Research On Collaborative Phase-shifted Space Time Code For High Speed Visible Light Communications

Combining lighting with communication, Visible Light Communication(VLC) is capable of defusing the frequency spectrum tension, the poor coverage problem and the energy consumption problem in wireless communication. In this paper, collaborative phase-shifted space-time coding techniques for high-speed VLC are considered, the main research contents and results are summarized as follows:1. AC and DC characteristics of commercial lighting Light Emitting Diodes(LEDs) are measured and analyzed. The voltage–current and current-optical power characteristics of LED are measured. Based on this, the nonlinearity of LED is modeled and analyzed with polynomial. The impact on higher order nonlinear modulation signal is also considered. The amplitude-frequency response of RGB LED is measured, with the time-domain and frequency-domain pre-equalization methods being given.2. Aiming at the high-speed transmission capability, a multi-LED phase-shifted space-time superimposition transmission method is proposed to increase transmission rate. The chips inside a LED lamp are used to transmit in parallel. As the superposition characteristics of natural light is linear, the multiple LED chips with one reciever construct an equivalent multi-input single-output transmission system. Then a Multi-LED Phase-Shifted Space Time Code(MP-SSC) is designed and its rapid detection algorithm is given. Theoretical analysis shows that the spectral efficiency is proportional to the number of the LED chips. The code can achieve full diversity in random fading channel. Based on MP-SSC, the Multi-LED Phase-Shifted Cascade Space Time Code(MP-SCSC) is constructed. For MP-SCSC, the bandwidth efficiency is approximate N times that of conventional single-input single-output transmission system(N is the number of LED chips). Then the precoding that can prevent the spread of errors is designed and its BER performance is analyzed. Simulation comparisions with VLC Orthogonal Frequency Division Multiplexing(OFDM) show that MP-SCSC based on On-off Keying(OOK) modulation can achieve a higher bit rate and a better anti-nonlinearity performance.3. After combining Uniquely Factorable Constellation Pair(UFCP) technology with VLC, the constellation collaborated VLC OFDM modulation and the constellation collaborated MP-SCSC are proposed. Based on existing VLC OFDM, we propose the design of the constellation collaborated VLC OFDM by introducing correlation into symbols on different subcarrier frequencies. Then the maximum likelihood detection method based on the constellation collaborated VLC OFDM is given. The average Euclidean distance gain after using two commonly used VLC OFDM constellation associated with acquired technique is analyzed. Combining UFCP with MP-SCSC, we proposed the constellation collaborated MP-SCSC. It introduces correlation into symbols on different branchs. Numerical simulations show that the introduction of the UFCP technology can get 0.8-1.5 d B SNR gain.4. For typical indoor VLC applications, two VLC baseband transmission systems are designed. First, concerning with low-speed communication applications, a 50 Mbit/s baseband chip prototype system is designed and implemented. The system structure, the function parameters, and details of the implementation process on FPGA are given. Secondly, to meet the demand for high-speed VLC applications, we combine the proposed constellation collaborated MP-SCSC with RGB LED and achieve an 800 Mbit/s offline processing system.