Signal Transmission and Detection Scheme for Energy and Spectrally Efficient Indoor Optical Wireless Communications

Abstract

The tremendous growth of wireless data traffic and exhaustive utilization of conventional radio frequency (RF) spectrum have motivated the telecom industry and researchers in the field to investigate the potential of alternative spectrum bands for wireless communication. In response to the looming conventional RF spectrum crisis, a lot of attention is given to utilize the spectrum in the range of optical domain for wireless communication. Optical wireless communication (OWC) is widely considered and believed to be the best complement of the conventional RF based wireless communication. In optical spectrum, a huge unregulated frequency resource is available for OWC which is many more times higher than that of the conventional RF spectrum. Intensity modulation and direct detection (IM/DD) type of communication has been confirmed as the most cost effective candidate for OWC. In IM/DD based OWC systems, the information signal has to be real and non-negative. Therefore, further signal processing needs to be carried on the complex bipolar orthogonal frequency division multiplexing (OFDM) signal to generate a real positive unipolar OFDM signal. Orthogonal frequency divisions multiplexing (OFDM) is a widely accepted multicarrier transmission schemes for high speed OWC due to its efficacy of combating inter symbol interference (ISI) and its efficiency of delivering a more efficient communication capacity. Among proposed OFDM schemes for OWC in literatures, asymmetrically clipped optical OFDM (ACO-OFDM) and direct-current-biased optical OFDM (DCO-OFDM) have got a wide acceptance. In spite of their wide acceptance, DCO-OFDM scheme has energy inefficiency issues and the spectral efficiency of ACO-OFDM is not attractive for high speed OWC. In this research work, a stratified asymmetrically clipped optical OFDM (STACO-OFDM) is proposed and investigated for IM/DD based OWC. STACO-OFDM utilizes both even and odd subcarriers for transmission of information bits in stratified fashion. STACO-OFDM follows an optimal design strategy and architecture to improve both spectral and energy efficiency of the system at the same time. Detailed theoretical framework of STACO-OFDM over both linear additive white Gaussian noise (AWGN) and frequency selective multipath channels is given on this thesis. The performance of STACO-OFDM is investigated and compared with both ACO-OFDM and DCO-OFDM schemes with the aid of theoretical and simulation analysis. For equal data rate systems, the theoretical and simulation results have shown that STACO-OFDM scheme can offer better energy efficiency compared to both ACO-OFDM and DCO-OFDM schemes over v linear AWGN and frequency selective multipath channels. Furthermore, for the same used QAM modulations, the spectral efficiency of STACO-OFDM is equivalent to the spectral efficiency offered DCO-OFDM and better than the spectral efficiency of ACO-OFDM scheme. In this thesis, the potential of adaptive optical OFDM to enhance the spectral efficiency of OWC on frequency selective optical wireless channel is also investigated. In particular, the performance of frequency selective rate adaptive M-QAM ACO-OFDM scheme is evaluated using theoretical and simulation analysis. In comparison to fixed M-QAM ACO-OFDM schemes, rate adaptive M-QAM ACO-OFDM scheme has shown better spectral efficiency performances for non-outage OWC systems over frequency selective channels.