Radio frequency (RF) spectrum congestion motivates the search for alternative communication techniques to complement radio systems. Visible light communications (VLC) is an emerging technology that exploits the recent and ever-growing increase in the usage of energy-efficient light emitting diodes (LEDs) to imperceptibly modulate the optical power output of LEDs to enable communication and augment RF networks.
Orthogonal frequency-divison multiplexing (OFDM) has been proposed as a modulation scheme for VLC due to its high spectral efficiency, ease of channel estimation and equalization, resistance to inter-symbol interference (ISI) and frequency-selective fading, efficient implementation using the Fast Fourier Transform (FFT), and compatibility with RF and power-line communication (PLC) standards that use OFDM.
One of the major drawbacks of conventional OFDM techniques is the high peak-to-average power ratio (PAPR) of OFDM signals. The peaks of the OFDM signals are clipped due to the limited dynamic range of the LED, which translates the high PAPR of the OFDM signal into non-linear distortion (NLD). This signal distortion causes bit-error rate (BER) performance degradation, especially at high optical signal-to-noise ratios (SNRs) typical of indoor VLC scenarios.
In this thesis, a new family of modulation techniques, termed spatial optical OFDM (SO-OFDM), is proposed with the aim of reducing the PAPR of conventional DC-biased optical OFDM (DCO-OFDM) by making use of the large number of LEDs typically available in indoor lighting settings. Each LED group signal is a narrowband signal consisting of a small number of subcarriers, and thus has a smaller PAPR than the original OFDM signal.
Firstly, SO-OFDM is introduced and its two key concepts of frequency-to-space mapping and spatial summing are explained. Frequency-to-space mapping is achieved by allocating a subset of OFDM subcarriers to each LED. Each LED group signal is a narrowband signal consisting of a small number of subcarriers, and thus has a smaller PAPR than the original OFDM signal. Several design variations of the subcarrier assignment to LEDs are introduced and are shown through simulations, to reduce PAPR, and NLD noise due to clipping, and improve the BER performance at high SNRs as compared to DCO-OFDM. In addition, luminous efficacy is identified as an important lighting design parameter that is impacted by modulation. Relative luminous efficacy is defined as the ratio of the luminous efficacy of a modulated LED to that of an LED driven by a DC signal, and is introduced as a metric to assess the impact of modulation on LED lighting. Relative luminous efficacy links communication parameters such as signal variance to lighting design requirements.
Secondly, a low-complexity amplify-and-forward (AF) scheme is proposed for an integrated power-line communication/visible-light communication (PLC/VLC) where SO-OFDM is used for the VLC link. Frequency translation of the incoming PLC signal is used to increase the usable bandwidth of the LED. The use of both frequency translation and SO-OFDM leads to capacity gains over DCO-OFDM in the high SNR regime.
Finally, a low-complexity variant of SO-OFDM, termed square-wave SO-OFDM (SW-SO-OFDM), is proposed. Square-wave SO-OFDM uses square-wave carriers instead of sinusoidal waves to modulate a single OFDM subcarrier signal per LED. By using square-wave carriers, SW-SO-OFDM eliminates the need for digital-to-analog converters (DACs), digital predistortion (DPD), and the FFT operation. Squarewave SO-OFDM is also shown, through simulations, to achieve BER performance gains over SO-OFDM and DCO-OFDM. In addition, an experimental demonstration of SW-SO-OFDM with 64 QAM modulation on subcarriers is described. / Thesis / Doctor of Philosophy (PhD) / Visible-light communications (VLC) is an emerging technology that exploits the increasingly widespread use of light-emitting diodes (LEDs) for indoor lighting, and modulates the optical power output of the LED for data transmission.
Among the various modulation techniques that have been proposed for VLC, orthogonal frequency-division multiplexing (OFDM) offers high data rates, resistance to channel impairments, and simple channel estimation and equalization. However, OFDM signals suffer from a high peak-to-average power ratio (PAPR) which degrades the efficiency of the power amplifier in the transmitter and hinders the communication performance.
In this thesis, a new multiple-LED modulation technique, termed spatial optical OFDM (SO-OFDM), is proposed to reduce the PAPR. Using a frequency-to-space mapping, SO-OFDM divides the wideband high-PAPR OFDM signal into multiple narrowband low-PAPR signals and assigns each signal to a group of LEDs. Spatial summing of the transmitted signals occurs at the receiver allowing for the use of a conventional OFDM receiver. Several variations of SO-OFDM are introduced and are shown, using simulations, to reduce the PAPR, combat non-linear distortion (NLD), and improve the bit-error rate (BER) performance at high signal-to-noise ratios (SNRs), typical of VLC systems.
Spatial optical OFDM is also applied to a practical scenario where its PAPR reduction capability is used to improve the overall capacity of a proposed system that integrates power-line communication (PLC) and VLC.
A low-complexity variant of SO-OFDM, that uses square-wave carriers and simplifies the transmitter design by eliminating the need for digital predistortion (DPD) and digital-to-analog converters (DACs) is also proposed, and tested experimentally.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/26363 |
| Date | January 2021 |
| Creators | Mossaad, Mohammed |
| Contributors | Hranilovic, Steve, Electrical and Computer Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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