The present invention is directed to a method including determining an appropriate power level for a phase modulator for an optimum number of subcarriers; and applying the determined appropriate power level via a controller to produce the optimum number of subcarriers, wherein the optimum number of subcarriers enables an optical-orthogonal frequency division multiplex O-OFDM based variable rate transmitter with automatic control by a controller to produce an optimum setting based on a required rate.