Transmitter diversity in indoor optical wireless communications

© 2021 Tingting SongOptical wireless communications use optical signals or light over the air for the transmission of information. With optical fibre components becoming mature, optical wireless communications operating at several tens of gigabits per second data rates have been explored recently for indoor applications, as an alternative to the most popular forms of wireless communications using radio frequencies. Unlike radio frequency transmission, optical wireless communications provide the unique advantages of offering higher, scalable and license-free communication bandwidth without RF interference. It is a promising solution to support the ever-increasing demand for high-bandwidth across indoor wireless applications, as highlighted by bandwidth challenges faced by people using Wi-Fi networks and mobile broadband networks during the recent COVID-19 pandemic. To realise optical wireless communication link operating at high data rates up to multi Gb/s, the line-of-sight link is preferred due to its high optical power efficiency and low multipath distortion that leads to high signal-to-noise-ratio performance. However, such line-of-sight links are susceptible to the obstructions of the optical beams due to objects or users shadowing the transmission of beams. The primary objective of this thesis is to address these challenges by investigating specific transmitter diversity techniques where multiple transmitters are introduced to improve the indoor optical wireless system performance. For optical wireless links with transmitter diversity, this thesis also investigates the delay-tolerant coding scheme to address the inter-symbol interference caused by multiple signals from transmitters and studies the spatial modulation techniques using multiple-inputs-single-output OWC links to increase the achievable system throughput with transmitter diversity. The thesis starts with the channel gain modelling of the indoor intensity modulation/direct detection-based line-of-sight optical wireless system as the foundation for the transmitter diversity investigation. The systematic investigation of two popular coding schemes, i.e., space-time-block-coding and repetition-coding, is then performed theoretically and experimentally to achieve transmitter diversity. These two schemes provide a fundamental framework to address the optical beam obstruction issue in the line-of-sight channel. Based on the superior performance of the repetition-coding scheme, a generalised delay-tolerant coding technique using the orthogonal-filters-based repetition-coding scheme with on-demand equalisation is further developed. This includes the development of the principle, a detailed theoretical analysis, and experimental demonstration, to tackle the channel delay caused by multi-channel optical wireless transmission. Moreover, to further increase the achievable system throughput in addition to the transmitter diversity, spatial modulation with signal space diversity is further proposed. The transmitter diversity performance of both signal space diversity scheme and the spatial modulation channel index detection impact are investigated using theoretical analysis and experimental demonstration. The thesis is finalised by reviewing all coding schemes’ bit-error-rate performance under different total channel gain distributions emulating various indoor line-of-sight optical wireless channel obstructions. In conclusion, the optical wireless communications with transmitter diversity principles have been investigated via analytical modelling and experimental demonstrations in this thesis. The transmitter diversity coding schemes proposed and investigated here have shown the capability of improving the communication performance and hence, provide a promising pathway to realise resilient optical wireless communications