| 摘要 |
An apparatus and a method for transmitting digital information over an optical channel are disclosed. The apparatus includes a signal generator adapted to process input digital information bits to generate a corresponding electrical orthogonal frequency division multiplexed (OFDM) signal waveform; an optical transmitter having an input adapted to receive the OFDM signal waveform, and configured to apply a modulation to intensity of an optical carrier corresponding with the electrical OFDM signal waveform to produce an optical signal and an optical output port for transmitting the optical signal over the optical channel to a receiver which includes an optical intensity detector. The modulation has an amplitude and a bias level selected such that negative excursions of a waveform of the optical signal are clipped at a minimum output optical power level and the optical signal waveform is asymmetrically-limited in amplitude. The modulation amplitude and bias level are selected such that an increase in the bias level and/or a decrease in the amplitude to reduce clipping at the minimum output optical power level results in a corresponding reduction of quality of an OFDM signal of equivalent optical power detected at the receiver by the optical intensity detector. Further disclosed are an apparatus and a method for receiving digital information transmitted over an optical channel via an optical signal to which an intensity modulation is applied corresponding with an electrical OFDM signal waveform carrying the digital information. The modulation has an amplitude and a bias level selected such that negative excursions of a waveform of the optical signal are clipped at a minimum output optical power level and the optical signal waveform is asymmetrically-limited in amplitude. The apparatus includes an optical intensity detector, configured to detect the optical signal to produce a received electrical signal corresponding with the asymmetrically-limited optical signal waveform; a signal processor operatively coupled to the Intensity detector and adapted to: apply a transformation to the received electrical signal from the time domain to the frequency domain to generate a corresponding plurality of received OFDM symbol values; perform an equalisation of the received OFDM symbol values to produce estimates of corresponding transmitted OFDM symbol values; and recover the digital information from the estimates of transmitted OFDM symbol values. The modulation amplitude and bias level are selected such that an Increase In the bias level and/or a decrease in the amplitude to reduce clipping at the minimum output optical power level results in a corresponding reduction of accuracy of the estimates of transmitted OFDM symbol values for a detected optical signal of equivalent optical power.
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