Advanced modulation and detection techniques for indoor visible light communication systems
Date of Issue2017-11-30
School of Electrical and Electronic Engineering
Illuminating light-emitting diodes (LEDs) enabled visible light communication (VLC) has attracted ever-increasing attention in recent years, due to the rapid development of solid-state lighting technology. The LEDs in general VLC systems play a dual role of providing simultaneous illumination and high-speed wireless data communication in typical indoor environments. Compared with traditional radio-frequency (RF) systems, VLC has many inherent advantages including license-free spectrum, high data rate, cost-effective front-ends, high security, electro-magnetic interference (EMI)-free operation, etc. By exploiting high-speed, bidirectional and fully networked VLC in indoor environments, light-fidelity (Li-Fi) can be realized which is envisioned as a promising complementary technology to the widely used wireless-fidelity (Wi-Fi). Nevertheless, the development and deployment of high-speed and large-coverage indoor VLC systems face many critical issues, such as the small 3-dB modulation bandwidth of commercially available off-the-shelf white LEDs, the limited coverage of each LED access point (i.e., optical attocell) due to the constraints of both illumination and communication, the inter-cell interference (ICI) in indoor multi-cell VLC systems, and the seamless integration of VLC with the last-mile optical access networks for hybrid wired and wireless indoor optical access. The main objectives of this thesis are to address the above issues, and propose and develop advanced transmission, modulation and detection techniques to improve the performance of VLC systems. Multiple-input multiple-output (MIMO) transmission and orthogonal frequency division multiplexing (OFDM) are two effective technologies to improve the capacity of indoor VLC systems. Due to the intensity modulation and direct detection (IM/DD) nature of LEDs based VLC systems, Hermitian symmetry (HS) is usually imposed before performing the inverse fast Fourier transform (IFFT), so as to obtain LED-compatible real-valued OFDM signals. However, imposing HS doubles the size of IFFT/FFT and hence the complexity of OFDM transmitters/receivers. To address this problem, firstly, this thesis proposes a non-HS OFDM (NHS-OFDM) scheme for indoor MIMO-VLC systems. By transmitting the real and imaginary parts of a complex-valued OFDM signal via a pair of white LEDs, NHS-OFDM circumvents the HS constraint. Analytical and experimental results show that an indoor MIMO-VLC system using NHS-OFDM achieves superior bit error rate (BER) performance than the same system using conventional HS based OFDM (HS-OFDM), resulting in improved communication coverage. Moreover, the impact of LED pairing on the performance of NHS-OFDM based indoor MIMO-VLC systems is also analyzed. In indoor MIMO-VLC systems, line-of-sight (LOS) transmission is usually dominant and the optical channel gains between one LED and two closely placed photo-detectors (PDs) could be very similar, leading to high spatial correlation that might severely degrade the performance of MIMO-VLC systems. In order to reduce channel correlation and improve multiplexing gain, imaging receivers (ImRs) have been applied in MIMO-VLC systems to replace widely used non-imaging receivers (NImRs). However, the field-of-view (FOV) of conventional ImRs is relatively small which limits the coverage of an indoor MIMO-VLC system. Therefore, an imaging angle diversity receiver (ImADR) is further proposed and investigated for indoor MIMO-VLC systems. By using angle diversity PDs instead of vertically oriented PDs, the proposed ImADR has a much wider FOV and achieves higher optical channel gain than the conventional ImR. Analytical and simulation results reveal that an indoor MIMO-VLC system using the proposed ImADR can achieve significantly improved communication coverage than the same system employing a conventional ImR. In practical scenarios, an indoor VLC system usually consists of multiple cells, i.e., optical attocells, so as to achieve full coverage of a typical indoor environment. However, ICI is a major issue that could greatly degrade the performance of indoor multi-cell VLC systems. So far, many frequency division based ICI mitigation techniques have been proposed such as the RF subcarrier allocation technique, OFDM or discrete multi-tone (DMT) enabled dynamic subcarrier allocation technique, etc. Although ICI can be mitigated by using these schemes, spectrum partitioning is required which could substantially reduce the achievable capacity of each cell. In order to efficiently mitigate ICI without losing the achievable cell capacity, this thesis also proposes an angle diversity multi-element receiver (ADMER) based ICI mitigation technique for indoor multi-cell VLC systems. Compared with the conventional frequency division based ICI mitigation techniques, the proposed ADMER enabled ICI mitigation technique enjoys three main advantages including improved signal-to-interference-and-noise ratio (SINR), reduced SINR fluctuation and higher cell capacity. Next generation access networks are expected to provide high-speed hybrid wired and wireless services for end users. OFDM based passive optical network (OFDM-PON) has been considered as a promising candidate for high-speed wired access due to its low cost, high spectral efficiency, robustness to chromatic dispersion, and flexibility of bandwidth allocation. As an alternative and complementary technology to RF based indoor wireless communication, VLC can provide high-speed and EMI-free indoor optical wireless access. Therefore, in order to integrate VLC with OFDM-PON systems for indoor hybrid wired and wireless optical access, an integrated VLC and OFDM-PON system is proposed and investigated, by using an adaptive envelope modulation technique.