| Author | Sohail, Muhammad |
| Call Number | AIT Diss no.TC-14-01 |
| Note | A dissertation submitted in partial ful llment of the requirements for the
degree of Doctor of Philosophy in Telecommunications, School of Engineering and Technology |
| Publisher | Asian Institute of Technology |
| Abstract | Signal voltage modulation has traditionally been used to support data transmis-
sions in Optical Wireless Communications (OWC). However, for Orthogonal Frequency
Division Multiplexing (OFDM) which is a continuous modulation scheme, the trans-
mission performance relies on a linear modulation range. For a Light Emitting Diode
(LED)-based OWC transmitter, the relationship between the input modulating voltage
and the output light intensity is nonlinear and may degrade the Bit Error Rate (BER)
performance. Instead of voltage modulation for OFDM-based OWC, which requires
signal distortion removal, this thesis focuses on current modulation. For current mod-
ulation, the output light intensity from an LED is linearly proportional to the input
current over a modulating range that is typically much wider than that for voltage
modulation. BJT-based and MOSFET-based transconductance circuits are considered
to implement current modulation on an LED-based transmitter. In particular, BJT-
based transconductance circuits with feedback are analyzed, yielding a circuit design
that can be used for current modulation with a suciently wide linear range and is
insensitive to variation in the BJT's forward current gain. System level simulation is
carried out to show the bene ts of current modulation over voltage modulation. To fur-
ther investigate the use of OFDM, three unipolar OFDM schemes, namely DC-biased
OFDM (DCO-OFDM), Asymmetrically Clipped Optical OFDM (ACO-OFDM), and
ip-OFDM are investigated for OWC. In particular, for Visible Light Communications
(VLC) with strong illumination, the signal Dynamic Range (DR), which is constrained
by the linear modulation range, is the main limitation instead of the transmit optical
power. For the considered unipolar OFDM schemes, BER expressions in terms of the
DR are derived. Numerical results suggest that DCO-OFDM is more attractive then
ACO-OFDM and
ip-OFDM when the DR is the main limitation. Finally, light dim-
ming is considered to identify the illumination level below which DCO-OFDM is no
longer attractive for VLC. |
| Year | 2014 |
| Type | Dissertation |
| School | School of Engineering and Technology (SET) |
| Department | Department of Information and Communications Technologies (DICT) |
| Academic Program/FoS | Telecommunications (TC) |
| Chairperson(s) | Poompat Saengudomlert ;Sterck, Karel Leopold ; |
| Examination Committee(s) | Teerapat Sanguankotchakorn;Mongkol Ekpanyapong;Sibley, Martin ; |
| Scholarship Donor(s) | University of Balochistan, Quetta,
Pakistan ;AIT Fellowship; |