| Wireless Communication has gone through several paradigm shifts starting from the smoke and torch signal (pre-industry age), to the discovery of electromagnetic waves and the invention of the radio system. In modern day wireless communication systems, the most popular communication media is the Radio Frequency (RF) electromagnetic wave (such as WiFi, LTE, etc.) as it can enable long-distance and rapid transmission with better quality and smaller, cheaper devices. However, there are some limitations of the RF technology. First, the shortage of RF spectrum in the wireless communication has been identified along with the exponential increase of the mobile wireless data traffic in the past two decades. The current RF spectrum is scarce to meet the ever-increasing traffic demand even with the efficient frequency and spatial reuse. The strict regulation of RF spectrum clearly constrains the RF based wireless system in terms of flexibility in allocating spectrum resources, which makes the situation even worse. Second, due to the long wavelength of RF, it can easily penetrate through most objects and walls. This inherent characteristic of RF leads to the severe interference among multiple transceivers and the vulnerability to eavesdropping attacks. Third, many kinds of mobile application cannot be achieved solely with RF, such as fine-granularity indoor localization, proximity detection, etc.;To overcome the aforementioned constraints of the RF-only wireless network, our research focuses on improving the performance and enhancing the security of wireless network through alternative transmission medium -- visible light and inaudible sound. In this thesis, we discuss three systems which either utilizes the visible light or inaudible sound to provide better services for mobile devices. In the first work, we present ColorBars, a Visible Light Communication system that utilizes Color Shift Keying (CSK) to modulate the data using different colors transmitted by the LED. It exploits the increasing popularity of Tri-LEDs (RGB) that can emit a wide range of colors. We show that commodity cameras can efficiently and accurately demodulate the color symbols. ColorBars ensures the flicker-free and reliable communication even in the presence of inter-frame loss and diversity of rolling shutter cameras. In the second work PCASA, we utilize the acoustic communication for secure proximity estimation with sub-meter level accuracy. PCASA uses Differential Pulse Position Modulation scheme that modulates data through varying the silence period between acoustic pulses to ensure the energy efficiency even when the authentication operation is being performed once every second. To augment the existing WiFi network, we propose PLiFi, a hybrid WiFi-VLC network architecture through the use of Power Line Communication (PLC). PLiFi creates a high-speed interconnection between LEDs themselves and Internet at low cost, and also seamlessly integrates WiFi and VLC networks.;With the alternative transmission medium, we could (1) relieve the overcrowded Radio Frequency spectrum imposed by the evergrowing interconnection demand of smart devices and IoT, (2) provide some novel services which are difficult to achieve by the RF-only wireless communication, such as visual association, fine-grained location specific advertisement, etc., (3) ease the inherent security vulnerability of RF. |
