Rate and Performance Enhancements of Indoor Optical Camera Communications in Optical Wireless Channels

Abstract

The wireless communication has undergone a massive increase in research and development for the past decade. The trending demand for higher communication density has been consistent over the past five years. Radio Frequency (RF) based communication technologies can somehow fulfill the current demands of high speed data communication by utilizing advanced techniques such as OFDM and MIMO. However, massively increased demands in multimedia and network services within the limited frequency spectrum will cause spectrum congestion and lead to service degradation in wireless communication. Optical Wireless Communication (OWC) schemes have been proposed as an alternative to the conventional RF based wireless communication to overcome the increasing demand for high data rate and high connectivity wireless communication by utilizing light spectrum as its communication medium.

Two major subsets of OWC have emerged for indoor communications, i.e., Visible Light Communication (VLC) and Optical Camera Communication (OCC). VLC uses light emitting diodes (LED) as the transmitter and photodiodes as the receiver, while OCC utilizes cameras for the receiver instead of photodiodes (PD). The major motivation behind the development of OCC is a pragmatic use of cameras that are available on many devices, including mobile devices such as smartphones. OCC is standardized by the IEEE 802.15.TG7r1 Optical Camera Communications Task Group. OCC is included as a part of the IEEE 802.15.7-2011 IEEE Standard for Local and Metropolitan Area Networks – Part 15.7: Short-Range Wireless Optical Communication Using Visible Light.

OCC offers an additional advantage over VLC by employing a two-dimensional image sensor (camera) compared with the one-dimensional PD sensor in VLC. In this regard, OCC exploits the spatial domain of the image sensor to modulate bits that can be distributed over space on top of the intensity and color. The extra information carried over the spatial coordinate is currently feasible due to the fact that a digital camera has millions of pixels that work similar to millions of PDs. Furthermore, OCC is considered a pragmatic version of VLC because of wide accessibility and availability in daily used smartphones. However, OCC has a critical disadvantage, i.e., the capture rate of the camera (which is a sampling rate for OCC) that is generally limited to 60 frames per second (fps) or lower. The sampling rate/capture rate of the camera in OCC is considered slow compared with the sampling rate of PD in VLC that can achieve up to tens of gigahertz (GHz). The disparity is mainly caused by the image processing nature of the camera that processes millions of pixels similar to utilizing millions of PDs simultaneously. This critical disadvantage causes the data rate of OCC to be mostly limited to a bit/s scale.

The motivation of the current dissertation is to investigate data rate improvements and performance enhancements for several potential downlink and uplink channels in OCC. One of the main objectives is to overcome the critical disadvantages of OCC and to investigate several compensations for its drawbacks by improving the transmitter, the receiver, and the modulation techniques. The first chapter introduces OWC and its subsets including VLC and OCC. The second chapter of the dissertation elaborates on the principles of OCC, its current working standards, the potential of OCC, and challenges of OCC. The third chapter and fourth chapter then investigate the rate of the downlink channel and the rate of the uplink channel in OCC, respectively. The fifth chapter investigates performance enhancements in OCC. The sixth chapter finally concludes the findings in the dissertation.

The focus of the studies discussed in the third chapter is to improve the rate of downlink channels in OCC by splitting the capture frame of the camera and implementing a high-density modulation (HDM). The split-capture-frame technique is useful to efficiently increase the capture rate of the camera since the image being processed is halved, thus reducing the processing time by 50%. The split-capture-frame technique has achieved a data rate of up to 11.52 Kbit/s with a global shutter camera. The HDM, on the other hand, is presented, combining cell, color, intensity, and shape to carry the information bits. In this regard, the HDM provides a denser modulation to effectively utilize the 2D space of the image captured by the camera. The HDM work, which is experimented through a device-to-device scheme, has obtained a maximum data rate of 2.66 Mbit/s with a distance of 20 cm. This data rate is approximately threefold compared with the previous reports.

The fourth chapter then investigates the uplink channel for OCC that can be provided using either visible light or invisible light such as infrared. A brief investigation of uplink OCC is provided using smartphone screen-based OCC. It is, however, considered impractical for general mobile use since the screen is dedicated for uplink functionality. The potential infrared-based camera communication is also investigated for a more practical uplink scheme since the infrared LED is also readily available on smartphones. The potential infrared based uplink on OCC, utilizing rolling shutter patterning demodulation, has obtained a data rate of 6.72 Kbit/s at a distance of 100 cm.

The fifth chapter proposes additional performance improvements besides the increased data rate. The first performance improvement is associated with light metering and focus assistance of the camera receiver by including additional illumination LEDs on the transmitter side. The illumination LEDs act as references to assist the camera in focusing and defining the light intensity for capture. The second improvement is an experimented transmitter design for providing a much wider coverage in OCC. The design enables an orientation-free OCC scheme that covers line-of-sight (LOS) and non-line-of-sight (NLOS) links up to 180° orientation in any direction. In addition, a positioning scheme is also investigated in this chapter to provide a positioning utilizing the presence of infrared LED on smartphones as a beacon.

The last chapter concludes and summarizes the rate and performance enhancements for the OCC discussed in the previous chapters. The future scope of OCC is also elaborated in the conclusion.