Proposal of visible light OFDM system with CAZAC equalization

Abstract

We have proposed a new visible light Orthogonal Frequency Division Multiplex (OFDM) system using the Constant Amplitude Zero Auto-Correlation (CAZAC) equalization. In visible light systems, LEDs or LDs in transmitters have small dynamic range of input signal magnitude for linear operation and the bias current must be carefully set to consider the maximum allowable forward current of LEDs or LDs, to reduce magnitude distortion and to control signal clipping. The OFDM enables the limited frequency bandwidth of LEDs or LDs to be fully-used and the CAZAC equalization drastically reduces magnitude fluctuation of OFDM time-domain signal. We found that one CAZAC sequence in cooperation with IFFT process converts the PAPR of the M-Quadrature Amplitude Modulation (QAM) OFDM signal into the PAPR of an M-QAM single-carrier signal. This fact is our new discovery. Moreover, since the CAZAC equalization has potential capacities of frequency diversity effect, the system has an advantage in improvement of the BER performance under frequency selective fading environments, which also includes the frequency-dependent attenuation due to the frequency response of LEDs and LDs. Furthermore, we verified for 16QAM-OFDM and 16PSK-OFDM, and confirmed that the communication distance is much extended by the CAZAC.