This thesis develops and investigates enhanced techniques for carrierless amplitude and phase
modulation (CAP) in optical communication systems. The CAP scheme is studied as the
physical layer modulation technique due to its implementation simplicity and versatility, that
enables its implementation as a single carrier (CAP) or multi-carrier technique (m-CAP).
The effect of timing jitter on the error performance of CAP is first investigated. The
investigation indicates that synchronization is a critical requirement for CAP receiver and as
a result, a novel low-complexity synchronization algorithm is developed with experimental
demonstration for CAP-based visible light communication (VLC) systems. To further reduce
the overall link complexity, a fractionally-spaced equalizer (FSE) is considered to mitigate the
effects of inter-symbol interference (ISI) and timing jitter. The FSE implementation, which
eliminates the need for a separate synchronization block, is shown through simulation and
VLC experimental demonstration to outperform symbol-spaced equalizers (SSE) that are
reported in literature for CAP-based VLC systems.
Furthermore, in this thesis, spectrally-efficient index modulation techniques are developed for
CAP. The proposed techniques can be divided into two broad groups, namely spatial index CAP
(S-CAP) and subband index CAP (SI-CAP). The proposed spatial index techniques leverage
the fact that in VLC, multiple optical sources are often required. The spatial CAP (S-CAP)
transmits CAP signal through one of Nt available LEDs. It is developed to reduce equalization
requirement and improve the spectral efficiency of the conventional CAP. In addition to the bits
transmitted through the CAP symbol, the S-CAP encodes additional bits on the indexing/spatial
location of the LEDs. The generalised S-CAP (GS-CAP) is further developed to relax the
S-CAP limitation of using a single LED per symbol duration. In addition to the S-CAP scheme,
multiple-input multiple-output (MIMO) techniques of repetitive-coded CAP (RC-CAP) and
spatial multiplexing CAP (SMux-CAP) are investigated for CAP. Low-complexity detectors
are also developed for the MIMO schemes. A key challenge of the MIMO schemes is that they
suffer power penalty when channel gains are similar, which occur when the optical sources are
closely located. The use of multiple receivers and power factor imbalance (PFI) techniques
are proposed to mitigate this power penalty. The techniques result in significant improvement
in the power efficiency of the MIMO schemes and ensure that the spectral efficiency gain is
obtained with little power penalty.
Finally, subband index CAP (SI-CAP) is developed to improve the spectral efficiency of
m-CAP and reduce its peak-to-average power ratio (PAPR). The SI-CAP encodes additional
information bits on the selection of ‘active’ subbands of m-CAP and only modulate data
symbols on these ‘active’ subbands. The error performance of the proposed SI-CAP is
evaluated analytically and verified with computer-based simulations. The SI-CAP technique is
also experimented for both VLC and step-index plastic optical fibre (SI-POF) communication
links. The experimental results show that for a fixed power efficiency, SI-CAP achieves higher
data rate compared tom-CAP. For example, at a representative bit error rate (BER) of 10-5, the
SI-CAP achieves a data rate and power efficiency gain of 26:5 Mb/s and 2:5 dB, respectively
when compared to m-CAP. In addition, an enhanced SI-CAP (eSI-CAP) is developed to
address the complexity that arises in SI-CAP at higher modulation order. The results of the
experimental demonstrations in VLC and 10 m SI-POF link shows that when compared with
m-CAP, eSI-CAP consistently yields a data rate improvement of between 7% and 13% for
varying values of the SNR
