Average Power Reduction Using Time-Varying Bias with Bandlimited Pulses in Multiple-Subcarrier Intensity-Modulated Optical Systems

We present a technique for average power reduction in intensity-modulated optical systems using multiple BPSK or QPSK subcarriers with bandlimited pulses, which are, necessarily, not time-limited. In order to insure the non-negativity of the transmitted intensity signal, we use a time-varying bias signal, which is a baseband pulse-amplitude modulation signal also using these pulses. We focus on the problem of designing this time-varying bias signal, which is complicated by the fact that the pulses are not time-limited. We consider pulses whose Fourier transform is a root-raised cosine with rolloff factor , where . We use a heuristic approach in which, during a given symbol interval, the bias amplitude is computed considering the multiple-subcarrier amplitudes during that symbol interval, as well as several neighboring symbol intervals. We consider three cases in detail: with BPSK, with BPSK, and with QPSK. We show that, although using these pulses requires slightly more power than using rectangular pulses, the use of a time-varying bias yields a large gain in average power efficiency over a fixed bias, especially when the number of subcarriers is large.