Interference mitigation techniques for optical attocell networks
The amount of wireless data traffic has been increasing exponentially. This results in the shortage of radio frequency (RF) spectrum. In order to alleviate the looming spectrum crisis, visible light communication (VLC) has emerged as a supplement to RF techniques. VLC uses light emitting diodes (LEDs) for transmission and employs photodiodes (PDs) for detection. With the advancement of the LED technology, LEDs can now fulfil two functions at the same time: illumination and high-speed wireless communication. In a typical indoor scenario, each single light fixture can act as an access point (AP), and multiple light fixtures in a room can form a cellular wireless network. We refer to this type of networks as ‘optical attocell network’. This thesis focuses on interference mitigation in optical attocell networks. Firstly, the downlink inter-cell interference (ICI) model in optical attocell networks is investigated. The conventional ray-tracing channel model for non-line-of-sight (NLOS) path is studied. Although this model is accurate, it leads to time-consuming computer simulations. In order to reduce the computational complexity, a simplified channel model is proposed to accurately characterise NLOS ICI in optical attocell networks. Using the simplified model, the received signal-to-interference-plus-noise ratio (SINR) distribution in optical attocell networks can be derived in closed-form. This signifies that no Monte Carlo simulation is required to evaluate the user performance in optical attocell networks. Then, with the knowledge of simplified channel model, interference mitigation techniques using angle diversity receivers (ADRs) are investigated in optical attocell networks. An ADR typically consists of multiple PDs with different orientations. By using proper signal combining schemes, ICI in optical attocell networks can be significantly mitigated. Also, a novel double-source cell configuration is proposed. This configuration can further mitigate ICI in optical attocell networks in conjunction with ADRs. Moreover, an analytical framework is proposed to evaluate the user performance in optical attocell networks with ADRs. Finally, optical space division multiple access (SDMA) using angle diversity transmitters is proposed and investigated in optical attocell networks. Optical SDMA can exploit the available bandwidth resource in spatial dimension and mitigate ICI in optical attocell networks. Compared with optical time division multiple access (TDMA), optical SDMA can significantly improve the throughput of optical attocell networks. This improvement scales with the number of LED elements on each angle diversity transmitter. In addition, the upper bound and the lower bound of optical SDMA performance are derived analytically. These bounds can precisely evaluate the performance of optical SDMA systems. Furthermore, optical SDMA is shown to be robust against user position errors, and this makes optical SDMA suitable for practical implementations.