Optimum Signal Shaping in OFDM-Based Optical Wireless Communication Systems

In this paper, a framework for optimum signal shaping in multi-carrier modulation is presented for optical wireless communications (OWC). The two fundamental multi-carrier transmission schemes based on orthogonal frequency division multiplexing (OFDM), direct-current-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM), are studied. The optimum signal shaping is defined as optimum biasing and optimum scaling of the time domain signal within the optical power constraints of the transmitter front-end. These include the boundaries of the limited linear dynamic range, such as minimum and maximum radiated optical power, and the desired average optical power level. As a result, the minimum required electrical signal-to-noise ratio (SNR) to maintain a target bit-error ratio (BER) is obtained for a desired multi-level quadrature amplitude modulation (M-QAM) scheme and a given combination of optical power constraints. The average optical power is varied over dynamic ranges of 10 dB, 20 dB and 30 dB. With the increase of the dynamic range and for a major portion of the average optical power levels, DCO-OFDM demonstrates a lower minimum electrical SNR requirement for a target BER as compared to ACO-OFDM for modulation orders with similar spectral efficiencies.