Background Optical Noises Circumvention in LED Optical Wireless Systems Using OFDM

Abstract

Background optical noises generated by the conventional fluorescent light sources or AC-power light-emitting diode (LED) can significantly affect the performance of the LED optical wireless communication and produce challenges to implementation. We demonstrate using orthogonal frequency division multiplexing (OFDM) to effectively circumvent the optical background noises. Besides, by using simple equalization at the receiver side, the transmission capacity can be extended from 1 Mb/s to 12 Mb/s. The theory and analysis of the equalization are presented. Experiments at different data rates and different OFDM subcarriers are preformed, and results show that, by adjusting the number of OFDM subcarriers, the influence of the background optical noises can be significantly circumvented.

Using specific and adaptive arrangement of grid-type pilot in channel estimation for white-lightLED-based OFDM visible light communication system

“…1). When high optical background noise, which usually occupies zero-frequency and low-frequency bands [20], is added in the VLC channel, some affected OFDM subcarriers (with reduced SNR) can be neglected. Hence the grid-type pilot channel estimation scheme can still work.…”

“…Hence, by using spectral efficient OFDM modulation format, the data rate of white-light LED OWC can be enhanced. Besides, according to our previous studies, OFDM can also mitigate the background optical noises, such as that generated from fluorescent lamp and AC-LED [20]; as well as providing an adaptive scheme for maintaining the performance of the in-homewhite-LED [4]. Besides, we have also demonstrated a practical real-timewhite-lightphosphor-LED OWC system for video communication using OFDM format at a transmission distance of 2 m [21].…”

“…Hence, it is regarded as one of the promising access solutions in the future. In the literatures, besides, conventional non-return-to-zero (NRZ) modulation [9][10][11], several modulation schemes can be applied for the LED-based OWC [12][13][14][15][16][17][18][19][20], and they have their merits. For example, Ref.…”

“…Pulse position modulation [14][15][16] can be used for LED-based VLC. Among these modulation schemes, orthogonal-frequency division multiplexing (OFDM) [19,20] is a highly spectral efficient modulation scheme that is suitable for the LED-based OWC since the direct modulation bandwidth of the white-light LED is about 1 MHz [21]. Hence, by using spectral efficient OFDM modulation format, the data rate of white-light LED OWC can be enhanced.…”

“…The idea of training symbol is to use known subcarriers inserted in a symbol to estimate the channel response. Then the parameters obtained from the channel estimation are used at the receiver (Rx) for signal equalization [4,20]. The other channel estimation scheme can by performed by using grid-type pilot scheme.…”