultipath effect is aggravated with the increase of the distance between CAP andLAAP, or the distance between LAAP and LED, and the maximum data transferrate is also constrained by the topology of LED arrays. Based on the channelmodel proposed, the strategies to eliminate inter-symbol interference (ISI) causedby LED array light source lighting inconsistent are studied, and the advantages anddisadvantages of each strategy are also analyzed.Secondly, in order to increase the data transfer rate, and considere with the limitation of the modulation bandwidth of LED, this dissertation focuses onmodulation and demodulation technologies for indoor VLC systems. The priciplesof classic modulation schemes used in VLC systems is analyzed, and specialrequirements including flicker mitigation and dimming control support areintroduced, and an novel M-ary variable period modulation (MVPM) scheme forVLC is proposed. The principle of MVPM and the key parameter selection guideare described in details, and the performance of flicker mitigation and dimmingcontrol choices, symbol srror rate and bandwidth efficiency are analyzed.Theoretical analysis and experimental results indicate that MVPM can be used toachieve a higher data transfer rate of an indoor VLC system than classicmodulation schemes, including On-Off Keying (OOK), Pulse Position Modulation(PPM), Differential Pulse Position Modulation (DPPM), Overlapping PulsePosition Modulation (OPPM). Meanwhile, two improved Duty CycleFixed-MVPM (DCF-MVPM) scheme and M×N-ary Variable Width PeriodModulation (MNVWPM) scheme are proposed, where DCF-MVPM can be used tomitigate light source flickering during data transfer process, and MNVWPM canbe used to achieve a higher data transfer rate than MVPM and suits for highersignal to noise ratio (SNR) application. The resistances of modulation schemesproposed to the inter-symbol interference (ISI) caused by channel multi-path effecare also analyzed.Thirdly, in order to better adapt to the channel environment and to improvethe transmission performance of the system, this dissertation carry out a researchon channel code technology in indoor VLC systems, and propose L-DimensionalSerial Concatenated Code (LDSCC) based on RS (Reed-Solomon) code. Theprinciples of LDSCC and the structures of encoding and decoding process arepresented in details. Theoretical analysis and experimental results indicate that,LDSCC can be used to improve the data transfer performance and reduce thesymbol error rate (SER) with the increase of L, but at the cost of a complexity ofdata process and a higher requirement of memory. Meanwhile, considering the datatransmission requirements of an indoor VLC system, a Rate Adaptive LDSCC(RALDSCC) is proposed to support different data transfer rate while theillumination intensity is changed. Theoretical analysis indicates that RALDSCC isan effective way to meet the data transfer rate variation requirements of customer.Finally, in order to study the application of MVPM, DCF-MVPM, MNVWPMand LDSCC in an indoor VLC system, an experimental system is developed in thisdissertation. The transmitter and receiver structure of experimental system, keycomponents selection and key circuit design are introduced indetails. In theexperimental hardware platform, the data transfer performance using MVPM,DCF-MVPM and MNVWPM are summarized by changing the parameter of MVPM, DCF-MVPM and MNVWPM with different communication distance, therelationship between the SER and communication distance under variousexperimental conditions are provided. The performance of LDSCC are also studied,the improvement of the SER with LDSCC used or without verified the validity ofthe LDSCC on the correct channel error.