SPECTRUM AND POWER EFFICIENT OPTICAL OFDM FOR VISIBLE LIGHT COMMUNICATION SYSTEMS

Abstract

The need for wireless connectivity is ever increasing while conventional radio frequency (RF) communications are limited by the amount of available spectrum. Visible light communications (VLC) are emerging as a promising complementary to the RF wireless, thanks to the enormous available bandwidth in the visible spectrum. Moreover, VLC integrates into the ubiquitous illumination infrastructures to satisfy the need for wireless connectivity indoors.

Commercially available light-emitting diodes (LEDs) are low-cost, simple, and have a small modulation bandwidth. For the small modulation bandwidth, VLC systems must enjoy high spectral efficiency to achieve high-rate transmission. Additionally, VLC systems must have high power efficiency to help preserve the critical advantage of LEDs for illumination applications. Furthermore, since LED luminaires are constrained by cost, deployed VLC systems must be low-complexity. Indoor VLC channels are dispersive due to multipath propagation indoors and due to the limited bandwidth of the optoelectronics. However, time-domain equalization on such channels can be prohibitively expensive for long serials. These challenges motivate extensive research on optical orthogonal frequency division multiplexing (OFDM).

Given those problems and challenges, this thesis introduces novel spectrum- and power-efficient optical OFDM modulation schemes to implement with low complexity in VLC systems. Firstly, absolute value layered asymmetrically clipped optical OFDM (ALACO-OFDM) is presented to achieve high spectral efficiency and high power efficiency with fewer layers, thus requiring low complexity. Compared to its counterparts, ALACO-OFDM can achieve higher spectral efficiency and information rate even with fewer layers.

Antisymmetry-constructed clipped optical OFDM (AC-OFDM) is then introduced as a novel low-complexity modulation scheme. To enhance the spectral efficiency and retain low-complexity, layered AC-OFDM (LAC-OFDM) is introduced, consisting of several layers of AC-OFDM signals. LAC-OFDM is shown to be less complex compared to its state-of-the-art counterparts.

Concerning a practical VLC dispersive channel, low-complexity layered ACO-OFDM (L-LACO) is introduced with simple equalization. Mathematically, L-LACO generates identical signals to the existing layered asymmetrically-clipped optical OFDM (LACO-OFDM); however, it requires only half arithmetic operations at both the transmitter and the receiver.

For a practical bandlimited VLC dispersive channel, the previous optical OFDM modulation schemes will be no longer non-negative after interpolation with sinc(t) pulse. This thesis presents Kramers-Kronig (KK) optical OFDM (KKO-OFDM) to enhance the spectral efficiency and power efficiency for such bandlimited VLC channels. The KKO-OFDM transmit signal is constructed to be real-valued, non-negative, and strictly bandlimited. Numerical results show that KKO-OFDM outperforms DCO-OFDM and LACO-OFDM in bandlimited VLC channels in terms of optical power efficiency.