Abstract
Visible light communications (VLC) is an energy efficient and cost-effective solution for indoor wireless multiple access and a candidate technique to provide high-speed data transmissions. VLC systems are built as dual systems (illumination and data transmission) and have potentially higher privacy than RF communication systems due to the natural character of light. Light emitting diodes (LEDs) that work as transmitters in VLC systems have many advantages, such as ease of modulation, high power efficiency, and long life expectancy. Since the radio frequency (RF) spectrum is so congested, and the data transmission rate of RF communications cannot satisfy the huge demand for a high data transmission, VLC has emerged as a possible new technology for the next generation communications.
In this dissertation, we introduce a multi-LED transmitter model and a multi-detector receiver model. Based on these models and the Lambertian law, we derive the impulse response of the indoor channel and the optical power distribution in space.
To support multiple access using VLC, we propose a centralized and four decentralized power allocation algorithms. In the centralized power allocation algorithm, all the LED lamps in the room are coordinated and controlled by a central controller; each LED lamp supports all the users within the indoor area. For standard indoor office illumination level (400 lx), about 40 users can be supported with bit error rates less than $10^{-3}$ using on-off keying and $70$ MHz bandwidth of receivers at $5\times 10^{-7}$ W/Hz noise spectral density. The decentralized power allocation algorithms proposed have similar bit error rate performance and less computational burden compared to the centralized algorithm. Compared with the centralized algorithm, the running time of decentralized algorithms is less than $10\%$ of the centralized algorithm. In addition, some practical considerations, such as shadowing effects, illumination requirements, dimming control and transmitted power quantization are taken into account. From numerical results, the proposed adaptive power allocation algorithms can adjust the transmitted power to reduce shadowing effects and provide an excellent communication performance.
High-speed data transmission is required by modern communication systems. For VLC systems, the transmitted bit rate is also an essential consideration. An adaptive M-ary pulse amplitude modulation (M-PAM) scheme is proposed to provide high bandwidth efficiency for different channel qualities. Given the bandwidth and the power limit characteristics of LEDs, a waveform design algorithm with adaptive M-PAM modulation can be applied for high-speed transmissions. When the 3 dB bandwidth of the LEDs is 20 MHz, and the peak transmitted power is 3 Watts for 3 users, the system can achieve about 200 Mbps bit rate per user using the proposed waveform design algorithm. Channel uncertainty is considered, which can be modeled as a Gaussian random process. Together with the minimum mean squared error filters at the receivers, the optimized waveforms can reduce intersymbol and multiple access interferences together. We then propose an off-line waveform design algorithm to diminish the computational time. For the off-line algorithm, a waveform lookup table can be established in advance, and the proper waveforms can be selected from the table based on the real channel gains in real time. The performance of the off-line algorithm can be estimated by using the channel uncertainty model. Compared with DC-biased optical orthogonal frequency division multiplexing, M-PAM with optimally designed waveforms can provide an $80\%$ higher data rate for asingle user.
Given the power distribution, we analyze the potential vulnerability of the system from eavesdropping outside the room. By setting up a signal to noise ratio threshold, we define a vulnerable area outside of the room through a window. We compute the receiver aperture needed to capture the signal and what portion of the space is most vulnerable to eavesdropping. Based on the analysis, we propose a solution to improve the security by optimizing the modulation efficiency of each LED in the indoor lamp. The simulation results show that the proposed solution can improve the security considerably while maintaining the indoor communication performance.
