High Speed Optical Wireless Communications using Multiple Input Multiple Output Transmission

Abstract

In recent years, the demand for high data rate mobile communication systems has increased dramatically. To satisfy this demand for high data rate communication, new methods need to be discovered by exploiting the limited resources, such as bandwidth and power, as efficient as possible. Multiple input multiple output (MIMO) system, with multiple antenna elements at both link ends, is an efficient solution for wireless communications systems. By exploiting the spatial domain, MIMO systems provide high data rates under the constraints of restricted bandwidth and transmit power. The multiple-antennas at the transmitter and/or at the receiver in a wireless communication link make a new dimension for reliable communication, which can substantially improve the system performance. The idea behind MIMO based communication system is that the multiple antennas at both ends (transmitter and receiver) are "connected and combined” in such a way that the quality (the bit error rate (BER), or the data rate) for each user is improved.

In the first study, a simplified generation scheme of Quasi Orthogonal Space Time Block Code (QOSTBC) for massive number of antennas is presented. However, QOSTBC is able to perform of achieving a full rate transmission for up to two transmit antennas, its generation entails complex algorithm to create the transmission matrix for a very large number of antennas. In addition, under a realistic massive MIMO with up to 16 transmit and receive antennas using a simple algorithm for generation of the matrix. We verify the proposed scheme via performance evaluation using BPSK and QPSK modulation under Rayleigh flat-fading channel. The proposed scheme can be readily extended to a massive MIMO with highly distributed antennas.

Although, Radio Frequency (RF) based communication technologies can somehow fulfil the current needs of high speed data communication by using advanced techniques like OFDM and MIMO. However, enormous increase demand in multimedia and other services within the limited frequency spectrum will cause spectrum congestion, thus, leading to service degradation and hence the reduced activity of the technology in the long term. The solutions to this problems of current RF based communication system includes utilization of unlicensed 60 GHz band, currently being proposed by Wireless Gigabit Alliance, but due to the problem of high path loss it is not suitable as an alternate to conventional RF based communication technologies. Another solution, which can become a better candidate for is optical wireless communication (OWC) networks. It is when appropriately studied, developed, and optimized, could provide a reliable, high-security, interference-insensitive, and especially for elders and health-sensitive people, biologically friendly indoor communication and monitoring network. This network would allow the creation and expansion of seamless computing applications, telemetry, and medical sensor monitoring using large bandwidth high-frequency pulsed light instead of RFs and microwaves. Using visible light as the communication medium, visible light communication (VLC) has many strong points over conventional RF communications, such as abundant unlicensed spectral bandwidth, usability at RF-prohibited areas.

In VLC, since the data transmission is done via light emitting diodes (LEDs) using intensity modulation techniques, VLC is able to provide faster data rates than LED switching rate, which is a several hundred of Megahertz. In order to provide high-speed data communication for the users, this dissertation addresses some of important issues in indoor VLC with experimental studies. Moreover, the dissertation provides a fundamental analysis of the VLC system in the aspects of indoor illumination, received power distribution and performance analysis in the indoor environment.

While keeping various advantages of VLC in mind, studies are carried out to enhance the performance and speed of VLC link in indoor environment. In an attempt for obtaining high-speed and high-performance, we merged the basic MIMO scale with VLC system in indoor environment. According to that concept, we propose a novel color clustered optical RGB MIMO system, as the second study of this thesis. Since visible light can be divided into three basic regions of red, green and blue, hence the data can be modulated and transmitted separately using these three color clusters. Each color cluster consist of 30 RGB LEDs is modulated using OOK and selection combining (SC) is performed at the receiver, producing diversity effect within that color cluster. The simulations result shows that the color clustered RGB MIMO-VLC system provides high performance via a relatively simple and efficient design of MIMO-VLC system.

The next study is a time frequency color cluster (TFCC) MIMO based on blue, cyan, yellow, and red (BCYR) LEDs in indoor VLC system. BCYR LEDs are employed to form four different color clusters. The data transmission using the four color clusters is performed in MIMO so that the scheme achieves very high speed data transmission. Moreover, the scheme employs the TFCC strategy to yield high performance in terms BER. TFCC operates in such a way that the original data and the two delayed versions of the data are multiplied by orthogonal frequency and then transmitted by using a specific color of the BCYR LED. In the receiver, color filters are employed to detect the transmit data from the desired cluster. SC is also performed to produce diversity effect within each color cluster to further improve the performance. Performance evaluation demonstrates that the proposed TFCC MIMO-VLC offers a data rate of 2 Gbit/s and a bit error rate of 10-6 at a Eb/No value of merely 3.8 dB.

The multiuser (MU) MIMO-VLC system is designed in the last study, presenting a high-performance time division multiplexing (TDM) based MU MIMO for an efficient indoor VLC system. In this work, RGB LEDs based MIMO technique is utilized with SC for data transmission. That is, the proposed scheme employs the RGB LEDs for parallel transmission of the user data and transmits MU data in predefined slots of time frame with a simple and efficient design to schedule the transmission time for multiple users. The simulations demonstrate that the proposed MU MIMO-VLC system provides an efficient multiuser VLC platform with significant performances over various transmission scenarios. MU with MIMO scheme is an essential technique for high-speed and high-performance in optical wireless communication.