Hybrid diversity combined OFDM for optical wireless communication

Abstract

Optical Wireless Communication (OWC) in the visible spectrum provides the opportunity of high speed data transmission along with room illumination. In order to encode data for OWC, different variants of Orthogonal Frequency Division Multiplexing (OFDM) modulation are developed in the literature. Some of the most prominent ones are the direct current biased optical OFDM (DCO-OFDM), the asymmetrically clipped optical OFDM (ACO-OFDM), asymmetrically clipped DC biased optical OFDM (ADO-OFDM) and diversity-combined ACO-OFDM (DACO-OFDM). This thesis combines the aspects of DACO-OFDM and DCO-OFDM to develop a new modulation form termed as Hybrid Diversity Combined OFDM (HDC-OFDM) which has the benefits of power efficiency of DACO-OFDM and illumination capacity of DCO-OFDM. In HDC-OFDM transmitter, the lower subcarriers are modulated with DACO-OFDM, and the higher subcarriers with DCO-OFDM. When the DACO-OFDM and the DCO-OFDM signals are added, they impact one another. In order to remove the impact of DCO-OFDM clipping noise on DACO-OFDM component, a high DC bias is applied on DCO-OFDM which results in less clipping noise. On the other hand to reduce the effect of DACO-OFDM clipping noise on DCO-OFDM element, the DACO-OFDM clipping noise is estimated at the receiver and then deducted from the received signal of DCO-OFDM. In order to obtain the optimum bit error rate performance of HDC-OFDM, the percentage of subcarriers and the ratio of power levels for DACO-OFDM and for DCO-OFDM components are varied. Simulation results show that the BER performance of HDC-OFDM is the best when the portion of DACO-OFDM and DCO-OFDM elements are the same. Since the HDC-OFDM has a DCO-OFDM component, the dimming facility is easily achieved in HDC-OFDM by only increasing or decreasing the DC bias level without using any other additional modules or methods. Simulation results demonstrate that a dimming as low as 10% is also possible in HDC-OFDM at a 20 dB optical power degradation with respect to full brightness conditions. Simulation results also show that for a given data rate, the BER performance of HDC-OFDM is 5 dB better than DCO-OFDM, 1 dB better than ACO-OFDM, but only 1.5 dB inferior to DACO-OFDM. Therefore, the proposed HDC-OFDM has slightly less power efficiency than DACO-OFDM but achieves the simple dimming capability of DCO-OFDM.