Orthogonal frequency division multiplexing for indoor visible light communication links

Visible light communications (VLC) is an attractive technique that uses white light emitting diodes (WLEDs) to provide illumination, data communications and indoor localization. The modulation bandwidth of WLEDs is limited to a few MHz, due to the long photoluminescence lifetime of yellow phosphors. Orthogonal frequency division multiplexing (OFDM) has been extensively investigated in VLC systems to compensate the limited LED bandwidth, along with the capability of improving the performance at frequencies beyond the 3dB modulation bandwidth. In intensity modulation and direct detection (IM/DD) based VLC systems, the complex and bipolar signal formats, such as the traditional OFDM, cannot be used. Therefore, a DC-biased optical OFDM (DCO-OFDM), and asymmetrically clipped optical OFDM (ACO-OFDM) are proposed. However, the use of DCO-OFDM comes at the cost of reduced power efficiency, while the adoption of ACO-OFDM results in reducing the spectral efficiency. Despite the OFDM advantages, its performance is adversely affected by non-linear distortion (from LEDs) due to its high peak to average power ratio (PAPR). Recently, interleaving frequency division multiple access (IFDMA) used in IM/DD is been proposed as a modified optical OFDM scheme to address the PAPR issue. However, this scheme is still not satisfying the PAPR reduction level as compared with the single carrier modulation such as OOK. Therefore, the two main aims of this research are to increase the ACO-OFDM data rates as well as to further improve the IFDMA PAPR for IM/DD based VLC systems. The first aim is achieved by reducing the ACO-OFDM symbol duration, where only the first half of the asymmetrical ACO-OFDM time domain symbol is used to carry the data samples, after inverting its negative samples to positive ones to meet the IM/DD requirements. Two schemes have been proposed in this research to identify the positions of the inverted samples at the receiver side. The schemes are known as the pilot aided ACO-OFDM (PA-ACO-OFDM) scheme and the position encoded-ACO-OFDM (PE-ACO-OFDM) scheme. Simulation results show that data rates of the proposed schemes are enhanced by up to 33% compared to the traditional ACO-OFDM scheme. The second aim is achieved by adapting the IFDMA scheme for IM/DD based VLC systems in three innovative ways, resulting in a new unipolar modified OFDM scheme known as unipolar pulse amplitude modulation frequency division multiplexing (U-PAM-FDM), and two bipolar modified OFDM schemes known as the bipolar pulse amplitude modulation frequency division multiplexing (B-PAM-FDM), and the optical interleaved frequency division multiplexing (OIFDM) schemes. As such UPAM-FDM proposed to address the PAPR of the unipolar OFDM schemes (i.e. ACO-OFDM) while the B-PAM-FDM and OIFDM are presented to reduce the PAPR of the bipolar OFDM schemes (i.e. DCO-OFDM). Simulation results show that the PAPR of ACO-OFDM is reduced by 3.8 dB when U-PAM-FDM is considered. They also demonstrate that the PAPR value of DCO-OFDM is reduced by 7 dB and 10 dB when B-PAM-FDM and OIFDM are implemented respectively. Furthermore, the reduction in the PAPR value for U-PAM-FDM and B-PAM-FDM has been experimentally investigated, where the results show that the B-PAM-FDM and U-PAM-FDM can provide 2 dBm and 3 dBm higher average transmitted power compared to ACO-OFDM and DCO-OFDM respectively. For B-PAM-FDM, further investigation has been experimentally carried out in terms of the achievable maximum distance between the transmitter (Tx) and the receiver (Rx). The results show that a 44% increase in the distance between the Tx and the Rx can be attained in comparison with DCO-OFDM.