Average-power reduction techniques for multiple-subcarrier optical intensity modulation

We describe two classes of simple, effective techniques for reducing the average optical power requirement in intensity-modulated optical systems using multiple BPSK or QPSK-modulated subcarriers. The first class of techniques involves alteration of the mapping between information bits and the symbol amplitudes modulated onto the subcarriers, in order to increase the minimum value of the MSM electrical waveform. The second class of techniques involves replacing the fixed d.c. bias by a bias signal that varies on a symbol-by-symbol basis. These two classes of techniques can be applied separately or in tandem. The reduction in power requirement increases with the number of subcarriers, and with eight subcarriers, can be as high as about 3.6 dB and 3.2 dB with BPSK and QPSK, respectively. The techniques described here are applicable as long as all subcarriers originate from a single transmitter and are symbol-synchronized.