Enhancing communication link performance in visible light communication
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Date
10/07/2017Item status
Restricted AccessEmbargo end date
31/12/2100Author
Li, Yichen
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Abstract
With data throughput increasing exponentially in wireless communication networks, the limited
radio frequency (RF) spectrum is unable to meet the future data rate demand. As a promising
complementary approach, optical wireless communication (OWC) has gained significant attention
since its licence-free light spectrum provides a considerable amount of communication
bandwidth. In conventional OWC systems, the information-carried signal has to be real-valued
and non-negative due to the incoherent light output of the conventional optical transmitter,
light emitting diode (LED). Therefore, an intensity modulation and direct detection (IM/DD)
system is used for establishing the OWC link. Some modified orthogonal frequency division
multiplexing (OFDM) schemes have been proposed to achieve suitable optical signals. In previous
research, three OFDM-based schemes have been presented, including DC-biased optical
orthogonal frequency division multiplexing (DCO-OFDM), asymmetrically clipped optical orthogonal
frequency division multiplexing (ACO-OFDM) and unipolar orthogonal frequency
division multiplexing (U-OFDM).
Basic concepts of SPAD receivers are studied and a novel application in OWC is proposed
for a permanent downhole monitoring (PDM) system in the gas and oil industry. In this thesis,
a complete model of the SPAD-based OWC system is presented, including some related
SPAD metrics, the photon counting process in SPAD and a specific nonlinear distortion caused
by passive quenching (PQ) and active quenching (AQ) recharged circuits. Moreover, a practical
SPAD-based visible light communication (VLC) system and its theoretical analysis are
presented in a long-distance gas pipe with a battery-powered LED and a basic on-off keying
(OOK) modulation scheme.
In this thesis, two novel optical orthogonal frequency division multiplexing (O-OFDM) technologies
are proposed: non-DC-biased orthogonal frequency division multiplexing (NDCOFDM)
and OFDM with single-photon avalanche diode (SPAD). The former is designed for
optical multiple-input multiple-output (O-MIMO) systems based on the optical spatial modulation
(OSM) technique. In NDC-OFDM, signs of modulated O-OFDM symbols and absolute
values of the symbols are separately transmitted by different information carrying units. This
scheme can eliminate clipping distortion in DCO-OFDM and achieve high power efficiency.
Furthermore, as the indices of transmitters carry extra information bits, NDC-OFDM gives a
significant improvement in spectral efficiency over ACO-OFDM and U-OFDM.
In this thesis, SPAD-based OFDM systems with DCO-OFDM and ACO-OFDM are presented
and analysed by considering the nonlinear distortion effect of PQ SPAD and AQ SPAD. A comprehensive
digital signal processing of SPAD-based OFDM is shown and theoretical functions
of the photon counting distribution in PQ SPAD and AQ SPAD are given. Moreover, based on
Bussgang theorem, a conventional method for analysing memoryless distortion, close-formed
bit-error rate (BER) expressions of SPAD-based OFDM are derived. Furthermore, SPAD-based
OFDM is compared with conventional photo-diode (PD) based OFDM systems, and a gain of
40 dB in power efficiency is observed.